Science Opportunities Provided by NASA's Constellation System

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Launching Science: Science Opportunities Provided by NASA's Constellation System

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Launching Science Science Opportunities Provided by NASA’s Constellation System

Committee on Science Opportunities Enabled by NASA’s Constellation System

Space Studies Board Aeronautics and Space Engineering Board

Division on Engineering and Physical Sciences

THE NATIONAL ACADEMIES PRESS 500 Fifth Street, N.W. Washington, DC 20001

NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

This study is based on work supported by Contract NNH06CE15B between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the agency that provided support for the project.

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COVER: Images courtesy of NASA. Design by Tim Warchocki.

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OTHER REPORTS OF THE SPACE STUDIES BOARD AND THE AERONAUTICS AND SPACE ENGINEERING BOARD

Assessing the Research and Development Plan for the Next Generation Air Transportation System: Summary of a Workshop (Aeronautics and Space Engineering Board [ASEB], 2008) A Constrained Space Exploration Technology Program: A Review of NASA’s Exploration Technology Devel- opment Program (ASEB, 2008) Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft: Elements of a Strategy to Recover Measurement Capabilities Lost in Program Restructuring (Space Studies Board [SSB], 2008) Final Report of the Committee for the Review of Proposals to the 2008 Engineering Research and Commercial- ization Program of the Ohio Third Frontier Program (ASEB, 2008) Final Report of the Committee to Review Proposals to the 2008 Ohio Research Scholars Program of the State of Ohio (ASEB, 2008) Managing Space Radiation Risk in the New Era of Space Exploration (ASEB, 2008) NASA Aeronautics Research: An Assessment (ASEB, 2008) Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity (SSB, 2008) Review of NASA’s Exploration Technology Development Program: An Interim Report (ASEB, 2008) Science Opportunities Enabled by NASA’s Constellation System: Interim Report (SSB with ASEB, 2008) Space Science and the International Traffic in Arms Regulations: Summary of a Workshop (SSB, 2008) United States Civil Space Policy: Summary of a Workshop (SSB with ASEB, 2008) Wake Turbulence: An Obstacle to Increased Air Traffic Capacity (ASEB, 2008)

Assessment of the NASA Astrobiology Institute (SSB, 2007) An Astrobiology Strategy for the Exploration of Mars (SSB with the Board on Life Sciences [BLS], 2007) Building a Better NASA Workforce: Meeting the Workforce Needs for the National Vision for Space Explora- tion (SSB with ASEB, 2007) Decadal Science Strategy Surveys: Report of a Workshop (SSB, 2007) Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond (SSB, 2007) Exploring Organic Environments in the Solar System (SSB with the Board on Chemical Sciences and Tech nology, 2007) Grading NASA’s Solar System Exploration Program: A Midterm Review (SSB, 2007) The Limits of Organic Life in Planetary Systems (SSB with BLS, 2007) NASA’s Beyond Einstein Program: An Architecture for Implementation (SSB with the Board on Physics and Astronomy [BPA], 2007) Options to Ensure the Climate Record from the NPOESS and GOES-R Spacecraft: A Workshop Report (SSB, 2007) A Performance Assessment of NASA’s Astrophysics Program (SSB with BPA, 2007) Portals to the Universe: The NASA Astronomy Science Centers (SSB, 2007) The Scientific Context for Exploration of the Moon (SSB, 2007)

Limited copies of these reports are available free of charge from

Space Studies Board National Research Council The Keck Center of the National Academies 500 Fifth Street, N.W., Washington, DC 20001 (202) 334-3477/[email protected] www.nationalacademies.org/ssb/ssb.html

NOTE: These reports are listed according to the year of approval for release, which in some cases precedes the year of publication.

COMMITTEE ON SCIENCE OPPORTUNITIES ENABLED BY NASA’S CONSTELLATION SYSTEM

GEORGE A. PAULIKAS, The Aerospace Corporation (retired), Chair KATHRYN C. THORNTON, University of Virginia, Vice Chair CLAUDIA ALEXANDER, Jet Propulsion Laboratory STEVEN V.W. BECKWITH, University of California System MARK A. BROSMER, The Aerospace Corporation JOSEPH BURNS, Cornell University CYNTHIA CATTELL, University of Minnesota ALAN DELAMERE, Ball Aerospace and Technology Corporation (retired) MARGARET FINARELLI, George Mason University TODD GARY, Tennessee State University STEVEN HOWELL, National Optical Astronomy Observatory ARLO LANDOLT, Louisiana State University FRANK MARTIN, Martin Consulting SPENCER R. TITLEY, University of Arizona CARL WUNSCH, Massachusetts Institute of Technology

Staff DWAYNE A. DAY, Study Director VICTORIA SWISHER, Research Associate CATHERINE A. GRUBER, Assistant Editor RODNEY N. HOWARD, Senior Project Assistant LEWIS GROSWALD, Lloyd V. Berkner Space Policy Intern

SPACE STUDIES BOARD

CHARLES F. KENNEL, Scripps Institution of Oceanography, University of California, San Diego, Chair A. THOMAS YOUNG, Lockheed Martin Corporation (retired), Vice Chair DANIEL N. BAKER, University of Colorado STEVEN J. BATTEL, Battel Engineering CHARLES L. BENNETT, Johns Hopkins University YVONNE C. BRILL, Aerospace Consultant ELIZABETH R. CANTWELL, Oak Ridge National Laboratory ANDREW B. CHRISTENSEN, Dixie State College and Aerospace Corporation ALAN DRESSLER, The Observatories of the Carnegie Institution JACK D. FELLOWS, University Corporation for Atmospheric Research FIONA A. HARRISON, California Institute of Technology JOAN JOHNSON-FREESE, Naval War College KLAUS KEIL, University of Hawaii MOLLY K. MACAULEY, Resources for the Future BERRIEN MOORE III, University of New Hampshire ROBERT T. PAPPALARDO, Jet Propulsion Laboratory JAMES PAWELCZYK, Pennsylvania State University SOROOSH SOROOSHIAN, University of California, Irvine JOAN VERNIKOS, Thirdage LLC JOSEPH F. VEVERKA, Cornell University WARREN M. WASHINGTON, National Center for Atmospheric Research CHARLES E. WOODWARD, University of Minnesota ELLEN G. ZWEIBEL, University of Wisconsin

MARCIA S. SMITH, Director

AERONAUTICS AND SPACE ENGINEERING BOARD

RAYMOND S. COLLADAY, Lockheed Martin Astronautics (retired), Chair CHARLES F. BOLDEN, JR., Jack and Panther, LLC ANTHONY J. BRODERICK, Aviation Safety Consultant AMY L. BUHRIG, Boeing Commercial Airplanes PIERRE CHAO, Center for Strategic and International Studies INDERJIT CHOPRA, University of Maryland, College Park ROBERT L. CRIPPEN, Thiokol Propulsion (retired) DAVID GOLDSTON, Princeton University JOHN HANSMAN, Massachusetts Institute of Technology PRESTON HENNE, Gulfstream Aerospace Corporation JOHN M. KLINEBERG, Space Systems/Loral (retired) RICHARD KOHRS, Independent Consultant IVETT LEYVA, Air Force Research Laboratory, Edwards Air Force Base EDMOND SOLIDAY, United Airlines (retired)

MARCIA S. SMITH, Director

vii

Preface

In January 2004 NASA was given a new policy direction known as the Vision for Space Exploration. That plan, now renamed the United States Space Exploration Policy, called for sending human and robotic missions to the Moon, Mars, and beyond. In 2005 NASA outlined how to conduct the first steps in implementing this policy and began the development of a new human-carrying spacecraft known as Orion, the lunar lander known as Altair, and the launch vehicles Ares I and Ares V. Collectively, these are called the Constellation System. In November 2007 NASA asked the National Research Council (NRC) to evaluate the potential for new science opportunities enabled by the Constellation System of rockets and spacecraft: The Space Studies Board, in conjunction with the Aeronautics and Space Engineering Board, will establish an ad hoc committee to assess potential space and Earth sciences mission concepts that could take advantage of the capabilities of the Constellation System of launch vehicles and spacecraft that is being developed by NASA. The ad hoc committee will first analyze mission concepts provided by NASA, and later, mission concepts submitted in response to a request for information [RFI] from the committee to the space and Earth science communities. The committee will analyze the following information for each mission concept considered: 1. Scientific objectives of the mission concept; 2. A characterization of the mission concept insofar as the maturity of studies to date have developed it; 3. The relative technical feasibility of the mission concepts compared to each other; 4. The general cost category into which each mission concept is likely to fall; 5. Benefits of using the Constellation System’s unique capabilities relative to alternative implementation approaches; and 6. Identification of the mission concept(s) most deserving of future study.

The time horizon for the survey of possible missions should extend from 2020 to approximately 2035.

For the interim report the committee will assess the mission concepts provided by NASA and group them into two categories: more deserving and less deserving of future study. For the final report the committee will assess the set of mission concepts submitted in response to an RFI and group them into similar categories. The final report should then compare the mission concepts in the more-deserving categories for the interim and final reports and recommend a consolidated list of the mission concept(s) it deems most deserving of future study for launch in the 2020-2035 time frame.

PREFACE

The NRC formed the Committee on Science Opportunities Enabled by NASA’s Constellation System to address this task. The committee released its interim report in May 2008, evaluating 11 future space science mission concepts provided by NASA. The committee has since evaluated 6 additional mission concepts, submitted in response to its request for information, and produced an integrated list of all of the missions that it evaluated (provided in Table S.1 in the Summary in this report). The committee chose 12 of the 17 to evaluate as types of missions that could be conducted using Constellation (see Chapter 2). The remaining 5 missions are summarized in Appendix B. During the course of its deliberations, the committee concluded that the Constellation System’s capability would be enhanced for many missions by technology developments such as aerocapture. In addition, several of the missions evaluated may be possible without employing the Constellation System, provided that alternative propulsion technologies are developed. Because many of these missions share technology requirements (such as improved communications capabilities), the committee addressed these issues in this report. The committee acknowledges the assistance that it received from NASA, particularly from Marc Allen, Marc Timm, Tibor Kremic, and Phil Sumrall, and those working on the Ares V launch vehicle, in providing materials for this study. The committee also thanks those who presented their mission-concept studies at the various committee meetings. The committee held four meetings in 2008, as follows: on February 20-22 in Washington, D.C.; on March 17- 19 in Irvine, California; on June 9-11 in Boulder, Colorado; and on August 4-6 in Woods Hole, Massachusetts.

National Research Council, Science Opportunities Enabled by NASA’s Constellation System: Interim Report, The National Academies Press, Washington, D.C., 2008. Committee member Steven Beckwith was a member of a team that contributed to one of the proposals submitted in response to the request for information. He had not actively participated on the team for nearly a year prior to the submission of the proposal and was unaware that it was being submitted. After learning about the submission of the proposal, he resigned from the team and recused himself from any deliberations on the proposal. Aerocapture is an orbit insertion maneuver that takes advantage of a planet’s atmosphere to decelerate a spacecraft sufficiently to allow it to be placed into its intended orbit.

Acknowledgment of Reviewers

This report has been reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council’s (NRC’s) Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the institution in making its published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their participation in the review of this report:

Peter Banks, Astrolabe Ventures, Robert Braun, Georgia Institute of Technology, Margaret Kivelson, University of California, Los Angeles, Richard Kohrs, NASA (retired), Emery Reeves, United States Air Force Academy (retired), Norman Sleep, Stanford University, and Charles Woodward, University of Minnesota.

Although the reviewers listed above have provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations, nor did they see the final draft of the report before its release. The review of this report was overseen by Martha P. Haynes, Cornell University. Appointed by the NRC, she was responsible for making certain that an independent examination of this report was carried out in accordance with institutional procedures and that all review comments were carefully considered. Responsibility for the final content of this report rests entirely with the authoring committee and the institution.

Contents

SUMMARY 1

1 THE CONSTELLATION SYSTEM AND OPPORTUNITIES FOR SCIENCE 9 The Relationship Between Launch Vehicle Size and Mission Cost, 10 Opportunities for Science, 13 Other Possible Missions, 14 The Constellation System and Earth Science, 15 International Cooperation, 16 Cautionary Tales: The Voyager-Mars Mission and Project Prometheus, 17

2 ANALYSIS OF SPACE SCIENCE MISSION STUDIES 20 Background and Approach, 20 Evaluation Criteria and Results, 22 The Sun-Earth Lagrangian Point, 26 Astronomy and Astrophysics Missions, 26 Advanced Technology Large-Aperture Space Telescope (ATLAST), 26 8-Meter Monolithic Space Telescope, 29 Dark Ages Lunar Interferometer (DALI), 32 Generation-X, 36 Modern Universe Space Telescope, 38 Astronomy and Astrophysics/Heliophysics Mission, 41 Stellar Imager, 41 Heliophysics Missions, 45 Solar Probe 2, 45 Interstellar Probe, 48 Solar Polar Imager, 52 Solar System Exploration Missions, 56 Exploration of Near Earth Objects via the Crew Exploration Vehicle, 56 Neptune Orbiter with Probes, 60 Titan Explorer, 63

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xiv CONTENTS

3 TECHNOLOGY REQUIREMENTS FOR FUTURE SPACE MISSIONS 67 In-Space Propulsion Technologies, 70 Electric Propulsion, 70 Solar Sails, 75 Aerocapture, 76 Propulsion System Technology Summary, 81 The Deep Space Network, 81

4 HUMAN AND ROBOTIC SERVICING OF FUTURE SPACE SCIENCE MISSIONS 83 Human Servicing of Spacecraft, 84 Robotic Servicing of Spacecraft, 91 Successful Robotic Servicing Missions, 91 Future Servicing Capabilities, 92 Future Human and Robotic Servicing Options, 93 Advantages of Servicing Missions, 94

5 LAUNCH VEHICLE AND SPACECRAFT OPTIONS FOR FUTURE SPACE 97 SCIENCE MISSIONS The Orion Spacecraft, 97 Opportunities for Small Space Science Payloads Aboard Constellation, 98 Ares I, 99 Ares V, 102 Delta IV, 103 Atlas V, 105 Ares Development Risks, 105 Vehicle Performance Comparisons, 108 Ares I, 108 Ares V, 109 Concerns Regarding the Ares V Shroud, 110

APPENDIXES

A Letter of Request from NASA 115 B Summary Analysis of Mission Concepts That Would Not Benefit from the Constellation System 117 C Request for Information 133 D Definitions for Technology Readiness Levels 136 E Committee and Staff Biographical Information 138

Summary

In 2004 NASA began implementation of the first phases of a new space exploration policy. This implementation effort included the development of a new human-carrying spacecraft, known as Orion; the Altair lunar lander; and two new launch vehicles, the Ares I and Ares V rockets—collectively called the Constellation System (described in Chapter 5 of this report). The Altair lunar lander, which is in the very preliminary concept stage, is not discussed in detail in this report. In 2007 NASA asked the National Research Council (NRC) to evaluate the science opportunities enabled by the Constellation System. To do so, the NRC established the Committee on Sci- ence Opportunities Enabled by NASA’s Constellation System. In general, the committee interpreted “Constellation- enabled” broadly, to include not only mission concepts that required Constellation, but also those that could be significantly enhanced by Constellation. The committee intends this report to be a general overview of the topic of science missions that might be enabled by Constellation, a sort of textbook introduction to the subject. The mission concepts that are reviewed in this report should serve as general examples of kinds of missions, and the committee’s evaluation should not be construed as an endorsement of the specific teams that developed the mission concepts or of their proposals. Additionally, NASA has a well-developed process for establishing scientific priorities by asking the NRC to conduct a “decadal survey” for a particular discipline. Any scientific mission that eventually uses the Constellation System will have to be properly evaluated by means of this decadal survey process. The committee was impressed with the scientific potential of many of the proposals that it evaluated. However, the committee notes that the Constellation System has been justified by NASA and selected in order to enable human exploration beyond low Earth orbit—not to enable science missions. Virtually all of the science mission concepts that could take advantage of Constellation’s unique capabilities are likely to be prohibitively expensive. Several times in the past NASA has begun ambitious space science missions that ultimately proved too expensive for the agency to pursue. Examples include the Voyager-Mars mission and the Prometheus program and its Jupiter Icy Moons Orbiter spacecraft (both examples are discussed in Chapter 1).

Finding: The scientific missions reviewed by the committee as appropriate for launch on an Ares V vehicle fall, with few exceptions, into the “flagship” class of missions. The preliminary cost estimates, based on mission concepts that at this time are not very detailed, indicate that the costs of many of the missions analyzed will be above $5 billion (in current dollars). The Ares V costs are not included in these estimates.

See http://www.whitehouse.gov/space/renewed_spirit.html.

LAUNCHING SCIENCE

All of the costs discussed in this report are presented in current-year (2008) dollars, not accounting for potential inflation that could occur between now and the decade in which these missions might be pursued. In general, preliminary cost estimates for proposed missions are, for many reasons, significantly lower than the final costs. Given the large cost estimates for many of the missions assessed in this report, the potentially large impacts on NASA’s budget by many of these missions are readily apparent.

SCIENCE MISSIONS THAT ARE ENABLED OR ENHANCED BY THE CONSTELLATION SYSTEM The committee evaluated a total of 17 mission concepts for future space science missions (11 were “Vision Missions” studied at the initiation of NASA between 2004 and 2006; the remaining 6 were submitted to the committee in response to its request for information). The committee based its initial evaluation of each mission concept on two criteria: (1) whether the concept offered the potential for a significant scientific advance and (2) whether or not the concept would benefit from the Constellation System. The committee determined that all of the concepts offered the possibility of a significant scientific advance, but it cautions that such an evaluation ultimately must be made by the NRC’s decadal survey process referred to above. This report’s evaluations should not be considered to be an endorsement of the scientific merit of these proposals, which must of course be evaluated relative to other proposals. The committee determined that 12 of the 17 mission concepts would benefit from the Constellation System, whereas 5 would not. See Table S.1 for a summary of the mission concepts, including their cost estimates, technical maturity, and reasons why they might benefit from the Constellation System. The five mission concepts that the committee deemed not worthy of further study as Constellation missions according to its evaluation criteria simply do not require, or do not appear to benefit highly from, use of the Con- stellation System (see Table S.1). In several cases they should easily fit within existing launch vehicles. In one case, that of Super-EUSO (Extreme Universe Space Observatory), the committee questions the cost-effectiveness of a flagship-class space mission as compared with the expansion of existing ground-based facilities. Notably, the committee did not receive any proposals in the Earth sciences. The committee lacked sufficient data to determine why it did not receive any such proposals, although it notes that the Vision Mission effort that sponsored many of the mission concepts evaluated in this study did not include Earth science, which at the time was separated organizationally within NASA from space science. It is possible that, if invited to consider the matter, the Earth science community may find uses for Constellation that are not readily apparent.

Finding: The committee did not receive any Earth science proposals and found it impossible to assess the potential of the Constellation System to meet the future needs of Earth-oriented missions.

The mission concepts reviewed during this study lacked the level of detail necessary for a full evaluation. In particular, the cost estimates were extremely rough. The lack of Earth science concepts also concerned the committee. NASA is still in the early stages of identifying the potential benefits of the Constellation System to the space science program and has not made a dedicated effort to evaluate the potential of the Constellation System for space and Earth science missions. As a result, the committee determined that the agency needs to continue efforts to attract and advance ideas for space and Earth science missions in general, and should develop a method for soliciting potential mission concepts.

Recommendation: NASA should solicit further mission concepts that are most likely to benefit from the capabilities of the Constellation System in each of the space and Earth science disciplines: astronomy and astrophysics, Earth science, heliophysics, and planetary science. The agency should seek mission concepts that are studied in a uniform manner with regard to design, system engineering, and costing.

In its interim report, the committee selected 7 of the 11 Vision Mission concepts as “worthy of further study as a Constellation mission.” See National Research Council, Science Opportunities Enabled by NASA’s Constellation System: Interim Report, The National Academies Press, Washington, D.C., 2008.

SUMMARY

The committee focused on the 12 mission concepts that, as shown in Table S.1, it determined are worthy of further study as Constellation missions. Because the committee was charged with determining which studies are “most deserving” of further study, it divided the list of 12 mission concepts into “more deserving” and “deserving” categories. All 12 of these concepts show great promise, but the committee determined that, as indicated in the recommendations below, several in particular serve as examples of what Constellation could provide to space science. The committee’s criteria for determining if a mission concept is more deserving or simply deserving of further study are as follows:

• Criterion 1: Mission Impact on Science in the Field of Study—The mission concept must present well- articulated science goals that the committee finds compelling and worthy of the investment needed to develop the technology. • Criterion 2: Technical Maturity—The mission concept must be sufficiently mature in its overall conception and technology. If the technology for accomplishing the mission does not currently exist at a high technology readiness level, the mission must provide a clear path indicating how it will be developed.

If a mission concept satisfied both criteria to a moderate or high degree, it was designated more deserving of further study. (These criteria are fully explained in Chapter 2.) As a result of these evaluations, the committee identified five missions that it determined are more deserving of further study.

Recommendation: NASA should conduct further study of the following mission concepts, which have the most potential to demonstrate the scientific opportunities provided by the Constellation System: 8-Meter Monolithic Space Telescope, Interstellar Probe, Neptune Orbiter with Probes, Solar Polar Imager, and Solar Probe 2.

Several of the missions named above, particularly the heliophysics missions, are well defined scientifically and do not require significant study of instruments or related issues. Further study should focus primarily on the relationship between the Ares V capabilities and the missions’ propulsion requirements. Because these are narrow requirements, NASA may have the ability to give further study to other possible Ares V science missions that the committee placed in the “deserving” category. The seven missions in the “deserving” category are also promising and offer great potential science return, but greater amounts of effort will be required to bring them to a similar level of maturity.

Recommendation: NASA should consider further study of the following mission concepts: Advanced Technology Large-Aperture Space Telescope, Dark Ages Lunar Interferometer, Exploration of Near Earth Objects via the Crew Exploration Vehicle, Generation-X, Modern Universe Space Telescope, Stellar Imager, and Titan Explorer.

Two missions that were placed in the category of “deserving” to be considered for further study did not receive the higher rating (i.e., they were not placed in the “more deserving” category) for reasons largely beyond the control of the proposing teams. Exploration of Near Earth Objects using astronauts is an intriguing and exciting potential future use of the Constellation System. This mission also has significant exploration benefits. Because exploration benefits were not part of the evaluation criteria, the committee could not place this mission in the “more deserving” category despite its strengths. Similarly, the Titan Explorer mission concept evaluated for this report was developed before Cassini reached Saturn, so it reflects an older series of science assumptions and questions; Ares V has great potential for Titan missions.

MISSION COSTS The committee accepted the cost estimates provided in the proposals themselves or by the study representatives who presented the proposals to the committee, but with some modifications based on the expertise of the

LAUNCHING SCIENCE

TABLE S.1 Summary of Mission Concepts Evaluated by the Committee Cost Worthy Estimatea of Further (billions of Study as a current-year Constellation Mission [2008] $) Technical Maturityb Mission? Notes Advanced ∼1 Medium No This mission does not benefit from the Constellation Compton System. It can fit in an existing Evolved Expendable Telescope (ACT)c Launch Vehicle (EELV). Advanced >5 Low for mirror Yes The 16-meter folded telescope design can only fit in an Technology technology Ares V payload fairing. Large-Aperture (including mass) Space Telescope Medium for detectors (ATLAST)d and thermal control Dark Ages Lunar >5 Medium for rovers and Yes The large antennas must be landed on the lunar farside. Interferometer interferometrics This requires both the Ares V launch vehicle and the (DALI)d Low for reducing mass Altair lunar lander. and for deploying and operating in a remote location 8-Meter 1-5 High for mirror and Yes The 8-meter-diameter telescope can only fit inside an Monolithic Space structure Ares V payload fairing. Telescoped Low for coronagraphic observation Exploration >5 High for instruments Yes The Orion vehicle is the only U.S. spacecraft envisioned of Near Earth Low for human factors that will be capable of operating beyond low Earth Objects via the such as radiation orbit. The mission also will require substantial payload Crew Exploration capability. This mission fits better within the purview of Vehicled the Exploration Systems Mission Directorate than as a mission of the Science Mission Directorate. Generation-X >5 Low for mirror Yes One Ares V launch of one 16-meter telescope (Gen-X)c development and is significantly simpler than the early proposed operations configurations. The cost estimates are weak. The additional mass capability could significantly reduce mirror development costs. Interstellar Probec 1-5 High for science, Yes Further study is needed of the benefits of Ares V—in instruments, and particular, of alternative propulsion options. mission concept Kilometer-Baseline >5 Low No This mission should be able to fit on an existing Far-Infrared/ EELV; therefore the need for Constellation is Submillimeter questionable, except for human servicing. Interferometerc Modern Universe >5 High for instruments Yes A large, one-piece central mirror rather than a Space Telescope Low for coronagraph robotically assembled mirror is possible with Ares V. (MUST)c and mirror assembly Neptune Orbiter >5 High for mission Yes Ares V could possibly obviate the need for aerocapture with Probesc concept and and/or nuclear-electric propulsion. instruments Low for propulsion and possibly lander Palmer Questc >5 Low No This mission does not benefit from Constellation. It can fit in an existing EELV.

SUMMARY

TABLE S.1 Summary of Mission Concepts Evaluated by the Committee Cost Worthy Estimatea of Further (billions of Study as a current-year Constellation Mission [2008] $) Technical Maturityb Mission? Notes Single Aperture >5 Medium for mission No This mission does not benefit from Constellation. It Far Infrared concept can fit in an existing EELV. However, it could benefit (SAFIR) Low for cooling and from human servicing. Telescopec detectors Solar Polar ∼1 High for instruments Yes Propulsion options enabled by Ares V should be Imagerc Propulsion not studied considered. in sufficient detail Solar Probe 2d 1-5 High for science, Yes Ares I and Ares V launch vehicles could enable instruments, and spacecraft to be placed in an orbit that could bring it mission concept close to the Sun, accomplishing the major science goals. Stellar Imagerc >5 Low for formation Yes Larger mirrors (2 meters versus 1 meter) and a second flying hub could be launched on a single Ares V launch. Super-EUSO 1-5 Low for mirror No This mission does not benefit from Constellation. (Extreme Significant advances in this science can be made using Universe Space ground-based and alternative approaches. Observatory)d Titan Explorerc >5 High for instruments Yes Launch on Ares V may enable propulsive capture Medium for blimp rather than aerocapture and may shorten transit time. NOTE: The mission concepts are listed in alphabetical order. All of the missions listed are robotic missions, with the exception of the proposal for Exploration of Near Earth Objects via the Crew Exploration Vehicle. aCost estimates are based on data estimates provided to the committee, with modifications based on expertise within the committee. bTechnical maturity is based on data provided to the committee. cThis is 1 of 11 Vision Mission studies initiated by NASA between 2004 and 2006. dThis study proposal was submitted in response to the committee’s request for information.

committee. Nevertheless, the committee concluded that these cost estimates are preliminary and are likely to be significantly lower than the actual cost of the missions. The committee is concerned that even according to the preliminary estimates, the costs of these missions will be as high as those of flagship-class missions (i.e., several billion dollars each), if not substantially higher than previous flagship-class missions. The committee was asked to consider missions that could be flown during the period 2020 to 2035; very few such large missions could possibly be funded during that period. However, the committee also heard arguments that the larger payload capability of the Ares V could also possibly balance increased costs by simplifying mission design. Many of the mission concepts evaluated in this study do not require the full mass capabilities of the Ares V, and it is therefore possible that mission concepts could make use of these capabilities to reduce mission cost. This subject remains conjectural and therefore requires further study.

Recommendation: NASA should conduct a comprehensive systems-engineering-based analysis to assess the possibility that the relaxation of weight and volume constraints enabled by Ares V for some space science missions might make feasible a significantly different approach to science mission design, development, assembly, integration, and testing, resulting in a relative decrease in the cost of space science missions.

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INTERNATIONAL COOPERATION Virtually all of the mission concepts evaluated by the committee are large, complex, and costly. Several are similar to studies currently being undertaken by traditional international partners of the U.S. space program in space science and exploration. As a result, there are opportunities for NASA to undertake joint missions in some of these areas.

Finding: International cooperation could provide access to international scientific expertise and technology useful for large, complex, and costly mission concepts and could reduce costs through provision of instruments and infrastructure by international partners.

TECHNOLOGY ISSUES The committee was charged with identifying the benefits of using the Constellation System’s unique capabilities relative to alternative implementation approaches. Such approaches include technologies that may allow a mission to be accomplished without the Constellation System, such as the Atlas and Delta launch vehicles that were used as the baseline for many of the Vision Mission studies that the committee evaluated. Such approaches also include technologies like in-space propulsion that might not be necessary if a launch vehicle such as the Ares V is available. The committee notes that the majority of mission concepts evaluated in this study (the NASA-funded Vision Missions) were originally designed to use launch vehicles—the Atlas and Delta—often in combination with technology options (such as ion propulsion) that were necessary because of the lack of mass or change in velocity provided by those launch vehicles. The Constellation System may offer an alternative to those launch vehicles and technologies. During this study, the committee concluded that even the Constellation System alone might be insufficient for some of the missions that it evaluated, and that additional technological developments would be required. NASA currently lacks a technology development strategy for science missions, a gap previously identified by the NRC as a shortcoming, and the committee concluded that some of the missions would be enhanced with the availability of additional technology developments.

Finding: Advanced in-space propulsion technology may be required for some science missions considered for using the Constellation System.

Virtually all of the missions evaluated in this report would introduce substantial new demands on the Deep Space Network (DSN). The committee was briefed on the current demands and plans for the DSN and became concerned about the future of the DSN, but determined that this subject was beyond the committee’s base of expertise or purview. Nevertheless, future Constellation science missions will have a major impact on the DSN. (Technology issues are further discussed in Chapter 3.)

Finding: Science missions enabled by the Constellation System will increase the strain on the capabilities of the Deep Space Network.

HUMAN AND ROBOTIC SERVICING Various proposers of observatory mission concepts suggested to the committee that large, expensive observatories might benefit from servicing, which would allow them to operate for decades and to be upgraded with the latest instruments. The Orion spacecraft, unlike the space shuttle, offers the possibility of human servicing of spacecraft beyond low Earth orbit, although it lacks the mass and volume required to conduct such missions alone. However, recent developments in robotic servicing also demonstrate that this technology is now reaching a

National Research Council, Grading NASA’s Solar System Exploration Program: A Midterm Review, The National Academies Press, Washington, D.C., 2008, pp. 11 and 59-61.

SUMMARY

mature stage and could provide an alternative method of servicing future spacecraft. (Human and robotic servicing issues are discussed in Chapter 4.)

Finding: The Constellation System and advanced robotic servicing technology make possible the servicing and in-space assembly of large spacecraft.

Finding: Designing spacecraft components for accessibility is essential for in-space servicing and is also advantageous for preflight integration and testing.

The committee was informed that one of the lessons that NASA has learned from decades of spacecraft servicing is that it is far easier to service spacecraft specifically designed for access and easy replacement of equipment. This approach has other benefits as well, such as prelaunch servicing and maintenance that may be required during integration and testing. However, because NASA largely abandoned the concept of the human servicing of spacecraft and because robotic servicing was not a developed technology, for many years the agency did not consider designing new spacecraft that could benefit from servicing. The new capabilities provided by the Constellation System and robotic servicing technologies highlight the importance of devoting new attention to this subject.

Recommendation: NASA should study the benefits of designing spacecraft intended to operate around Earth or the Moon, or at the libration points for human and robotic servicing.

SPACECRAFT AND LAUNCH VEHICLES The alternative implementation approaches that the committee was charged with evaluating include technologies that allow the use of launch vehicles smaller than Ares V. Although the Ares V offers significant capabilities not available from other vehicles, the Ares I launch vehicle does not offer capabilities significantly different from those currently available with the Evolved Expendable Launch Vehicle (EELV) family of launch vehicles for science missions. (Launch vehicles are discussed in Chapter 5.) The Ares I is required for launching the Orion spacecraft, and so any science missions that require astronauts will use the Ares I.

Finding: The Ares I will not provide capabilities significantly different from those provided by existing launch vehicles.

Although the Orion spacecraft is being designed primarily for transporting astronauts to and from the Interna- tional Space Station and to and from the Moon, it will possess additional capabilities, such as the ability to carry secondary payloads, including deployable satellites. During the Apollo program, the Apollo service module was equipped with a bay for carrying science instruments for use while the spacecraft was in orbit around the Moon. NASA is currently seeking to incorporate a similar capability in the Orion spacecraft and has provided for mass and volume reserves in its current design. Although the Ares V offers the greatest potential value to science, the launch vehicle must be made capable of accommodating science payloads. Science missions are more likely to take advantage of the Ares V if these capabilities are designed into the vehicle rather than their needing to be added later. A potentially serious issue for using Ares V for planetary missions concerns the need for a dedicated upper stage to provide high excess escape velocities for spacecraft (velocity squared per second squared, known as C3). Neither the current most likely upper stage, the Atlas V Centaur III Dual Engine Configuration, nor the previous Titan IV Centaur would make efficient use of the Ares V payload shroud volume and may present other design problems such as load (weight)-bearing capability (see Figure S.1). Planetary missions could better use an upper stage that is shorter and takes advantage of the full width of the Ares V; however, the development of such a stage

C3 is km2/s2 the square of the hyperbolic excess velocity—in other words, the amount of velocity that the vehicle can provide to the spacecraft beyond that needed to escape Earth’s gravitational field.

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4.44 m [ 14.6

7.50 m [ 24.6

12.4 m [ 40.8 ft] 9.70 m [ 31.8

8.80 m 8.80 m [ 28.9 [ 28.9 ft] Baseline Extended Shroud Shroud

FIGURE S.1 Two possible configurations of the Ares V shroud—the current baseline shroud and a proposed extended shroud. Shown inside the shrouds are two possible Centaur upper-stage configurations: the Titan IV Centaur (left) and the Atlas V Centaur III Dual Engine Configuration (right). Any spacecraft carried atop an upper stage would have severely restricted volume constraints. Neither shroud option takes advantage of the width of the Ares V shroud. SOURCE: Adapted courtesy of NASA.

could be expensive. In order for Ares V to be attractive for future science missions, vehicle designers will have to consider the requirements of potential science missions.

Recommendation: If NASA wishes to use the Constellation System for science missions, it should preserve the capability for Orion to carry small scientific payloads and should ensure that the Ares V development team considers the needs of scientific payloads in system design.

The Constellation System offers great potential for space science missions, but the costs of the types of missions evaluated in this report may be unaffordable. Many of these missions have such large costs that they might require that funds be taken from numerous other, smaller science missions, which could create imbalances in the science programs in the individual disciplines. These missions will have to be evaluated carefully within the NRC’s decadal survey process. NASA will have to proceed with caution as it develops these new capabilities.

1 The Constellation System and Opportunities for Science

NASA asked the National Research Council (NRC), through the Space Studies Board and the Aeronautics and Space Engineering Board, to establish an ad hoc committee to assess potential space and Earth science research concepts that could take advantage of the capabilities of the Constellation System of launch vehicles and spacecraft and could launch in the period between 2020 and 2035. The Constellation System is being developed by NASA to implement the initial phases of the Vision for Space Exploration. The system consists of the Orion spacecraft, the Altair lunar lander, and the Ares I and Ares V launch vehicles (for an overview of the system, see Chapter 5). In response to NASA’s request, the NRC established the Committee on Science Opportunities Enabled by NASA’s Constellation System. The committee conducted its assessment in two phases: (1) an interim report was released in May 2008, (2) followed by the completion of this final report. This report is not intended as a review or endorsement of the Constellation System as a whole or of any of its elements, nor is the report intended to make any suggestions regarding potential changes to the Constellation System. Rather, this report accepts the Constellation System as it was defined for the committee, and seeks to assess a set of proposed science missions and to identify those that could benefit from Constellation’s capabilities and would potentially fly sometime after the next decade. In addition, this report does not prioritize science goals for NASA, a responsibility that lies with the NRC’s decadal survey process. This report consists of five chapters. Chapter 1 provides introductory and background information and a discussion of potential costs and benefits of Constellation science missions. Chapter 2 consists of summaries and evaluations of 12 different science mission concepts reviewed by the committee. It should be noted that this report treats these concepts as examples of potential missions, rather than as specific mission proposals. They are as follows:

• Advanced Technology Large-Aperture Space Telescope (ATLAST), • Dark Ages Lunar Interferometer (DALI),

National Research Council, Science Opportunities Enabled by NASA’s Constellation System: Interim Report, The National Academies Press, Washington, D.C., 2008. Marc Timm, NASA, “Constellation Overview,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 2008. The decadal survey process is the method by which the National Research Council, through its study committees, advises NASA, the National Science Foundation, and other government agencies on science priorities in particular disciplines at roughly one-decade intervals, looking out through the next decade. The astronomy and astrophysics decadal survey has been conducted since the 1960s. The first decadal surveys in solar system exploration, heliophysics, and Earth science were commissioned within the past decade.

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• 8-Meter Monolithic Space Telescope, • Exploration of Near Earth Objects via the Crew Exploration Vehicle, • Generation-X (Gen-X), • Interstellar Probe, • Modern Universe Space Telescope (MUST), • Neptune Orbiter with Probes (2 studies), • Solar Polar Imager, • Solar Probe 2, • Stellar Imager, and • Titan Explorer.

Chapter 3 provides a discussion of technology that may be required by many of these missions and an overview of NASA technology development efforts relevant to the mission concepts that were evaluated. Chapter 4 addresses the potential of human and robotic servicing of space missions, discussing relatively new capabilities that Constellation and other projects will provide. Finally, Chapter 5 provides an overview of the Orion spacecraft, the Constellation launch vehicles, and alternative launch vehicles available to space science missions. The committee chose the 12 concepts listed above to evaluate as types of missions that could be conducted using Constellation (see Chapter 2). The committee determined that the 5 mission concepts listed below would not be enabled by Constellation (these 5 concepts are summarized in Appendix B):

• Advanced Compton Telescope (ACT), • Kilometer-Baseline Far-Infrared/Submillimeter Interferometer, • Single Aperture Far Infrared (SAFIR) Telescope, • Solar System Exploration/Astrobiology Vision Mission (Palmer Quest), and • Super-EUSO (Extreme Universe Space Observatory).

THE RELATIONSHIP BETWEEN LAUNCH VEHICLE SIZE AND MISSION COST The kinds of proposed science missions presented to the committee for evaluation for this report are relatively large and ambitious. They all fall into a category that is generally referred to as flagship-class missions. The committee grouped the missions into three “cost bins” for the purpose of this study: those with preliminary cost estimates of less than $1 billion, of $1 billion to $5 billion, and of more than $5 billion. Of the mission concepts evaluated, only one was considered to be marginally in the “less-than-$1 billion” category, and seven were considered to be in the “more-than-$5 billion” category, which would make them larger than any other space science mission developed by NASA to date.

The committee notes that expensive space science programs will place a great strain on the space science budget, which has been essentially flat for several years and is already under strain from an ambitious slate of 85 flight missions. To estimate the costs of potential, large space science missions, the committee used NASA’s Advanced Mis- sions Cost Model and estimated the costs of three new-design planetary science missions representing the three

NASA’s science program as of Spring 2008 consisted of 94 flight missions—53 in operation plus 41 in development (Alan Stern, NASA Science Mission Directorate, presentation to the Space Studies Board, March 10, 2008).

THE CONSTELLATION SYSTEM AND OPPORTUNITIES FOR SCIENCE 11

FIGURE 1.1 Estimated spacecraft costs as a function of payload dry mass for three classes of difficulty for solar system exploration missions, using NASA’s Advanced Missions Cost Model. NOTE: For comparison, the Cassini-Huygens mission’s dry mass was approximately 2.5 metric tons (the cost of the Cassini-Huygens mission was approximately $2 billion in 1997 dollars, and may not reflect the total cost of the Figuremission owing 1.1.eps to European Space Agency involvement). Note that the Ares V capabilities would actually spanBitmap a number of payload image masses - in Lo this figure.w resolutio For example, Aresn V could launch 10 metric tons to Uranus or Neptune and significantly more to Jupiter and Mars.

levels of technological difficulty used in NASA’s model. Illustrated in Figure 1.1, the results indicate the correlation between rising payload mass and higher cost. The committee was not provided cost estimates of either the Ares I or the Ares V launch vehicle, but Ares V can be expected to be more expensive than even the largest Evolved Expendable Launch Vehicle (EELV) currently in the U.S. inventory—the Delta IV Heavy, with a launch cost of about $250 million. The committee notes that the combined effect of expensive payloads and expensive launchers would distort the balance of the space science program. The advent of the Ares V boosters promises to enable very large, very heavy science payloads to be delivered to orbits of interest. The Ares V lift capability is expected to be about five times larger than that of the current largest U.S. booster—the Delta IV Heavy. Similarly, the very large payload shroud of the Ares V will be capable of housing payload volumes some three times larger than are currently feasible with a Delta IV. Detailed comparisons between the capabilities expected of the boosters in the Constellation System and those possible with the current family of boosters are provided in Chapter 5.

The Advanced Missions Cost Model is available at http://cost.jsc.nasa.gov/AMCM.html. The committee bases the conclusion that the Ares V will be significantly more expensive than the Delta IV Heavy on several factors, but one of the most straightforward is counting the number of engines: the Delta IV Heavy has three RS-68 main engines, whereas the Ares V is expected to have six RS-68s, plus two 5.5-segment solid rocket boosters.

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The various mission proposals discussed in this report have identified multiple ways to take advantage of the increased lift capability and payload volume afforded by the Ares V. Those benefits can be loosely grouped in three areas: simplification of spacecraft or mission design; elimination of the reliance on advanced, immature technologies; and increased scientific capabilities. The Large Ultraviolet/Optical Modern Universe Space Telescope (LUVO-MUST) and the 8-Meter Monolithic Space Telescope are examples of reduced complexity of spacecraft design, where the increased payload volume of the Ares V would obviate the need for complex packaging and on- orbit system deployment. For Generation-X, the Ares V would eliminate the need for multiple launches, complex packaging, and on-orbit assembly. The reliance on technologies such as aerocapture, nuclear electric propulsion, solar electric propulsion, or solar sails can potentially be eliminated from missions such as the Solar Polar Imager, Interstellar Probe, Titan Explorer, and Neptune Orbiter with Probes. The 16-meter telescope identified in the ATLAST proposal would take advantage of the added capabilities of the Ares V to increase the scientific capability of the mission, whereas the DALI mission and Exploration of Near Earth Objects via the Crew Exploration Vehicle could only be accomplished with the Ares V system. The variety of ways in which the scientific teams that developed the mission concepts took advantage of the Ares V calls into question the long-held belief that increased payload capability leads to larger payloads resulting in higher mission costs. Long experience has demonstrated that there is a relationship between the mass of a payload and its cost—bigger payloads inevitably cost more than smaller payloads do. In addition, payload mass often expands to fill the available launch vehicle capability. However, it is likewise clear that complexity of spacecraft design is also a critical cost driver. These relationships are captured in NASA’s Advanced Missions Cost Model. The question is then whether programs can be carefully managed within cost budgets, or whether the appetite for increased scientific knowledge will drive system growth to the physical limits of the launch system. If the former is true, the curves depicted in Figure 1.1 would no longer apply. An increased weight allowance, for example, could be used to reduce complexity, thus changing the slope of the cost-versus-weight curve drastically, and possibly resulting in a negative slope. The three ultraviolet (UV)/optical telescope options presented to the committee are clear examples of the cost-versus-capability tug-of-war. The baseline concept for MUST, a 10-meter-diameter telescope to be flown on a Delta IV Heavy, required complex packaging with on-orbit, robotic assembly. The 8-Meter Monolithic Space Telescope built on this concept, optimizing the design to take full advantage of the payload volume and lift capability of the Ares V to minimize the design complexity and reduce reliance on advanced technologies in order to minimize total development costs. On the opposite end of the cost-versus-capability spectrum is ATLAST. Recognizing the significant increase in scientific value afforded with larger space telescopes, the ATLAST team developed a concept for a 16-meter segmented mirror design that takes full advantage of both the lift capability and payload volume of the Ares V (see Figure 1.2). The added design complexity associated with packaging and deployment, as well as the lightweight mirrors, would significantly increase the cost of the mission. According to the Advanced Missions Cost Model, a very-low-complexity design that fully utilizes the lift capability of the Ares V (such as the 8-Meter Monolithic Space Telescope) would cost on the order of $3 billion, whereas a highly complex design (such as ATLAST) could easily cost in excess of $10 billion. Virtually all of the mission concepts evaluated by the committee are large, complex, and costly. The capabilities of the Ares V will enable even larger, more complex, and more capable systems than these—systems that can dramatically increase scientific return. With the advent of the Ares V, the challenge for program managers will be to temper the appetites of scientists who will clearly recognize the dramatic scientific benefits enabled by the launch system. There will need to be an enforced paradigm shift where cost, rather than launch system capability, is the design limiter. The committee was unable to locate any systematic engineering analysis that tested the proposition that relaxation of weight and volume constraints will lead to relatively cheaper payloads. Prior spaceflight experience demonstrates that payloads fill all available weight and volume budgets (and power and telemetry budgets as well)

LUVO is considered to be a class of telescopes, and MUST is a specific mission-concept proposal within that class. See http://cost.jsc.nasa.gov/AMCM.html.

THE CONSTELLATION SYSTEM AND OPPORTUNITIES FOR SCIENCE 13

FIGURE 1.2 An example of the growth in space telescope apertures. The Hubble Space Telescope (HST) has been in orbit since 1990. The James Webb Space Telescope (JWST) has substantially increased area. Two proposals, the 8-Meter Monolithic Space Telescope and the Advanced Technology Large-Aperture Space Telescope (ATLAST) 16-meter telescope, demonstrate the significant growth in aperture made possible by theFigure Ares V launch 1.2.eps vehicle. SOURCE: Courtesy of NASA. Bitmap image - Low resolution

and that heroic measures are frequently undertaken to reduce weights sufficiently to fit within the launch vehicle’s capabilities. However, the differences between the capabilities of a Delta IV Heavy, for example, and those of an Ares V are so large that, indeed, a new regime of payload construction might be possible. The testing of this proposition by means of a comprehensive, systems-engineering-based analysis merits NASA’s attention.

OPPORTUNITIES FOR SCIENCE Exciting new science may be enabled by the increased capability of Ares V. The larger launch mass, large volume, and increased C3 capability are only now being recognized by the science community. (C3 is km2/s2 the square of the hyperbolic excess velocity, in other words, the amount of velocity that the vehicle can provide to the spacecraft beyond that needed to escape Earth’s gravitational field.) During the course of this study, NASA sponsored several workshops on the potential of Ares V for future science missions. The first workshop, Ares V Astronomy Workshop, sponsored by the NASA Ames Research Center and held April 26-27, 2008, focused on the Ares V and possible astronomy missions, the majority of which are also evaluated in this study. The second workshop, Ares V Solar System Science Workshop, also sponsored by Ames and held August 16-17, 2008, focused on the Ares V and potential solar system exploration.

See http://event.arc.nasa.gov/aresv/ and http://event.arc.nasa.gov/aresv-sss/index.php?fuseaction=home.home. The Astronomy Workshop is summarized in S. Langhoff, D. Lester, H. Thronson, and R. Correll, eds., Workshop Report on Astronomy Enabled by Ares V, NASA/CP-2008- 214588, August 2008, available at http://event.arc.nasa.gov/main/home/reports/CP-2008-214588-AresV.pdf. The Solar System Workshop is summarized in S. Langhoff, T. Spilker, G. Martin, and G. Sullivan, eds., Workshop Report on Ares V Solar System Science, NASA/CP-2008- 214592, August 2008, available at http://event.arc.nasa.gov/main/home/reports/CP-2008-214592-AresV-SSS_Print.pdf.

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The mission concepts that the committee evaluated, along with the criteria used to rank them, are described in Chapter 2. In general, the mission concepts fall into two broad groups—those involving the universe and those involving our solar system. The universe group includes large-aperture telescopes or arrays of telescopes leading to the “Greater Observatories”—that is, the astronomical instruments that will succeed the Hubble Space Telescope, the Chandra X-ray Observatory, the Spitzer Space Telescope, and the Compton Gamma-Ray Observatory. The primary factors for most of the missions in the universe group are the volume and mass availability of the Ares V. For the solar system group of missions, the primary driver is the C3 capability. The astronomy and astrophysics community has long desired increasingly larger apertures for its observatories and naturally had several concepts for large-aperture space telescopes that could take advantage of the capabilities of a large launch vehicle like the Ares V (for instance, see Figure 1.2). However, other space science communities have been more constrained by the capabilities of the existing family of launch vehicles and have only recently begun to consider the potential of the Ares V. Because NASA is only beginning to study the potential of this new launch vehicle for science missions, other mission concepts may emerge that are as exciting as the ones identified in this report. The committee notes that it received three separate proposals for large UV/optical telescopes to be flown at the Earth-Sun Lagrangian point L2, essentially an eventual successor to the highly successful Hubble Space Telescope. Two of these proposals, ATLAST and MUST, presented a science case that listed several general astrophysics themes as their goals. (For example: How are galaxies formed and how do they evolve?) However, the mission concepts did not provide detailed answers to show how the specific observations would be made and the scientific results that their respective missions would uniquely provide to answer these questions. The committee observes that in the past, merely stating general science goals for a larger telescope, suggest- ing that the well-known “discovery space” results will be enough to justify the mission cost and launch, has been insufficient to elevate UV/optical telescopes to high-priority science missions. A more effective approach requires specifying high-priority science questions to be answered and directly identifying how a proposed mission and instruments will answer the questions. As indicated in its evaluations in Chapter 2, the committee ranked the 8- Meter Monolithic Space Telescope higher than ATLAST and MUST because it provides the best possibility of extrasolar planet imaging and spectroscopy, a high-interest topic with paradigm-changing possibilities. This mission has the additional benefit that many new “discovery space” science results will also occur. If the scientific community seeks to develop a new large UV/optical space telescope to follow Hubble, it must clearly identify significant scientific questions that the telescope will answer and a path for answering them, and not rely on the vague assertion, however accurate, that a larger aperture will automatically result in further discoveries.

OTHER POSSIBLE MISSIONS As mentioned above, after this study had started, NASA undertook several workshops that paralleled the work of the committee.10 The Ares V Astronomy Workshop focused on the Ares V and possible astronomy missions, most of which this study also evaluates. The Ares V Solar System Science Workshop focused on the Ares V and potential solar system exploration. With the exception of the mission concept for sending an Orion spacecraft to a near-Earth object (evaluated in this study), there were few well-defined mission concepts discussed by the participants. This highlights the fact that the planetary science community has had less incentive or opportunity to consider the capabilities presented by a significantly larger launch vehicle than has the astronomy and astrophysics community. There are several reasons for this, but one primary explanation is that astronomers are always interested in larger-aperture telescopes and therefore ready for larger launch vehicles, whereas planetary science missions are more tightly constrained not only by payload shrouds but also by the energy required to reach distant targets. The planetary scientists at NASA’s Ares V Solar System Science Workshop discussed ideas that generally split into two different themes: (1) maximizing the science capabilities of a mission (i.e., filling the available volume and mass of the Ares V) and (2) finding methods of adding relatively inexpensive capabilities to missions in the current cost categories. One of the latter ideas was the concept of “cheap, dumb mass” that could be added

10 See http://event.arc.nasa.gov/aresv/ and http://event.arc.nasa.gov/aresv-sss/index.php?fuseaction=home.home.

THE CONSTELLATION SYSTEM AND OPPORTUNITIES FOR SCIENCE 15

to a spacecraft, such as additional fuel, radiation shielding, or an impactor such as that used for the Deep Impact and the Lunar Crater Observation and Sensing Satellite (LCROSS) missions. Other participants proposed adding moderate additional capabilities to missions, such as increasing the aperture size of an imager or adding a relatively inexpensive icy moons gravity-measuring spacecraft to a Europa or Jupiter system mission. Many of the comments suggested that although the additional C3 capability of an Ares V would be beneficial in a number of cases, some of this capability could be traded for incremental improvements in instrumentation as well. Ares V’s mass capabilities also offer potential for sample-return missions. Mars seems to be the most likely beneficiary, although only one Mars sample-return concept was discussed at the workshop. The Ares V Solar System Science Workshop participants were particularly intrigued by the possibility that Ares V could enable sample-return missions to the outer solar system. Although the participants discussed Europa and Titan missions, several participants noted that one possibility was a mission to Enceladus.11 In 2007, NASA sponsored a study of a potential Enceladus flagship-class mission that included a possible sample-return option. But the incredibly long duration of this mission (18 to 20 years), plus the high velocity at which a collection spacecraft would fly through the vapor plumes at Enceladus (probably destroying the samples), made this concept unlikely. Ares V offers the possibility of flying a fast trajectory to Enceladus, slowing down to collect the samples, and then accelerating back to Earth. What both the Ares V Astronomy Workshop and Ares V Solar System Science Workshop also highlighted was the requirement for incorporating certain features into the Ares V that would enable science missions. The astronomy community stressed the need for a clean launch shroud environment (so as not to contaminate optics, either on the ground, during launch, or at fairing separation) and noise and vibration levels equivalent to or better than those on the space shuttle. The planetary science community emphasized requirements for removing the heat produced by radioisotope thermoelectric generators (RTGs) inside the shroud, as well as on-pad access (i.e., shortly before launch) to the spacecraft inside the shroud. Because virtually all planetary missions require some sort of upper stage, and because NASA recently increased the overall length of Ares V, the Ares V Solar System Science Workshop participants were particularly concerned about the amount of usable volume inside the shroud. According to one rough estimate, adding a Centaur upper stage to Ares V would leave only 4 meters from the top of the Centaur to the top of the shroud—significantly less than the length of a flagship-class spacecraft such as Cassini. (The Ares V height cannot be significantly increased because of size limitations in the Vehicle Assembly Building.)

THE CONSTELLATION SYSTEM AND EARTH SCIENCE The mission concepts evaluated in this report represent only a subset of the potential missions that could benefit from the Constellation System. During the course of this study, no Earth science mission concepts were proposed to the committee. A proposed NASA workshop on the Constellation System and Earth science was canceled due to lack of interest, and only the most basic ideas about using Constellation for Earth science were discussed at the Ares V Solar System Science Workshop. The majority of the mission concepts evaluated in this study are the result of NASA’s Vision Mission effort, which did not include Earth science because at that time Earth science was separated organizationally within NASA from space science. Had NASA included Earth sciences in the Vision Mission studies it might have generated advanced concepts that could also benefit from the Constellation System. However, the committee lacked data for determining why it did not receive any Earth science responses to its request for information. The committee suspects that although there may be potential Earth science uses for the Constellation System, no such uses are currently apparent. The explanation may simply be that there were no concepts sufficiently mature for presentation to the committee.

Finding: The committee did not receive any Earth science proposals and found it impossible to assess the potential of the Constellation System to meet the future needs of Earth-oriented missions.

11 See http://www.lpi.usra.edu/opag/Enceladus_Public_Report.pdf.

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INTERNATIONAL COOPERATION The original NASA solicitation for the Vision Missions—mission concepts for future space science missions studied at the initiation of NASA between 2004 and 2006—did not require that respondents address opportunities for international cooperation. In reviewing many of the individual Vision Missions, however, the committee noted parallel or complementary activities on the part of potential international partners. For example, the Advanced Compton Telescope (see Appendix B), which does not require an Ares V-class launch vehicle, does require a low- inclination orbit such as could be provided by the European Space Agency’s (ESA’s) Ariane V launch vehicle. Similarly, both the Interstellar Probe and the Solar Polar Imager correspond to missions that are under study by the ESA. It is beyond the scope of this report to make recommendations regarding international cooperation, but the committee nevertheless considers it important to point out that due to cost, many of these extremely large missions would probably require some degree of international cooperation or collaboration in order to be feasible. The cost, schedule, and performance implications of such international cooperation are not considered in this report.

BOX 1.1 Cautionary Tale 1: The Voyager-Mars Mission

In 1960, NASA began establishing long-term plans for lunar and planetary missions. The Voyager-Mars program arose from these early concepts, prompted by a desire to explore the solar system and reach Mars before the same goal was achieved by the then-USSR. The Voyager-Mars mission was a Jet Propulsion Labo- ratory (JPL)-led program (not to be confused with the Voyager outer planets program of the 1970s).1 NASA selected Avco and General Electric to propose ambitious robotic Mars exploration spacecraft. While their proposals differed, both companies proposed spacecraft equipped with biological equipment for life detection, geophysical-geological tools to explore seismic activity and planetary composition, and atmospheric devices. General Electric proposed two landers and one orbiter, whereas Avco suggested one lander and one orbiter. In 1964, NASA managers decided to set a launch date for Voyager. A test flight would take place in 1969, followed by two launches in 1971 and four in 1973. The total projected cost of the mission was $1.25 billion, making it the most expensive robotic spacecraft program proposed by NASA at the time. Mariner 4 conducted a Mars flyby in 1964. Contrary to NASA’s expectations about the red planet, Mariner 4 gave scientists a bleak prognosis with respect to the chances that there had been life on Mars. In addition, atmospheric occultation data indicated a surface pressure of 5 to 7 millibars, whereas Voyager had been designed for a surface pressure of 10 or 11 millibars. The thinner atmosphere necessitated reworking the lander’s atmospheric entry method. From the beginning there was friction between NASA Headquarters and JPL over the design of the mission. NASA Headquarters ordered that the Saturn IB-Centaur rocket that was part of the original design proposal be changed to a Saturn V. However, the Saturn V possessed substantially more payload capability than that required for the Voyager spacecraft. The cost of the Saturn V rocket was also projected to be the same as that for constructing two Saturn IB-Centaur rockets. To take advantage of this additional capability, spacecraft designers decided to launch two independent Voyager craft on the same Saturn V launch vehicle. (See Figures 1.1.1 and 1.1.2.) NASA’s decision made program costs soar, and NASA representatives developed a new estimate for program costs—$2.2 billion through fiscal year 1977, in addition to about $95 million in associated costs. Al- though it is difficult to calculate these projected costs in current-year dollars, it is clear that the Voyager-Mars Senate Appropriations Committee made its final allocations. The cancellation set back NASA’s Mars explora- mission would have cost over $10 billion in today’s dollars. tion by several years. Eventually NASA launched two Viking missions to Mars in the mid-1970s to accomplish With most of NASA’s funding dedicated to the Apollo program, Congress was skeptical about the rising many of the goals of Voyager. Although significantly less expensive than the Voyager-Mars mission, the Viking costs of a Voyager mission. In the latter half of the 1960s, NASA canceled the 1969 test flight and the 1971 missions remain NASA’s most expensive planetary spacecraft. flight owing to lack of funds. These setbacks spelled the end for Voyager. The Vietnam War’s demands on the U.S. budget, the growing cost of the Apollo program, and widespread civil unrest made Congress reprioritize 1E.C. Ezell and L. Neuman Ezell, On Mars: Exploration of the Red Planet 1958-1978, NASA, Washington, D.C., 1984, how the government should spend its money. In 1967, the Voyager-Mars mission was slashed entirely as the pp. 85-86.

THE CONSTELLATION SYSTEM AND OPPORTUNITIES FOR SCIENCE 17

CAUTIONARY TALES: THE VOYAGER-MARS MISSION AND PROJECT PROMETHEUS Of all the components of the Constellation System, the Ares V rocket offers the greatest potential benefit to science. However, there is anecdotal evidence from NASA’s own past about the pitfalls of designing large science missions to fit on large launch vehicles. See Boxes 1.1 and 1.2.

BOX 1.1 Cautionary Tale 1: The Voyager-Mars Mission

In 1960, NASA began establishing long-term plans for lunar and planetary missions. The Voyager-Mars program arose from these early concepts, prompted by a desire to explore the solar system and reach Mars before the same goal was achieved by the then-USSR. The Voyager-Mars mission was a Jet Propulsion Labo- ratory (JPL)-led program (not to be confused with the Voyager outer planets program of the 1970s).1 NASA selected Avco and General Electric to propose ambitious robotic Mars exploration spacecraft. While their proposals differed, both companies proposed spacecraft equipped with biological equipment for life detection, geophysical-geological tools to explore seismic activity and planetary composition, and atmospheric devices. General Electric proposed two landers and one orbiter, whereas Avco suggested one lander and one orbiter. In 1964, NASA managers decided to set a launch date for Voyager. A test flight would take place in 1969, followed by two launches in 1971 and four in 1973. The total projected cost of the mission was $1.25 billion, making it the most expensive robotic spacecraft program proposed by NASA at the time. Mariner 4 conducted a Mars flyby in 1964. Contrary to NASA’s expectations about the red planet, Mariner 4 gave scientists a bleak prognosis with respect to the chances that there had been life on Mars. In addition, atmospheric occultation data indicated a surface pressure of 5 to 7 millibars, whereas Voyager had been designed for a surface pressure of 10 or 11 millibars. The thinner atmosphere necessitated reworking the lander’s atmospheric entry method. From the beginning there was friction between NASA Headquarters and JPL over the design of the FIGURE 1.1.2 NASA’s Saturn V launch vehicle— mission. NASA Headquarters ordered that the Saturn IB-Centaur rocket that was part of the original design the largest launch vehicle ever built—inside the proposal be changed to a Saturn V. However, the Saturn V possessed substantially more payload capability than Vehicle Assembly Building in the late 1960s. In that required for the Voyager spacecraft. The cost of the Saturn V rocket was also projected to be the same as FIGURE 1.1.1 Test of the aeroshell for the Voy- addition to launching the Apollo missions to the that for constructing two Saturn IB-Centaur rockets. To take advantage of this additional capability, spacecraft ager-Mars lander in the mid-1960s. Voyager was Moon, NASA planned on using a Saturn V to designers decided to launch two independent Voyager craft on the same Saturn V launch vehicle. (See Figures to be a large spacecraft launched atop a Saturn V launch the Voyager spacecraft to Mars. SOURCE: rocket. SOURCE: Courtesy of NASA. 1.1.1 and 1.1.2.) Courtesy of NASA. NASA’s decision made program costs soar, and NASA representatives developed a new estimate for program costs—$2.2 billion through fiscal year 1977, in addition to about $95 million in associated costs. Al- though it is difficult to calculate these projected costs in current-year dollars, it is clear that the Voyager-Mars Senate Appropriations Committee made its final allocations. The cancellation set back NASA’s Mars explora- mission would have cost over $10 billion in today’s dollars. tion by several years. Eventually NASA launched two Viking missions to Mars in the mid-1970s to accomplish With most of NASA’s funding dedicated to the Apollo program, Congress was skeptical about the rising many of the goals of Voyager. Although significantly less expensive than the Voyager-Mars mission, the Viking costs of a Voyager mission. In the latter half of the 1960s, NASA canceled the 1969 test flight and the 1971 missions remain NASA’s most expensive planetary spacecraft. flight owing to lack of funds. These setbacks spelled the end for Voyager. The Vietnam War’s demands on the U.S. budget, the growing cost of the Apollo program, and widespread civil unrest made Congress reprioritize 1E.C. Ezell and L. Neuman Ezell, On Mars: Exploration of the Red Planet 1958-1978, NASA, Washington, D.C., 1984, how the government should spend its money. In 1967, the Voyager-Mars mission was slashed entirely as the pp. 85-86.

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BOX 1.2 Cautionary Tale 2: Project Prometheus

Project Prometheus was a program to develop a nuclear fission reactor and high-power electric propulsion systems for a range of outer-planet applications. The plan first circulated as early as 2001, but the program was not formally started until March 2003. Initially it was a science program, but it was subsequently transferred to the NASA Exploration Systems Mission Directorate when that directorate was established in February 2004. Although the overall project was named Prometheus, the initial emphasis of the program was to develop a spacecraft known as the Jupiter Icy Moons Orbiter (JIMO). (See Figure 1.2.1.) JIMO’s mission was to search for evidence of global subsurface oceans on Jupiter’s three icy moons: Europa, Ganymede, and Callisto. JIMO was intended to launch in 2011, with an approximate 12-year lifespan (the proposed launch date was later slipped to 2015). It would reach Jupiter 8 years after launch and have an operational mission time frame of 40 months. JIMO would have more than 10,000 watts of available power, compared with 885 watts for the radioisotope-powered Cassini mission (633 watts at the end of the mission). The available power, higher in comparison with that of radioisotope thermoelectric generators, would have allowed JIMO to carry a more extensive suite of instruments and also to transmit significantly more data back to Earth—some 500 gigabits of science data volume returned to Earth, over five times more data than received from Cassini, Voyager, and Galileo combined. To develop the technologies required for Prometheus, other agencies partnered with NASA, including the Department of Energy, the U.S. Navy’s Naval Reactors Division, and the Department of Defense’s Office of the Director of Defense Research and Engineering. As Prometheus progressed, mission designers realized that prior to launching the JIMO mission, they would first have to launch a demonstration mission to prove that the reactor design would operate successfully for the many years required for outer-planetary missions. This demonstration mission became known as Prometheus-1. In the summer of 2005, NASA canceled the Prometheus program after the expenditure of nearly $464 million (this did not include the costs to the contractors that prepared expensive bids to win the Prometheus contract—costs that were in the tens of millions of dollars each). The project was officially discontinued effective October 2, 2005. Of the $100 million allocated in fiscal year 2006, $90 million was spent to pay closeout costs on canceled contracts. The final report on Project Prometheus, released on October 1, 2005, provided a detailed cost analysis of a Jupiter Icy Moons Orbiter, which would have cost $16 billion through Phase E (i.e., science data collection), not counting the additional estimated $5 billion for the launch system.1 This $21 billion for a single solar system exploration mission presumably did not include the development costs for the nuclear reactor and associated systems, which would have been developed to support the Prometheus-1 demonstration mission. Prometheus serves as an example of the risks associated with pursuing ambitious, expensive space science missions. Despite the expenditure of hundreds of millions of dollars over a relatively short period of time, no spacecraft was ever developed.

1Jet Propulsion Laboratory, Prometheus Project Final Report, 982-R120461, October 1, 2005, p. 178.

THE CONSTELLATION SYSTEM AND OPPORTUNITIES FOR SCIENCE 19

BOX 1.2 Cautionary Tale 2: Project Prometheus

Project Prometheus was a program to develop a nuclear fission reactor and high-power electric propulsion systems for a range of outer-planet applications. The plan first circulated as early as 2001, but the program was not formally started until March 2003. Initially it was a science program, but it was subsequently transferred to the NASA Exploration Systems Mission Directorate when that directorate was established in February 2004. Although the overall project was named Prometheus, the initial emphasis of the program was to develop a spacecraft known as the Jupiter Icy Moons Orbiter (JIMO). (See Figure 1.2.1.) JIMO’s mission was to search for evidence of global subsurface oceans on Jupiter’s three icy moons: Europa, Ganymede, and Callisto. JIMO was intended to launch in 2011, with an approximate 12-year lifespan (the proposed launch date was later slipped to 2015). It would reach Jupiter 8 years after launch and have an operational mission time frame of 40 months. JIMO would have more than 10,000 watts of available power, compared with 885 watts for the radioisotope-powered Cassini mission (633 watts at the end of the mission). The available power, higher in comparison with that of radioisotope thermoelectric generators, would have allowed JIMO to carry a more extensive suite of instruments and also to transmit significantly more data back to Earth—some 500 gigabits of science data volume returned to Earth, over five times more data than received from Cassini, Voyager, and Galileo combined. To develop the technologies required for Prometheus, other agencies partnered with NASA, including the Department of Energy, the U.S. Navy’s Naval Reactors Division, and the Department of Defense’s Office of the Director of Defense Research and Engineering. As Prometheus progressed, mission designers realized that prior to launching the JIMO mission, they would first have to launch a demonstration mission to prove that the reactor design would operate successfully for the many years required for outer-planetary missions. This demonstration mission became known as Prometheus-1. In the summer of 2005, NASA canceled the Prometheus program after the expenditure of nearly $464 million (this did not include the costs to the contractors that prepared expensive bids to win the Prometheus contract—costs that were in the tens of millions of dollars each). The project was officially discontinued effective October 2, 2005. Of the $100 million allocated in fiscal year 2006, $90 million was spent to pay closeout costs on canceled contracts. The final report on Project Prometheus, released on October 1, 2005, provided a detailed cost analysis of a Jupiter Icy Moons Orbiter, which would have cost $16 billion through Phase E (i.e., science data collection), not counting the additional estimated $5 billion for the launch system.1 This $21 billion for a single solar system exploration mission presumably did not include the development costs for the nuclear reactor and associated FIGURE 1.2.1 Artist’s conception of the Jupiter Icy Moons Orbiter spacecraft. SOURCE: Courtesy of NASA. systems, which would have been developed to support the Prometheus-1 demonstration mission. Prometheus serves as an example of the risks associated with pursuing ambitious, expensive space science missions. Despite the expenditure of hundreds of millions of dollars over a relatively short period of time, no spacecraft was ever developed.

1Jet Propulsion Laboratory, Prometheus Project Final Report, 982-R120461, October 1, 2005, p. 178.

Analysis of Space Science Mission Studies

This study evaluates a total of 17 mission concepts for advanced space science missions in the fields of astronomy and astrophysics, heliophysics, and solar system exploration. The study did not receive any proposals for Earth science missions. Although NASA had proposed a workshop to help identify potential Earth science missions that could make use of the Constellation System, the workshop was canceled for lack of interest. The evaluated mission concepts are examples of the types of science missions that could be conducted using the Constellation System. The committee is not endorsing any specific mission team or approach and recognizes that, because of their preliminary nature, any mission concept that is pursued may require significant revision.

BACKGROUND AND APPROACH In 2004, to extend analyses of potential future space science missions and to identify precursor technology requirements, NASA funded studies for a variety of advanced missions referred to as the space science Vision Missions. These missions were inspired by a series of NASA roadmap activities conducted in 2003 and were not connected to the Vision for Space Exploration announced by President George W. Bush in January 2004. The Vision Mission concepts were predicated on the launch vehicles that were available at the time—the Evolved Expendable Launch Vehicles (EELVs) such as Atlas V and Delta IV in their various configurations. NASA funded studies of 14 potential Vision Missions, and final reports on 11 of these studies were prepared and delivered to NASA in 2006 and 2007. The Vision Mission studies fell into three broad categories: astronomy and astrophysics, heliophysics, and solar system exploration (i.e., planetary exploration), with some overlap. The mission concepts studied were the following:

• Advanced Compton Telescope (ACT), • The Big Bang Observer, • Generation-X (Gen-X),

Information describing the scientific objectives and current development status of the characteristics of each mission concept evaluated here is derived from the materials provided to the committee by the teams that developed the respective proposed mission concepts. These studies are summarized in Marc S. Allen, NASA Space Science Vision Missions, Progress in Astronautics and Aeronautics Series, No. 224, American Institute of Aeronautics and Astronautics, Reston, Va., 2008.

ANALYSIS OF SPACE SCIENCE MISSION STUDIES 21

• Innovative Interstellar Explorer, • Interstellar Probe, • Kilometer-Baseline Far-Infrared/Submillimeter Interferometer, • Modern Universe Space Telescope (MUST), • Neptune Orbiter with Probes (2 studies), • Palmer Quest, • Single Aperture Far Infrared (SAFIR) Telescope, • Solar Polar Imager, • Stellar Imager, • Titan Explorer, and • Titan Organics Exploration Study.

Of the studies listed above, the three that did not result in final reports were the Big Bang Observer, the Innova- tive Interstellar Explorer, and the Titan Organics Exploration Study. For the incomplete Innovative Interstellar Explorer mission study, the committee received a briefing that focused primarily on the science and the changes to the mission profile that could result from use of the Constellation System. The committee received briefings on and addressed the two Neptune mission studies in a single assessment. The National Research Council’s (NRC’s) Committee on Science Opportunities Enabled by NASA’s Con- stellation System asked the principal investigators or other representatives of the 14 Vision Mission studies to present their studies to the committee at its first meeting, on February 20-22, 2008. The principal investigators were asked to consider how their existing studies might benefit from the Constellation capabilities—primarily the larger payload capability and shroud dimensions of the Ares V rocket and the possibility of the human servicing of spacecraft. Because of the scheduling requirements of the committee’s work, the presenters had only limited time to assess how the Constellation capabilities might affect their proposals. Some indicated that the Constellation System would have no appreciable effect on their concepts, and others indicated that it would serve as a substitute for required technology development. Some of the proposers also indicated that their science objectives might be enlarged and expanded if they had additional mass and volume available. In its interim report, the committee evaluated the 11 Vision Missions on which final studies had been completed., The committee also issued a request for information to the scientific community seeking proposals for additional space science missions (see Appendix C in the present report), resulting in 6 additional mission proposals:

• Advanced Technology Large-Aperture Space Telescope (ATLAST), • Dark Ages Lunar Interferometer (DALI), • 8-Meter Monolithic Space Telescope, • Exploration of Near Earth Objects via the Crew Exploration Vehicle, • Solar Probe 2, and • Super-EUSO (Extreme Universe Space Observatory).

For its final report, the committee evaluated these 6 additional mission concepts using the same criteria used for the Vision Mission studies.

National Research Council, Science Opportunities Enabled by NASA’s Constellation System: Interim Report, The National Academies Press, Washington, D.C., 2008. For the interim report, and for this final report, the committee chose to consider both the Vision Mission final reports and the presentations made to the committee during its February 2008 meeting. In some cases there were no or only minor differences between the reports and the presentations. In others, the differences primarily concerned speculation supported by limited analysis on how the mission would benefit from the Constellation capabilities. For the Interstellar Probe concept, the presentation was made by the team that did not produce a final report. However, the science objectives of both Interstellar Probe studies were identical, and the presenter focused primarily on how the mission concept would benefit from the Constellation System.

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EVALUATION CRITERIA AND RESULTS The committee analyzed each of the 17 missions—the 11 Vision Mission concepts in hand from NASA’s 2004 solicitation and the 6 additional concepts received from the scientific and technical community in response to its request for information—according to a standard format based on the committee’s statement of task, which specified analysis of the following information for each mission concept considered:

1. Scientific objectives of the mission concept; 2. A characterization of the mission concept insofar as the maturity of studies to date have developed it; 3. The relative technical feasibility of the mission concepts compared to each other; 4. The general cost category into which each mission concept is likely to fall; 5. Benefits of using the Constellation System’s unique capabilities relative to alternative implementation approaches; and 6. Identification of the mission concept(s) most deserving of future study.

In view of the Constellation System architecture, the committee evaluated the mission concepts using two criteria:

1. Does the concept offer a significant advance in a scientific field? “Significant” is defined here as providing an order-of-magnitude or greater improvement over existing or planned missions and enabling a qualitative new approach to the important scientific questions in the field. The committee judged the significance of the scientific advances of each concept on the basis of material submitted or presented to the committee. 2. Does the concept benefit from (or have a unique requirement for) Constellation System capabilities—for example: —Does use of the Constellation System’s elements make a previously impossible mission technically feasible? —Does use of the Constellation System’s elements reduce mission risk or enhance mission success for a previously complicated mission? —Does use of the Constellation System’s elements enhance performance? —Does use of the Constellation System’s capabilities offer a significant cost reduction (i.e., 50 percent or more) in the cost of accomplishing the mission?

The committee was tasked with assessing the relative technical feasibility of mission concepts compared to each other. “Relative technical feasibility” was judged on the basis of the material submitted to the committee, without any detailed independent analysis. The committee was not asked to assess the relative scientific merit of the missions. Such prioritization of missions is more appropriately the work of decadal surveys, which look at proposed missions in the context of others in each scientific field. However, the committee did take note of mission proposals that had been mentioned in previous NRC reports, particularly decadal surveys. Of the 17 mission concepts reviewed for this report, the committee determined that all could potentially offer a significant advance in their scientific fields (meeting criterion 1 above). However, of the 17, the committee determined that 5 mission concepts (ACT, SAFIR Telescope, Kilometer-Baseline Far-Infrared/Submillimeter Interferometer, Palmer Quest, and Super-EUSO; see Appendix B of this report) did not directly benefit from Constellation (thus not meeting criterion 2 above). Nevertheless, the committee was impressed by the scientific objectives and ambition of these 5 missions. The missions would undoubtedly be evaluated in the decadal surveys to which they are relevant and may perform well in those. That the committee did not identify several proposed missions for further study as Constellation-enabled missions should not be interpreted to imply anything about their future viability, only that they may have other paths to pursue. The reasons for the committee’s assessment that some of the mission concepts would or would not benefit from the Constellation System generally fall into two categories. The first concerns the mass, volume, and complexity of each proposed mission and whether it fits into the capabilities of the existing family of EELV rockets (if so, the mission would not need the greater capabilities of Constellation). The second category concerns the propulsion

ANALYSIS OF SPACE SCIENCE MISSION STUDIES 23

requirements of each mission and whether it could benefit from a powerful launch vehicle that could place it in its required mission orbit (propulsion required either to accelerate the mission or decelerate it into its proper orbit). The committee did not conduct detailed cost assessments of any of the mission concepts. Instead, it used the cost estimates provided by the proposers of the respective concepts (not adjusted for inflation) and relied on the experience of the committee members and comparisons with similar missions to adjust them. The mission concepts were judged by subject area, that is, astronomy and astrophysics, heliophysics, and solar system exploration. This was done for two reasons. First, NASA generally does not integrate its overall science mission priorities but treats them separately. Second, one of the committee’s goals was to produce a broadly representative sample of science opportunities offered by the Constellation System. For its final report, the committee focused on the 12 mission concepts it had determined would benefit from the Constellation System (see Chapter 2). Because the committee was charged with determining which concepts are “most deserving” of further study, it divided the list of 12 mission concepts into “more deserving” and “deserving” categories based on two considerations:

• Mission Impact on Science in the Field of Study—The mission concept must present well-articulated science goals that the committee finds compelling and worthy of the investment needed to develop the technology. The committee defined “compelling” in terms of the breadth of impact that the science would have on the field, the relative size of the community for that science, and whether or not it would provide an order-of-magnitude improvement in science return. • Technical Maturity—The mission concept must be sufficiently mature in its overall conception and technology. If the technology for accomplishing the mission does not currently exist at a high technology readiness level, the mission must provide a clear path indicating how it will be developed. The committee defined “sufficiently mature” as indicating a technology readiness level (TRL) of 5 or higher (see Appendix D) for the major components and a form of “integrated TRL” that considered the challenges of integrating multiple technologies, as well as the lowest TRL rating of the individual technologies.

If a mission concept satisfied both criteria to a moderate or high degree, it was designated “more deserving” of further study. In addition, the committee evaluated whether a mission was “enabled” or “enhanced” by Constel- lation. If a mission was “enabled,” it could not be accomplished without the Constellation System’s capabilities. If a mission was “enhanced,” it could be accomplished without Constellation (for instance, by employing a smaller launch vehicle), but it might better accomplish its goals using Constellation’s capabilities: for instance, the mission could return much greater amounts of data or could return useful data faster if “enhanced” by Constellation. Table 2.1 presents a summary of the committee’s evaluations of the 12 individual mission concepts that it identified as being worthy—that is, either “deserving” or “more deserving”—of further study as Constellation missions. As indicated in the table, the committee identified 5 missions that it determined are “more deserving” of further study.

Several of the above missions, particularly among the heliophysics missions, are well defined scientifically and do not require significant study of instruments or related issues. Further study with respect to those mission concepts should focus primarily on the relationship between the Ares V capabilities and their propulsion requirements. Additional study of scientific capabilities would primarily be confined to how the science would be affected by the additional Ares V capabilities. Because of these narrow requirements, NASA may have the ability to further study other possible Ares V science missions. The other seven “deserving” missions listed in Table 2.1 are also promising and offer great potential science return but will require more effort in order to be brought to a higher level of maturity.

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TABLE 2.1 Overall Assessment of Mission Concepts Deemed Worthy of Further Study as Constellation Missions Impact of Summary with Discipline/Mission Constellation Respect to Concept on Mission Impact of Mission on Science in the Field Technical Maturity Further Study

Astronomy and Astrophysics Missions Advanced Enabled Has several disparate ultraviolet (UV)/optical Low for mirror technology Deserving Technology science goals; unclear what the scientific (including mass) Large-Aperture impact will be. Medium for detectors and Space Telescope thermal control (ATLAST) 8-Meter Monolithic Enabled Monolithic mirror allows direct study of High for mirror and More deserving Space Telescope extrasolar terrestrial planets and the search structure for signs of life. Low for coronagraphic observation Dark Ages Lunar Enabled Unique capability for viewing the universe at Medium for rovers and Deserving Interferometer a distinctive period: between about 10 million interferometrics (DALI) and 300 million years after the big bang. Low for reducing mass and for deploying and operating in a remote location Generation-X Enabled Observing the birth and evolution of the Low for mirror Deserving (Gen-X) earliest black holes and galaxies. development and operations Modern Universe Enabled Has disparate UV/optical science goals; High for instruments Deserving Space Telescope unclear what the scientific impact will be. Low for coronagraph and (MUST) mirror assembly

Astronomy and Astrophysics/Heliophysics Mission Stellar Imager Enhanced Study of stellar magnetism and activity cycles Low for formation flying Deserving to understand solar weather and its potential human implications. Limited lifetime of the mission may not accomplish the stated goals.

Heliophysics Missions Solar Probe 2 Enabled Allows first measurements of the acceleration High More deserving of the solar wind and coronal heating. (focused on Ares V) Interstellar Probe Enabled Accomplishes new science on the interaction High for science, More deserving (ISP) of the Sun and the interstellar medium. instruments, and mission (focused on concept Ares V) Solar Polar Imager Enhanced Accomplishes the first close and repeated High for instruments More deserving imaging of the Sun’s poles, providing Propulsion not studied in (focused on comprehensive information on the structure sufficient detail Ares V) and dynamics of the Sun.

Solar System Exploration Missions Exploration of Enabled Allows sample return in context and in situ High for instruments Deserving Near Earth Objects measurements of the near-Earth asteroid, Low for human factors via the Crew providing information on the interior such as radiation Exploration Vehicle properties. Neptune Orbiter Enhanced Permits first detailed exploration of an ice High for instruments More deserving with Probes giant planet, with implications for clarifying Low for propulsion the origins of the solar system. Titan Explorer Enhanced Provides great potential for high-value future High for instruments Deserving science, but did not account for Cassini Medium for blimp results. NOTE: The mission concepts are listed by discipline. All of the concepts are robotic missions, with the exception of the proposal for Exploration of Near Earth Objects via the Crew Exploration Vehicle.

ANALYSIS OF SPACE SCIENCE MISSION STUDIES 25

(a)

(b)

FIGURE 2.1 (a) Illustration of the Sun-Earth Lagrangian points and the Earth-Moon Lagrangian points. Sun-Earth L2 is the proposed site of several large astronomical and heliophysics observatories. One possible option for spacecraft servicing is moving an observatory from Sun-Earth L2 to an Earth-Moon21. b from Lagrangian Lester point.eps for easier access. (b) Change in velocity (delta-v) requirements for different locations in the Earth-Moon system and Lagrangian points. NOTE: E-M L1, Earth-Moon libration point L1; GEO, geostationary orbit; GTO, GEO transfer orbit; LEO, low Earth orbit; LLO, low lunar orbit; LTO, lunar transfer orbit; Low-T, low thrust; High-T, high thrust; S-E L1, Sun-Earth libration point L1; S-E L2, Sun-Earth libration point L2; ΔV, delta-v. SOURCE: Courtesy of NASA.

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After this study began, NASA initiated several workshops to explore the potential benefits of the Ares V launch vehicle for science missions. The committee believes that during the brief time of this study, the enormous potential capabilities of the Ares V, for example, were not recognized by the scientific community at large. The committee also believes that the short deadlines associated with the second phase of this study impaired the ability of the scientific community to respond with mission concepts, especially in terms of the weight and volume capabilities planned for Ares V in particular. The Ares V workshops sponsored by NASA were a valuable means for facilitating broader involvement of the scientific community in assessing the potential of Constellation for science.

THE SUN-EARTH LAGRANGIAN POINT Several of the mission concepts evaluated in this report (e.g., ATLAST, 8-Meter Monolithic Space Telescope, MUST, Gen-X, Stellar Imager) would operate at the Sun-Earth Lagrangian point L2. The Wilkinson Microwave Anisotropy Probe is already in orbit around the Sun-Earth L2 point, and this is the location planned for the James Webb Space Telescope (JWST) as well as other space telescopes. The value of this location is that spacecraft remain in a stable orbit, they have near-continuous sunlight, and their view of the full sky is not obstructed by Earth. See Figure 2.1 on page 25.

ASTRONOMY AND ASTROPHYSICS MISSIONS Advanced Technology Large-Aperture Space Telescope (ATLAST) Scientific Objectives of the Mission Concept New observational capabilities, such as those needed to find faint traces of life on extrasolar planets, will depend on the use of space-based telescopes with large apertures. The Advanced Technology Large-Aperture Space Telescope will be able to explore the nearest ~1,000 stars capable of harboring life for Earth-size planets and characterize their spectra. ATLAST will have up to 10 times the resolution of JWST and up to 300 times the sensitivity of the Hubble Space Telescope (HST). ATLAST will also be a next-generation ultraviolet (UV)/optical Great Observatory, in the model established by the Hubble Space Telescope, capable of achieving breakthroughs in a broad range of astrophysics and adaptable to addressing scientific investigations yet to be conceived.

The first workshop, Ares V Astronomy Workshop, sponsored by the NASA Ames Research Center and held April 26-27, 2008, focused on the Ares V and possible astronomy missions, the majority of which are also evaluated in this study. The second workshop, Ares V Solar Sys- tem Science Workshop, also sponsored by Ames and held August 16-17, 2008, focused on the Ares V and potential solar system exploration. See http://event.arc.nasa.gov/ aresv/ and http://event.arc.nasa.gov/aresv-sss/index.php?fuseaction=home.home. The Astronomy Workshop is summarized in S. Langhoff, D. Lester, H. Thronson, and R. Correll, eds., Workshop Report on Astronomy Enabled by Ares V, NASA/CP-2008- 214588, August 2008, available at http://event.arc.nasa.gov/main/home/reports/CP-2008-214588-AresV.pdf. The Solar System Workshop is summarized in S. Langhoff, T. Spilker, G. Martin, and G. Sullivan, eds., Workshop Report on Ares V Solar System Science, NASA/CP-2008- 214592, August 2008, available at http://event.arc.nasa.gov/main/home/reports/CP-2008-214592-AresV-SSS_Print.pdf. The first two subsections are based on material in M. Postman, T. Brown, A. Koekemoer, and the ATLAST Team, “An Advanced Technol- ogy Large Aperture UV/Optical Space Telescope: A NASA Astrophysics Strategic Mission Concept and Technology Development Study,” in response to a request for information by the Committee on Science Opportunities Enabled by NASA’s Constellation System, May 2008. See footnote 1 above.

ANALYSIS OF SPACE SCIENCE MISSION STUDIES 27

The goals of the ATLAST mission concept are the following:

1. Search for life on exoplanets. 2. Study star formation histories in the local universe. 3. Probe the environments near supermassive black holes.

Characteristics of the Mission Concept as Developed to Date ATLAST is a proposal for a 16-m diameter telescope (see Figure 2.2) that would be unfolded in space in a similar manner to the approach employed by JWST, the latter currently being scheduled for launch in 2013. Like JWST, ATLAST would operate at the Sun-Earth L2 point. The ATLAST mirror would be folded along two chords and stacked vertically within the Ares V payload shroud. This mission follows on some of the technologies developed for HST and JWST, particularly in terms of large-mirror technology and construction and placement at L2. The ATLAST mirror would have a mass of approximately 10,000 kilograms (kg). The delta-v required to transport it from low Earth orbit to Sun-Earth L2 is approximately 4 kilometers per second (km/s). The combination of high angular resolution and high sensitivity is, increasingly, a science driver in astrophysics. The impressive capabilities anticipated for ground-based observatories in the coming decade will redefine the existing synergy between ground and space telescopes. Space will, however, remain an optimal environment for optical observations that require any combination of very high angular resolution over fields of view larger than ∼1 arc minute (arcmin) or at wavelengths shorter than ∼1 micron, very high sensitivity, very stable point spread function performance across the field of view, high photometric precision and accuracy in crowded fields, and very

36, 2.4 Meter Hex Mirror Deployable Stray Light Baffle Petals (3 Ring)

2.4 Meter Diameter ISIM Tower 19.4 m

Telescope Positioning and Isolation Boom 16.8 m

Deployable Solar Array

4.4 Meter Diameter Primary Deployable Solar Sail Central Cylinder Structure

103 m

FIGURE 2.2 Illustration of the ATLAST 16-meter telescope. SOURCE: Courtesy of Northrop Grumman Space Technology. Figure 2.2.eps Includes low resolution bitmap images

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high stability of all of these performance parameters over tens to hundreds of hours of exposure time. Of course, UV observations can only be accomplished in space. ATLAST would provide an unprecedented combination of reproducible high sensitivity and angular resolution at wavelengths from 0.11 to 1 micron over a field of view of at least 25 square arcmin.

Relative Technical Feasibility of the Mission Concept A UV/optical space telescope with an aperture of at least 8 m will accommodate some of the science goals discussed above, while some of the more stringent drivers point to a 16-m segmented-aperture telescope. Both would incorporate the following baseline requirements: a wavelength range of 0.11 to 2.5 microns with angular resolution diffraction limited at 0.5 micron and a field of regard of the entire sky over the course of 1 year. Both would occupy a halo orbit at Sun-Earth L2. The James Webb Space Telescope currently uses a segmented-mirror design. Expanding the aperture size to a 16-m UV/optical segmented space telescope such as that employed by ATLAST presents significant technical challenges. JWST is a 6.5-m telescope diffraction-limited at 2 microns. The technical feasibility of aligning and operating a 16-m diffraction-limited (0.5 micron) space-based telescope is beyond that demonstrated by JWST, and the challenges must be identified and addressed in feasibility studies.

General Cost Category in Which This Mission Concept Is Likely to Fall The phase A-D costs (i.e., development costs but not including science operations) associated with the ATLAST point design for the 8-meter-class mission (see the subsection below entitled “8-Meter Monolithic Space Tele- scope”) will be in the multibillion-dollar range, comparable to JWST. Because the 16-m mission is larger and more technically complex than the 8-Meter Monolithic Space Telescope, it is expected to cost significantly more.

Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Ares V cargo launch vehicle, with its upmass capacity and fairing volume, is one of the key enabling technologies for future large UV/optical space telescopes with apertures of 8 m or more. The vehicle would vastly reduce the risk associated with launching and deploying ATLAST. Ares V would enable a folded, segmented telescope with an aperture up to 24 m to be flown in a single launch, and it would enable the use of high-TRL telescope components. For the segmented primary design, the Ares V would enable the use of JWST (2 petal) chord fold deployment for telescopes with apertures up to 16.8 m. The ATLAST mission could also potentially benefit from either human or robotic servicing.

Assessment of the Mission Concept for Further Study The ATLAST mission is enabled by the Constellation System. The Ares V cargo launch vehicle is the key enabling technology for ATLAST. The large telescope requires the large mass and volume capacity of Ares V and its ability to deliver such a payload to Earth-Moon L1 or Sun-Earth L2. A telescope with an aperture up to 24 m can be flown in a single launch on the Ares V cargo launch vehicle. The potential scientific impact of this mission concept was unclear. The proposal listed several broad goals cutting across the ultraviolet and optical spectral regions. A detailed science case for ATLAST and trade-off studies of its various options could better define this concept. For example, the goals of imaging and spectroscopy of exoplanets present challenges in terms of scattered light for a segmented-mirror telescope, whereas other

Space is an optimal environment for optical observations for several reasons, including the following: (1) weather—clouds and wind can affect observations on the ground; (2) point spread function stability and the fact that low-order tip-tilt systems do nothing if seeing reaches near 1 degree; (3) day/night cycles; (4) scattered light from the Moon or a nearby star can affect observations; (5) sky brightness and background noise are problems on the ground; (6) thermal environment and instrument and detector stability are degraded for ground observations; and (7) cleanliness—dust and mechanical particulates affect ground-based telescopes.

ANALYSIS OF SPACE SCIENCE MISSION STUDIES 29

science questions require the larger aperture that ATLAST can provide compared with other telescope designs. The proposal suggested that several mirror aperture sizes are possible, but it does not clearly explain the science advantages and disadvantages of each. Several key technologies for ATLAST are immature at this time. The production, deployment, mass, and stability of a large segmented mirror presents significant challenges. The diffraction limit required for ATLAST is currently beyond the capabilities demonstrated by the Hubble Space Telescope or the James Webb Space Tele- scope. Other items, such as the detectors and thermal control system, appear to be at reasonable TRLs to anticipate development in time for such a mission. In summary, the ATLAST mission has a medium/low rating for the mission’s impact on science in the field and for its technical maturity, and therefore it is placed in the “deserving of further study” category. This program could build on JWST’s technology base and significantly advance UV/optical astronomy. However, in order for this mission to move forward, the science goals for ATLAST must be better defined.

8-Meter Monolithic Space Telescope Scientific Objectives of the Mission Concept The 8-meter-class UV/optical space observatory is a concept for a powerful telescope with very high angular resolution and sensitivity, broad spectral coverage, and high performance stability that would afford the opportunity to answer compelling science questions: How did the universe come into existence? What is it made of? What are the components of the formation of today’s galaxies? How does the solar system work? What are the conditions for planet formation and the emergence of life? Are we alone? Periodic robotic servicing would allow for extended mission life of 20 or 30 years. The first 5 years of the mission could be focused on UV science, with a narrow-field-of-view UV spectrometer and a wide-field-of-view UV imager. After a servicing mission, the next 5 years could be dedicated to visible science such as terrestrial planet finding, with either an external occulter or an internal coronagraph.

Characteristics of the Mission Concept as Developed to Date The 8-Meter Monolithic Space Telescope mission concept has three main subsystems: telescope, support structure, and spacecraft (see Figure 2.3). The telescope consists of a 6- to 8-m primary mirror, secondary mirror, and forward structure/baffle tube. Active thermal management by means of 14 heat pipes is required to hold the primary mirror temperature at a constant 300 kelvin (K) for all Sun angles with less than 1 K of thermal gradient. The spacecraft provides all normal spacecraft functions and houses the science instruments. The support structure supports the primary mirror and carries the observatory mass (of the primary mirror, telescope forward structure, and spacecraft), providing the interface of this mass to the Ares V for launch. If manufactured using conventional ground-based telescope techniques in order to reduce cost, the mirror mass would be approximately 44,000 kg. The delta-v required to transport the telescope to Sun-Earth L2 from low Earth orbit is only 4 km/s. The Ares V is capable of transporting more than 55,000 kg to Sun-Earth L2. A feasibility study conducted by NASA’s Marshall Space Flight Center (MSFC) considered two different telescope optical systems. An F/15 Ritchey-Chrétien design was examined, with its on- and off-axis image quality, compact size, and ultraviolet throughput. Unfortunately, this optical design has only a relatively narrow, 1-arcmin field of view that is diffraction limited at 500 nanometers (nm). Another option is to use a three-mirror anastigmatic telescope with fine steering mirror design, with a wide, 100-arcmin (8.4 by 12 arcmin) field of view that is diffraction limited at 500 nm. But it also has lower UV throughput because of its two additional reflections. A potential

The first two subsections are based on material in H. Philip Stahl, “Design Study of 8 Meter Monolithic Mirror UV/Optical Space Tele- scope,” in response to request for information by the Committee on Science Opportunities Enabled by NASA’s Constellation System, May 2008. This proposal and the ATLAST proposal were produced by many of the same people, and the 8-m telescope is occasionally referred to as the 8-Meter Monolithic ATLAST. See footnote 1 above.

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FIGURE 2.3 Illustration of the 8-Meter Monolithic Space Telescope inside its payload fairing atop an Ares V rocket. SOURCE: Courtesy of NASA.

solution is to implement a dual pupil configuration, where UV and narrow-field-of-view instruments operate at the Cassegrain focus, and wide-field-of-view instruments operate off-axis providing their own tertiary mirror. Due to the large lift ability of the Ares V, the primary mirror in this telescope can be a single piece of glass and does not have to be “space light-weighted.” The mirror can be of standard ground-based technology such as those produced for the Very Large Telescope or the Large Binocular Telescope. Mirror masses of 12,000 kg to 20,000 kg would not be too heavy for this mission and could in fact be considerably heavier. The proposed observatory has two separate spacecraft: a telescope bus that houses the optical telescope element and a replaceable spacecraft/instrument bus. The spacecraft propulsion system is sized to get the observatory from roughly a geostationary transfer orbit into a halo orbit around the Sun-Earth L2 point and performs all station-keeping operations. The spacecraft has a dual-mode hydrazine-NTP bipropellant/hydrazine monopropellant propulsion system with 5 years of propellant and redundant thrusters.

Relative Technical Feasibility of the Mission Concept The NASA MSFC study indicates that it is feasible to launch a 6- to 8-meter-class monolithic primary mirror UV/visible observatory with a 10-m Ares V launch vehicle, have it survive launch, and place it into a halo orbit around the Sun-Earth L2 point. Specific technical areas studied included structural, thermal, and optical design and analysis; launch vehicle performance and trajectory; spacecraft; operations and servicing; mass and power budgets; and system cost. Structural design and analysis were performed for the spacecraft using standard NASA guidelines. No technical problems were identified. The use of existing technology, such as ground-based mirror production, has the advantages of technical maturity and low risk. It has been demonstrated that one can polish an 8-meter-class ground-based telescope mirror to a surface figure of better than 8 nm root mean square. The Ares V launch environment was analyzed by the MSFC Advanced Concepts Office to determine whether an 8-meter-class ground-based telescope mirror could survive launch. To do so, 66 axial support points would keep the stress level on an 8.2-meter-diameter 175-millimeter (mm) thick meniscus primary mirror below 1,000

ANALYSIS OF SPACE SCIENCE MISSION STUDIES 31

pounds per square inch (psi). A key design element of the structural concept is that all the mass of the observatory is carried through the main support structure to an interface ring, which attaches by means of another support system to the Ares V launch vehicle. This allows the use of a completely conventional spacecraft. Although the basic overall design and technology for this observatory are straightforward, for the most exciting science that can be performed by this mission—the detection of exoplanets—the technical readiness is low and the coronagraph technology needs to be further developed.

General Cost Category in Which This Mission Concept Is Likely to Fall This preliminary study claims that by eliminating complexity, it should be possible to design and build an 8-Meter Monolithic Space Telescope with seven times the collecting area of HST for less cost than JWST. This mission concept proposes to use existing ground-based mirror technology rather than ultralightweight mirror technology required for a large telescope to be launched on an EELV. While this adds 20,000 kg to the mass, this approach is estimated to save $0.7 billion to $2 billion in total program cost. The total cost for a 6- to 8-m observatory (excluding science instruments and operations) is estimated by its proposers to be $1 billion to $1.5 billion. The committee judges that the cost of this concept is likely to lie in the $1 billion to $5 billion range.

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The unprecedented mass and volume capabilities of NASA’s planned Ares V cargo launch vehicle enable entire new mission concepts. First, the baseline 10-m fairing has an 8.8-m internal dynamic envelope diameter. This diameter is sufficient to accommodate a 6- to 8-meter-class monolithic circular primary mirror without the need for segmentation—which is compelling for specific science cases such as occultation studies of nearby stars performed to directly image host planets. A single primary mirror also provides a more uniform, symmetric, and stable point spread function. The payload mass of 55,800 kg to an L2 transfer orbit enables design simplicity, allowing the use of current-day technology for mirror and telescope design. The Ares V capacities allow the use of mass to reduce performance, cost, and schedule risk. This mission could also potentially benefit from either human or robotic servicing.

Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The 8-Meter Monolithic Space Telescope offers the possibility of a relatively easier and faster scientific use of the Ares V launch vehicle compared with other, more complex telescope designs. The reason for this fast-track ability is the use of present-day, proven, low-cost (compared with lightweight space design) technologies. The mission concept should therefore be studied further as a Constellation-enabled science mission. The various telescope designs, the initial and future instruments, the servicing concept, and schedule are examples of the details that are required before any final space telescope design and mission concept can be developed.

Assessment of the Mission Concept for Further Study The 8-Meter Monolithic Space Telescope mission is enabled by the Constellation System. No existing launch vehicle has sufficient fairing volume or lift capacity to launch an 8-m monolithic space telescope into orbit. The scientific impact of this mission is similar to that for the ATLAST proposal in many respects. However, compared with ATLAST, an 8-m monolithic telescope potentially enables the direct observation of Earth-like planets around other stars in order to search for signs of life. However, to achieve this capability, the telescope must

To study extrasolar planetary systems directly, coronagraphs will have to achieve contrast ratios of nearly 10 billion to 1. These ratios are several orders of magnitude beyond the current state of the art and may not be possible with some kinds of telescope designs, for example those using segmented mirrors.

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be used as a coronagraph with a high rejection factor. Although the proposed mission concept mentioned but did not stress observations of Earth-like planets, the successful characterization of even one extra-solar-system planet would revolutionize astronomy and is seen by the committee as a compelling goal for science and a discriminat- ing factor in comparing the 8-Meter Monolithic Space Telescope with the two similar telescopes—ATLAST and MUST—both of which have more difficult technical challenges to achieving this science goal. Furthermore, an 8-m space telescope is a larger version of HST, one of NASA’s most successful missions, and it would bring some of the same benefits to science as those from Hubble. Even if technical or operational problems on orbit prevented the characterization of Earth-like planets, this mission would still be an enormous scientific success for its impact on astronomy. The construction of large monolithic telescopes is a well-proven technology for ground-based observatories. It is likely that only modest improvements are required to produce space-qualified telescopes, allowing the mirror and structure to be rated as technically mature. The technology needed to create a coronagraph with a rejection factor on the order of 1010 is still immature and rated low. This technology is required for the most compelling science. Nevertheless, a monolithic mirror is at present foreseen as the best platform to enable a high-performance coronagraph, which makes the 8-Meter Monolithic Space Telescope a good candidate for further study. In summary, the 8-Meter Monolithic Space Telescope is currently seen as the best platform from which to observe Earth-like planets directly to search for signs of life, and the inherent capabilities of a large optical space telescope guarantee an enormous science return even if planet characterization falls short of its ambitious goals. For these reasons, the committee believes this project is “more deserving of further study.”

Dark Ages Lunar Interferometer (DALI)10 Scientific Objectives of the Mission Concept The Dark Ages Lunar Interferometer is a mission concept for a long-wavelength radio telescope on the lunar farside (see Figure 2.4). It is designed to probe the Dark Ages—the cosmic epoch between post-big bang recom- bination and the formation of the first luminous objects. It was during this interval that baryons—neutral hydrogen atoms—first began collapsing into matter-dominated, dense regions, eventually leading to the formation of stars and galaxies. DALI will detect, using interferometric techniques, neutral hydrogen signals (H I) in absorption against the cosmic microwave background through the highly redshifted 21-centimeter (cm) line. The primary science goal is to see into the Dark Ages—acquiring a three-dimensional view of the evolution of a large fraction of the universe. It should be possible to see the cosmic evolution of the H I line excitation temperature in absorption against the cosmic microwave background, thereby producing a precision probe of cosmology and of the first large-scale structures of the universe. DALI would allow for imaging of the H I line at different redshifts, producing a history of the growth of structure formation. The most powerful application of H I line observations is tomography, in which imaging spectral-line observations are acquired, allowing structures to be distinguished as a function of the redshift, z. Additionally, detection of radio emissions from extrasolar planetary magnetospheres may be possible. Some solar system planets such as Jupiter are significant radio emitters as a result of interactions between the solar winds and the planet’s magnetosphere. With sufficient sensitivity, extrasolar planetary radio emissions should be detectable over interstellar distances.

Characteristics of the Mission Concept as Developed to Date DALI will be a radio interferometer on the lunar farside to be shielded at night from both human-generated and solar radio emissions. Its design is hierarchical, with individual antennas grouped into “stations,” and the signals from stations brought to a central processing facility.

10 The first two subsections are based on material in J. Lazio, for the DALI team, “The Dark Ages Lunar Interferometer (DALI),” in response to request for information by the Committee on Science Opportunities Enabled by NASA’s Constellation System, May 2008. See footnote 1 above.

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FIGURE 2.4 Successively closer views of a Dark Ages Lunar Interferometer (DALI) concept instrument on the surface of the Moon. The image at lower right depicts a single instrument. Many instruments would be deployed in a crater on the lunar surface. SOURCE: Courtesy of NASA Goddard Space Flight Center.

In simple interferometry, signals from two receivers are brought to a common location and combined. The resulting correlation coefficient is a sample of the visibility function (the Fourier transform of the sky brightness) seen by that pair of receivers. The sky brightness distribution can be determined by Fourier inversion of multiple samples, obtained by combing the signals from many receivers. Processing these observations to obtain numerous frequency channels across a large wavelength range is standard practice for Earth-based radio interferometers; it provides spectral analysis of the incident signal. Table 2.2 summarizes key technical requirements for the DALI mission concept. A key requirement is the collecting area, with preliminary estimates for imaging tomography on the order of 10 km2, although the concept may be sufficiently flexible to allow for the gradual buildup to reach the full aperture. A significant challenge for DALI will be the removal of foreground signals. The required dynamic range is at least 105 (10 millikelvin [mK] versus 1,000 K galactic emission). The dominant foreground will be the nonthermal (synchrotron) galactic emission.

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TABLE 2.2 Dark Ages Lunar Interferometer (DALI): Scientific and Technical Requirements Parameter Requirement Technical Capability Comments Redshift z ∼ 15-150a 10 < v < 100 MHz H I spin temperature decouples from cosmic microwave background at z ∼ 200a H I begins to be destroyed by first luminous objects at z ~ 20a Sensitivity ∼10 mK Collecting area ∼ 10 km2 Expected strength of H I absorption Mission lifetime ∼ 5 yr Imaging tomography is the most demanding collecting area No RFI—terrestrial or locally generated requirement Observations only during lunar night Angular ∼3 arcmin Baselines 0.5-50 km Expected size of H I structures resolution az = (square root (1 + v/c))/(square root (1 − v/c)) − 1. NOTE: H I, neutral hydrogen; RFI, radio frequency interference. SOURCE: Joseph Lazio, Naval Research Laboratory, “Dark Ages Lunar Interferometer,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, June 10, 2008.

The fundamental receiving element for celestial radio signals will be stations, each composed of 100 individual dipole antennas. The baseline mission concept has 1,000 antenna stations, to provide the imaging quality required. The equipment would be landed on a cargo variant of the planned Altair lunar lander and would be remotely deployed by one or more robotic rovers. The stations are distributed on the Moon over a region approximately 50 km in diameter. Each antenna station is conceived to be deployed by a mini-rover, which moves from a lunar lander to the desired location; the mini-rover remains as a station controller, or hub, after antenna station deployment. Signals from the individual dipoles imprinted on polyimide film are sent through imprinted strip lines to a hub to be summed and stored through the lunar night. The stored data are then relayed to a central location for cross- correlation with signals from all other stations. The correlated data are relayed to Earth, where imaging processing and analysis will proceed using methods already in use (or being developed) for ground-based arrays.

Relative Technical Feasibility of the Mission Concept Most of the technology challenges for the DALI mission involve the engineering challenge of scaling up current components to large numbers and high complexity. In addition, the DALI components will need to survive very harsh lunar environments (temperature variations from approximately 100 to 400 K), and will need to be suitable for deployment by remote means on the lunar farside. Several key technologies have high TRLs because of substantial space experience or the Earth-based experience (50 years) with radio interferometers, although in smaller scale in numbers of individual antennas and receivers. Sustained investments over the coming decade will likely mature a number of major technology areas. Ongoing work under NASA’s Astrophysics Strategic Mission Concept Study program could be expected to produce a technology roadmap for DALI in the near future.

General Cost Category in Which This Mission Concept Is Likely to Fall The DALI mission concept departs significantly from mission concepts whose costs can be estimated using standard methodologies. Because this mission is at least 15 years in the future, there are major uncertainties in the costs. The current mission concept envisions an imaging tomography baseline mission, with 1,000 stations and a dedicated data-relay satellite. Two Ares V launches would be required to deliver the complete payload to the surface of the lunar farside, including the rovers that would deploy the arrays. Although each element of this concept is, in itself, not particularly complex, the deployment of a large number of receivers on the lunar farside remotely makes this overall concept very complex, challenging, and costly. DALI will be a mission whose cost is likely to be above $5 billion.

ANALYSIS OF SPACE SCIENCE MISSION STUDIES 35

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The DALI observatory needs to be located in a radio-quiet region. The best candidate for such a radio-quiet environment is the lunar farside, out of view of terrestrial radio emissions. This makes Constellation eminently suitable to execute such a mission. The Constellation System, and specifically the Ares V and the Altair (cargo) lander, is an additional key enabling capability for the DALI concept. The estimated mass for the approximately 1,000 stations needed to receive the faint H I signal produces a large mass budget of around 35,000 kg, and an Ares V is needed to get such a mass to the Moon. Development of a cargo lander with the capability to land approximately 15,000 kg on the Moon would be required as a means for delivering DALI components to the lunar surface. Three DALI subsystems dominate the baseline design mass budget: the mini-rover/station/antenna subsystem, the correlator/central processor, and the power-storage subsystem. The estimated mass for a mini-rover, including antennas embedded in the polyimide film, is 25 kg. Each mini-rover has a battery mass of 10 kg, giving a total mass (for 1,000 stations) of 35 metric tons. While the exact number of antenna stations in the array may be modified as studies and analysis of relevant data progress, the number of antennas is likely to be large. The number depends on the redshift for which DALI is optimized, which will be based on results obtained from ground-based radio telescopes under construction, operating at higher frequencies albeit in a much noisier background environment, and on studies that may lead to much smaller arrays deployed during early expeditions to the Moon. At the present time, there is no information indicating that it is possible to significantly reduce the mass required to be delivered to the lunar surface while still meeting the scientific goals.

Assessment of the Mission Concept for Further Study The DALI mission is enabled by the Constellation System, requiring several Constellation capabilities, including the Ares V and lunar lander technology. Only the Constellation System is projected to have the capability of placing interferometric arrays on the lunar farside, which is currently considered to be the quietest (from a radio- frequency point of view) accessible location on or near Earth. The scientific impact for this mission is high, providing the unique capability for viewing the universe at a distinctive period between about 10 million and 300 million years after the big bang. This period, known as the Dark Ages, represents the last frontier in cosmology, the era between the genesis of the cosmic microwave background and the formation of the first stars and galaxies. The technical maturity for the DALI mission is rated as medium for the rovers and interferometrics and low for the need to reduce array mass and for deploying and operating in a remote location. Although the rover and interferometric technologies exist and are advancing, integrating them so that the rovers could emplace large arrays on the lunar farside would be extremely challenging. Adapting existing technology to operate in the harsh lunar thermal environment would also be difficult. A mission of this complexity to the lunar farside is likely to occur later rather than earlier in the queue of lunar missions employing Ares V, as NASA focuses on lunar near-side operations. In summary, although the implementation of the DALI mission has many daunting elements, the science is unique, and it cannot be addressed other than by placing radio arrays on the lunar farside. Therefore it is placed in the “deserving of further study” category. The primary near-term challenge will be reducing the mass of the antenna arrays in order to enable significant numbers to be landed on the Moon.

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FIGURE 2.5 Artist’s conception of one possible version of the Generation-X x-ray telescope. SOURCE: Courtesy of Roger Brissenden, Smithsonian Astrophysical Observatory. Figure 2.5.eps Bitmap image - Low resolution Generation-X11 Scientific Objectives of the Mission Concept The proposed Generation-X space-based x-ray telescope (Figure 2.5) complements all the other wavelength future-generation telescopes, such as the Atacama Large Millimeter Array (ALMA), JWST, the Square Kilometre Array (SKA), and Thirty Meter Telescope (TMT). The Gen-X telescope is designed to study the new frontier of astrophysics: the birth and evolution of the first stars, galaxies, and black holes in the early universe. X-ray astronomy offers an opportunity to detect these by means of the activity of the black holes and the supernova explosions and gamma-ray burst afterglows of massive stars. The Generation-X Vision Mission is based on an x-ray observatory with a 50 m2 collecting area at 1 kilo- electronvolt (keV) (1,000 times larger than the Chandra X-ray Observatory) and 0.1-arc second (arcsec) angular resolution (several times better than Chandra and 50 times better than the Constellation-X12 resolution goal). Such a high-energy observatory will be capable of detecting the earliest black holes and galaxies in the universe and will study the chemical evolution of the universe and extremes of density, gravity, magnetic fields, and kinetic energy, which cannot be created in laboratories. The goal of Gen-X is to observe the first black holes and stars at redshift z ∼ 10-20. The idea is to search for the effects of the early black holes on the formation of galaxies and to trace through time the evolution of galaxies, black holes, and the chemical elements.

Characteristics of the Mission Concept as Developed to Date Various configurations have been studied for Gen-X: a single 20-m mirror, six 8-m mirrors, and a single 16-m mirror. The committee considers the 20-m concept and the six 8-m mirror concepts to be exceedingly complex, and it decided to evaluate only the 16-m mirror configuration employing the Ares V launch vehicle.13 The Gen-X mission operates at Sun-Earth L2. A 16-m telescope will require either robotic or human-assisted

11 The first two subsections are based on material in R. Brissenden, Smithsonian Astrophysical Observatory, and the other members of the Gen-X Study Team, “Generation-X Vision Mission Study Report,” March 2006; and R. Brissenden, Smithsonian Astrophysical Observatory, “The Generation-X Vision Mission,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 21, 2008. See footnote 1 above. 12 Constellation-X is a proposed x-ray telescope. 13 There are numerous problems for the other configurations. For instance, the six 8-m mirror concept would require six launches of Delta IV Heavy rockets. The Delta IV Heavy requires 3 months of ground preparation prior to launch; therefore the launches alone would require 18 months of preparation, during which no other Delta IV rockets—necessary for national security payloads—could be launched.

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in-flight assembly (see Chapter 4 for a discussion of these technologies). The required effective area, 50 m2, implies that extremely lightweight grazing incidence x-ray optics must be developed. To achieve the required areal density, at least 100 times lower than in Chandra, technology development is needed to produce 0.2-mm- thick mirrors (approximately the width of two human hairs) that have active on-orbit figure control. The suite of detectors includes a large-field-of-view high-angular-resolution imager (wide-field imager), a cryogenic imaging spectrometer (microcalorimeter), and a reflection grating spectrometer. The technology relating to the mirrors would clearly require early development and investment. The mission concept proposes the delivery of a detailed technology development plan for the next decade that will get key optics and detector technologies evolved to TRL 6. The proposal suggests the need for a decade of development effort so that the mission could be realized in the following decade. The use of very thin mirrors with active control is a technological challenge. The success of the mission depends on this development effort. Initial adjustment and alignment, and then the maintaining of the alignment, will be challenging. The space assembly of the 16-m mirror has not been defined in detail. The proposers have suggested using robotic assembly, but human assembly remains another possibility.

Relative Technical Feasibility of the Mission Concept The Chandra X-ray Observatory has a collecting area of 0.08 m2 and uses glass shell technology. Gen-X has a 50 m2 collecting area and uses thin foils with on-orbit figure adjustment of shape to reduce the mass. The Ares V launch vehicle would simplify the mission concept because of the large payload fairing and mass to L2 capability. It has the potential of further simplifying the mirror development by allowing thicker mirror shells than were studied for the earlier configurations. The technical challenges are the mirror development and in-space assembly.

General Cost Category in Which This Mission Concept Is Likely to Fall A November 2007 Gen-X proposal that would employ the Ares V estimated a $4 billion cost (excluding launch costs).14 The committee believes that this is a low estimate. The technology development of the mirrors is the large unknown in the cost estimates. The committee believes that the cost of this mission will exceed $5 billion.

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The Ares V capability results in a simplified and more cost-effective baseline mission concept for Gen-X. It would be able to deliver Gen-X directly to the Sun-Earth L2 point. A single launch would allow a 16-meter- diameter mirror to be placed into position, would permit piezoelectric control of the mirror’s optical surface for final figure on-orbit to achieve ∼0.1-arcsec angular resolution, and would enable placing into position a 60-m extendable optical bench between the optics and the science instrumentation package. The estimated mass for the Gen-X Ares V configuration is 22,000 kg. The Ares V is capable of delivering 55,900 kg to translunar injection trajectory (essentially equivalent to that required to reach the Sun-Earth L2 point). This mass margin possibly could be used to reduce the risk in the mirror development by allowing the use of thicker mirrors.

14 R. Brissenden, Smithsonian Astrophysical Observatory, “A Concept Study of the Technology Required for Generation-X: A Large Area and High Angular Resolution X-ray Observatory to Study the Early Universe,” submitted in response to NNH07ZDA001N-ASMCS, No- vember 20, 2007.

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Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Gen-X mission concept is worthy of further study as a Constellation-enabled science mission. The major issue is the mirror development. The extra mass capability of the Ares V may enable the use of thicker mirrors and therefore significantly reduce risk.

Assessment of the Mission Concept for Further Study In its interim report,15 the committee determined that the Gen-X mission would be enhanced by the Constella- tion System, but it has reevaluated that decision and determined that it is enabled by Constellation. The proposal, as defined several years ago, required a future version of the Atlas V that does not currently exist. The mission can only be accomplished with a significantly larger launch vehicle—that is, an Ares V—than exists in the current EELV family. In addition, in order to fit on a smaller launch vehicle, the mission concept would require small and fragile mirrors that may be impossible to develop. The Ares V launch vehicle would simplify the mission concept because of its large payload fairing and mass capability to the L2 point. The scientific impact for this mission is high. The mission’s science goals are clearly defined. Such a high- energy observatory would be capable of detecting the earliest black holes and galaxies in the universe and could study the chemical evolution of the universe and extremes of density, gravity, magnetic fields, and kinetic energy, which cannot be created in laboratories. The technical maturity is rated as low for development of the mirror segment. One potential option is in-space assembly, which also has a low level of technical maturity. In summary, although the potential scientific impact of Gen-X is high, very challenging technical maturity issues place the mission in the “deserving of further study” category.

Modern Universe Space Telescope16 Scientific Objectives of the Mission Concept The Modern Universe Space Telescope is a 10-m, diffraction-limited optical-ultraviolet telescope (Figure 2.6) that establishes a new threshold in terms of sensitivity, imaging resolution, and scientific return.17 MUST would consist of a single large (4- to 6-m) central optical mirror surrounded by a number of ∼2-m petal-like segments. The segments would be robotically assembled around the primary mirror at the Sun-Earth L2 point, enabling a telescope architecture that would be scalable to larger mirror concepts in the future. The observatory would consist of four elements: the 10-m telescope, four or more science instruments, a spacecraft bus, and a sunshield/baffle. The proposed telescope would cover the wavelengths from 1,150 angstrom (Å) in the ultraviolet to approximately 2.5 μm in the near-infrared. Capabilities investigated for the canceled robotic Hubble Space Telescope servicing mission and developed for the International Space Station (ISS) would be used to assemble the observatory in space at the Sun-Earth L2 point. MUST’s resolution in the optical and ultraviolet wavelengths would isolate individual stars in other galaxies and extend the evaluative techniques that are used with objects in our own galaxy to distant ones. The telescope will focus on the following key scientific questions:

15 National Research Council, Science Opportunities Enabled by NASA’s Constellation System: Interim Report, The National Academies Press, Washington, D.C., 2008. 16 The first two subsections are based on material in J. Green, J. Bally, M. Beasley, R. Brown, D. Ebbets, W. Freedman, J. Grunsfeld, J. Huchra, S. Kilston, R. Kimble, J. Morse, R. O’Connell, K. Sembach, M. Shull, O. Siegmund, and E. Wilkinson, “The Modern Universe Space Telescope: A Vision Mission Concept Study for a Large UV-Optical Space Telescope,” Center for Astrophysics and Space Astronomy, Uni- versity of Colorado, Boulder, Colo.; J. Green, University of Colorado, Boulder, “The Next Large UV-Optical Space Telescope: The Modern Universe Space Telescope,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 21, 2008. See footnote 1 above. 17 This mission concept has been referred to as the Large Ultraviolet/Optical Modern Universe Space Telescope, or LUVO-MUST. LUVO is considered to be a class of telescopes, and MUST is a specific proposal within that class.

ANALYSIS OF SPACE SCIENCE MISSION STUDIES 39

ATLAS X 6.5 m DIA BY 20.4 m LONG COMPOSITE PAYLOAD FAIRING A A

USABLE PAYLOAD STATIC ENVELOPE 6500 mm 255.9 in MUST SECONDARY MIRROR STOWED

MUST MIRROR HUB

VIEW A 14561.8 mm MUST PRIMARY MIRROR 573.3 in SEGMENTS STOWED

AVAILABLE VOLUME FOR PROPULSION MODULE

LVA (SIP)

B B VIEW B

EELV 4394 -5 PAF BOLTED I/F

FIGURE 2.6 Artist’s illustration of the Modern Universe Space Telescope (MUST) inside its launch vehicle prior to assembly in space. SOURCE: Courtesy of NASA. Figure 2.6.eps Includes low resolution bitmap images

Text is too small • How are metals created and distributed through the modern universe? • How are galaxies assembled, and how do they evolve? • How do stars and planetary systems form, and how does this affect their likelihood of supporting life? • Where are the baryons in the modern universe, and how are they distributed?

The committee believes that this mission offers a significant advance in its scientific field. However, to receive broad community support to enable such a multibillion-dollar mission, any such telescope should address one or more of the emerging science areas that will have an impact that is qualitatively different from the impact of other contemporaneously proposed missions.

Characteristics of the Mission Concept as Developed to Date MUST would be a “next-generation Hubble Space Telescope” in many ways. Some of the technologies follow on from JWST in terms of large-mirror technology and construction and placement at L2. Coronagraphic performance at contrast levels needed for exoplanet science objectives (10−9 to 10−10) are likely to be limited owing to scattered light from the segmented mirrors. The currently planned detectors and optics requirements require minor advances from those available today. The mission is proposed to have a 10-year start-to-flight timescale. The only

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available mass estimate for this mission concept refers to the proposed Atlas V (with a 7.2-m fairing) variant, not an Ares V variant, and totals 16,500 kg (spacecraft bus [wet], 12,000 kg; dispenser for robotic assembly, 4,500 kg). MUST concentrates on science themes that are addressed largely by other observatories, such as JWST and ALMA, and that were the main themes of a similar mission, Space Telescope 2010, proposed during the 2001 astronomy and astrophysics decadal survey.18 The Space Telescope 2010 project was not highly ranked, owing to the limited advances of UV/optical performance when compared with less mature fields such as far-infrared astronomy, and also owing to the relative maturity of UV-unique observations when compared with emerging fields. The segmented-mirror design for MUST may preclude direct observation of extrasolar planets (one of its stated goals)—a scientific area that is one of the most exciting new possibilities in optical/infrared astronomy. This issue will merit further study for segmented-mirror optical telescopes in general.

Relative Technical Feasibility of the Mission Concept The key technologies identified for MUST are large-format, buttable high-quantum-efficiency charge-coupled devices (CCDs) and high-resolution, large-format anodes for improved microchannel plate detectors. None of these is likely to be challenging. The current baseline design of a segmented primary mirror might have low-enough scattered light to allow progress for observations of Earth-like planets in habitable zones. On-orbit assembly and verification for the large optics may benefit from astronaut involvement (presumably at a location closer to Earth than the operating location of the Sun-Earth L2 point; see Chapter 4). Optic coat- ings and tunable filters need to be developed as well. However, these do not represent substantial technological improvements from current technology.

General Cost Category in Which This Mission Concept Is Likely to Fall No cost estimate was provided for the MUST mission concept. However, extrapolating from the costs of HST and JWST, the committee estimates that MUST will cost more than $5 billion.

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches As indicated above, MUST was proposed for launch on an Atlas V Phase II launch vehicle with a 7.2-m fairing. Although this vehicle configuration has been identified as a potential growth option, it does not currently exist, and there are no plans at the moment to develop this upgraded capability. Current mission plans would use robotic assembly at the L2 point. Robotic assembly tools would be part of the launched telescope assembly payload. The 10-m primary mirror (4- to 6-m segment and surrounding 2-m petals) would be deployed using space shuttle and ISS robot arm technology. Reduction of risk and cost may be likely if astronaut participation in assembly at the Sun-Earth L2 point is used. Because the baselined launch vehicle does not exist, executing MUST would require that it be scaled back to fit within the existing capabilities of the Delta IV Heavy, or reconfigured to use the Ares V launch vehicle. The benefits of using the Constellation suite of launch vehicles would be the following: (1) an Ares V would allow a large (single) primary mirror (or a fully deployable larger single mirror) to be placed at the L2 point and (2) the Orion spacecraft, launched atop an Ares I rocket and then boosted out of low Earth orbit with the assistance of a departure stage (see Chapter 4), might provide the ability to assemble and/or service and upgrade the telescope at the Earth-Moon L1 or Sun-Earth L2 point. Although this would enhance the capabilities of the telescope, it would also increase costs. The long history of human servicing of HST offers lessons in the costs and advantages of this approach.

18 National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001.

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Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The MUST mission concept is worthy of further study as a Constellation-enabled science mission. The robotic assembly technique proposed in this mission concept could be simplified, enhanced, or eliminated with the capabilities of the Constellation System, particularly a launch vehicle with a larger shroud diameter. The remarkable success of HST and its continued importance to many fields of astronomy indicate that there should be further study of a next-generation large UV/optical space-based telescope.

Assessment of the Mission Concept for Further Study In its interim report,19 the committee determined that the MUST mission would be enhanced by the Constel- lation System, but the committee has reevaluated that decision and determined that it is enabled by Constellation. The proposal, as defined several years ago, required a future version of the Atlas V that does not currently exist. The mission can only be accomplished with a significantly larger launch vehicle—that is, Ares V—than exists in the current EELV family. It is possible that a smaller version of this telescope could be flown on an EELV, but that is not what was proposed. As is the case with the ATLAST proposal, the scientific impact of this mission is unclear. The mission proposal listed several broad goals cutting across the ultraviolet and optical spectral regions. A detailed science case for this mission should be developed, with trade-off studies of its various options examined. For example, the goals of imaging and spectroscopy of exoplanets present challenges in terms of scattered light for a segmented-mirror telescope such as MUST. The proposal suggests that several mirror aperture sizes are possible, but it does not clearly explain the science advantages and disadvantages of each. Several key technologies for MUST are immature at this time. The production, deployment, mass, and stability of a large segmented mirror that would be assembled in space presents significant challenges. As with ATLAST, other items appear to be at reasonable TRLs to allow anticipation of development in time for such a mission. In summary, because the MUST mission has medium/low ratings for the mission’s impact on science in the field and for its technical maturity, it is placed in the “deserving of further study” category. This program could significantly advance UV/optical astronomy. However, in order for this mission to move forward, the science goals for MUST have to be better defined.

ASTRONOMY AND ASTROPHYSICS/HELIOPHYSICS MISSION Stellar Imager20 Scientific Objectives of the Mission Concept The complex dynamics of the interaction of solar magnetic fields and plasma can result in the release of coronal mass ejections (CMEs), large clouds containing up to a few billion tons of plasma with the embedded solar magnetic field and moving away from the Sun at speeds up to a few thousand kilometers per second. These eruptions are most common during the maximum of the solar activity cycle. The associated accelerated relativistic particles can damage satellites and be harmful to astronauts and airline passengers. When the CME-associated shock waves impact Earth’s magnetic field, the effects of the resulting magnetic storms affect satellites and Earth- based technology such as navigation systems, communications systems, and power grids. The goals of the Stel- lar Imager are to build on current solar physics and heliospheric physics research in order to understand stellar

19 National Research Council, Science Opportunities Enabled by NASA’s Constellation System: Interim Report, The National Academies Press, Washington, D.C., 2008. 20 The first two subsections are based on material in K.G. Carpenter and the Stellar Imager Vision Mission Team, “SI—The Stellar Imager: A UV/Optical Deep-Space Telescope to Image Stars and Observe the Universe with 0.1 Milli-arcsec Angular Resolution,” NASA Goddard Space Flight Center, Greenbelt, Md., September 2005; K.G. Carpenter, NASA Goddard Space Flight Center, “Stellar Imager (SI): Reveal- ing Our Universe at High Resolution,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 20, 2008.

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FIGURE 2.7 The Stellar Imager, which uses a large array of formation-flying mirrors (left) to focus light on a common beam- combining hub (center) to produce ultrahigh-resolution images of stars and other celestial targets (right). SOURCE: Courtesy of K. Carpenter, NASA Goddard Space Flight Center.

magnetic activity and magnetic processes and their roles in the origin and evolution of structure and the transport of matter throughout the universe. The proposed Stellar Imager (Figure 2.7) will enable general astrophysics research. Its long-baseline interferometric capabilities would benefit astrophysics in many ways—allowing unprecedented images of active galactic nuclei, quasi-stellar objects, supernovae, interacting binary stars, supergiant stars, hot main-sequence stars, and protoplanetary disks. The Stellar Imager would provide information on the interiors of stars using astroseismology to contribute to the understanding ot internal structure, differential rotation, and large-scale circulations. Plans are for the Stellar Imager to use formation flying 1-m mirrors to form a Fizeau interferometer providing 0.1-milliarcsec resolution or, in “light bucket mode” (reflecting all of the photons from all of the elements into one focus), providing an equivalent 11-meter-mirror UV/optical telescope. The Stellar Imager would cover the ultraviolet and optical wavelengths (1,200-6,600 Å). It would provide images of dozens of stars over a period of up to a decade each. Stellar Imager would be composed of 20 1-m primary mirrors on a virtual surface (formation flying) that can have their baseline varied from 100 m up to 1,000 m depending on the target requirements. Stellar Imager would orbit at the Sun-Earth L2 point and would provide significant advances in the observation of stellar interiors and magnetic variability.

Characteristics of the Mission Concept as Developed to Date The general idea of interferometry from many ground-based instruments is quite mature. Space-based interferometric platforms are still immature, and Stellar Imager would benefit greatly from early-demonstration

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interferometric missions such as the Fourier-Kelvin Stellar Interferometer (FKSI) or Pegase (a proposed space mission to build a double-aperture interferometer composed of three free-flying satellites). The major technology requirements related to formation flying for Stellar Imager (see the subsection below) are all immature at the present time. Some small efforts in development have been undertaken through NASA and Air Force funding, but these efforts would need to be further developed in a timely manner to realize Stellar Imager operation in the 2020 time frame. Depending on the choice of the number of mirror satellites and other options, the payload mass ranges from 4,000 to 9,000 kg.

Relative Technical Feasibility of the Mission Concept The Stellar Imager is a technically challenging mission, but it would be a natural follow-on to ground-based interferometers (e.g., Center for High Angular Resolution Astronomy [CHARA], European Southern Observatory Very Large Telescope Interferometer [VLTI]), and space-based interferometers (e.g., FKSI, Pegase). The Fizeau interferometer mode will reposition the same mirrors to form the interferometer. Baselines from the central hub (containing the secondary mirror and focal-plane instrument) would range from 100 to 1,000 m. Because each Stellar Imager operating mode requires all-formation flying of the many mirrors, there is a significant technology challenge for all the options. The Stellar Imager is a somewhat easier concept than the Space Interferometer Mission or the Terrestrial Planet Finder Interferometer (TPF-I) (these two being astrometric or nulling missions) because of its less stringent tolerances for the interferometry. Both the SIM and TPF-I missions may offer technology development useful for the Stellar Imager. The main technology challenge and/or development areas for the Stellar Imager are as follows:

1. Formation flying of 20 spacecraft • Launch to and deployment at the Sun-Earth L2 point • Positioning and control of formation elements to 3 nm • Aspect control to tens of microseconds of arc • Need for variable, noncondensing, continuous-use micro-Newton thrusters • Large dynamic range of motion control required • Autonomous real-time analysis and/or real-time wavefront sensing control needed 2. Onboard and ground-based methods for dynamic control testing and validation.

There are additional, although less severe, challenges in technologies for mirror fabrication, spacecraft construction, and detector development. Key specific issues such as uncertainties about propellant (what propellant, how much propellant) and contamination issues involved with many free-flying mirrors (constantly firing thrusters) would need to be addressed as well. The Stellar Imager proposal team has a well-thought-out technology plan, at this early stage, for approaches to the needed technology development. However, this plan requires technology development by other programs that have not yet been flown.

General Cost Category in Which This Mission Concept Is Likely to Fall The Stellar Imager is a large, flagship-class mission estimated by the proposers to cost approximately $3 billion if flown with approximately 20 1-m mirrors and a single hub. Larger formation-flying mirrors and an additional hub (to increase scientific and operational efficiency as well as provide redundancy for risk mitigation) would be possible with a single Ares V launch vehicle, but it is currently not possible to estimate cost. As discussed above, Stellar Imager is technically challenging, and as such, the initial cost estimate is likely to be too low. The mission depends critically on earlier testbed missions for technology development in certain areas. Thus, it is likely that the mission falls in the higher-than-$5 billion category. It is hard to determine any cost savings that may occur if a single Ares V launch is accomplished compared

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with the use of a pair of Delta IV Heavy launches. The trade-offs between larger fairing and weight limits versus the complexity of initiating formation flying once the missions reach the Sun-Earth L2 point are difficult to assess at present. However, it seems highly likely, based on past experience, that mission complexity and risk (and thus cost) are far reduced by the use of a single launch because it would allow the integration of as much of the hardware as possible prior to launch.

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The scientific benefit and reliability of the Stellar Imager mission would be enhanced if an Ares V were available as the launch vehicle. Larger formation-flying mirrors could be easily accommodated in the fairing at only a modest cost increment for the mission. Larger mirrors would increase the sensitivity and science productivity of the observatory by increasing the amount of light gathered and enabling much faster seismic measurements of the observed star (and therefore more stars to be studied in less time). Flying two central hubs would also be possible, thereby providing risk mitigation and easing the complexity of deployment. Cost savings may occur as well using an Ares V because the Stellar Imager could be flown in a single launch, placing the entire mission payload in orbit and at the Sun-Earth L2 point. The Stellar Imager could be launched using the current fleet of EELVs. However, two launches using the Delta IV launch system would be required for the full mission concept.

Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Stellar Imager mission concept is worthy of further study as a Constellation-enhanced science mission. A large launch vehicle such as the Ares V could allow for the full concept to be placed in orbit with a single launch, thereby reducing launch risks. This mission would allow a dramatic improvement in the scientific understanding of our Sun and similar stars.

Assessment of the Mission Concept for Further Study This mission is enhanced by the Constellation System. Although a spacecraft could be launched on multiple smaller launch vehicles, this would increase risk and complexity. The Ares V launch vehicle would allow the spacecraft to be placed in orbit in a single launch. The scientific impact of this mission could be high, providing unprecedented study of stellar magnetism and activity cycles, potentially leading to a better understanding of the Sun and space weather. Additional science goals would study extreme physics environments unable to be re-created in laboratories. However, the developed science plan, as described, is fairly limited in scope and would likely only appeal to a small science community. An increase in scientific programs and goals to be investigated would greatly benefit the Stellar Imager mission concept. Additionally, it is unclear how a mission of limited duration—that is, less than typical solar-like stellar cycles (the Sun’s solar cycle is 11 years, and the Stellar Imager’s lifetime is 10 years)—would accomplish the primary mission goals. The technical maturity of this mission is low for the free-flying mirrors required to accomplish the scientific goals. Although interferometry as practiced on the ground is a mature technology, controlling many mirrors in a constellation in space has not been achieved yet. The development of onboard and ground-based methods for the dynamic control, testing, and validation of the mirrors involves technical challenges also at low maturity. Stellar Imager will benefit from early demonstration of interferometric missions if they are developed and flown. Technology investment in the formation flying of multiple spacecraft and their metrology and control is needed. Additional challenges include mass production of the free-flying mirror spacecraft, development of larger-format detectors, and methods to ensure a long mission lifetime. In summary, although the potential scientific impact of Stellar Imager could be higher, it is limited by the developed science plan and the limited lifetime of the mission, and the technical maturity of controlling a large number of free-flying mirrors is low, placing the mission in the “deserving of further study” category.

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FIGURE 2.8 Illustration of the Solar Probe 2 mission concept. SOURCE: NASA Goddard Space Flight Center.

HELIOPHYSICS MISSIONS Solar Probe 221 Scientific Objectives of the Mission Concept The proposed Solar Probe 2 would be the first spacecraft to directly explore with high time resolution in situ measurements of the solar corona at altitudes above the photosphere as low as 4 solar radii (perihelia of 5 solar radii), the source region of the solar wind. This mission would provide the necessary observations to solve the long-standing question of solar wind acceleration and coronal heating. Frequent and close encounters with the Sun would provide sufficient data to address the equally puzzling phenomena of energetic particle acceleration that result in an adverse near-Earth space environment for astronauts and space systems (see Figure 2.8). A solar probe mission to explore the source regions of the solar wind has been studied for many decades; however, it has been limited by technology requirements. The most recent decadal survey by the NRC for solar

21 The first two subsections are based on material in A. Szabo, D.C. Folta, J.P. Downing, and C. Roberts, NASA Goddard Space Flight Center, “Solar Probe 2,” in response to request for information by the Committee on Science Opportunities Enabled by NASA’s Constellation System, May 2008. See footnote 1 above.

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TABLE 2.3 Orbit Characteristics of Ballistic Solar Probe Solutions Mission Parameter Ares V Direct Ares I Multiple Venus Gravity Assists Ares V Multiple Venus Gravity Assists Perihelion (Rs) 18.5 7.7 5.0 Aphelion (AU) 0.91 0.73 0.74 Final orbit period (days) 129 87 86 Launch C3 (km2/s2) 275 87 200 Payload mass (kg) 500 2,500 5,800 Cruise phase (years) 0.16 15.0 7.3 Number of gravity assists 1 (Venus) 11 (3, Earth; 8, Venus) 8 (all Venus) SOURCE: Information about the vehicle’s capabilities with a Centaur V2 upper (third) stage was obtained from Marc Timm, Ares Program Executive, Exploration Systems Mission Directorate, NASA, “Constellation Overview,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 29, 2008.

and space physics, The Sun to the Earth—and Beyond, gave this mission high priority for the 2003-2013 decade.22 In its fiscal year (FY) 2009 budget, NASA initiated the start of the Solar Probe Plus mission, which would accomplish some of the goals of the decadal survey. The Solar Probe 2 mission would employ a spacecraft similar to that proposed for Solar Probe Plus, but could send it closer to the Sun by taking advantage of the propulsive capabilities of an Ares I rocket with an upper stage (and nuclear electric propulsion), or, more likely, by using the Ares V rocket. The proposed European Space Agency (ESA) Solar Orbiter/NASA Sentinels (Heliophysical Explorers [HELEX]) mission would explore the inner heliosphere, providing solar photospheric and coronal remote sensing observations along with multipoint in situ measurements of the heliosphere.23 Solar Probe 2 would provide short- duration but frequent dips into the solar corona, testing and validating the understanding and models developed on the basis of the long-duration systematic studies of HELEX. Thus, Solar Probe 2 would be part of the spacecraft fleet charged to develop the critical forecasting capability of the space radiation environment in support of human and robotic exploration. The 2005 Solar Probe Science and Technology Definition Team established the following detailed Solar Probe science objectives: determine the structure and dynamics of the magnetic fields at the sources of the solar wind, trace the flow of the energy that heats the solar corona and accelerates the solar wind, determine what mechanisms accelerate and transport energetic particles, and explore dusty plasma phenomena and their influence on the solar wind and energetic particle formation.

Characteristics of the Mission Concept as Developed to Date The Solar Probe 2 mission concept retains the technological advancements developed for the Solar Probe and Solar Probe Plus missions. The primary innovation of the present concept is the exploitation of Ares I and Ares V launch capabilities, which might provide both a low perihelion distance (as low as 5 solar radii) and frequent revisit times without the use of radioisotope thermoelectric generators. The Constellation System offers several options for Solar Probe 2 to reach a low perihelion orbit. These are summarized in Table 2.3. In summary, several options exist using the Ares vehicles to obtain low perihelion orbit with short orbital periods, thus meeting the requirements to penetrate deeply into the solar corona and shorten the revisit times of this region. The Solar Probe 2 spacecraft bus design as proposed will follow very closely the Solar Probe Science and

22 National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003. 23 See http://hubble.esa.int/science-e/www/object/index.cfm?fobjectid=41396.

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Technology Definition Team Report concept. It is a three-axis-stabilized spacecraft designed to operate in the extreme thermal environment of the solar corona. The spacecraft bus will be protected by a 2.7-meter-diameter carbon-carbon conical primary heat shield with a secondary shield attached to its base. The bus consists of a hex- agonal equipment module and a cylindrical adapter. The equipment module will house the spacecraft subsystems and all of the instruments. The Solar Probe 2 instrumentation is identical to the 2005 concept, and it consists of both in situ and remote sensing instruments serviced by a common data processing unit and low-voltage power supply. The in situ instrumentation includes the following: fast ion analyzer, two fast electron analyzers, ion composition analyzer, energetic particle instrument, magnetometer, plasma wave instrument neutron/gamma-ray spectrometer, and coronal dust detector. The remote sensing package consists of a polar source region imager for extreme ultraviolet and magnetographic imaging of the solar wind source regions and a hemispheric imager for the white-light imaging of coronal structures. To facilitate data telemetry and commanding, the spacecraft will be equipped with one high-gain antenna, one medium-gain antenna, and two low-gain antennas for emergencies. X band will be used for both data downlink and commanding. Optionally, the higher-telemetry-rate Ka band can be employed for larger-data-volume downlink. The estimated total power requirement of the spacecraft is ∼230 watts (W). Solar Probe 2 would be powered by retractable solar panels. During solar close encounters, the primary solar panels will be retracted and the spacecraft will rely on the combination of smaller high-temperature secondary solar panels and batteries. The total spacecraft mass is estimated to be ∼900 kg, well inside the lift capacities of the Ares I and V launch vehicles.

Relative Technical Feasibility of the Mission Concept The mission concept and instruments required for the Solar Probe 2 mission do not present any technical challenges. The thermal control technology is apparently well developed. The ability of Ares V in particular to achieve the mission goals still requires further review but appears feasible.

General Cost Category in Which This Mission Concept Is Likely to Fall Precise cost estimation of this mission without a detailed study is difficult. Previous Solar Probe studies estimated the total cost of the mission at between $700 million and $1.4 billion, including the launch vehicle. The present estimate does not include the cost of the launch vehicle: based on earlier studies of Solar Probe missions, the total mission cost would be expected to be more than $1 billion.

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The use of Constellation System vehicles as described in this mission concept proposal would enable a Solar Probe 2 mission. Ares I or Ares V launch vehicles, together with gravity assists by Venus, would get the probe into the 4 to 7 solar radii position. This would result in an orbital period around the Sun of under 100 days, thus allowing multiple passes through the lower solar corona. This can be accomplished without the need for radioisotope thermoelectric generators or electric propulsion, although adding these would provide enhancement of the mission. The currently developed Constellation System uniquely enables a Solar Probe 2 mission with an orbital configuration that combines the advantages of the 2005 and 2008 Solar Probe studies, significantly enhancing the returned science data compared with the data from any of the previous mission designs.

Assessment of the Mission Concept for Further Study The Solar Probe 2 mission is enabled by the Constellation System, which allows insertion of the satellite into a short-period orbit as close as 5 solar radii to the surface of the Sun. The mission would be enhanced by the use of additional propulsion, which would be accommodated with the large Ares V lift capabilities.

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The potential impact of the Solar Probe 2 on scientific knowledge of the Sun and solar wind is very high. The science goals of Solar Probe 2 have been a high priority in the heliophysics community for many decades, and the mission has continually been endorsed by decadal surveys and NASA roadmaps. For budgetary and programmatic reasons, a version of the mission known as Solar Probe Plus, which only reaches 10 solar radii but has a shorter orbital period than that of the original design, is currently being planned by NASA. Solar Probe 2 would provide the opportunity to recover the fundamental science discoveries enabled by reaching the much closer approach to the Sun (4 to 5 solar radii, depending on the specific implementation) with the short (less than ∼100-day) orbital period. In addition, some implementations of Solar Probe 2 could reach approximately 50 degrees latitude, thus recovering the higher-latitude observations. The Constellation System would provide the propulsion needed to enable the mission without the need for new technologies. The technical maturity for this mission is rated as high. The instrument payload is based on existing instruments. Technology development, including heat shields, has already been done. A number of different mission concepts were examined. The mission concept employing only conventional propulsion and power is well developed and at high TRL. There are also implementations requiring nuclear electric propulsion, which provide significant improvements over the conventional implementations. These have low technology readiness levels. Because the mission payload is based on one developed many years ago, it would be useful to revisit the optimal payload in light of recent instrument development. It seems likely an enhanced payload could be accommodated with the Constellation capability. In summary, Solar Probe 2’s combination of clearly defined, high-priority science goals of major interest to the community and mature instrument and spacecraft technology places this mission in the “more deserving of further study” category. Further study should be narrowly focused on evaluating the ability to use the Ares V to obtain the desired orbit.

Interstellar Probe24 Scientific Objectives of the Mission Concept The heliosphere, the bullet-shaped bubble created by the interaction of the solar wind and solar magnetic field with the interstellar medium, shields our solar system from most interstellar plasma, cosmic rays, and dust. The Interstellar Probe (Figure 2.9) will travel beyond the heliopause (the boundary of the heliosphere) to study cosmic rays, plasma, neutral particles, and dust that constitute the galactic environment close by our solar system. It is designed to answer the following fundamental questions:

1. What is the nature of the nearby interstellar medium? 2. How do the Sun and galaxy affect the dynamics of the heliosphere? 3. What is the structure of the heliosphere? 4. How did matter in the solar system and interstellar medium originate and evolve?

The heliosphere moves through the interstellar medium at supersonic velocity relative to the medium creating a shock bounding the heliosphere at about 180 to 200 astronomical units (AU). The Interstellar Probe would explore interstellar space located beyond this boundary. To reach interstellar space in a reasonable amount of time will require a spacecraft with unprecedented propulsion capability. The initial NASA-funded Vision Mission concept studies included two implementations of

24 The first two subsections are based on material in T.H. Zurbuchen, University of Michigan, and the ISP [Interstellar Probe] Vision Mis- sion Team, “Nuclear-Powered Interstellar Probe, Final Report 2006,” 2006; and R.L. McNutt, Jr., Johns Hopkins University Applied Physics Laboratory, “Enabling a Faster, Better Innovative Interstellar Explorer with NASA’s Constellation System,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 21, 2008. See footnote 1 above. Note that the presentation to the committee was not made by a member of the team that delivered the final report. However, the presentation was made at the recommendation of the principal investigator for “Nuclear-Powered Interstellar Probe, Final Report 2006,” and both studies had nearly identical scientific recommendations.

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FIGURE 2.9 One possible Interstellar Probe configuration during its flyby of Jupiter. SOURCE: Reprinted with permission of the Johns Hopkins University Applied Physics Laboratory.

the Interstellar Probe, one employing nuclear reactor technology25 and one using solar sails.26 Although a final report was written only for the nuclear option, the committee received a presentation from R.L. McNutt on three others: a Delta IV launch vehicle with a spacecraft using a solar sail (the original Vision Mission proposal prepared by McNutt), a Delta IV launch vehicle with a spacecraft using radioisotope electric propulsion, and an Ares V launch vehicle with a spacecraft also using radioisotope electric propulsion.27 These three versions (collectively referred to as Innovative Interstellar Explorer) differ primarily only in propulsion options. The spacecraft would weigh approximately 1,230 kg. All four approaches have almost identical science goals and very similar instrument payloads. The main difference is that, because of its large size, the nuclear reactor implementation included

25 T.H. Zurbuchen, University of Michigan, and the ISP [Interstellar Probe]Vision Mission Team, “Nuclear-Powered Interstellar Probe, Final Report 2006,” 2006. 26 R.L. McNutt, Jr., Johns Hopkins University Applied Physics Laboratory, “Enabling a Faster, Better Innovative Interstellar Explorer with NASA’s Constellation System,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, Febru- ary 21, 2008. 27 R.L. McNutt, Jr., Johns Hopkins University Applied Physics Laboratory, “Enabling a Faster, Better Innovative Interstellar Explorer with NASA’s Constellation System,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, Febru- ary 21, 2008.

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remote sensing instruments in the visible, ultraviolet, and infrared wavelengths, allowing it to observe the universe from a unique vantage point beyond the zodiacal light and far from Earth. Because of the similarity of the primary instruments and science goals, the concept maturity, technology readiness, and advantages gained through use of the Constellation System, the four approaches are collectively discussed together below. The Interstellar Probe (previously called Interstellar Explorer) was endorsed by the 2003 NRC decadal survey The Sun to the Earth—and Beyond.28 The mission concept is included in the 2005 Heliophysics Roadmap as a Vision Mission. It has also been discussed in earlier roadmaps and decadal surveys. In addition, an ESA Technol- ogy Reference Studies report, funded through the Science Payload and Advanced Concepts Office29 and published in April 2007, describes a solar sail mission with two close flybys of the Sun. This mission may therefore provide an opportunity for international cooperation. The Interstellar Probe mission logically follows on the limited in situ observations by the Voyager spacecraft and the upcoming remote sensing observations to be obtained by the Interstellar Boundary Explorer (IBEX), which will image the large-scale shape of the boundaries separating the heliosphere from the interstellar medium. Because of its complete instrument payload and data rate, the Interstellar Probe would provide a very significant improvement over existing missions, potentially enabling revolutionary discoveries about our local galaxy.

Characteristics of the Mission Concept as Developed to Date The scientific concept, instrument package, and mission plan of the Interstellar Probe are quite mature. The goal is to leave the solar system within ∼20° of the incoming interstellar wind and to reach 200 AU in as short a time as possible. Most of the scientific instruments on the Interstellar Probe have substantial heritage. These include a fast plasma instrument to measure the solar wind, pickup ions, and interstellar plasma; a magnetometer to measure the quasi-static magnetic field; and a plasma/radio-wave receiver. There are also instruments to measure energetic particles, an instrument to measure the composition of the dust in the interstellar medium, and a neutral imager. The Interstellar Probe requires either a nuclear reactor or radioisotope thermoelectric generator/radioisotope power system (RTG/RPS). The mission also requires high-capability propulsion. Four options were considered: conventional, nuclear reactor electric, radioisotope electric, and solar sail. Given the current lack of technology development programs for nuclear propulsion and for solar sails, the conventional propulsion or the radioisotope electric propulsion (REP) provides the most mature mission concept. The major disadvantages are longer flight time to 200 AU, ∼30 years with Jupiter gravity assist versus 20 years for nuclear, or 15 years for solar sail. There are also trade-offs in the use of a Jupiter assist, which requires the use of radiation-hardened parts and shielding.

Relative Technical Feasibility of the Mission Concept The scientific concept, instrument package, and mission plan for the Interstellar Probe are quite mature. For all four implementations, the primary technology issues are (1) the need for radioactive power sources, (2) qualifying parts and ensuring instrument operations for a 30- to 50-year lifetime, and (3) a Deep Space Network upgrade to a phased array of many antennas. All four implementations have propulsion technology issues. The nuclear reactor option described in the Vision Mission report by Zurbuchen and the ISP Vision Mission Team is the more complex of the missions because of the low technology readiness level of the nuclear reactor and the relatively low technology readiness level of the nuclear electric propulsion options. The mission plan also incorporated a mother spacecraft with the nuclear reactor and remote sensing instruments and two identically instrumented daughter spacecraft with in situ particles and fields instruments that would be released after most of the reactor burnout. This approach provides additional science capability, but at a higher cost. Because there is no nuclear reactor development program at this time, this option is not considered in the next subsections.

28 National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003. 29 See http://sci.esa.int/sciencee/www/object/index.cfm?fobjectid=36022.

ANALYSIS OF SPACE SCIENCE MISSION STUDIES 51

General Cost Category in Which This Mission Concept Is Likely to Fall Conventional propulsion (using the Ares V with an additional Centaur upper stage) and the REP place this mission in the $1 billion to $5 billion category.30

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches Although three different options were considered, the solar sail option was not presented to the committee in great detail because it requires the most new technology development. The other two options were described in greater detail (see Table 2.4). One was a Delta IV Heavy with two solid rocket motors as upper stages; the other was an Ares V with Centaur upper stage (a capability that NASA has no plans for at the moment). Both options require an RTG power source and electric propulsion. For the same spacecraft dry weight, the Ares option reached 200 AU in ∼23 years, compared to 30 years for the Delta IV option, and was traveling 1.9 AU/year faster at that time. The benefits of using Constellation are avoiding nonconventional propulsion (solar sail or a nuclear reactor with electric propulsion). The Delta IV option is a unique configuration that has never been flown. It would require further study to determine if it is feasible and, if so, the cost relative to the Ares V option.

Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Interstellar Probe mission concept is worthy of further study as a Constellation-enabled science mission. Using an Ares V, the mission may provide a very significant improvement over earlier mission concepts. The use of the Ares V may enable the mission to be flown using only conventional propulsion, thus removing the need for solar sail technology or REP, and it could shorten the time to reach 200 AU. Shortening the mission lifetime would reduce costs and risk. Additional capability may also enable instrument package redundancy, further reducing risk. The science motivation for the mission has continued to receive the endorsement of decadal surveys and other studies for two decades.

Assessment of the Mission Concept for Further Study In its interim report, the committee determined that the Interstellar Probe would be enhanced by the Constel- lation System, but it has reevaluated that decision and determined that it is enabled by Constellation. Reaching interstellar space in a reasonable time requires a spacecraft with extremely high propulsion capabilities. One of the two original Vision Mission studies used a nuclear reactor, and the other used solar sails. Preliminary studies indicate that using the Constellation System will enable the mission to reach the desired distance of 200 AU in a reasonable time (approximately 25 years) using only conventional propulsion. Without the propulsion capability of the Ares V launch vehicle, such a mission would take from 30 to 50 years, a time period that the committee determined to be unrealistic for effective science return. The potential impact of the Interstellar Probe mission on science in the field is very high. The Interstellar Probe would travel outside the heliopause (the boundary of our solar system) to study cosmic rays, plasma, neutral particles, and dust that constitute the galactic environment close by our solar system. It is designed to determine the nature of the nearby interstellar medium, the structure of the heliosphere, how the galaxy affects the dynamics of the heliosphere, and how matter in the solar system and interstellar medium originated and evolve. The Inter- stellar Probe has been endorsed for more than 20 years by both NRC and NASA panels, and its science goals—to provide fundamental new observations of the interstellar medium—are highly rated. The technical maturity for this mission is relatively high. The planned instrument payload has substantial heritage, and similar instruments have flown on numerous heliophysics missions. The primary technology issues

30 This estimate is based on the presentation to the committee by R.L. McNutt, Jr., Johns Hopkins University Applied Physics Laboratory, “Enabling a Faster, Better Innovative Interstellar Explorer with NASA’s Constellation System,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 21, 2008.

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TABLE 2.4 Comparison of the Two Innovative Interstellar Explorer Options Using the Delta IV Heavy and Ares V Delta IV Heavy + 2 Star 48As—Option 1 Ares V + Centaur—Option 1 Launch date October 22, 2014 December 3, 2015 Gravity assist body Jupiter Jupiter Gravity assist date February 5, 2016 October 10, 2016 Gravity assist altitude 75,150 km 128,855 km Gravity assist radius 2.05 Rj 2.80 Rj Gravity assist 2∆ 23.8 km/s 25.1 km/s Burnout date October 13, 2032 July 3, 2030 Burnout distance 104 AU 116 AU Burnout speed 7.9 AU/year 9.8 AU/year Date 200 AU reached December 31, 2044 February 14, 2039 Minimum trip time to 200 AU 30.2 years 23.2 years Speed at 200 AU 7.8 AU/year 9.7 AU/year Right ascension at 200 AU 263.8° 248.7° Declination at 200 AU 0.0° 0.58° Launch mass 1,230 kg 1,230 kg Propellant mass 440 kg 440 kg Final mass 790 kg 790 kg Power 1.0 kW 1.0 kW Isp (specific impulse) 3,800 s 3,410 s EP system efficiency 53.8% 53.5% Total stack C3 123.3 km2/s2 270.0 km2/s2 EP 2∆ 16.5 km/s 14.8 km/s Thrust time 18.0 years 14.6 years NOTE: Both spacecraft would also require radioisotope electric propulsion. SOURCE: Adapted from R.L. McNutt, Jr., Johns Hopkins University Applied Physics Laboratory, “Enabling a Faster, Better Innovative Interstellar Explorer with NASA’s Constellation System,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 21, 2008.

are (1) the need to have radioactive power sources, (2) the qualifying of parts and ensuring of instrument operations for a longer-than-20-year lifetime, and (3) a Deep Space Network upgrade to a phased array of many antennas. In summary, the Interstellar Probe’s combination of clearly defined, high-priority science goals and mature instrument technology places this mission in the “more deserving of further study” category. Further study should be primarily narrowly focused on evaluating the ability to use the Ares V to obtain the desired orbit.

Solar Polar Imager31 Scientific Objectives of the Mission Concept The current understanding of the Sun and its atmosphere is severely limited by the lack of good observations of the polar regions. The Solar Polar Imager is a spacecraft (Figure 2.10) in a 0.48-AU circular orbit around the Sun with an inclination of 75° to the ecliptic plane, intended to provide remote sensing observations of the Sun and in situ observations (measurements of the local properties of the plasma and electromagnetic fields) of this critical region. Observations of the Sun’s poles, not possible from the usual ecliptic viewpoint, may revolutionize

31 The first two subsections are based on material in P.C. Liewer, D. Alexander, J. Ayon, L. Floyd, G. Garbe, B. Goldstein, D. Hassler, A. Kosovichev, R. Mewaldt, N. Murphy, M. Neugebauer, A. Sandman, D. Socker, S. Suess, R. Ulrich, M. Velli, A. Vourlidas, and T.H. Zurbu- chen, “Solar Polar Imager: Observing Solar Activity from a New Perspective, Vision Mission Study Final Report,” Jet Propulsion Laboratory, Pasadena, Calif., December 2005; and D. Socker, U.S. Naval Research Laboratory, for the Solar Polar Imager Vision Mission Study Team, “Solar Polar Imager: Observing Solar Activity from a New Perspective,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 21, 2008. See footnote 1 above.

ANALYSIS OF SPACE SCIENCE MISSION STUDIES 53

FIGURE 2.10 Illustration of the Solar Polar Imager spacecraft. SOURCE: Courtesy of NASA, the Jet Propulsion Laboratory, and California Institute of Technology.

scientific understanding of the internal structure and dynamics of the Sun and its atmosphere. Through the use of remote sensing and in situ instrumentation, the Solar Polar Imager would investigate the physical connection between the Sun, the solar wind, and solar energetic particles. The primary science questions motivating the Solar Polar Imager mission are the following:

1. What is the relationship between the magnetism and dynamics of the Sun’s polar regions and the solar cycle? 2. What is the three-dimensional global structure of the solar corona, and how is this influenced by solar activity and coronal mass ejections? 3. How are variations in the solar wind linked to the Sun at all solar latitudes? 4. How are solar energetic particles accelerated and transported in radius and latitude? 5. How does the total solar irradiance vary with solar latitude? 6. What advantages does the polar perspective provide for space weather prediction?

Because the Solar Polar Imager will make the first close-up remote sensing measurements of the polar regions of the Sun, the possibility exists for surprising discoveries and greatly improved understanding. The Solar Polar Imager would serve as a pathfinder for permanent high-ecliptic-inclination solar sentinels, if it is deemed that the polar perspective is important for space weather prediction. The Solar Polar Imager will have a Doppler-magnetograph imager, a white light coronagraph, an extreme ultraviolet imager, a total solar irradiance monitor, an ultraviolet spectrograph, a magnetometer, a solar wind ion composition and electron spectrometer, and an energetic-particle instrument. In the original Vision Mission study, the spacecraft used a Delta IV launch vehicle and required 6.7 years of flight time to reach its science orbit. Once it reached its intended orbital distance from the Sun, it would use a solar sail to achieve the highly inclined orbit of 75°. The use of the solar sail determines the mission duration, the choice of launch vehicle, the spacecraft configuration and control subsystems, and the operations cost during the long cruise. It also complicates the thermal, power, and telecommunications designs. Use of the Ares V might permit use of solar-electric propulsion or perhaps direct injection with consequent simplification and shortening of the time to arrive at the science orbit. The science goals for the mission require simultaneous observations in the ecliptic plane for the helioseismology and magnetographic studies, and the cost for these is not included in the Solar Polar Imager mission description.

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The only measurements obtained to date of the Sun’s polar regions are the observations of the solar wind made by the Ulysses spacecraft (in a polar orbit around the Sun) at 2 to 3 AU. The structure of the solar wind at the poles was very different at solar maximum compared with solar minimum, but Ulysses’s long (8 year) orbital period permitted only snapshots of the solar wind speed and magnetic field at the poles. Ulysses did not have any remote sensing instruments to probe solar structure and magnetic fields. In comparison, the Solar Polar Imager orbit would provide measurements of the poles every 4 months, closer to the Sun, and with more complete diagnostics. Although Solar Polar Imager is not mentioned by name in the NRC’s 2003 solar and space physics decadal survey, a multi-spacecraft mission to provide imaging of the poles of the Sun is described as an important next step in providing understanding of the solar structure, magnetic field, and dynamics, and a multi-spacecraft mission is a possible enhanced version of the Solar Polar Imager mission.32 The committee believes that this mission does offer a significant advance in its scientific field, but it will have to be further reviewed in a future decadal survey. The ESA Solar Orbiter mission is planned to reach latitudes of approximately 32°. ESA has performed a preliminary study of the Solar Polar Orbiter, making this mission a candidate for international cooperation.33

Characteristics of the Mission Concept as Developed to Date The scientific concept, instrument package, and mission plan of the Solar Polar Imager are mature. The primary technology issues are (1) the need to use solar sails to reach the desired polar orbit, (2) the weight and power requirements for a total solar irradiance monitor, and (3) the flight test needed for the Doppler magnetograph and the UV spectrograph. The mission concept as described depends on the use of solar sails, which are an untested technology that currently has no development path. If use of the Ares V would allow the high-inclination orbit to be attained with solar-electric propulsion or conventional propulsion, it would greatly simplify the mission.

Relative Technical Feasibility of the Mission Concept The instrument payload of the Solar Polar Imager is relatively mature, with essentially all instruments, except the Doppler magnetograph imager, having heritage from many missions, including Solar and Heliospheric Observa- tory, Solar Terrestrial Relations Observatory, Ulysses, Cassini, Fast Auroral Snapshot/Time History of Events and Macroscale Interactions during Substorms (FAST/THEMIS), Mars Reconnaissance Orbiter, and Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER). The Doppler magnetograph imager design is based on a ground instrument that has received study as a flight instrument. There is no existing total irradiance monitor that can meet the low weight and power requirements, but there is an ongoing instrument development program. The telemetry assumes the use of X band and a new Deep Space Network using 180 12-meter-diameter antennas, although the Solar Polar Imager could use Ka band and 30-m antennas with an increase in power and mass. The mission concept for the Solar Polar Imager is mature. Propulsion to enable the spacecraft to reach the desired high-inclination orbit is the only element that is not mature.

General Cost Category in Which This Mission Concept Is Likely to Fall The cost estimated for the initial Delta IV version of the Solar Polar Imager was less than $1 billion. The cost of this mission is likely to be approximately $1 billion. Although there are cost benefits in building multiple spacecraft, adding two extra spacecraft would increase the overall mission cost.

32 National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003. 33 See http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=36025.

ANALYSIS OF SPACE SCIENCE MISSION STUDIES 55

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The Solar Polar Imager would require the development of solar sail technology to reach the desired orbit using a Delta IV launch vehicle. However, with a launch on Ares V, the mission may be achievable without solar sails, but using an upper stage and solar-electric propulsion with advanced ion thrusters. The result would be a shorter time to mission orbit (5 years compared with 8 years). Eliminating the solar sails would also simplify the mission. Because power and mass were drivers in the initial design of instruments, there may also be some simplification in instruments if the mission switches to the Ares V approach. However, the benefits of using Ares V are still not fully studied. Another interesting possibility would be the launch of multiple satellites to monitor solar activity simultaneously. The baseline Solar Polar Imager concept observes a particular point on the Sun every 4 months; however, activity on the Sun occurs on much shorter timescales. The optimum would be to insert three identical satellites into equally spaced polar orbits, thereby ensuring continuous coverage of the Sun. Given the projected launch capability of the Ares V with a dual-engine Centaur, it appears that two identical spacecraft could be inserted into complementary orbital inclinations, providing enhanced coverage of the Sun. (Note that NASA has no current plans to adapt the Centaur to the Ares V.) Additional study is required to determine if a third spacecraft could be launched on a single mission in order to provide more continuous coverage of the polar region. The cost of the project would increase primarily by the cost of the additional spacecraft. Some of the Solar Polar Imager science goals require simultaneous observations from the ecliptic plane, which could possibly be provided by these additional spacecraft. The Vision Mission report suggests that astronauts could assemble the satellite for the solar sail implementation, an activity that would not be necessary if this mission is launched on an Ares V.

Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Solar Polar Imager mission concept is worthy of further study as a Constellation-enhanced science mission. The Ares V launch vehicle could allow the elimination of the unproven solar sail technology. The Solar Polar Imager would make unique observations of the Sun’s polar regions, dramatically improving and possibly changing current understanding of the behavior of the star closest to Earth. This better understanding could be applied to the study and understanding of stars elsewhere in the universe. The enhanced understanding may enable predictive capabilities about the behavior of the Sun, needed for space weather and climate prediction.

Assessment of the Mission Concept for Further Study The Solar Polar Imager mission is enhanced by the Constellation System. Without Ares V, the desired orbit could be achieved through the use of solar sails, an unproven technology (discussed further in Chapter 4). Ares V offers the potential to place the spacecraft (one or more) into the proper orbit without solar sails. The potential impact of this mission on science in the field is large. The Solar Polar Imager would provide the first imaging of the Sun’s poles with repeated observations at short intervals due to the close-in orbit, yielding comprehensive diagnostics that may dramatically improve understanding of the structure and dynamics of the Sun and solar magnetic field. Although Solar Polar Imager has not specifically been endorsed by the decadal surveys or roadmaps, the need for imaging of the polar regions of the Sun was clearly elucidated by the 2003 solar and space physics decadal survey. The science goals are important and compelling. The Solar Polar Imager provides a significant improvement over currently available observations of the Sun. The technical maturity for this mission is relatively high overall. The instrument payload has strong heritage; most instruments have flown on previous heliophysics missions. Reaching the desired polar orbit over the Sun with a short orbital period requires large propulsion capabilities. Two possibilities for accomplishing this involve using solar-electric propulsion or undeveloped solar sail technology. Using the Constellation propulsion capabilities and therefore eliminating the solar sails would simplify the mission. However, the required solar-electric propul-

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sion systems are currently at TRL 6. A number of options were developed by the original Vision Mission team to employ the Constellation System. However, the use of Ares V with Centaur and the additional propulsion needed from solar-electric propulsion required additional study. There are options to launch more than one satellite that could significantly enhance the science return of the mission, but these options also require additional study. In summary, the Solar Polar Imager’s combination of clearly defined, high-priority science goals and mature instrument technology places this mission in the “more deserving of further study” category. Further study should be primarily narrowly focused on evaluating the ability to use the Ares V to obtain the desired orbit and on answering the question of whether additional satellites could be launched simultaneously to provide multipoint observations.

SOLAR SYSTEM EXPLORATION MISSIONS Exploration of Near Earth Objects via the Crew Exploration Vehicle34 Scientific Objectives of the Mission Concept Near-Earth objects (NEOs) are asteroids and comets whose orbits come close to Earth’s orbit around the Sun. There is a chance that on occasion a NEO will impact Earth. NEOs range in size from hundreds of meters to tens of kilometers in diameter. In recognition of this potential threat, NASA started the Spaceguard project and, more recently, Congress directed NASA to detect, track, catalogue, and characterize 90 percent of all NEOs down to 140 meters in diameter by 2020. Planetary scientists have long speculated about sending a spacecraft to the vicinity of an asteroid, particularly one whose orbital path brought it near Earth. A spacecraft can easily approach and escape from an asteroid’s weak gravitational field. In fact, landing equipment on and retrieving samples from an asteroid’s surface has already been accomplished robotically. A study sponsored by the Advanced Projects Office within NASA’s Constellation Program examined the feasibility of sending an Orion spacecraft to a NEO (see Figure 2.11). An ideal mission of 90 to 180 days would take a crew of two or three astronauts for a 7- to 14-day stay in the vicinity of a NEO. Compelling reasons for sending humans in spacecraft to a NEO include being able to directly see and acquire data to characterize surface characteristics, morphology, mineral composition, and so on for the NEO. Human direction of sample collecting would enable quick, informed decisions in acquiring samples of different locations on the NEO’s surface and could allow the geological context of those samples to be determined. Such samples could lead to major advances in the areas of geochemistry, impact history, thermal history, isotope analyses, mineralogy, space weathering, formation ages, thermal inertias, volatile content, source regions, solar system formation, and so on. Samples directly returned from a primitive body may provide the same kind of breakthrough that the Apollo samples have done for the understanding of the Moon and its formation history. The main goal of the Exploration of Near Earth Objects via the Crew Exploration Vehicle mission would be to collect macroscopic samples from various terrains on the NEO’s surface by means of astronaut extravehicular activities (EVAs). This would enable sample collection in geological context. Intact samples of the surface would also be used to evaluate space weathering and surface alteration effects in a deep space environment. In addition, supplemental robotic collection of samples from either different or difficult-to-reach sites on the NEO would expand the sample suite. Another primary goal of this mission would be to investigate and determine the interior characteristics of the target NEO. This would place some constraints on the macroporosities that may be found among this popu- lation of objects and help scientists understand the accretion and impact history of the early solar system. Such investigations could be combined with a detailed examination of any features or structures associated with crater formation in microgravity environments to further refine impact-physics models. Active detonation of a kinetic

34 The first two subsections are based on material in P.A. Abell, D.J. Korsmeyer, R.R. Landis, T.D. Jones, D.R. Adamo, D.D. Morrison, L.G. Lemke, A.A. Gonzales, R. Gershman, T.H. Sweetser, and L.L. Johnson, “Exploration of Near-Earth Objects via the Crew Exploration Vehicle,” NASA Johnson Space Center, Houston, Tex., in response to request for information by the Committee on Science Opportunities Enabled by NASA’s Constellation System, May 2008. See footnote 1 above.

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FIGURE 2.11 Illustration of an Orion spacecraft approaching a near-Earth object. SOURCE: Hyabusa image courtesy of JAXA. Orion image courtesy of NASA. Photoillustration by Tim Warchocki.

energy experiment after the deployment of a seismic network would also serve to measure the internal properties of the NEO, while gaining insights into the effects of crater excavation. Momentum transferred to the NEO orbit after charge detonation could be measured, and the change in orbital motion of the NEO could be observed. Such information might provide useful insights for future hazard-mitigation scenarios.

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Characteristics of the Mission Concept as Developed to Date This study of a mission to an asteroid has not progressed as far as have some of the Vision Mission studies. This study focused on the feasibility of sending a piloted mission to a NEO employing the basic hardware described in the Constellation System. This initial study was constrained to limited modifications to the Orion spacecraft and did not consider additional equipment, particularly a dedicated habitation module, that may be required. The four launch options that were assessed were the following: (1) the “lower-bookend” option, (2) the midvolume Ares IV single-launch option,35 (3) the midvolume Ares V single-launch option, and (4) the “upper-bookend” option. The lower-bookend option consists of a dual-launch of an EELV, such as the Atlas V or Delta IV Heavy, carrying an unmanned Centaur upper stage and an Ares I rocket carrying an Orion spacecraft. The midvolume Ares IV single launch is a modified Ares V with an Ares I upper stage carrying a Crew Exploration Vehicle (CEV). Similarly, the midvolume Ares V single launch is an Ares V with an Orion on top. The upper-bookend option is a dual-launch scenario most like the proposed lunar architecture, with spacecraft similar to an Altair Lunar Surface Access Module atop an Ares V vehicle and an Ares I rocket carrying an Orion. Defining studies need to be made to enable the proper choice of a launch vehicle or vehicles for this mission concept. Work needs to be done on understanding the physical characteristics of the target asteroid: determining surface morphology and properties, studying the asteroid’s gravitational field structure, determining the rotation period, estimating the mass and density, and attempting to learn something about its general mineral composition. Such information will aid in the planning process for exploration once the spacecraft arrives at the asteroid. Efforts are needed to maximize mission efficiency for proximity and surface operations and for sample collection. A robotic precursor mission would be launched before the human mission to better characterize the target asteroid and reduce the risk for the astronauts traveling there. The Orion spacecraft would require several basic capabilities in order to complete the scientific and technical objectives of the mission, which would involve equipment and techniques supporting remote sensing, deployment and redeployment of surface experiment packages, and surface sampling. The majority of Orion operations should take place during close proximity (from a few to several hundred meters) to the NEO. The crew of the Orion should be able to match the rotation of the NEO and maintain a stable orbit from which they can conduct a detailed scientific exploration of the target’s surface. This would require station-keeping capability. In terms of remote sensing capability, the Orion should have a high-resolution camera for detailed surface characterization and optical navigation. A light detection and ranging (lidar) system would be mandatory for hazard avoidance during close-proximity operations and for detailed topography measurements. In addition, the Orion has in its current design a radar transmitter that could be outfitted to perform radar tomography of the object. This would allow a detailed examination of the interior structure of the NEO. Given that several NEOs appear to have a high degree of porosity, it is important to measure this physical characteristic of the target NEO. Such information has implications for the understanding of the formation and impact history of the NEO, and may also have implications for future hazard-mitigation techniques of such objects.

Relative Technical Feasibility of the Mission Concept Much of the technology needed for this mission is already under development for other missions, by NASA or elsewhere. Therefore, the expected new technology development required for crewed missions to NEOs is minimal in terms of the overall cost of the Orion and Ares infrastructure. In terms of technology development, a relatively small amount of new technology would be required for the mission. Some technologies associated with onboard operations automation, radiation shielding, microgravity EVA equipment, inflatable habitats, and NEO surface science packages would be required. However, most of these technologies are already being considered within NASA and the private sector for other space exploration missions. The expected new technology development required for crewed missions to NEOs is minimal in terms of the overall cost of the Orion and Ares infrastructure.

35 The Ares IV is a theoretical hybrid of the Ares V first stage and the Ares I upper stage. It is not under active consideration by NASA.

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The ideal scenario for the mission would involve launching an Orion spacecraft atop an Ares V rocket.36 NASA has no current plans for this vehicle configuration, although the agency is preserving the option of developing it in the Ares V design. However, the most significant technology challenge may be that of providing radiation shielding for the crew. Methods of accomplishing this depend in part on the mass available for the mission. The Ares launch vehicles and Orion CEV will have undergone rigorous testing and will presumably have flown astronauts multiple times to the ISS and, possibly, to the Moon. Hence, these missions will be using the best-understood space transportation system infrastructure for human scientific and exploration missions outside of low Earth orbit.

General Cost Category in Which This Mission Concept Is Likely to Fall Both an Ares I with Orion and an Ares V with an Earth departure stage are assumed for the Exploration of Near Earth Objects via the Crew Exploration Vehicle mission concept. The proposer’s cost estimate is that one should expect to spend funds similar to those spent for an extended lunar sortie—that is, $2 billion to $4 billion. However, given the unique nature of this mission, the risk-mitigation measures likely required, and the development of proximity operation techniques around the asteroid, the committee places this mission in the more-than- $5 billion category.

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches By definition, this mission involves sending humans to a nearby asteroid and therefore requires the Orion spacecraft. Because Constellation hardware already is under design and development for trips to the ISS and the lunar surface, the incremental development costs would be minimized for the Orion and the Ares family of launchers that would be needed for science missions to NEOs.

Assessment of the Mission Concept for Further Study The Exploration of Near Earth Objects via the Crew Exploration Vehicle mission is enabled by the Constel- lation System. Each of the four mission concept options considered requires the Orion crew vehicle in conjunction with some combination of Ares I and Ares V launch vehicles. No other method of sending humans to such distances currently exists or is planned. The potential mission impact on science in the field is moderate. The main scientific goal of a NEO mission would be to collect macroscopic samples within the context of their surroundings in order to evaluate any space weathering and surface alteration. These samples would be returned to sophisticated terrestrial laboratories for studies in geochemistry, isotopic analysis, mineralogy, and volatile content, as well as impact and thermal history. The examination of these samples would yield significant knowledge about their origin, age, and history, just as with typical meteorites, but now realizing the precise source of the samples. Furthermore, knowledge of the physical properties of objects that may be on a collision course with Earth is essential for the success of any strategies that call for changing the object’s trajectory. The emplacement of an array of suitable instruments would provide an understanding of the object’s internal structure, which would be valuable for understanding the accretion processes that brought this small solar system body together, but also for recognizing appropriate mitigation strategies to deflect any Earth-threatening NEO. Although the committee found the argument that a crewed mission to a NEO is the best, and possibly the only, way to gather a sample within the context of its surroundings and return a significant amount of material to Earth, the primary focus of scientific research in this field at the present time is devoted to characterizing as many diverse bodies as possible.

36 The Ares V and Orion spacecraft launch option remains a potential future development for NASA. See Steve Cook, “Lunar Program Industry Briefing: Ares V Overview,” September 25, 2008, available at http://www.nasa.gov/pdf/278840main_7603_Cook-AresV_Lunar_ Ind_Day_Charts_9-25%20Final%20rev2.pdf.

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The technical maturity for this mission concept is closely tied to the development of lunar mission architecture. For instance, the most capable of the four options considered for the NEO mission requires the same hardware as a lunar mission (an Ares V with an Earth Departure Stage and Altair lunar lander, and Ares I with an Orion crew vehicle), all of which are part of the Constellation System. Once those vehicles are developed, the technical maturity for the asteroid mission would be relatively high. Additional modifications to first-generation Constella- tion hardware would be required for a human mission to a NEO, such as an extended-duration Orion spacecraft, radiation protection, EVA capability, and robotics. These capabilities will also be needed to meet NASA’s long- term Mars exploration goals as well as for human servicing missions of robotic spacecraft in cislunar space (see Chapter 4). In summary, although a human NEO mission would be expected to yield interesting science, it is not as compelling as the other mission concepts that the committee evaluated; therefore, this mission is placed in the “deserving of further study” category. However, the most compelling rationale for a human NEO mission is for its value as an intermediate step between lunar exploration and interplanetary travel—to prove new hardware and to maintain the momentum of the human exploration program by showing sustained progress. Although the committee did not rate this mission as “more deserving of study” because of its scientific limitations, the committee does believe that this mission is an intriguing and exciting mission concept and deserves serious consideration for further study by NASA’s Exploration Systems Mission Directorate.

Neptune Orbiter with Probes37 Scientific Objectives of the Mission Concept The planets in our solar system fall into three classes: terrestrial planets, gas giants, and ice giants. Only the last category has not been studied comprehensively, meaning that the Neptune Orbiter (Figure 2.12) would be the first detailed exploration of such a body. The Voyager 2 flyby of the Neptune system nearly two decades ago produced most of the knowledge of this distant planet. It revealed a surprisingly dynamic atmosphere, surrounded by a displaced and highly distorted magnetosphere. Many of the extrasolar planets detected to date are similar in terms of mass and size. A Neptune mission would also survey the almost-planet-sized, geologically active satellite Triton, which—owing to its retrograde orbit—is suspected to be a captured Kuiper Belt Object. Within the context of comparative planetary studies, these objects have significant scientific interest. The Neptune system was ranked as one of the three “other important objects,” after the top four choices, in the NRC’s 1994 report An Integrated Strategy for the Planetary Sciences 1995-2010, and was the focus for two of the nine “deferred high-priority flight missions” listed in the 2003 NRC solar system exploration decadal survey.38 A Neptune mission was also recommended for further science definition in the next solar system decadal survey by the 2007 NRC report Grading NASA’s Solar System Exploration Program: A Midterm Review.39 A comprehensive study of an ice giant would be a logical mission to follow the detailed explorations of Jupiter by Galileo (mid- 1990s) and of Saturn by Cassini-Huygens (ongoing).

37 The first two subsections are based on material in B. Bienstock, The Boeing Company, and the Neptune Orbiter with Probes Team, “NASA Vision Mission Neptune Orbiter with Probes,” Contract No. NNH04CC41C, Final Report, Volumes 1 and 2, Revision 1, September 2005; A.P. Ingersoll, California Institute of Technology (Caltech), and T.R. Spilker, Jet Propulsion Laboratory (JPL), “Study of a Neptune Orbiter with Probes Mission, Final Report,” May 2006; T.R. Spilker, JPL, and A.P. Ingersoll, Caltech, “Aerocapture Implementation of NASA’s ‘Neptune Orbiter with Probes’ Vision Mission,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 20, 2008; and B. Bienstock, The Boeing Company, and D. Atkinson, University of Idaho, “Neptune Orbiter with Probes,” presentation to the Committee on Planetary and Lunar Exploration, July 21, 2005 (presented by T. Spilker, JPL, to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 20, 2008). Note that both Neptune reports were presented by T. Spilker, a lead author of one of the studies. See footnote 1 above. 38 National Research Council (NRC), An Integrated Strategy for the Planetary Sciences 1995-2010, National Academy Press, Washington, D.C., 1994; NRC, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003. 39 National Research Council, Grading NASA’s Solar System Exploration Program: A Midterm Review, The National Academies Press, Washington, D.C., 2007.

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FIGURE 2.12 Illustration of a possible Neptune Orbiter mission employing aerocapture. The aerocapture stage is on the left; the coast stage, including two probes, is on the right. SOURCE: Used with permission, courtesy of T.R. Spilker, Jet Propul- sion Laboratory. Figure 2.12.eps Bitmap images - Low resolution

Characteristics of the Mission Concept as Developed to Date Neptune is 30 times farther from the Sun than Earth is, so any mission will take many years to reach its target; for example, the most energetically efficient orbit takes 30 years. An EELV-based mission using chemical propulsion first to reach Neptune and then to decelerate into orbit provides either too little payload mass or too long a flight time. In order to reduce flight times to an acceptable duration, new technology is required to slow down the spacecraft on approach to its target. In the proposed non-nuclear version of this mission, solar-electric propulsion is employed to speed the interplanetary transit, and then aerocapture reduces the orbital energy, yielding an elliptical orbit about Neptune. In the alternative version, nuclear-electric propulsion is used to accelerate and then decelerate the spacecraft to allow a conventional, powered capture. Both of these proposed Neptune missions, with launch in 2016, would perform a Jupiter flyby before arriving at Neptune in 2029. Early on, just before and just after capture of the mother spacecraft into a Neptune orbit, a probe would be released for insertion into Neptune’s atmosphere (one at the equator and another at high latitude). Four years later the spacecraft would rendezvous with Triton on its retrograde orbit in order to allow deployment of an orbiter, or a lander, on the satellite. The mission would end in 2033. Since the gravity assist by Jupiter makes this mission feasible, similar missions can be accomplished only every dozen years, meaning that another opportunity will occur late in the 2020s with the mission’s end happening about 2045. Any Neptune orbiter mission requires strategic investments in power sources, transportation, communication, and sensor technology. In order to have reasonable trip times while carrying a comprehensive payload, some new technology is required, either a significantly powered flight (via nuclear-electric propulsion) or aerocapture. Maneuvering within the Neptune system is accomplished in one case by solely using gravity assists by Triton, whereas a mission carrying nuclear-electric propulsion would be able to use that capability to supplement gravity assists when switching orbits. Of the two missions reviewed by the committee, only the non-nuclear version can fly on a Delta IV Heavy launch vehicle, according to the current Delta IV Payload Planners Guide.40 Using the performance characteristics given in the 2002 Delta IV Payload Planners Guide,41 the proposers concluded that, if a lander were to be deployed, a Delta IV Heavy would be inadequate. However, the more recent Payload Planners Guide claims an increased payload capability, and this should be sufficient for all stated mission options. This proposed mission uses both

40 Delta IV Payload Planners Guide, 06H0233, United Launch Alliance, Littleton, Colo., September 2007, available at http://www.ulalaunch. com/docs/product_sheet/DeltaIVPayloadPlannersGuide2007.pdf. 41 Delta IV Payload Planners Guide, The Boeing Company, Huntington Beach, Calif., 2002.

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solar-electric propulsion and aerocapture. Aerocapture represents a technology challenge. Currently NASA does not plan to develop or demonstrate aerocapture. The Neptune mission would carry at least two entry probes. In order to better define formation models of Neptune, the probes would measure the elemental abundances of He, Ne, Ar, Kr, Xe, C, and S, and the isotopic ratios 15N/14N and D/H. By combining these data with similar available information for Jupiter and possibly Saturn, it should be possible to determine whether the gas giants and the ice giants are born from the same material in the same general locale. One of the probes would carry a gas chromatograph mass spectrometer (GCMS) that is critical to the mission goals. The probes would study temperature and abundance data from Neptune’s stratosphere down to pressure levels of ∼100 to 200 bar. Complementary measurements of winds, structure, composition, cloud- particle size, and lightning would also be obtained. The orbiter would carry the usual broad array of instruments to observe the planet, its satellites, rings, and magnetosphere. These include imaging instruments at ultraviolet, optical, infrared, and radio wavelengths, as well as detectors to measure the magnetosphere, both in situ and remotely. Dust and plasma wave detectors might be included in the payload. Any Neptune orbiter would also have the ability to map Triton’s atmosphere and surface globally in order to establish the composition and age of the surface, the inventory of volatiles, the satellite’s current geologic activity, and the atmosphere’s character. A Triton surface lander would enable direct measurement and sampling of the atmosphere and surface for purposes of the study of surface-atmosphere interactions, and might allow direct seismic probing of Triton’s interior.

Relative Technical Feasibility of the Mission Concept The technical maturity of the instrument complement for the Neptune Orbiter with Probes mission concept is high, whereas the necessary propulsion options are relatively immature. Each version of the basic Neptune mission with probes concept (i.e., aerocapture or nuclear-electric propulsion) has an aspect that would require significant further development.

General Cost Category into Which This Mission Concept Is Likely to Fall The Cassini-Huygens mission, an ongoing comprehensive exploration of the Saturn system that dropped a single probe into Titan’s atmosphere in early 2005, is an acceptably close analog to this proposed flight. Cassini- Huygens cost $3 billion to $4 billion. Given inflation and the long duration of the Neptune missions and their somewhat more complex mission profiles, the proposed Neptune missions likely would cost more than $5 billion.

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The nuclear-electric propulsion mission concept significantly exceeds the capabilities of the largest rocket currently in any nation’s inventory. By contrast, Ares V would be able to lift the 36 metric tons identified for this concept. However, the nuclear-electric propulsion required for this mission represents significant technical challenges. In the case of the mission concept using RTGs and no electric propulsion, an Ares V rocket would likely allow such a mission to be flown without the requirement for aerocapture at Neptune. Because there is currently no development program for either aerocapture or nuclear reactors, conventional chemical propulsion may provide the most mature option. However, this would severely limit the amount of payload that could be carried to Neptune and would also eliminate some important scientific objectives, such as a Triton lander.

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Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Neptune Orbiter with Probes mission concept is worthy of further study as a Constellation-enhanced science mission. The Ares V launch vehicle could eliminate the need for aerocapture in Neptune’s atmosphere. The study of an ice giant planet is pivotal to understanding the origins of the solar system. Neptune and Triton are also important for comparative planetary studies, in particular for understanding what drives the activity of these bodies in the frigid outer solar system. Finally, understanding an ice giant would be useful for calibrating models of extrasolar planets.

Assessment of the Mission Concept for Further Study The Neptune Orbiter with Probes mission is enhanced by the Constellation System. This mission’s science goals can only be achieved with a very capable spacecraft that, in most cases, requires more mass and volume than can be accommodated on conventional launch vehicles. However, such carrying capacity will be readily available with the Ares V. Ares V would allow the mission to carry more instruments, possibly both atmospheric probes and a Triton lander (a mission employing a smaller launch vehicle would be limited to only one option). In addition, because Neptune is so far from Earth, conventional missions would only be completed in several decades, a duration that many consider unacceptable. An Ares V could launch a mission to Neptune at higher velocity and therefore a shorter travel time. The potential impact on science for such a mission is high. The mission would explore both an ice giant planet and an almost-planet-sized, active satellite that is believed to be a captured Kuiper Belt object. Knowledge of Neptune would add significantly to the scientific understanding of the formation of the solar system. The science goals for a Neptune-Triton mission are those of the second stage of planetary exploration: thorough examination of a planetary system employing a broad suite of instruments over an extended interval. These objectives are similar to those that the Galileo mission sought at Jupiter and the Cassini-Huygens mission is pursuing at Saturn. The technical maturity of this mission is high. The instrument package, which is similar to that developed for Galileo and Cassini, has a high TRL rating and requires no new technology. A Triton lander/rover would borrow technology developed for Mars, although it would require adaptation to operate in the new environment. How- ever, the committee believes that aerocapture still offers a potentially significant enhancement even to an Ares V launched mission, although aerocapture at Neptune has a relatively low TRL. In summary, its combination of clearly defined, high-priority science goals and mature instrument technology places this mission in the “more deserving of further study” category. Further study should be primarily focused on evaluating the ability to use the Ares V to obtain the desired orbit and additional capabilities.

Titan Explorer42 Scientific Objectives of the Mission Concept Knowledge and understanding of Titan, Saturn’s largest moon, have increased significantly as a result of remote sensing measurements obtained from the Cassini spacecraft following its orbital insertion around Saturn on June 30, 2004, and more recently with measurements from the descent of the Huygens probe through the atmosphere and onto the surface of Titan on January 14, 2005. More than 3 years of analysis of the Huygens data and more than 4 years of analysis of the Cassini data have dramatically improved the understanding of Saturn and its moons, particularly Titan and Enceladus. In the search for life in the solar system, Titan is in a unique position. Its density suggests that it is composed of a mixture of rock and ice in almost equal amounts, and Titan’s atmosphere

42 The first two subsections are based on material in J.S. Levine, NASA Langley Research Center, “Titan Explorer: The Next Step in the Exploration of a Mysterious World,” Final Report for NASA Vision Mission Study per NRA-03-OSS-01, June 2005; and J.S. Levine, NASA Langley Research Center, “Titan Explorer: The Next Step in the Exploration of a Mysterious World,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 21, 2008. See footnote 1 above. Note: This study should not be confused with the Titan Explorer study conducted for NASA during 2007. However, the two studies had similar science goals. The latter study can be found at http://www.lpi.usra.edu/opag/Titan_Explorer_Public_Report.pdf.

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3.75 m diameter rigid biconic aeroshell TUFROC Forebody TPS –Peak heat rate = 308 W/cm2 (56 convective and 252 radiative) – 6.3 cm thick 5 m dia. Conical Ribbon Parachute

FIGURE 2.13 Illustration of the deployment sequence for the Titan Explorer airship. NOTE: TUFROC, Toughened Uni-piece Fibrous Reinforced Oxidation-resistant Composite; TPS, Thermal Protection System; El, elevation; T, time; h, height; M, mass. SOURCE: Courtesy of Joel S. Levine, NASA. Figure 2.13.eps Includes low resolution bitmap image

may reveal answers to questions about the chemical evolution on early Earth. The Titan Explorer mission focuses on nearly a dozen scientific questions, including the following:

• What is the chemical composition of the atmosphere, including the trace gases? • What is the isotopic ratio of the gases in the atmosphere? • What prebiological chemistry is occurring in the atmosphere/surface of Titan today and what is its relevance to the origin of life on Earth? • What is the nature, origin, and composition of the clouds and haze layers?

However, the mission’s objectives were developed before Cassini reached Titan, and they have not been altered significantly to take into account the results of Cassini and Huygens.

Characteristics of the Mission Concept as Developed to Date The Titan Explorer mission (Figure 2.13), with a launch mass of 5,961 kg, would launch on a Delta IV Heavy rocket. The spacecraft consists of a Titan orbiter and a Titan airship that will traverse the atmosphere of Titan and can land on its surface. The airship is designed to have an operational lifetime of 4 months after entry, and the orbiter is designed to have a lifetime of 40 months after arriving in orbit around Titan. To answer the questions posed above, instruments on the Titan orbiter include a solar occultation spectrometer to measure atmospheric composition and isotopic ratios, a radar mapper to measure the nature of Titan’s surface, a magnetometer to search both for a planetary dipole field and surface magnetism, an ultraviolet spectrometer to

ANALYSIS OF SPACE SCIENCE MISSION STUDIES 65

measure the escape of gases from Titan’s upper atmosphere, and a visual and infrared mapping spectrometer to measure cloud and haze layers and the nature of the satellite’s surface. Instruments on the Titan Explorer airship include an imager, mass spectrometer, surface composition spectrometer, haze and cloud particle detector, Sun-seeking spectrometer, and a surface science package similar to that flown on the Huygens mission. Titan’s surface would be explored from an airship. Whereas other Titan exploration proposals would utilize a balloon, an airship offers the possibility of navigation to specific sites of interest. Success with this technique requires the development of cryogenically capable envelope materials for the airship.

Relative Technical Feasibility of the Mission Concept The airship instruments (the imager, the mass spectrometer, and the surface composition spectrometer) and the orbiter instruments (the magnetometer and the radar mapper) all need additional development to result in improved resolution. The orbiter would require a second-generation multimission RTG, and for the airship a second-generation Sterling radioisotope generator. The spacecraft will be captured into orbit around Titan by means of aerocapture, which is one of the key enabling technologies for the mission. Owing to the lower entry velocity, aerocapture at Titan is not as challenging as at other planetary destinations. The mission also uses solar-electric propulsion, although mission requirements involve greater capabilities in this area than have previously been flown.

General Cost Category in Which This Mission Concept Is Likely to Fall The project, as described in the final Vision Mission report, would have a total cost in FY 2008 dollars in the range of $3 billion to $4 billion. The mission was planned around the use of an existing EELV. Given recent estimates of outer-planet flagship missions significantly in excess of $2 billion, and considering that this mission would be significantly larger and more complex than those missions, the mission cost is likely to exceed $5 billion.

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The Ares V with a dual-engine Centaur has significant excess capability to enable various mission design options. It has more than enough capability to deliver the Titan Explorer, without the use of Earth gravitational assist or the solar-electric propulsion module, within the proposed 6-year mission time line outlined in the baseline mission architecture. The excess capacity (nearly 10 metric tons) could be used to carry a conventional propulsion system to reduce or eliminate reliance on aerocapture. Further, the elimination of the solar-electric propulsion module is projected to save ∼$500 million, offsetting much of the cost differential between the Delta IV Heavy and Ares V launch systems. Alternatively, the basic Ares V (without the Centaur) could potentially eliminate the need for Earth gravity assist. It may also be possible to eliminate or dramatically reduce the reliance on the solar-electric propulsion module. Further study of the various mission design options is required to answer these questions.

Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Titan Explorer mission concept is worthy of further study as a Constellation-enhanced science mission. The use of an Ares V launch vehicle could eliminate the requirement for aerocapture. The Titan Explorer would provide a significant improvement in the knowledge of the evolution of prebiological chemistry. The committee notes that NASA is already conducting ongoing studies of possible Titan missions, although the most recent one studied uses a balloon instead of an airship to study Titan’s atmosphere and surface.

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Assessment of the Mission Concept for Further Study This mission is enhanced by the Constellation System. The Ares V with a dual-engine Centaur upper stage would have significant excess capability to allow various Titan exploration options, including reduced travel times and/or elimination of Earth gravity assist. Excess mass capability provided by the Ares V could be used to simplify the spacecraft design and therefore lower cost. The potential impact on science for the proposed mission is mixed. Although a Titan science mission could have significant impact on science, the science goals of the Titan Explorer as presented to the committee are obsolete, having become outdated following a new understanding of Titan with the results of the Cassini mission and the Cassini Equinox Mission (Cassini’s extended mission). As currently conceived, this mission will require aerocapture, a technology that has not been used before. Also as currently conceived, the mission will require solar-electric propulsion and next-generation ion engines. Furthermore, the proposal did not address the planetary protection issues surrounding taking Advanced Sterling Radioisotopic Generators to Titan’s surface on the airship.43 By studying Cassini’s experience, it will be possible to improve on the science definition, justify the instrument selection, and make a compelling case for advanced technology such as a blimp(s), balloon(s), or other method for moving through Titan’s atmosphere. NASA is currently undertaking studies for a Titan mission that would use an EELV based on science-definition teams established in the wake of the Cassini mission. These efforts would naturally support any potential Ares V-based Titan exploration effort.44 The technical maturity of the Titan Explorer mission concept is relatively high. The instrument package, although scientifically outdated, requires no new technology and has a high TRL rating. This mission would require aerocapture, a technology that has not been used before and is at TRL 3 to 6. The mission would require solar-electric propulsion and next-generation ion engines. In summary, although the potential scientific impact of a Titan Explorer mission using the Constellation System is potentially high, this particular mission concept was weakened because it did not incorporate recent Cas- sini scientific results, placing the mission in the “deserving of further study” category. The committee concluded that with an appropriate science discussion the concept would be more highly ranked, and the basic concept of a Titan mission should not be penalized for a poor presentation. NASA can revisit the potential for Ares V Titan missions after the agency has made a decision concerning the next outer-planets flagship mission.

43 For example, the warm radioisotope generator could melt part of the surface and create its own mini-environment. It is possible, albeit remotely so, that microorganisms brought from Earth on the spacecraft could then flourish in this environment, contaminating it. 44 J. Leary and the Titan Explorer Team, “Titan Explorer NASA Flagship Mission Study,” Johns Hopkins University Applied Physics Laboratory, Laurel, Md., August 2007; R.D. Lorenz, “A Review of Balloon Concepts for Titan,” Journal of the British Interplanetary Soci- ety 61:2-13, 2008; R.D. Lorenz, “Titan Bumblebee: A 1-kg Lander-Launched UAV Concept,” Journal of the British Interplanetary Society 61:118-124, 2008.

Technology Requirements for Future Space Missions

The committee’s charge requested that the benefits of using the Constellation System be compared with those of using “alternative implementation approaches,” meaning other technologies and launch vehicles. Some large payloads can only be accommodated by the Ares V rocket. However, even if the Ares V is used, some of the future space mission concepts evaluated by the committee (see Chapter 2 and Appendix B) would require advanced technologies that have not been fully developed. In some cases the technologies, such as advanced sensors, are required in order for a specific spacecraft to accomplish its mission and therefore would likely be developed as part of the flight program for that mission. However, the committee noted that several technologies were required for multiple mission concepts and were essentially “mission enabling,” meaning that the mission could not be accomplished without them. These include propulsion technologies that might allow an alternative to the use of a heavy-lift launch vehicle such as the Ares V and are applicable to multiple missions (for example, aerocapture, which can be used at Venus, Mars, Titan, and Neptune; and solar sails, which can be used for the Solar Polar Imager and Interstellar Probe missions). If NASA develops these technologies, an Ares V launch vehicle might not be required for these missions but might enhance them. However, there are also missions for which the advanced propulsion technologies are complementary to the Constellation System, such as use of the combination of conventional propulsion and aerocapture/aerobraking for orbit insertion and adjustment at the outer planets. This approach would maximize the mass fraction of the scientific payloads. Because the committee was asked to evaluate alternative implementation approaches, because advanced technology development is so important to the majority of these missions, and because many of the technologies affect several mission concepts, the committee sought more information on the status of NASA’s overall technology development program for space science missions, particularly in the area of advanced in-space propulsion technologies and deep space communications. Of the mission concepts that the committee considered, the Interstellar Probe, Neptune Orbiter with Probes, Solar Polar Imager, and Titan Explorer could all directly benefit from some form of in-space propulsion technology even if the Ares V is available. Although NASA had a vigorous in-space propulsion technology development program in the past, that program has been reduced in recent years. Many of the in-space propulsion development efforts that the committee reviewed were approaching flight-readiness testing but are now in the stage of being shut down due to lack of funding. The committee does not endorse any of these in-space technologies or development efforts. They are discussed in detail here in order to serve as an introduction to what in-space propulsion options may be required for many of these missions. However, it is clear from recent experience that NASA cannot afford all of them and

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BOX 3.1 Technology Readiness Levels

NASA employs a technology readiness level (TRL) process to assess the flight readiness of a System Test, Launch & Operations given technology under development on the basis of TRL 9 its level of maturity (see Appendix D in this report). The TRL is rated on a scale of 1 to 9 depending on the degree of concept development, analytic and System/Subsystem TRL 8 empirical validation, and ultimately, flight validation Development (see Figure 3.1.1). TRL 7 TRL 9: Actual system “flight proven” through successful mission operations Technology Demonstration TRL 6 TRL 8: Actual system completed and “flight qualified” through test and demonstration (ground or space) TRL 5 Technology TRL 7: System prototype demonstration in a space Development environment TRL 4 TRL 6: System/subsystem model or prototype dem- TRL 4 onstration in a relevant environment (ground Research to Prove or space) Feasibility TRL 3 TRL 5: Component and/or breadboard validation in relevant environment TRL 4: Component and/or breadboard validation in Basic Technology TRL 2 Research laboratory environment TRL 3: Analytical and experimental critical formula TRL 1 and/or characteristic proof-of-concept TRL 2: Technology concept and/or application formulated FIGURE 3.1.1 Technology readiness level scale. TRL 1: Basic principle observed and reported SOURCE: Courtesy Figureof NASA. 3.1.1.eps

SOURCE: NASA In-Space Propulsion Web site at http://www.grc.nasa.gov/WWW/InSpace/when.html.

must choose carefully how and what to fund so as to expand its mission capabilities. The importance of a tightly focused technology program to support future advanced missions such as those that would be conducted using the Constellation System led the committee to recommend that NASA develop a comprehensive strategic plan for advanced propulsion system development (see the section below entitled “Propulsion System Technology Summary”). The overall technology requirements for the mission concepts studied in this report are summarized in Table 3.1. The technology readiness levels (TRLs) used by NASA to assess the flight readiness of a given technology are listed in Box 3.1. For a fuller description of these TRLs, see Appendix D.

69 Ares V Upper Stage ✓ ✓ ✓ ✓ ✓ ✓ Solar/ Nuclear- Electric Propulsion ✓ ✓ ✓ ✓ ✓ ✓ Solar Sails ✓ ? ? ✓ ? Technologies Enabling Alternatives or Enhancing Performance of Ares V Aerocapture ✓ ✓ ✓ b Human Assembly and Servicing ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Robotic Assembly and Servicing ✓ ✓ ✓ ✓ ✓ ✓ Space Nuclear Reactors ✓ ✓ ✓ a a a Next- Generation DSN ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ? ✓ ? ? ? ✓ Tethered Flight ✓ Enabling/Enhancing Technologies Free-flying Constellations ✓ ” signifies that missions may benefit, in the opinion of the committee. Aerocapture, solar sails, and solar/nuclear-electric propulsion all fall into ? Correlation Needsof Technology with the Mission AnalyzedConcepts by the Committee

Significant mission enhancement. Only if solar sail is implemented. a b TABLE 3.1 TABLE Mission Concept Advanced Compton Telescope (ACT) Advanced Technology Large-Aperture Space Telescope (ATLAST) Dark Ages Lunar Interferometer (DALI) 8-Meter Monolithic Space Telescope Exploration of Near Earth Objects via theExploration Crew Vehicle Generation-X (Gen-X) Interstellar Probe Kilometer-Baseline Far-Infrared/Submillimeter Interferometer Modern Universe Space Telescope (MUST) Neptune Orbiter with Probes Nuclear Neptune Orbiter with Probes Palmer Quest Single Aperture Far Infrared (SAFIR) Telescope Solar Polar Imager Solar Probe 2 Stellar Imager Super-EUSO (Extreme Universe Space Observatory) Titan Explorer NOTE: Basic enabling technologies are those Aresthat areexist thosein concept whose at availabilitypresent but wouldwhosewould enhancedevelopmentprovide the optionsiscapabilities needed for for of thesethesome missions.missionsof theDSN, Some missions.currentlyDeep of theSpace Technologiesmissions, conceivedNetwork; enablinglike “ as the Advanced beingalternatives Comptonexecuted the toTelescope, categorywithout would ofnot requiring directlyin-space benefitpropulsionan Ares from V. Humanthesetechnologies. technologyassembly developments.and servicing

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Finally, the committee also learned that some of the missions anticipate upgrades to the Deep Space Network (DSN) that are not currently funded or planned. Many of the potential Constellation System mission concepts reviewed in this report would have a major impact on the DSN. At the very least, the development of gigapixel detector arrays for future telescopes would vastly increase the amount of data to be relayed to the ground. Because advanced in-space propulsion technology and deep space communications were identified as so important to so many of these mission concepts, they are discussed in greater detail in this chapter.

IN-SPACE PROPULSION TECHNOLOGIES A number of advanced propulsion systems are being developed by NASA’s In-Space Propulsion Technology Program Office. Located at the NASA Glenn Research Center, this office’s stated objective is to “develop in-space propulsion technologies that can enable or benefit near to mid-term NASA science missions by significantly reducing travel times required for transit to distant bodies, increasing scientific payload capability or reducing mission costs.” The In-Space Propulsion Technology Office primarily focuses on technology development and demonstration, which represents a TRL range from 3 to 6. This provides the foundation for flight system development and technology implementation by the various science missions, including several of the missions reviewed in this study. Current development efforts include electric propulsion, solar sails, aerocapture, and advanced chemical propulsion. A brief overview of electric propulsion, solar sails, and aerocapture and their development status is provided below.

Electric Propulsion Electric propulsion technologies generate thrust by using electrical energy to accelerate an onboard propellant, such as xenon gas, to very high ejection velocities. These velocities may reach 20 times those of traditional chemical propulsion systems. This is characterized by the specific impulse (abbreviated as Isp), which is the product of the thrust and firing duration of the engine, divided by the mass of the propellant expended. This increased efficiency means that less propellant needs to be carried onboard for a given mission. Electrical propulsion systems are subdivided by the sources of the electrical energy, such as solar electric propulsion (SEP), nuclear electric propulsion (NEP), and radioisotope electric propulsion (REP).

Solar Electric Propulsion Solar electric propulsion is a technology that uses electrical energy from solar arrays to accelerate a propellant to very high velocities and obtained thrust. In its simplest form, SEP consists of a propellant supply, power processing unit (PPU), and an ion thruster (see Figure 3.1). NASA has already flown SEP successfully, and the Dawn spacecraft currently heading toward a rendezvous with the asteroid Vesta employs this technology. The Titan Explorer, Solar Polar Imager, and Neptune Orbiter with Probes all have mission design options that include the use of SEP.

NASA’s Evolutionary Xenon Thruster. NASA’s Evolutionary Xenon Thruster (NEXT) project builds on the technology developed for the NASA SEP Technology Application Readiness (NSTAR) system that served as the primary ion propulsion system for the Deep Space 1 (DS-1) spacecraft launched in 1998. The NSTAR system used gridded ion thrusters. During the DS-1 mission, the NSTAR engine operated for a total of 16,246 hours, or 6,477 days, easily surpassing the previous record of 161 days established by NASA’s Space Electric Rocket Test 2 (SERT 2), which flew in 1970.

Much of the information in this chapter was provided in a briefing to the committee during its June 2008 meeting: Tibor Kremic, “NASA Investment in In-Space Propulsion Technologies,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, June 9-11, 2008. Rae Mayer, “In-Space Propulsion Technology Program Office (ISPT) Overview,” February 2006.

TECHNOLOGY REQUIREMENTS FOR FUTURE SPACE MISSIONS 71

FIGURE 3.1 Electrostatic ion thruster. SOURCE: Courtesy of NASA.

As does NSTAR, the NEXT system uses a gridded ion thruster, which uses two oppositely charged grids to accelerate and eject high-speed ions. The NEXT project incorporates advances in the gridded ion thruster, PPU, and propellant management system designs for increased system performance and life. It is a 7-kilowatt (kW) system, compared with the 2.3-kW NSTAR system. The NEXT thrusters have higher efficiency and operate with a higher Isp over a broader throttling range compared with the NSTAR thrusters. The NEXT thrusters also have increased propellant throughput, which reduces the number of thrusters required for equivalent mission requirements. A comparison of the state of the art and NEXT thrusters is provided in Table 3.2. The PPU and propellant management system are similarly designed to be highly efficient, with lower specific mass. The propellant management system provides tighter propellant flow-control capability, reducing the propellant residuals at the end of the mission and increasing overall mission performance. The first phase of the NEXT project was completed in 2003, during which a breadboard PPU, a breadboard propellant management system, and engineering model thrusters were built, and their component and system- level testing were completed (see Figure 3.2). In 2005, three engineering-model thrusters were tested as an array,

TABLE 3.2 Comparison of Current State-of-the-Art Thrusters and NASA’s Evolutionary Xenon Thruster (NEXT), Under Development Thruster Attribute State of the Art NEXT Maximum input power (kW) 2.3 Up to 6.9 Throttle range 4:1 >12:1 Isp (s) 3,170 4,190 Efficiency (%) 62 71 Propellant throughput (kg) 150 >300 Specific weight (kg/kWe) 3.6 1.8

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FIGURE 3.2 NASA’s Evolutionary Xenon Thruster (NEXT), a gridded ion thruster. SOURCE: Courtesy of NASA.

and an extended-duration test of a single engineering-model thruster was initiated. The extended-duration test exceeded 15,000 hours of operation with a 325 kg throughput. (See Figure 3.3.) Knowledge gained from the tests was incorporated into the prototype-model (PM-1) thruster and engineering-model PPU and propellant management system for the current phase of development. With the exception of the life testing, the NEXT systems have completed all component and system-level testing, achieving TRL 6. The life testing is in process and will continue through 2010.

High Voltage Hall Accelerator Thruster. The High Voltage Hall Accelerator (HiVAC) Thruster project is intended to develop a low-power, long-life Hall thruster. In contrast to the gridded ion thrusters, where two oppositely charged grids are used to accelerate and eject ions to generate thrust, Hall thrusters use a radial magnetic field to accelerate low-density plasma to high velocities. The Isp of Hall thrusters is generally in the range of 1,000 to 3,000 seconds (s). They can operate over a wide range of input power, allowing them to be used far from the Sun, where little solar energy is available. The major advantage of Hall thrusters is the potential of lower costs compared with costs for the gridded ion thrusters. Recent advances include the design and fabrication of a flight-prototype thruster, which is intended to be able to operate for more than 7,500 hours, with a specific impulse of 2,800 s at a maximum power level of 3.5 kW. A laboratory version has been designed to operate up to 15,000 hours under similar conditions. Thus far, 4,100 s of operation with 88-kg throughput have been demonstrated. At the 3.5-kW design power level, 55 percent total efficiency has been demonstrated. The design and fabrication of a full-engineering-model Hall thruster is to be completed this year. At this time, NASA has not funded the development of other system elements such as the PPU or propellant management system.

Energetics Program Development. Until 2005, the Energetics Program was funded to develop SEP technologies to TRL 3, setting the stage for future development. Areas of interest included electrostatic ion thruster technology,

The NASA Web page is available at http://www.grc.nasa.gov/WWW/RT/2006/RP/RPP-manzella.html.

TECHNOLOGY REQUIREMENTS FOR FUTURE SPACE MISSIONS 73

FIGURE 3.3 NASA-94M, a state-of-the-art ion thruster. SOURCE: Courtesy of NASA.

electrostatic Hall-effect thruster technology, pulsed-plasma thruster technology, high-power electric propulsion technology, and electric propulsion systems. This effort primarily focused on longer life, higher reliability, and higher power and thrust.

Nuclear Electric Propulsion Missions to the outer planets drive the need for high-power, high-specific-impulse propulsion systems. Such missions include the Neptune Orbiter with Probes and the Interstellar Probe. Nuclear electric propulsion has the greatest potential to meet the propulsion needs of these missions. In addition to the high power levels, nuclear power plants provide a constant source of electrical energy throughout the mission, which is in direct contrast to SEP systems, for which the available solar energy declines dramatically as a spacecraft travels to the outer planets. NASA’s Prometheus program was developing NEP systems as a building block to space exploration. The initial mission focus was the study of three ice-covered moons of Jupiter: Ganymede, Callisto, and Europa. It was intended to study the vast oceans of water under the icy surface of the moons, looking for indications of or opportunities for life. The Jupiter Icy Moons Orbiter (JIMO) spacecraft (see Figure 3.4) was to be propelled by high-power electric propulsion (HiPEP) ion propulsion thrusters, with the electrical power supplied by a nuclear fission reactor. The reactor was to be positioned at the tip of a boom extended away from the spacecraft, with a strong radiation shield protecting the electronics and sensors on the spacecraft. As described in Box 1.2 in Chapter 1 of this report, due to a shift in NASA priorities and significant technical challenges associated with the JIMO mission, as well as the high costs of the reactor program, funding was cut, ending the Prometheus program. In support of the Prometheus program and future NEP systems, the In-Space Propulsion Technology Program Office was developing high-energy thruster technologies. The challenge for NEP system design is the increased power, performance, and life requirements of the thrusters. The NSTAR and NEXT thrusters developed for SEP systems provide 2.3 kW and 6.9 kW, respectively, compared with power requirements in excess of 20 kW for deep space missions. Similarly, the Isp requirement of 7,000+ s is significantly greater than the 3,170 s and 4,190 s provided by NSTAR and NEXT. And, finally, where the NSTAR and NEXT thrusters demonstrated more than 15,000 hours of operational life, deep space missions will require in excess of 70,000 hours. Within the past 5 years, development efforts were completed for the HiPEP thruster and Nuclear Electric Xenon Ion System (NEXIS). The goal of NEXIS was to demonstrate a 20-kW system and a 2,000-kg xenon throughput providing an Isp of 6,000 s to 8,000 s, with a 7- to 11-year life. The HiPEP thruster was intended to provide a 25-kW system with a 2,000-kg xenon throughput and similar performance characteristics. A comparison of the objectives of NEXIS and HiPEP thrusters with the NSTAR and NEXT thruster designs is provided in Table 3.3.

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FIGURE 3.4 Artist’s concept of the nuclear-powered Jupiter Icy Moons Orbiter. SOURCE: Courtesy of NASA.

Both development efforts advanced the technology to the TRL 3 to 4 range, laying the foundation for future high-power, long-life thruster development, including the thrusters for the JIMO mission. The programs each demonstrated performance and preliminary life characteristics but require additional component, subsystem, and system verification and validation through modeling and testing to bring the technology to TRL 6.

TABLE 3.3 Comparison of Several High-Power Electric Propulsion Systems Under Development at NASA Performance Metric NSTAR NEXT NEXIS HiPEP Power (kWe) 2.3 7 20 23.5 Isp (s) 3,170 4,190 7,500 6,000-8,000 Thruster efficiency (%) 63 71 78 >74 Specific mass (kg/kWe) 3.6 1.8 1.5 <2 Throughput (kg) 150 >300 2,000 5,100 Life capability (thousand hr) 30 >15 93 243 NOTE: NSTAR, NASA SEP Technology Application Readiness; NEXT, NASA’s Evolutionary Xenon Thruster; NEXIS, Nuclear Electric Xenon Ion System; HiPEP, high-power electric propulsion.

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FIGURE 3.5 A solar sail design during ground test. SOURCE: Courtesy of NASA.

Solar Sails The Interstellar Probe and Solar Polar Imager mission design options included the use of solar sails. These devices use photons from the Sun, which apply pressure on thin, lightweight, highly reflective sheets or sails to propel the spacecraft, much like wind pushes a sailboat across a body of water (see Figure 3.5). The direction of the spacecraft is controlled by changing the angle of the solar sail relative to the Sun and spacecraft. Since the Sun provides all of the propulsive energy, there is no need for onboard propellants. Although the thrust associated with solar sail propulsion is very small—orders of magnitude less than traditional chemical propulsion systems—the continuous application of solar energy can increase the spacecraft velocity to levels in excess of chemical propulsion systems. The Solar Polar Imager concept uses a solar sail to approach and then increase the orbital inclination about the Sun, while the Interstellar Probe uses the technology to accelerate the vehicle to the edge of the solar system. Solar sails are most effective within 2 to 3 AU of the Sun. Since solar pressure is very low, sails that are very large and lightweight are required to produce appreciable thrust. The Jet Propulsion Laboratory studied the use of solar sails in the 1970s and determined that a 600,000-m2 sail would be required to rendezvous with Halley’s Comet, matching both its position and velocity. These very large sails must be stowed during launch and safely deployed once the vehicle has left Earth’s atmosphere. The support structures, or trusses, for the sail must also be very lightweight and need to be stowed in a small volume for launch. The sail material requires a highly reflective coating on the Sun-facing side in order to maximize the thrust, while retaining a highly emissive coating on the back side. The sail materials developed to date are 40 to 100 times thinner than a piece of standard writing paper. It is generally a durable polymer material, such as Mylar or Kapton, with a metalized surface to obtain the desired reflectivity.

See http://www.nasa.gov/vision/universe/roboticexplorers/solar_sails.html.

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In 2004 and 2005, ATK Space Systems and L’Garde, Inc., designed, fabricated, and tested 10-m and 20-m solar sails under thermal vacuum conditions for NASA’s In-Space Propulsion Technology Program Office. ATK was able to demonstrate greater than 90 percent reflectance across the solar spectrum and a backside emissivity of 0.3 or better, achieving the goals of the development program. Given the results of their study, ATK projected the area density and stowed-volume requirements to be met for the 100-m sail configuration. In addition, the company projected that emissivities of greater than 0.5 can be achieved with carbon-loaded clear polyimide 1 (CP1), a temperature-resistant material invented by the NASA Langley Research Center specifically for space applications. Similar work by L’Garde, Inc., demonstrated 86 percent reflectance, slightly below the 90 percent goal, and an emissivity of 0.4, with a potential for 0.62 with a thicker deposition of blackened chromium. Both projects demonstrated robust attitude control, with the thrust turning rate significantly greater than design goals. NASA terminated the development effort at TRL 5 because of funding limitations. Additional work remains in order to develop improved, automated manufacturing capabilities, characterize material capabilities, and model and test the materials and lightweight, deployable structures. Improved, high-fidelity ground simulators are also required to offset the effect of gravity on the large sails. In addition, in order to meet the requirements of the Interstellar Probe mission, an order-of-magnitude improvement in area density of the sails would be needed. NASA attempted to demonstrate the deployment and propulsion capability of a solar sail in orbit. The Nano- Sail-D was intended to be the first spacecraft to use a solar sail for attitude control. The sail is a four-segment square, approximately 10 ft on a side. Unfortunately, the launch vehicle, the SpaceX Falcon 1 booster, failed at the August 2008 launch during the boost phase, and the mission was lost. In June 2005, Cosmos Studios and the Planetary Society had also attempted to test a solar sail in space using a submarine-launched missile to place the Cosmos 1 spacecraft into Earth orbit. The Cosmos 1 concept employed eight triangular sails, each 15 m in length, to provide a total sail area of 600 m2. Unfortunately the launch vehicle failed, preventing Cosmos 1 from reaching orbit. The Planetary Society is working on Cosmos 2, which is intended to make a controlled flight using solar pressure. The society hopes to launch on another rocket, such as the Russian Soyuz-Fregat booster. Even as a secondary payload on a Soyuz mission, the launch capability of the booster enables a simpler design of the Cosmos 2 spacecraft compared with the design of Cosmos 1, therefore improving the probability of successful deployment of the solar sail upon reaching orbit.

Aerocapture Aerocapture ranges from being a strongly enhancing to an enabling technology for the Neptune Orbiter with Probes and Titan Explorer missions and has been identified as an enhancing technology for robotic exploration of Mars and enabling for human missions to Mars. It is an orbit insertion maneuver that takes advantage of a planet’s atmosphere to decelerate a spacecraft sufficiently to allow it to be placed into its intended orbit. This type of maneuver is intended to minimize the use of propellants. It begins with the spacecraft entering the atmosphere of a target planet or moon. It then uses the friction or drag of the atmosphere to slow the vehicle. Once sufficient energy has been bled off, the spacecraft exits the atmosphere and performs orbit adjustment maneuvers to place it into the desired circular or elliptical orbit. Figure 3.6 depicts a generic aerocapture trajectory. The only propellant required for the overall maneuver is a minimal amount used for attitude control during atmospheric flight and for the final circularization maneuvers. Aerocapture requires guidance. Too shallow of an entry could result in the spacecraft skipping off the atmosphere and continuing into space permanently. Too steep of a trajectory could result in excessive aerodynamic and aerothermal loads on the vehicle, potentially damaging or destroying it. It could also result in low apoapsis or impact with the target body (i.e., a crash). The guidance system, therefore, has to account for entry target and navigation errors, atmospheric uncertainties and variabilities, as well as aerodynamic uncertainties. Aerocapture guidance is analogous to that planned for use by Mars Science Laboratory and Orion to accomplish precision landing.

J. Masciarelli, Ball Aerospace and Technologies Corporation, “Aerocapture Overview,” presentation to the Committee on Science Op- portunities Enabled by NASA’s Constellation System, June 9-11, 2008. Rae Mayer, “In-Space Propulsion Technology Program Office (ISPT) Overview,” February 2006.

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Atmospheric Entry Interface Hyperbolic Approach Trajectory Begin Bank Angle Modulation

Periapsis

Circularization Periapsis Maneuver Raise Maneuver

End Bank Angle Modulation Science Orbit Atmosphere Exit

FIGURE 3.6 Aerocapture trajectory. SOURCE: Courtesy of Rae Mayer, In-Space Propulsion Technology Program Office, NASA. Figure 3.6.eps

There are two basic types of aerocapture systems: aeroshells and deployables. The aeroshell designs are the most mature, having been used extensively for atmospheric entry missions. The deployable designs offer the benefit of lighter weight, but are much less mature. The aeroshell design encases the spacecraft in a structure covered with a thermal protection material. The shell serves as an aerodynamic surface, providing both lift and drag as the spacecraft flies through the target atmosphere. The thermal protection materials protect the structure from the intense aerodynamic and radiative heating experienced during atmospheric flight. The shell is then jettisoned once the spacecraft achieves its final orbit. There are blunt- and slender-body aeroshell configurations, with the blunt-body configuration being the most mature (see Figure 3.7). Blunt-body designs have been used extensively for atmospheric entry missions, such as the Mercury, Gemini, and Apollo crewed missions, as well as the Viking, Pioneer, Galileo, and the more recent Mars Exploration Rover (Sprit and Opportunity) missions. The slender-body design is less mature (Figure 3.8). However, it provides greater control authority as a lifting body and is more tolerant of navigation and atmospheric uncertainties. Studies have demonstrated that the slender- body design may be required for the Neptune mission. There are also two general families of deployable designs: trailing ballutes and inflatable aeroshells (Figures 3.9 and 3.10). A “ballute” is an aerodynamic decelerator that is towed behind the vehicle. Inflatable aeroshells consist of a rigid shell with an attached inflated or deployed structure that increases the effective drag area. Bal- lutes rely strictly on aerodynamic drag for aerocapture, whereas the inflatable aeroshells also use lift for greater control during the atmospheric flight. Both types of deployable designs have the benefit of a drag area that can be significantly greater than the cross-sectional area of the spacecraft. As a result, they are effective at higher

NASA, Aerocapture Technology—In-Space Propulsion Technology Project, NASAfacts FS-2007-09-12-GRC Pub ACap001, NASA Glenn Research Center, Cleveland, Ohio.

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FIGURE 3.7 Mars Science Laboratory aeroshell, which has a blunt-body shape similar to that required for aerocapture missions at Mars and Titan. SOURCE: Courtesy of NASA.Figure 3.7.eps Bitmap image

FIGURE 3.8 A potential slender-body aeroshield with atmospheric probes on top. This is the type of shape required for aerocapture at Neptune. SOURCE: Courtesy of NASA.

TECHNOLOGY REQUIREMENTS FOR FUTURE SPACE MISSIONS 79

FIGURE 3.9 “Balloon-parachute” or “ballute.” SOURCE: Courtesy of NASA.

FIGURE 3.10 Inflatable aeroshell. SOURCE: Courtesy of NASA.

altitudes, where the atmosphere is less dense and the aerodynamic and thermal loading are significantly lower. They can therefore be built with lower-mass materials and require less thermal protection, allowing more mass allocation for scientific payloads. A variant of aerocapture is skip entry. In this mode, the atmospheric entry trajectory is such that the vehicle dips into the atmosphere multiple times, decreasing the velocity of the vehicle with each dip. This reduces the overall heat load and aerodynamic loads associated with steeper descents. It also allows the targeting of a larger number of landing sites, or conversely allows the targeting of a given landing site from a wide range of entry points. This capability was developed for the Apollo program to allow the astronauts to target alternate Earth landing sites in the event of adverse weather conditions, but it was never used. The Russians used skip-entry trajectories for the return to Earth of both Zond 6 and Zond 7, two lunar flyby probes. The implementation of aerocapture requires an understanding of the target atmosphere through which the spacecraft will fly, as well of the aerodynamics and aerothermal environment of the spacecraft in the atmosphere. The design of the various spacecraft systems, including the guidance, navigation, and control (GN&C) system and associated algorithms, thermal protection system, and structures must accommodate the associated environments and flight conditions. A summary of the readiness of these aerocapture technologies is provided in Figure 3.11 for several space exploration missions. With the exception of Neptune missions, the technology is relatively mature,

Tibor Kremic, NASA, “NASA Investments in In-Space Propulsion Technologies,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, June 9-11, 2008.

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Destination Venus Earth Mars Titan Neptune Subsystem Venus-GRAM based on Earth-GRAM validated by Mars-GRAM continuously Titan-GRAM based on Yelle Neptune-GRAM Atmosphere world-wide VIRA. Space Shuttle updated with latest atmosp. Accepted worldwide developed from Voyager, mission data. and updated with Cassini- other observations Goal: Capture Physics Huygens data Aerodynamics Heritage shape, well Heritage shape, well Heritage shape, well Heritage shape, well New shape; aerodynamics understood aerodynamics understood aerodynamics understood aerodynamics understood aerodynamics to be established. Goal: Errors = 2% GN&C APC algorithm captures Small delivery errors. APC APC algorithm captures APC algorithm captures APC algorithm with α Goal: Robust 96% of corridor algorithm captures 97% of 99% of corridor 98% of corridor control captures 95% of performance for 4-6 corridor corridor. DOF simulations More testing needed on Technology ready for ST9. ISPT investments have ISPT investments have Zoned approach for mass TPS efficient mid-density TPS. LMA hot structure ready for provided more materials provided more materials efficiency. Needs more Goal: Reduce SOA by Combined convective arrivals up to 10.5 km/s. ready for application to ready for application. investment. 30%+, expand TPS and radiative facility slow arrivals, and new choices needed. ones for faster entries. High-temp systems will High-temp systems will High-temp systems will High-temp systems will Complex shape, large Structures reduce mass by 31%. reduce mass by 14%-30%. reduce mass by 14%- reduce mass by 14%-30%. scale. Extraction difficult. Goal: Reduce SOA 30%. mass by 25% Convective models Environment fairly well- Convective models agree Convective models agree Conditions cannot be within 15%. Radiative: Aerothermal match within 20% known from Apollo, Shuttle. within 15%. Radiative duplicated on Earth in laminar, 45% with Models match within 15% predict models will agree models agree within 35- existing facilities. More within 50% where radiation Goal: Models match turbulence. Radiative 300% work on models needed. within 15% models agree within 50% is a factor. System Accomplishes 97.7% of Accomplishes 97.2% of ∆V Accomplishes 97.8% of Accomplishes 95.8% of Accomplishes 96.9% of Goal: Robust ∆V to achieve 300 x 300 to achieve 300 x 130 km ∆V to achieve 1400 x 165 ∆V to achieve 1700 x 1700 ∆V to achieve Triton performance with km orbit. No known orbit. No known technology km orbit. No known km orbit. No known tech observer. orbit. ready technology technology gaps. gaps. technology gaps. gaps. ENABLING ENABLING

Ready for Infusion Some Investment Needed Significant Investment Needed

FIGURE 3.11 Summary of aerocapture technology readiness for several different targets. Whereas aerocapture could be easily developed for use at Venus, Earth, Mars, and Titan, developing it for use at Neptune will require significant investment in several important areas (note that the chart indicatesFigure that extracting3.14.eps a spacecraft from its aeroshell will be difficult for the Neptune mission). According to this NASA source, aerocapture is required for Titan and Neptune missions and is therefore “enabling.” An Ares V launch vehicle could potentially alter that conclusion. NOTE: GRAM, Global Reference Atmospheric Model; VIRA, Venus International Reference Atmosphere; GN&C, guidance, navigation, and control; DOF, degrees of free- dom; APC, Analytic Predictor Corrector; TPS, thermal protection system; SOA, state of the art; ST9, Space Technology 9; LMA, Lockheed Martin Astronautics; ISPT, In-Space Propulsion Technology. SOURCE: Courtesy of NASA.

but additional development work is necessary. The unique environment of Neptune will require significant additional development effort to enable exploration. Significant maturity has been gained in the areas of atmospheric modeling, aerodynamics, and GN&C. The atmospheric composition and models have been well established and accepted by the community for all destinations. A simple blunt-body aeroshell design can be used for the majority of the destinations. These systems have a long heritage, dating back to the early crewed space program, and the associated aerodynamics have been well characterized. Missions to Neptune, however, may require a slender-body aeroshell. There has been less work in this area, so additional aerodynamic characterization is required. A robust guidance algorithm, the Analytic Predictor Corrector, has been developed and shown through simulation to be able to fly successfully through atmospheres of any of the destinations and deliver the spacecraft to the desired orbit. This algorithm is able to accommodate the uncertainties and variabilities of the various flight parameters, including atmospheric variations and perturbations and uncertainties in the aerodynamic conditions and spacecraft mass properties, as well as the navigation errors that build up in flight. Currently NASA has no plans to develop or demonstrate aerocapture beyond its current TRL and has recently determined that this technology will not be developed further. This decision will severely limit the science explo-

TECHNOLOGY REQUIREMENTS FOR FUTURE SPACE MISSIONS 81

ration of Neptune even with an Ares V capability (for instance, by preventing the mission from carrying both atmospheric probes and a Triton lander) and could have an adverse impact on missions to Venus and Titan.

PROPULSION SYSTEM TECHNOLOGY SUMMARY In the recent past, NASA undertook a number of impressive in-space propulsion technology development projects that reached moderate to high technology readiness levels and demonstrated significant promise for future missions. However, for various reasons the agency has eliminated much of this research. The committee concluded that at least some of these technologies will be required for future missions even if they use the capabilities of the Constellation System.

Finding: Advanced in-space propulsion technology may be required for some science missions considered for using the Constellation System.

THE DEEP SPACE NETWORK The Deep Space Network, developed during the Apollo era, consists of three major tracking sites spaced in longitude around the globe (in Spain, California, and Australia) to provide continuous communication and navigation support for deep space (beyond geosynchronous) missions. (See Box 3.2.) Many of the mission concepts that were evaluated by the committee would place additional demands on the DSN (see Table 3.1). These additional demands are likely to arise from several sources:

• The mission concepts are large, complex payloads with concomitant large outputs of data. For example, large-aperture telescopes, operating at the Sun-Earth L2 point with gigapixel focal planes, would produce a substantial amount of data at a relatively far distance from Earth. While precise definition of data transmission and uplink requirements are stated only in some of the mission concepts, it is clear that during the time period of this study (2020-2035), data transmission requirements will grow substantially. • A number of mission concepts envision servicing of the scientific payloads—either robotically or by way of human visits. This implies a need for substantial two-way communications to Earth uplinks and downlinks and increased data flow. Assured connectivity and link redundancy will be required if human repair and servicing activities on orbit are involved. In addition, a mission to send astronauts to a Near Earth Object would require substantial two-way communications for the same reasons. • Some of the concepts that the committee evaluated would stress the capabilities of DSN simply because of the very weak signals at the communication distances involved. Concepts of orbiters around Neptune at 30 AU or a mission to the boundary of interstellar space to about 200 AU are two examples of types of missions for which large-aperture antennas on Earth would be required.

The committee requested and received information from NASA about the planned course of evolution of the Deep Space Network. It learned that by 2025 the DSN may not reliably meet the current and projected uplink and downlink requirements. These current and projected requirements do not include any of the Constellation- enabled missions reviewed during this study, which nominally would fly in the 2020-2035 time interval. NASA is concerned with the deterioration of the existing infrastructure, particularly the 70-m antennas now more than 40 years old, and the lack of overall investment to modernize the DSN to accommodate the expected increase in data rates from a variety of missions, including missions to the Moon and Mars. Absent such modernization, the data return from the Constellation-enabled missions might be compromised and fail to fulfill the potential of these very expensive missions.

Finding: Science missions enabled by the Constellation System will increase the strain on the capabilities of the Deep Space Network.

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BOX 3.2 Brief Description of the Deep Space Network

The Deep Space Network (DSN) is an international network of antennas that supports interplanetary spacecraft missions, selected Earth-orbiting missions, and radio and radar astronomy observations for the exploration of the solar system and the universe. The network is a NASA facility managed by the Interplanetary Network Directorate (IND) within the Jet Propulsion Laboratory (JPL). (See Figure 3.2.1.) The origins of the DSN can be traced back to the late 1950s and the inception of NASA in 1958. The execution of several planetary robotic missions, managed by JPL, also contributed to the further development of the network, which expanded from a single tracking station to a worldwide network of large-dish antennas. The DSN consists of three deep space communications facilities strategically placed approximately 120 degrees from one another. The Goldstone Deep Space Communications Complex is located in California’s Mojave Desert; the Madrid Deep Space Communications Complex is in Spain, 37 miles to the west of Madrid at Robledo de Chavela; and the Canberra Deep Space Communications Complex is in the Australian Capital Territory, 25 miles southwest of Canberra near the Tidbinbilla Nature Reserve. Each complex is situated in semimountainous, bowl-shaped terrain for better shielding against radio-frequency interference, and consists of at least four deep space stations equipped with ultrasensi- tive receiving systems and large parabolic dish antennas. The system is made up of the following array of antennas: one 34-meter-diameter high-efficiency antenna, one or more 34-meter beam waveguide antennas (three at Goldstone, two in Madrid, and one in Canberra), one 26-meter antenna, and one 70-meter antenna. Every one of these is a steerable, high-gain, parabolic reflector antenna. All of the stations are remotely operated from a centralized Signal Processing Center at each complex, which house the electronic subsystems that control the antennas, receive and process the data, transmit commands, and generate the spacecraft navigation data. The data are then transmitted to JPL for further processing and distribution to science teams over a ground communications network. The antennas and data delivery systems make it possible to acquire telemetry data and transmit commands to spacecraft, track spacecraft position and velocity, perform very long baseline interferometry observations, measure variations in radio waves for radio science experiments, gather science data, and monitor and control the performance of the network. The strategic placement of these facilities is what characterizes the DSN as the larg- FIGURE 3.2.1 One of the 70-meter dishes of the Deep Space est and most sensitive scientific telecommu- Network. This system currently has long-term maintenance nications system in the world. Their position and usage challenges, but it would be significantly taxed by permits the constant observation of spacecraft many of the proposed mission concepts evaluated in this study. SOURCE: Courtesy of NASA. and enables continuous observation and suitable overlap for transferring the spacecraft radio link from one complex to the next.

Human and Robotic Servicing of Future Space Science Missions

The capability of the Orion spacecraft to travel beyond low Earth orbit together with recent developments in robotics creates new opportunities to service future large spacecraft, primarily observatories. The ability to service spacecraft enables scientists to repair missions and to add science capabilities to existing missions, even changing their scientific focus; to address new questions and changing scientific emphases; to avoid the 15- to 20-year wait- ing period for new missions; and to take advantage of advances in technology. Thus, telescopes would no longer be obsolete by the time they were launched, and making adjustments to existing missions would be cheaper than launching a new mission to replace outdated science and technology capabilities. The current NASA administrator has offered a general endorsement of making future observatories serviceable. When NASA first began operating the space shuttle, the agency had ambitious plans to use it to service numerous science spacecraft in orbit. The most notable science spacecraft regularly serviced by humans has been the Hubble Space Telescope (HST). Several Hubble servicing missions resulted in a much more capable instrument than was originally launched. Hubble serves as a testament to the potential to upgrade and improve scientific instruments placed in space. As science spacecraft, particularly observatories, are placed beyond low Earth orbit, new methods will be required to service them. The Constellation System has potential use for spacecraft servicing. Despite the fact that NASA currently conducts regular construction and repair of the International Space Sta- tion (ISS) and at the time of the writing of this report was preparing for a fifth and final servicing mission to the Hubble Space Telescope, the concept of regular servicing of spacecraft has not been a part of NASA’s future planning for numerous reasons. Both the Constellation System and new robotic capabilities can change that situation. However, the agency needs to take these capabilities into account and not simply to assume that they will develop automatically. This means not only actively considering the future servicing of spacecraft, but also considering new and novel approaches to servicing, such as the creation of way stations, or “servicing nodes,” in space. Designing spacecraft for human servicing increases cost. However, with careful planning, it should be possible to develop methods to enable the most likely servicing without adversely increasing costs. Because robotic servicing is only a recent development, it is unclear how designing spacecraft for robotic servicing would affect costs. The committee has determined that further study is required.

Orion is limited in its capabilities for servicing spacecraft. For example, it lacks an airlock and has insufficient extra payload capability for carrying equipment and replacement instruments to a servicing location. In response to its request for information, the committee received a proposal for the creation of a servicing node for future observatories. The committee also received briefings about the opportunities of servicing for several possible telescope designs.

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Recommendation: NASA should study the benefits of designing spacecraft intended to operate around Earth or the Moon, or at the libration points for human and robotic servicing.

HUMAN SERVICING OF SPACECRAFT NASA first began developing extravehicular activity (EVA) experience with the Gemini program in the mid-1960s. The early goals for these EVAs were to gain basic understanding of human operations in space and to develop better spacesuit designs for the purpose of eventually conducting EVAs on the lunar surface and for simple maneuvering and maintenance tasks in space. By the latter part of the Apollo program, astronauts conducted EVAs during the return from the Moon in order to retrieve film canisters from the Apollo Service Module’s Scientific Instrument Module. Although the idea of having humans repair and upgrade spacecraft in orbit had existed for a long time (and in fact were a staple of 1950s science fiction movies), it was not until the first Skylab mission in 1973 that the usefulness of having humans perform spacewalks to repair a spacecraft was actually demonstrated. Skylab, launched in May 1973, sustained severe damage during its ascent, including damage to its micrometeoroid/sun shade and one of its solar panels. The launch of the first piloted mission to Skylab was delayed in order to develop repair techniques for the ISS. The astronauts were eventually launched into orbit with a special repair kit. During a series of difficult and largely unrehearsed EVAs, they successfully made substantial repairs, including the deployment of a parasol sunshade that cooled interior temperatures enough so that Skylab was habitable. Subsequent Skylab missions installed the twin-pole sunshade, repaired a malfunctioning antenna, and replaced film in the solar observatory. The Skylab experience successfully demonstrated that humans could conduct complex, challenging, and unplanned repairs to spacecraft in low Earth orbit. From relatively early in its development, the space shuttle was designed to be capable of rendezvous with and capture of orbiting spacecraft, and the possibility of using shuttle crews to repair and upgrade spacecraft evolved out of these plans. The shuttle design evolved to include a robotic arm (the Robotic Manipulator System, or RMS, more commonly referred to as the Canadarm) and also equipment inside the payload bay to secure and support spacecraft as well as astronauts during EVA repair and maintenance missions. Many of the tools and techniques for servicing spacecraft were initially developed early in the shuttle program as an effort to demonstrate the shuttle’s ability to provide services that expendable launch vehicles could not. During the course of these operations, NASA learned a great deal about the development of EVA tools and techniques as well as about the importance of designing spacecraft for servicing. In November 1984, the space shuttle Discovery (STS-51A) performed the first retrieval and return to Earth of a satellite for repair and relaunch. Two astronauts wearing jet-propelled manned maneuvering units retrieved two malfunctioning satellites, Palapa-B2 and Westar-VI, which were in improper orbits due to kick motor malfunction. A “stinger” was used to capture Palapa-B2 and Westar-VI. This mission was the last time that the Manned Maneuvering Unit, a backpack that allowed an astronaut to maneuver untethered from the shuttle, was used. It was also the last time that astronauts performed EVAs without a tethering system (besides the 1994 Simplified Aid for EVA Rescue [SAFER] test on STS-64). The ability to improvise has proven to be a key asset in servicing satellites, as demonstrated with the rescue of Palapa-B2 and also with the attempted servicing of Leased Satellite (LEASAT) 3. LEASAT 3 was deployed in April 1985, and it immediately became apparent that its engines were not firing. Engineers theorized that an unreleased hook on the outside of the spacecraft was preventing the engines from working properly and advised the astronauts to push the hook down. No spacewalks had been planned for the mission, and there was the danger that upon releasing the hook the engines would accidentally immediately fire and injure anyone nearby. Further complicating matters was the fact that the satellite was rotating and could not be easily grabbed by the shuttle’s RMS. Four days after its deployment, the astronauts tried to repair LEASAT 3 by building a “fly swatter” (Figure 4.1)—created on the shuttle using a plastic document cover attached to the shuttle’s RMS. An astronaut was then able to carefully extend the arm toward LEASAT 3 and snag the hook in the “fly swatter.” Unfortu-

See http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/archives/sts-51A.html.

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FIGURE 4.1 On space shuttle mission STS-51D, April 1985, an improvised “fly swatter” is used by the Canadarm to try to activate the Leased Satellite (LEASAT) 3 satellite. SOURCE: Courtesy of NASA.

nately, snagging the hook at least twice did not fix the satellite’s problem, and engineers on the ground realized that something else must have been malfunctioning. The satellite was successfully repaired when it was rewired on a later shuttle mission. In May 1992, during the maiden flight of the space shuttle Endeavour during STS-49, NASA conducted the only three-person EVA in order to capture the INTELSAT VI communications satellite. The initial plan was for an astronaut to attach a “capture bar” to the satellite so that it could be grabbed with the RMS. However, after the attempt to attach the capture bar failed, the shuttle crew developed a backup plan. Mounted on footrests, the three astronauts simultaneously grabbed the satellite to stop its rotation (Figure 4.2). After capture, the satellite was fitted with a new perigee kick motor and subsequently released back into orbit, where the new motor successfully fired the spacecraft into a geosynchronous orbit. One of the primary lessons from these early years of spacecraft servicing was the requirement for spacecraft to be designed for capture. When the spacecraft did not have a grapple fixture, it was difficult and time-consuming for astronauts to capture the spacecraft and to move it to a location where it could be serviced. Before the space shuttle even began flying, NASA recognized that in addition to tools and techniques for servicing spacecraft, it would be easier to service spacecraft if they were designed for such servicing. This meant not only a grapple fixture, but also requirements that instruments and important systems be easily accessed by astronauts in their bulky spacesuits. However, NASA abandoned an effort to require that all future NASA Earth- orbiting spacecraft be modularly designed in order to facilitate astronaut servicing. One mission that was designed for servicing and benefited greatly from it was the Solar Maximum Mission

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FIGURE 4.2 STS-49 crew members complete successful capture of the INTELSAT VI into Endeavour’s payload bay. Left to right, mission specialists Richard J. Hieb, Thomas D. Akers, and Pierre J. Thuot, positioned on the remote manipulator system, have handholds on the satellite and prepare to attach the capture bar (tethered to Hieb). SOURCE: Courtesy of NASA.

(SMM), a joint endeavor of NASA Goddard Space Flight Center, the Jet Propulsion Laboratory, and NASA Marshall Space Flight Center, launched on February 14, 1980, to study the Sun during the period of greatest solar activity during the solar cycle (Figure 4.3). The SMM experienced a malfunction in 1981 that would have cut its mission short. However, the SMM had been fitted with a shuttle “grapple feature” in anticipation of unforeseen malfunctions, and this enabled a shuttle to use its robotic arm to capture the satellite for repair. On April 6, 1984, a servicing mission by space shuttle Challenger successfully repaired it. An astronaut using a Manned Maneuver- ing Unit approached the slowly spinning satellite and slowed its rotation, enabling the shuttle arm to capture it

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FIGURE 4.3 Mission specialist George Nelson approaches the damaged Solar Maximum Mission in a capture attempt during the April 6, 1984, servicing mission. SOURCE: Courtesy of NASA.

and place it in the payload bay where it could be repaired. This was the first capture with the arm of the shuttle, performed during the first on-orbit servicing mission in history. The Palapa, LEASAT, and SMM repair missions all demonstrated the ability of astronauts to adapt to unforeseen challenges, and SMM demonstrated the value of designing a spacecraft for modularity and repair. Both of these attributes were employed extensively for the best-known examples of spacecraft repair and upgrade—the four completed Hubble Space Telescope servicing missions (plus one planned) conducted over a 15-year period between 1993 and 2008. (See Box 4.1.) Designing spacecraft for planned human servicing increases costs. Spacecraft have to be designed to include handholds and foot-restraint sockets. They have to provide access to all the interfaces. They must also eliminate potential hazards such as sharp edges and dangerous fuels such as hydrazine. Furthermore, ground simulators must be produced and procedures developed. (See Figure 4.1.3 in Box 4.1.) However, as the history of human servicing demonstrates, astronauts have been able to perform repairs to spacecraft and spacecraft systems that were not

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BOX 4.1 The Hubble Experience

There have been four servicing missions to the Hubble Space Telescope, each bringing new technological advances to the telescope that have enabled some of the best-known and most breathtaking science in the field of astronomy. After the Hubble was launched in 1990 with an improperly focused mirror, the first planned servicing mission assumed significantly greater importance than it had had before. Servicing Mission 1 (SM1, STS-61) in December 1993 (Figure 4.1.1) enabled the installation of the Wide Field and Planetary Camera 2, Cor- rective Optics Space Telescope Axial Replacement (COSTAR), gyros, and solar arrays. The new scientific instruments led to the determination of the age of the universe, the observation of the birth and death of stars, and the observation of the formation of planets and stars. The Imaging Spectrograph, Near Infrared Camera, and fine-guidance sensor were installed during Servicing Mission 2 (SM2, STS-82) in February 1997. From these new scientific instruments came proof of black holes, the observation of Supernova 1As, the determination that the universe is accelerating, observations of the effects of dark matter and dark energy, and the Hubble Deep Field image. In De- cember 1999 (SM3A, STS-103), the gyros, advanced computer, and fine-guidance sensor were installed to improve Hubble operations. Finally, in March 2002 (SM3B, STS-109), the Advanced Camera, solar arrays, power control unit, and Near Infrared Camera and Multi-Object Spec- After the Columbia accident, the NASA administrator decided to cancel a planned servicing mission to trometer (NICMOS) were installed on Hubble (Figure Hubble. After a significant public outcry, NASA announced that it would attempt to service Hubble robotically, 4.1.2). Since their installation, Hubble has seen into the using a dedicated spacecraft and manipulator system that would be developed in a relatively short period of Hubble ultradeep field, seen the evolution of galaxies, time and launched before Hubble degraded too significantly to be repaired. However, Hubble was not designed confirmed the dark energy principle, created the first for robotic servicing, and such an operation had never been demonstrated before. direct mapping of dark matter, made the first detection Under congressional pressure, NASA sponsored a National Research Council (NRC) study on whether of an organic molecule in the atmosphere of a Jupiter- a robotic or a human servicing mission would accomplish the goals of a servicing mission most successfully.1 like planet in the Milky Way Galaxy, and detected over After thorough deliberation, the committee concluded that the state of robotic technology was not advanced 500 extremely old proto galaxies formed just after the enough to complete the mission and successfully overcome unanticipated risks that are bound to occur on any big bang. FIGURE 4.1.1 After replacing the Hubble Space Tele- given mission. Many of the findings in that NRC report focused on the fact that this complex work in space had The Hubble servicing missions have been so scope’s solar panels, astronaut Kathy Thornton releases never been done before by a robot. Since much of the technology had not yet been proven, more time would successful because of the methodical approach adopt- one of the old solar panels during the first Hubble Space have been required to test the technology and, in some cases, develop it from scratch. Preparing for a robotic ed by the engineers, astronauts, and project managers Telescope Servicing Mission in 1993. SOURCE: Courtesy repair mission would have exceeded the planned 39-month development schedule for the servicing mission. involved. While originally planned as a 15-year mission, of NASA. Besides this risk to the development schedule, other risks included successfully launching the vehicle to the by 2001 servicing Hubble had extended its lifetime to desired orbit and conducting a successful rendezvous, proximity operations and grappling by the robot with the 20 years. Mission planners have credited the follow- telescope, and successfully completing the necessary combination of complex autonomous and robotic activi- ing factors for the success of Hubble servicing: a focus on “EVA friendliness” in the telescope design, and the ties. The final risk lay in the inflexibility of a robotic mission to address unforeseen Hubble equipment failures extensive preplanning involved in the servicing missions themselves. that might occur before the mission was completed. To plan for such failures would have required even more Servicing Hubble requires the space shuttle’s robotic arm to grapple Hubble and to place it on and latch development time, and it would have added complexity to the already difficult robotic mission concept. it to a special carrier platform that can rotate and pivot the telescope in the shuttle payload bay.

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BOX 4.1 The Hubble Experience

There have been four servicing missions to the Hubble Space Telescope, each bringing new technological advances to the telescope that have enabled some of the best-known and most breathtaking science in the field of astronomy. After the Hubble was launched in 1990 with an improperly focused mirror, the first planned servicing mission assumed significantly greater importance than it had had before. Servicing Mission 1 (SM1, STS-61) in December 1993 (Figure 4.1.1) enabled the installation of the Wide Field and Planetary Camera 2, Cor- rective Optics Space Telescope Axial Replacement (COSTAR), gyros, and solar arrays. The new scientific instruments led to the determination of the age of the universe, the observation of the birth and death of stars, and the observation of the formation of planets and stars. The Imaging Spectrograph, Near Infrared Camera, and fine-guidance sensor were installed during Servicing Mission 2 (SM2, STS-82) in February 1997. From these new scientific instruments came proof of black holes, the observation of Supernova 1As, the determination that the universe is accelerating, observations of the effects of dark matter and dark energy, and the Hubble Deep Field image. In De- cember 1999 (SM3A, STS-103), the gyros, advanced computer, and fine-guidance sensor were installed to FIGURE 4.1.2 The Hubble Space Telescope in orbit. The spacecraft has been repaired and upgraded repeatedly during nearly two decades in orbit. SOURCE: Courtesy of NASA. improve Hubble operations. Finally, in March 2002 (SM3B, STS-109), the Advanced Camera, solar arrays, power control unit, and Near Infrared Camera and Multi-Object Spec- After the Columbia accident, the NASA administrator decided to cancel a planned servicing mission to trometer (NICMOS) were installed on Hubble (Figure Hubble. After a significant public outcry, NASA announced that it would attempt to service Hubble robotically, 4.1.2). Since their installation, Hubble has seen into the using a dedicated spacecraft and manipulator system that would be developed in a relatively short period of Hubble ultradeep field, seen the evolution of galaxies, time and launched before Hubble degraded too significantly to be repaired. However, Hubble was not designed confirmed the dark energy principle, created the first for robotic servicing, and such an operation had never been demonstrated before. direct mapping of dark matter, made the first detection Under congressional pressure, NASA sponsored a National Research Council (NRC) study on whether of an organic molecule in the atmosphere of a Jupiter- a robotic or a human servicing mission would accomplish the goals of a servicing mission most successfully.1 like planet in the Milky Way Galaxy, and detected over After thorough deliberation, the committee concluded that the state of robotic technology was not advanced 500 extremely old proto galaxies formed just after the enough to complete the mission and successfully overcome unanticipated risks that are bound to occur on any big bang. given mission. Many of the findings in that NRC report focused on the fact that this complex work in space had The Hubble servicing missions have been so never been done before by a robot. Since much of the technology had not yet been proven, more time would successful because of the methodical approach adopt- have been required to test the technology and, in some cases, develop it from scratch. Preparing for a robotic ed by the engineers, astronauts, and project managers repair mission would have exceeded the planned 39-month development schedule for the servicing mission. involved. While originally planned as a 15-year mission, Besides this risk to the development schedule, other risks included successfully launching the vehicle to the by 2001 servicing Hubble had extended its lifetime to desired orbit and conducting a successful rendezvous, proximity operations and grappling by the robot with the 20 years. Mission planners have credited the follow- telescope, and successfully completing the necessary combination of complex autonomous and robotic activi- ing factors for the success of Hubble servicing: a focus on “EVA friendliness” in the telescope design, and the ties. The final risk lay in the inflexibility of a robotic mission to address unforeseen Hubble equipment failures extensive preplanning involved in the servicing missions themselves. that might occur before the mission was completed. To plan for such failures would have required even more Servicing Hubble requires the space shuttle’s robotic arm to grapple Hubble and to place it on and latch development time, and it would have added complexity to the already difficult robotic mission concept. it to a special carrier platform that can rotate and pivot the telescope in the shuttle payload bay.

continued

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BOX 4.1 Continued

By contrast, the NRC committee that wrote this report on options for extending the life of the Hubble concluded that it was much simpler to do a human servicing mission, especially since all four previous human servicing missions to Hubble had fully met their objectives. Furthermore, that committee found that “space shuttle crews, in conjunction with their ground-based mission control teams, have consistently developed innovative procedures and techniques to bring about desired mission success when encountering unplanned for or unexpected contingencies on-orbit.”2 The same NRC committee discussed the findings of the Columbia Accident Investigation Board’s (CAIB’s) report and NASA’s reaction to the findings (which was full acceptance and the institution of even more stringent guidelines on shuttle missions), and it found that meeting both the CAIB and NASA requirements for a shuttle servicing mission to Hubble was possible, and that such a mission would be no riskier than one to the International Space Station. On October 31, 2006, NASA administrator Michael Griffin decided that Hubble would receive a final servicing mission before the retirement of the space shuttle. Dr. Griffin’s decision was based not only on the findings of the NRC report, but on his own knowledge of the robotic servicing option prior to his becoming NASA administrator. The final Hubble servicing mission, scheduled for October 2008, is to be performed by the crew of space shuttle Atlantis (STS-125). On this mission, astronauts will install the Cosmic Origins Spectrograph, Wide Field Camera 3, fine-guidance sensor, aft shroud cooling system batteries, and gyros; repair the Space Telescope Imaging Spectrograph and the Advanced Camera for Surveys; and add the Soft Capture Mechanism. One of the planned repairs to Hubble includes equipment and techniques originally developed for the robotic servicing mission and involves accessing a part of Hubble that was never designed for servicing. (See Figure 4.1.3.) With these new upgrades to the telescope’s capabilities, scientists will be able to track galaxy evolution, learn more about the formation of stars, measure the composition and structure of baryonic matter, study the characteristics of quasars, and investigate the nature of cold dark matter. These additions to the Hubble Space Telescope will illuminate both the large-scale structure of the universe and the progressive changes in chemical composition of matter as the universe has grown older.

FIGURE 4.1.3 Astronaut practices for the final Hubble Servicing Mission. He is using a device that was originally developed for the robotic servicing mission to prevent screws from floating free. This device will be used for a part of Hubble that was never intended for servicing. SOURCE: Courtesy of NASA.

1National Research Council, Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report, The National Academies Press, Washington, D.C., 2005. 2Ibid., p. 91.

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designed for such servicing. This indicates that it should be possible to develop some servicing capabilities into a spacecraft without adverse impact on design or cost. In addition, in recent years a new servicing option has also emerged—robotic servicing.

ROBOTIC SERVICING OF SPACECRAFT Robotic servicing is a capability that has yet to be fully proven, although substantial advances have been made in recent years. After NASA canceled the shuttle servicing mission for Hubble following the Columbia accident, the agency sought to develop a robotic servicing mission. This project was eventually canceled for several reasons, including its complexity and the short development time available. It would have been one of the most complex spacecraft ever developed, and there was insufficient time to build and fly it before the expected demise of Hubble. One of the major factors contributing to the spacecraft’s complexity was that although Hubble was designed to be serviced by humans, it was not designed to be serviced robotically. The Hubble servicing spacecraft would have had to rendezvous with what is known as a noncooperative target, increasing the complexity level. In addition, several repair tasks scheduled for Hubble were never even intended for humans, let alone robots. The challenge for robotic servicing can be divided into two primary areas: (1) automated rendezvous and docking and (2) the exchange of equipment and fuel. In the past several years, the United States and its European ISS partners have demonstrated significant advances in these areas. In 2007 the Orbital Express mission demonstrated the ability to automatically rendezvous and dock with another spacecraft and also to transfer fuel and replace various equipment such as batteries and electronic units. In 2008, the European Space Agency’s (ESA’s) Automated Transfer Vehicle (ATV) successfully delivered the Jules Verne cargo spacecraft to the ISS after performing an automated approach and docking.

SUCCESSFUL ROBOTIC SERVICING MISSIONS Orbital Express, a mission managed by the Defense Advanced Research Projects Agency, was launched on March 8, 2007, with the goal of demonstrating the technical feasibility of autonomous, on-orbit satellite servicing. The spacecraft’s two vehicles were the Autonomous Space Transfer and Robotic Orbiter (ASTRO), which was the servicer, and the Next Generation Satellite/Commodity Spacecraft (NEXTSat/CSC), which was the client (Figure 4.4). These two vehicles had the following goals: interface with a satellite, transfer propellant in orbit between a serviceable satellite and the servicing satellite, and transfer a battery and computer. In addition to successfully completing these requirements, the vehicles also demonstrated autonomous operations such as rendezvous, operation in close proximity to the target satellite, and capture of the target satellite. Four levels of supervised autonomy were demonstrated, showing vehicle operation in increasingly challenging environments. ESA’s ATV is another example of successful autonomous robotic technology. The ATV is a highly intelligent spacecraft that has an onboard high-precision navigation system that automatically guides ATV on a rendezvous trajectory with the ISS and docks it with the Russian service module Zvedzda. It is a resupply vehicle that also at regular intervals boosts ISS into higher orbit to offset the drag effects of oxygen molecules above Earth’s atmosphere. There are five planned ATV launches from 2008 until approximately 2018. The Jules Verne ATV launched on March 9, 2008, and docked with the ISS on April 3, 2008, providing fuel, water, oxygen, food, spare parts, and clothing. In the course of its launch and docking, Jules Verne demonstrated relative navigation with the ISS, its capability to execute the Collision Avoidance Maneuver, successful use of the relative Global Positioning System, close proximity maneuvering and control, and contingency maneuvers. The ATV can remain attached as a pressurized and integral part of the station for up to 6 months, at which point it will de-orbit and burn up in Earth’s atmosphere along with up to 14,000 pounds of waste from the ISS. The interior of the ATV consists of the service module and the pressurized integrated cargo carrier. Thus, the ATV combines full automatic capabilities with human spacecraft safety requirements. Although no crews will be launched in the ATV, once it is docked with the ISS, astronauts will be able to use its interior like any other habitable part of the ISS.

See ESA’s ATV Web site at http://www.esa.int/esaMI/ATV/index.html.

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FIGURE 4.4 Illustration of Orbital Express’s two vehicles: the Autonomous Space Transfer and Robotic Orbiter, or ASTRO (servicer) (left), and the Next Generation Satellite/Commodity Spacecraft, or NEXTSat/CSC (client) (right). SOURCE: Cour- tesy of the Defense Advanced Research Projects Agency.

FUTURE SERVICING CAPABILITIES The successes of Orbital Express and ESA’s ATV demonstrate how far technology has progressed and hint at a future when the robotic servicing of space science missions will not only be possible but also a practical and cost-effective solution to servicing spacecraft in orbit. If NASA investigates the value of on-orbit servicing using autonomous robotics, more advanced concepts can be tested and implemented. The basic requirements for servicing include the following: some degree of autonomous rendezvous capability, a capture mechanism, some kind of manipulator or arm to grapple with specific elements on the satellite, and finally, the satellite itself must be designed so that it can be serviced. The Hubble Space Telescope was designed to be serviced, but the James Webb Space Telescope, because it is being sent farther out, to the Sun-Earth L2 point, is not. To be serviceable, a satellite needs systems that are designed to be easily removed and replaced and needs to have an interface design that works with current or future tools. High-fidelity physical and electrical simulators on the ground are important for designing new instruments—allowing “fit checking” of devices to be installed and the development of contingency procedures. Many of the items repaired or replaced on Hubble were not designed to be serviced, but access to replicas of the spacecraft both in the underwater training facility and in a ground facility at the NASA Goddard Space Flight Center (see Figure 4.5) made it possible for engineers to

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FIGURE 4.5 Hubble Space Telescope high-fidelity simulators in the Goddard Space Flight Center clean room along with other flight items. At back center is the axial equipment bay where Corrective Optics Space Telescope Axial Replacement (COSTAR) and the other axial instruments were fit-checked. At back right is the equipment bay with real electrical and computer components for testing and checking out electronics boxes. The ability for astronauts to train on the ground with hardware similar to that in space is important for human servicing missions. SOURCE: Courtesy of NASA.

develop repair procedures and, in some cases, specialized tools that were used successfully to repair or replace failed or aging components.

FUTURE HUMAN AND ROBOTIC SERVICING OPTIONS The Orion spacecraft, although capable of traveling beyond low Earth orbit, has a number of limitations with respect to acting as a servicing mission. Orion has limited capability for change in velocity (delta-v) and would require additional propulsion to leave low Earth orbit. The spacecraft is not equipped with an airlock, which would therefore require that the entire cabin be depressurized or that an airlock be provided for the crew. In addition, Orion has limited extra mass and volume capacity for carrying equipment for a servicing mission, such as a robotic manipulator system, toolkits, and any equipment to be installed. Any plan to use the Orion for a servicing mission will have to address these limitations. Options include launching a second spacecraft to carry servicing equipment, designing spacecraft to include many of the necessary tools and even an airlock, and launching a dedicated servicing spacecraft to which Orion would rendezvous and dock before journeying to the spacecraft to be serviced. Several of the mission concepts evaluated in this report would operate at the libration points a significant distance from Earth (see Figure 2.1b in Chapter 2). Several telescopes currently operate at these locations, and others, such as the James Webb Space Telescope, are scheduled to be placed there. Although these sites are attrac-

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tive for operating heliophysics and astronomy observatories, they are less than ideal for servicing purposes owing to their distance and communications lag times and for other reasons. One possibility is to move the spacecraft closer to Earth for servicing purposes and then move it back to its operating location. Several proposals exist for establishing a servicing node or way station at a closer location that could be visited either by humans in an Orion spacecraft or by a robotic servicing spacecraft. Transferring the observatory from one Lagrangian point to another requires very little change in velocity and subsequently only small amounts of fuel. The James Webb Space Telescope will operate at the Sun-Earth L2 point, which maintains the orientation of the spacecraft with respect to the Sun and Earth and is an attractive location for astronomy missions because it is thermally stable (sources of heat and scattered light are in the same general direction), it has continuous communication capability with Earth, it can easily make use of solar power, and it has very low debris and dust. The James Webb Space Telescope was not designed for human or robotic servicing. Its delicate thermal shield could collapse if exposed to a spacecraft’s thruster jets, and rocket exhaust could condense on the telescope’s mirror. The instruments were also not designed for access. A servicing mission at the Earth-Moon L1 (or L2) point would allow easy access to the Sun-Earth L2 location. A servicing station at this location would be able to make use of the same advantages afforded by the Sun-Earth L2 location, but it would also provide easy access to lunar-capable architecture (approximately 4-day transfer time) and build on the lunar navigation-and-communication network. By contrast, low Earth orbit makes it difficult to service spacecraft because propulsive loads would be too large for the lightweight support structures of space telescopes. Additionally, these telescopes would be exposed to damage from the contaminants and debris in low Earth orbit. To get humans to the Earth-Moon L1 point, the Constellation System will be required. Other observatories (e.g., lunar surface missions, Single Aperture Far Infrared Telescope, Advanced Technol- ogy Large-Aperture Space Telescope) may also be operated at the Sun-Earth L2 and Earth-Moon L1 locations in the future. It is possible to have a servicing station for spacecraft at Earth-Moon L1, as illustrated in Figure 4.6. Such a servicing station would have a servicing node, which would remain in orbit and would require some additional avionics and propulsion capabilities beyond those found in a simple airlock. In addition to servicing spacecraft out at Earth-Moon L1, it could also possibly be used to aid lunar surface exploration. This servicing node could enable two types of robotic servicing: a robot operated by astronauts and a robot operated autonomously from Earth. Ultimately, a servicing node at the Earth-Moon L1 or L2 point would make it possible to construct large astronomical observatories that surpass even Ares V single-launch capabilities, as shown in Figure 4.7. It could also be used to facilitate lunar exploration goals, and in the far term, an Earth-Moon L2 point servicing mission could provide a stepping stone between lunar missions and Mars missions. It could be used as a test site for issues such as duration in space, distance from Earth, communication delays, and supply issues. However, these missions might prove to be expensive, and it would be reasonable to expect both NASA’s Science Mission Directorate and the Exploration Systems Mission Directorate to share these costs.

ADVANTAGES OF SERVICING MISSIONS Although the advent of the Orion spacecraft and the Constellation System’s other capabilities reopens the possibility of human servicing of spacecraft, it is clear that this possibility will create operational complexities greater than those that NASA has faced in the past. Large spacecraft deployed at distant locations may have to be moved nearer to Earth for servicing, and human servicing missions will require the development of major new pieces of equipment, such as airlocks, which are not currently planned by NASA. The recent development of robotic servicing options opens up additional exciting possibilities for future spacecraft servicing. It is possible to conceive of a robotic servicing node at the Earth-Moon L1 point similar to that discussed earlier. A robotic spacecraft could be designed to rendezvous with an observatory and to repair and

The L1 and L2 points are unstable, so a small amount of fuel is required to keep the spacecraft from drifting away. Harley A. Thronson, Daniel F. Lester, Adam Dissel, Jason Budinoff, and John Stevens, “On the Way to the Moon: Using NASA’s Constellation Architecture to Achieve Multiple Science Goals in Space,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, June 9, 2008.

HUMAN AND ROBOTIC SERVICING OF FUTURE SPACE SCIENCE MISSIONS 95

FIGURE 4.6 Concept for in-space servicing throughout the Earth-Moon system. This figure shows the stack after Earth-orbit rendezvous of the Orion with the Centaur and servicing node. The Centaur is ejected before rendezvous with the observatory. NOTE: EVA, extravehicular activity; LIDS, Low-Impact Docking System. SOURCE: Courtesy of NASA.

upgrade it. Another option could be a combination of human and robotic servicing, using robotic spacecraft for basic upgrading and other tasks and using human servicing for more complex tasks. For all of these robotic and human servicing capabilities to be realized, servicing capabilities must be considered as the telescopes and spacecraft themselves are built. This type of adjustment might involve additional costs when constructing the spacecraft. However, making spacecraft serviceable will provide numerous savings and benefits in the long run. Components with long lifetime ratings are more expensive than those with shorter ones. Servicing can mitigate this issue (since components need not have such long lifespans), and replacing components with new technology can increase capability and capacity. The ability to service a spacecraft also enables the correction of various mission-design problems and failures and the ability to increase the longevity of critical systems by transferring fuel and cryogenics. Additionally, while NASA’s space science budget has been stable in recent years, cost overruns and setbacks (e.g., the Columbia accident and its return to flight costs and effect on the shuttle launch schedule) have caused budgets for the various divisions in the Science Mission Directorate to become even more restrictive. Because of the expense of large space observatories, launching fewer platforms and gaining the ability to operate them longer may be a cheaper way to continue to do new science, by fitting new capabilities on already-orbiting spacecraft.

Finding: The Constellation System and advanced robotic servicing technology make possible the servicing and in-space assembly of large spacecraft.

Finding: Designing spacecraft components for accessibility is essential for in-space servicing and is also advantageous for preflight integration and testing.

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FIGURE 4.7 An illustration of the concept of operations for a servicing node at the Earth-Moon L1 point. The facility to be serviced, in this case an astronomical observatory, travelsFigure by means 4.7.eps of low-energy (i.e., velocity) transfer to the Earth-Moon L1 (or L2) location, where it is met by the Orion and servicing node module, which features an airlock. This system can remain onsite for 2 to 3 weeks and perhapsBitmap could carry image out other -tasks. Low On completion resolution of the servicing mission, the Orion crew module returns to Earth and the servicing node may remain at one of the libration points for further duties. NOTE: SM, service module; ΔV, delta-v; TEI, trans Earth injection. SOURCE: Courtesy of NASA.

Human servicing missions have already enabled fantastic advances in space science through the Solar Maxi- mum Mission and the Hubble Space Telescope. In the future, these advances can only improve as technology and science continue to progress at increasing rates. Future servicing missions may continue to be human servicing missions, or a combination of human and robotic capabilities, but as robotic technology advances, robotic autonomous missions to scientific spacecraft will be possible. ESA is already using autonomous robotics to service the ISS, and Orbital Express has proven that some of the technology already works. As robotic technology continues to improve, new servicing capabilities will become possible, creating opportunities for even greater science that better keeps up with evolving space technologies.

Launch Vehicle and Spacecraft Options for Future Space Science Missions

The Constellation System is being developed to enable human exploration beyond low Earth orbit, to include the Moon and Mars. It consists of the Orion Crew Exploration Vehicle (CEV), the Altair lunar lander, and the Ares I and V launch vehicles. Primarily envisioned to deliver cargo to the Moon, the Ares V has nearly five times the lift capability of the most capable launch vehicle currently in the U.S. fleet—the Delta IV Heavy. Recognizing that this additional lift capability could enable new science opportunities, NASA has identified alternative Ares V vehicle configurations to launch spacecraft for science missions. Similarly, several notional configurations of the Ares I have been identified for missions that are at or slightly exceed the capabilities of the Delta IV Heavy. This chapter provides a high-level overview of the Orion spacecraft and the Ares I and Ares V launch systems, as well as a comparison of the performance of these launch vehicles with that of the existing fleet of expendable launch vehicles employed by the U.S. government. The Altair lunar lander (technically the Lunar Surface Access Module) is in the very preliminary concept stage and is not discussed in this report.

THE ORION SPACECRAFT The Orion CEV is to be launched atop the Ares I rocket and consists of a launch-abort system (capable of pulling the spacecraft and its crew to safety in the event of an emergency), the crew module (CM), the service module (SM, which houses the power and propulsion modules), and the spacecraft adapter (to connect the Orion capsule and service module to the launch systems). (See Figure 5.1.) Orion has a configuration similar to that of the Apollo spacecraft but is significantly larger, with twice the internal volume. Orion will be 5 m in diameter and will have a mass of about 22.7 metric tons and a pressurized volume of 20 m3. Orion will boast a lunar return payload of 100 kg and a crew habitable volume of 11 m3, allowing it to support up to six crew members for low-Earth-orbit missions. The current baseline design of the service module includes a scientific instrumentation module (SIM) bay of 0.57 m3 volume, with power and data accommodations, capable of holding 382 kg. The Orion vehicle is specifically designed to support missions to the Moon as well as to the International Space Station (ISS). The standard lunar mission would involve the launch of four crew members aboard an Orion atop an Ares I, followed by the launch of the Altair lunar lander atop an Ares V. The Orion would then rendezvous and dock with the Altair atop its Earth Departure Stage and then conduct an approximately 4-day journey to the Moon. The four-person crew would descend to the lunar surface inside the Altair, leaving the Orion uncrewed in

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Crew Module (JSC) Crew and cargo transport

Launch Abort System (LaRC) Emergency escape during launch

Spacecraft Adapter (GRC) Structural transition to Ares launch vehicle

Service Module (GRC) propulsion, electrical power, fluids storage

FIGURE 5.1 The Orion spacecraft. The overall configuration is similar to that of the Apollo spacecraft. NOTE: GRC, Glenn Research Center; JSC, Johnson Space Center; LaRC, Langley Research Center. SOURCE: Courtesy of NASA.

Figure 5.1.eps low lunar orbit for up to 180 days. At the end of the lunar phase of the mission, the Altair would launch from the surface and rendezvous with OrionIncludes for the returnlow resolutionto Earth. The command bitmap module image would perform direct or skip reentry and make a precision water landing off the coast of California. The core capabilities for the ISS mission are launch by way of Ares I for a crew of up to six, automated rendezvous and docking with relative navigation sensors, and a low impact docking system (LIDS). Orion then remains at ISS for up to 180 days and undocks for return to Earth. Additional capabilities are available with further small- or large-scale design work. These include the delivery of unpressurized cargo to the ISS with volume of up to 2.92 m3 and mass up to 600 kg, the ability to configure the service module as a stand-alone element (ability to deliver a payload to a particular location), and the capability of being used in a modular fashion.

OPPORTUNITIES FOR SMALL SPACE SCIENCE PAYLOADS ABOARD CONSTELLATION Although the Orion spacecraft is still in a relatively early stage of development, NASA has reserved payload space and mass at the rear of the vehicle for potential science payloads (see Figure 5.2). These could include both attached payloads and small deployable satellites. The committee did not receive any such small-payload proposals in response to its request for information (all of the mission proposals that the committee evaluated are large, multiton experiments), but this payload capability offers the potential for interesting and worthwhile science to be conducted on Orion flights. This would enhance Orion’s capabilities. As of the writing of this report, opportunities appear to exist to attach experiments to the service module on lunar missions. Studies and analyses to evaluate various options are in progress. The Orion designers currently

LAUNCH VEHICLE AND SPACECRAFT OPTIONS FOR FUTURE SPACE SCIENCE MISSIONS 99

Service module includes a scientific instrumentation module (SIM) bay – 0.57 cubic meter volume – Capable of holding 382 kg – Power and data accommodations

FIGURE 5.2 NASA has reserved space on the Orion spacecraft for a scientific instrumentation module. This could carry instruments as well as small deployable payloads. FigureSOURCE: Courtesy5.2.eps of NASA. Includes low resolution bitmap image

envision relatively simple interfaces between such experiments and the service module. The experiments could ride along with the service module into orbit with the spacecraft, could be separated from the service module, or could also be propelled into a different orbit by a self-contained rocket motor. There may be similar opportunities for additional secondary payloads aboard the other elements of Constel- lation such as Orion and Altair and even the Ares V. Historically, as space launch systems mature, opportunities develop for secondary payloads to ride into space in addition to primary payloads. This has been the case for Atlas, Thor, Delta, and Titan, and one can expect a similar evolution as the Constellation System matures. Secondary experiments were carried aboard the Apollo program. Subsatellites were released during the Apollo 15 and 16 missions and orbited the Moon for periods much longer than the Apollo excursions to the lunar surface. (See Figure 5.3.) Similarly, many secondary payloads were carried into orbit aboard Department of Defense boosters. Such secondary experimental payloads had the virtue of being relatively inexpensive and provided flexible, quick-turnaround, rapid-response capabilities to seize scientific opportunities. For example, assuming that lunar missions are launched twice a year and assuming that secondary experiments capabilities also become available aboard Altair and Orion, there would be multiple opportunities each year for appropriate experiments to fly.

ARES I The Ares I will lift the Orion into low Earth orbit, either to dock with the ISS or to mate with the Earth Depar- ture Stage (EDS) for transport to the Moon (Figure 5.4). The Ares I is a two-stage, human-rated launch vehicle that employs proven technologies and hardware designs to increase vehicle safety and reliability and significantly reduce development cost and time. The first stage is a five-segment reusable solid-rocket motor derived from the four-segment Reusable Solid Rocket Motor (RSRM) currently used to boost the space shuttle into orbit. The

upper stage is a liquid oxygen/liquid hydrogen (LOX/LH2) system using a single J-2X engine. (See Figure 5.5.)

Flights to the International Space Station could accommodate a mass of about 600 kg and a volume of about 3 m3. One possibility discussed for Ares V is the inclusion of a device similar to that developed for the Evolved Expendable Launch Vehicle (EELV) family and known as the EELV Secondary Payload Adapter, or ESPA ring. This device enables small satellites to be carried and deployed. However, no formal proposal for such a device was presented to the committee, and there is no indication that NASA is actively considering such a device at this early stage of Ares V development.

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FIGURE 5.3 Artist’s impression of an Apollo spacecraft launching a subsatellite while in orbit around the Moon. The Orion spacecraft could have a similar capability. SOURCE: Courtesy of NASA.

This engine is a derivative of the J-2 used on the Saturn IB and Saturn V rockets during the Apollo program. The modern version incorporates technologies developed for the J-2S, a simplified version of the original J-2, and the XRS-2200 aero-spike engine developed for the X-33 Advanced Technology Demonstrator program in the late 1990s. It also employs modern manufacturing processes used for the Delta IV RS-68 engine. The first development test flight of a dynamically similar test version of the Ares I (designated the Ares I-X) is scheduled for the fall of 2009. The first flight validation test is scheduled for 2012 or 2013. To meet the needs of science missions, the Orion CEV and Encapsulated Service Module could be replaced with a 5-m shroud, derived from either the Atlas V or Delta IV composite fairing. This version could be used to place satellites into low Earth orbit. For higher orbital altitudes and interplanetary missions, a dual-engine Centaur (Centaur V2) could be added. Using heritage systems again reduces development costs and time. The primary development would focus on system integration and the interfaces between the Ares I booster and the Centaur or payload.

LAUNCH VEHICLE AND SPACECRAFT OPTIONS FOR FUTURE SPACE SCIENCE MISSIONS 101

FIGURE 5.4 Artist’s conception of the Ares I launchFigure vehicle on5.4.eps its pad. SOURCE: Courtesy of NASA. Bitmap image

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Instrument Unit Stack Integration Module (ESM) Panels • Primary Ares I control • 927.1 mT (2,044.0K lbm) avionics system gross liftoff mass • NASA Design / • 99.1 m (325.0 ft) in length Boeing Production ($0.8B) •NASA-led

Orion CEV First Stage • Derived from current Interstage Shuttle RSRM/B • Five segments/Polybutadiene Acrylonitrile (PBAN) propellant Upper Stage • Recoverable • 137.1 mT (302.2K lbm) LOX/LH2 prop • New forward adapter • 5.5-m (18-ft) diameter • Avionics upgrades • Aluminum-Lithium (Al-Li) structures • ATK Launch Systems ($1.8B) • Instrument unit and interstage • Reaction Control System (RCS) / roll control for first stage flight • Primary Ares I control avionics system • NASA Design / Boeing Production ($1.12B)

Upper Stage Engine • Saturn J–2 derived engine (J–2X) • Expendable • Pratt and Whitney Rocketdyne ($1.2B)

FIGURE 5.5 Ares I configuration. The first stage is derived from the space shuttle’s solid-rocket booster. The upper stage will

use the J-2X engine. NOTE: RSRM/B, Reusable Solid Rocket Motor/Booster; lbm, pound-mass; LOX/LH2, liquid oxygen/liquid hydrogen. SOURCE: Courtesy of NASA. Figure 5.5.eps Includes low resolution bitmap image

LinesARES are V squiggly The Ares V provides significant increased capability over the existing fleet of U.S. government launch vehicles, both in terms of payload mass and available payload volume. It is being designed to transport cargo to the Moon as well as to deliver the Earth Departure Stage to low Earth orbit to rendezvous with the Orion spacecraft for missions to the Moon. The Ares V consists of a pair of 5.5-segment solid-rocket boosters (SRBs) strapped to

an LOX/LH2 core stage, and an EDS. The SRBs are common to the first-stage motors used on the Ares I, with direct heritage to the shuttle RSRMs. The core stage will be propelled by six RS-68B engines derived from the RS-68 engines developed for the Delta IV launch vehicle. The EDS will employ the J-2X engine being developed for the second stage of the Ares I, as well as the Ares I instrumentation unit. (See Figure 5.6.) Given the use of hardware and systems being developed for Ares I, the development risks of the Ares V are significantly reduced. The first test flight of the Ares V is projected to occur in the 2018 time frame, with the first opportunity for a lunar mission occurring by 2020. The standard Ares shroud can be used for science missions. It has a usable volume of 860 m3, which is more than three times the volume of the Delta IV fairing. For larger payloads, the cylindrical portion of the baseline shroud could be extended by 9 m, to provide usable volume of 1,410 m3. To enhance the usefulness of Ares V for deep space missions, the Constellation System has identified a notional configuration that includes a dual-engine Centaur to improve mission capability.

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Composite Shroud

Launch Abort System Altair Lunar Lander

Orion Crew Exploration Vehicle Earth Departure Stage (Crew Module / Service Module) LOX/LH 2 Encapsulated Service 1 J–2X Engine Module Panels Al-Li Tanks Composite Structures Instrument Unit

Loiter Skirt Upper Stage

Interstage J–2X Upper Stage Engine

Interstage

Forward Frustum Core Stage LOX/LH 2 6 RS–68 Engines Al-Li Tanks/Structures First Stage (5-Segment RSRB)

2 5.5-Segment RSRBs

Ares I Ares V 25.5 mT (56.2K lbm) to 71.1 mT (156.7K lbm) to TLI (with Ares I) Low Earth Orbit (LEO) 62.8 mT (138.5K lbm) to TLI ~187.7 mT (413.8K lbm) to LEO FIGURE 5.6 Comparison of Ares V and Ares I configurations. Although significantly larger than the Ares I, it is planned that the Ares V will use a number of the Ares I components. NOTE: RSRB, Reusable Solid Rocket Booster; lbm, pound-mass;

LOX/LH2, liquid oxygen/liquid hydrogen. SOURCE: CourtesyFigure of NASA. 5.6.eps Includes low resolution bitmap images

DELTA IV The Delta IV is currently in service, primarily carrying national security payloads. (See Figure 5.7.) The Delta IV family of vehicles consists of five different configurations. The smallest is the Medium Launch Vehicle (MLV; referred to in several of the Vision Mission studies as a Delta 4040). It consists of a Common Booster Core, with a 4-m upper stage and payload fairing. A variant of this configuration includes the addition of two solid-rocket motors and is referred to as a M+(4,2) (or Delta 4240 in the various studies). The next variant uses either two or four solid-rocket motors with a 5-m upper stage and payload fairing. These variants are referred to as M+(5,2) and M+(5,4) vehicles, or Delta 4250 and 4450, respectively. The largest variant of the Delta IV is the Heavy Lift Vehicle (HLV). It consists of three boosters and a 5-m upper stage and payload fairing. (It is the variant identified in the majority of the Vision Mission studies and is referred to as the 4050 or 4050H.) The first Delta IV was launched in November 2002, with the first Heavy configuration launched in December 2004.

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FIGURE 5.7 Launch of a Delta IV Heavy launch vehicle carrying a military payload. Most of the Vision Mission concepts evaluated in this report used the Delta IV Heavy. SOURCE: Courtesy of U.S. Air Force.

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The launch vehicle performance estimates used in each of the Vision Mission studies were based on the original Delta IV Payload Planners Guide, published in 2000 and updated in 2002. United Launch Alliance (a joint venture formed by the Boeing Company and Lockheed Martin Corporation) released an updated version of the Payload Planners Guide in September 2007. The updated performance estimates for low Earth orbit are slightly lower (∼6 percent) than the 2002 estimates. The vehicle performance estimates across the reported range of C3 (hyperbolic excess speed over escape velocity) increased by 10 to 15 percent. Performance comparisons and assessments in the remainder of the discussion are based on the most recent values. The government is currently funding an upgrade program for the RS-68 engine used on the first stage of the Delta IV. The upgraded engine, known as the RS-68A, is projected to be completed to support a launch in the 2010 time frame. Various improvements will increase both the thrust and specific impulse (Isp) of the engine, increasing the overall vehicle performance. Since the results of the upgrade are not yet documented, the benefits of the performance improvements are not considered in this evaluation. A slightly modified version of the RS-68A engine, designated the RS-68B, is to be used on the Ares V launch vehicle. The modifications are intended to (1) take advantage of the performance enhancements currently in development, (2) reduce the use of helium for the five engines to levels comparable with the levels used by the three space shuttle main engines, and (3) minimize the hydrogen flame that rises around the Delta IV during the engine start sequence.

ATLAS V As with the Delta IV, the Atlas V consists of multiple vehicle configurations. All configurations use a standard common core booster, with up to five strap-on solid-rocket boosters, and a Centaur upper stage. The Centaur can be ordered with either a single or a pair of RL10 engines. A three-digit naming convention is used to identify the specific configuration of the highly modular Atlas V vehicle. The first digit indicates the payload fairing size (4- or 5-m), the second identifies the number of strap-on solid-rocket motors (from 0 up to 5), and the third identifies the number of RL10 engines (1 or 2). The Atlas V is currently in service carrying military, civilian, and commercial payloads, as illustrated in Figure 5.8. The first Atlas V flew in August 2002. The 400 and 500 series configurations provide a broad range of payload capabilities, bridging the gap between the M+ and Heavy configurations of the Delta IV. An HLV configuration of the Atlas V is currently under development but does not have a planned launch date. (The Super Heavy configuration identified in several of the Vision Mission studies is a notional growth option for the Atlas family and does not currently exist, nor is any work underway to develop it.)

ARES DEVELOPMENT RISKS Engines generally provide the greatest technical risks for launch systems. To address this risk, NASA is employing and enhancing engines with significant flight and development history (see Figure 5.9). A second significant risk is mass growth of the launch vehicle. This is a continual challenge during the development phase and often leads to engine performance upgrades or structural weight-saving measures. The RS-68 has a modest flight history (8 flights, 12 engines) and significant development history. The government is currently upgrading the performance of the engine to address unique requirements of a mission scheduled to fly in the 2010 time frame. NASA will use this upgraded engine, the RS-68A, as the foundation for the RS-68B to be used for the Ares V. The modifications planned for this upgrade are not necessarily simple modifications, but they are much simpler than a complete engine development activity, or even the performance upgrades currently in work by the U.S. Air Force and the National Reconnaissance Office.

Delta IV Payload Planners Guide, The Boeing Company, Huntington Beach, Calif., 2002. Delta IV Payload Planners Guide, 06H0233, United Launch Alliance, Littleton, Colo., September 2007, available at http://www.ulalaunch. com/docs/product_sheet/DeltaIVPayloadPlannersGuide2007.pdf.

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FIGURE 5.8 Launch of an Atlas V launch vehicle carrying NASA’s New Horizons spacecraft. SOURCE: Courtesy of NASA.

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122 m (400 ft)

Crew Altair

91 m Orion Lunar (300 ft) Earth Departure Lander

) Stage (EDS) (1 J–2X) 253.0 mT (557.7K lbm)

LOX/LH2 S–IVB (1 J–2 engine) Upper Stage 108.9 mT (1 J–2X) (240.0K lbm) 61 m 137.1 mT (200 ft) LOX/LH (302.2K lbm) 2 LOX/LH 2 S–II (5 J–2 engines) 453.6 mT 5-Segment Core Stage (1,000.0K lbm) Reusable (6 RS–68 Engines) Overall Vehicle Height, m (ft Solid Rocket 1,587.3 mT LOX/LH2 30 m (100 ft) Booster (3,499.5K lbm) S–IC (RSRB) LOX/LH2 (5 F–1) 2 5.5-Segment 1,769.0 mT RSRBs (3,900.0K lbm) LOX/RP–1

0 Space Shuttle Ares I Ares V Saturn V Height: 56.1 m (184.2 ft) Height: 99.1 m (325.0 ft) Height: 116.2 m (381.1 ft) Height: 110.9 m (364.0 ft) Gross Liftoff Mass: Gross Liftoff Mass : Gross Liftoff Mass : Gross Liftoff Mass : 2,041.1 mT (4,500.0K lbm) 927.1 mT (2,044.0K lbm) 3,704.5 mT (8,167.1K lbm) 2,948.4 mT (6,500K lbm) Payload Capability: Payload Capability: Payload Capability: Payload Capability: 25.0 mT (55.1K lbm) 25.5 mT (56.2K lbm) 71.1 mT (156.7K lbm) to TLI (with Ares I) 44.9 mT (99.0K lbm) to TLI to Low Earth Orbit (LEO) to LEO 62.8 mT (138.5K lbm) to TLI 118.8 mT (262.0K lbm) to LEO ~187.7 mT (413.8K lbm) to LEO FIGURE 5.9 Ares I and Ares V use flight-proven technologies adopted from other vehicles, including the space shuttle and

Delta IV. NOTE: lbm, pound-mass; LOX/LH2, liquid oxygen/liquid hydrogen; LOX/RP-1, Rocket Propellant-1. SOURCE: Courtesy of NASA. Figure 5.9.eps Includes low resolution bitmap images

The space shuttle SRB has significant flight history and demonstrated reliability. Nonetheless, increasing the number of segments and increasing the chamber pressure are significant modifications. A planned series of development and qualification test motors should provide confidence in the design, leading up to the first flight test of the five-segment variant by 2013. This development activity is well ahead of the need for the Ares V and therefore represents a low risk for the future superheavy lift booster. The most challenging engine development activity is the J-2X, which is a second-generation version of the J-2 engine last flown on the Saturn V. Although there is pedigree to the Saturn IB and Saturn V engines, the new design incorporates design features and manufacturing processes developed for various engines over the past 30 years. As such, this represents a significant development effort. Pratt & Whitney Rocketdyne is utilizing its proven development philosophy of incremental component, subsystem, and engine system testing, coupled with early identification and elimination of development risks. The first test flight of the J-2X is scheduled for 2012. Again, the development activity is well ahead of the need for the Ares V and represents a low risk for that vehicle. One of the most significant residual technical risks associated with the Ares V will be the mass of the tank structure/airframe, electronics, controls systems, and so on. If the mass exceeds expectations, the engines will require upgrades or mass will need to be shaved off the vehicle. Standard mass growth allowances are being used for the design, which should mitigate this concern.

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Vehicle Performance Comparison LEO Missions 25

yload (tonnes) yload 10 Pa Ares I Delta IV HLV 5 Delta IV M+(5,4) Delta IV (M+4,2) Delta IV MLV 0 200 300 400 500 600 700 800 900 1000 Circular Orbit Altitude (km)

FIGURE 5.10 Comparison of Ares I and Delta IV family vehicle performance. Figure 5.10.eps

VEHICLE PERFORMANCE COMPARISONS Ares I The capability of the Ares I is roughly equivalent to, and perhaps slightly greater than, that of the Delta IV Heavy to a low Earth orbit of 200 km. However, the performance of the Ares I drops off more dramatically with altitude than does the performance of the Delta IV. Figure 5.10 provides a comparison of the payload capability of the Ares I and the various Delta IV configurations, based on the data provided to the committee by NASA’s Exploration Systems Mission Directorate and the Delta IV Payload Planners Guide released in 2007. This figure shows that the Ares I does not provide any benefit over the current fleet of Evolved Expendable Launch Vehicle (EELV) vehicles for low-Earth-orbit missions. Figure 5.10 also shows that because Ares I is optimized to deliver the heavy Orion spacecraft to low Earth orbit, Ares I capability falls off rapidly above low Earth orbit. The performance of the Atlas V is not included in this figure, since the various Delta IV configurations envelope the lift capability of the Atlas configurations. In general, a dual-engine Centaur would be used to maximize performance for low-Earth-orbit missions, with the 532, 542, and 552 configurations filling in the gap between the Delta IV M+(5,4) and Heavy configurations. As indicated above, the Ares I/Centaur V2 configuration can be used to augment performance to low Earth orbit; however, its primary purpose is to provide capability for interplanetary missions. A comparison between the Delta IV Heavy and the Ares I is provided in Figure 5.11. The performance of the Ares I/Centaur and the Delta IV Heavy are roughly equivalent for C3 values up to roughly 20 km2/s2. However, the Ares I appears to provide a greater lift capability for higher C3 requirements. The Atlas V 551 can be fitted with a Star 48V Orbit Insertion

Delta IV Payload Planners Guide, 06H0233, United Launch Alliance, Littleton, Colo., September 2007, available at http://www.ulalaunch. com/docs/product_sheet/DeltaIVPayloadPlannersGuide2007.pdf.

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Vehicle Performance Comparison Interplanetary 12.0

10.0 Ares I - Centaur Delta IV HLV Atlas V 551 w/ Star 48 8.0 Atlas V 551 onnes)

(T 6.0 yload yload

Pa 4.0

2.0

0.0 0 20 40 60 80 100 120 140 C3 (km2/s2)

FIGURE 5.11 Comparison of escape performance of theFigure Delta IV 5. Heavy,11.eps Atlas V, and Ares I/Centaur V2.

Stage. In this configuration, it provides capability equivalent to that of the Delta IV for a C3 of 100 km2/s2 and approaches that of the Ares I with a Centaur for C3 values greater than 120 km2/s2. Since the Ares I would use the Atlas V extended payload fairing configuration with the dual-engine Centaur, the usable volume in the payload fairings are roughly equivalent for the Delta IV and Ares I. The available volume for the Atlas V 551 with the Star 48V will be reduced slightly to accommodate the Orbit Insertion Stage. This is a notional, nonstandard configuration for the Ares I and is not part of the Constellation System baseline. The performance estimates should therefore be used with caution. Nonetheless, the Ares I with a Centaur V2 may be a potential alternative to the Delta IV Heavy and Atlas V 551/Star 48V.

Finding: The Ares I will not provide capabilities significantly different from those provided by existing launch vehicles.

Ares V The Ares V provides significantly greater launch mass and C3 performance over the existing fleet of expendable launch vehicles in the U.S. inventory (Figure 5.12.). Figures 5.13 and 5.14 provide a comparison of the low Earth orbit and C3 performance for the Ares V, Ares I, and Delta IV. For low-Earth-orbit missions, the Ares V provides four to seven times the mass to orbit of the other systems. Similarly, the Ares V, with or without the Cen- taur, offers dramatically greater performance for interplanetary missions than does either the Delta IV or Ares I.

The performance curves for the Ares V are based on the latest data available to the committee and refer to the earlier Ares V version with five RS-68 engines and five-segment solid-rocket boosters. NASA has indicated that the performance of the latest baseline of the Ares V will be higher, but has not produced the data yet.

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FIGURE 5.12 Launch of an Ares V rocket. SOURCE: Courtesy of NASA.

CONCERNS REGARDING THE ARES V SHROUD Although the Ares V offers the greatest potential value to science, the launch vehicle has to be capable of accommodating science payloads. Astronomy and astrophysics payloads will require cleanliness and vibration and noise levels at least as low as the space shuttle. Ares V design goals currently include these cleanliness and vibration and noise-level targets. Planetary missions will require a method of removing waste heat from radioisotope power sources on spacecraft underneath the payload shroud and will also require late (i.e., prelaunch) access to the payload through the shroud. Science missions are more likely to take advantage of the Ares V if these capabilities are designed in to the vehicle rather than being added later.

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Vehicle Performance Comparison LEO Missions 160 Ares V 140 Ares I Delta IV HLV 120

) ) 100

yload (tonnes 60 Pa

0 200 300 400 500 600 700 800 900 1000 Circular Orbit Altitude (km) FIGURE 5.13 Comparison of payload capability to low Earth orbit for the Ares V, Ares I, and Delta IV Heavy Launch Ve- hicle. Figure 5.12.eps Vehicle Performance Comparison Interplanetary 70.0

60.0 Ares I - Centaur Delta IV HLV 50.0 Ares V Ares V / Centaur 2

40.0 onnes) (T

30.0 yload yload Pa 20.0

10.0

0.0 0 20 40 60 80 100 120 140 C3 (km2/s2)

FIGURE 5.14 Comparison of escape performance for DeltaFigure IV Heavy, 5.14 Ares.eps I/Centaur V2, and Ares V with and without an upper stage.

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A potentially more serious issue for using Ares V for planetary missions concerns the need for a dedicated upper stage to provide high excess escape velocities for spacecraft (known as C3). Several of the mission concepts evaluated in this report, including Solar Probe 2, Interstellar Probe, Solar Polar Imager, Neptune Orbiter with Probes, and Titan Explorer, would require some kind of upper stage. The current most likely upper stage, the Atlas V Centaur III Dual Engine Configuration, is relatively tall and thin compared with the Ares V. When placed under the current baseline launch shroud, it would leave relatively little room for a spacecraft (Figure 5.15). An extended shroud would provide more room, but it is unclear if this would be sufficient for many of these mission concepts. The Titan IV Centaur was shorter and wider than the current Centaur but has long been out of production. Neither upper stage takes advantage of the increased diameter of the Ares V payload shroud. Planetary missions could better use an upper stage that is shorter and takes advantage of the full width of the Ares V; however, development of such a stage could be expensive. In order for Ares V to be attractive for future science missions, vehicle designers will have to consider the requirements of potential science missions.

4.44 m [ 14.6

7.50 m [ 24.6

12.4 m [ 40.8 ft] 9.70 m [ 31.8

8.80 m 8.80 m [ 28.9 [ 28.9 ft] Baseline Extended Shroud Shroud

FIGURE 5.15 Two possible configurations of the Ares V shroud—the current baseline shroud and a proposed extended shroud. Shown inside the shrouds are two possible Centaur upper-stage configurations: the Titan IV Centaur (left) and the Atlas V Centaur III Dual Engine Configuration (right). Any spacecraft carried atop an upper stage would have severely restricted volume constraints. Neither shroud option takes advantage of the width of the Ares V shroud. SOURCE: Adapted; courtesy of NASA.

Appendixes

Letter of Request from NASA

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Summary Analysis of Mission Concepts That Would Not Benefit from the Constellation System

In its interim report, the Committee on Science Opportunities Enabled by NASA’s Constellation System determined that 4 of the 11 Vision Mission studies that it evaluated would not directly benefit from the Constel- lation System. (As described in Chapter 2 of the present report, Vision Missions are mission concepts for future space science missions studied at the initiation of NASA between 2004 and 2006.) Those four missions are the Advanced Compton Telescope (ACT), the Kilometer-Baseline Far-Infrared/Submillimeter Interferometer, the Single Aperture Far Infrared (SAFIR) Telescope, and the Solar System Exploration/Astrobiology Vision Mission (Palmer Quest) Mars lander. The committee did not receive any comments from the mission teams that developed these concepts challenging the committee’s conclusions. In its interim report, the committee also issued a request for information, in response to which it received six mission concepts (plus other responses that were not actual mission concepts). The committee determined that one of those mission concepts, Super-EUSO (Extreme Universe Space Observatory), would not benefit from Constellation. That mission concept, plus the four listed above, are summarized in this appendix. (See Chapter 2 for the committee’s evaluations of the 12 mission concepts that it deemed worthy of further study as Constel- lation missions.)

ADVANCED COMPTON TELESCOPE Scientific Objectives of the Mission Concept The Advanced Compton Telescope is a concept for a powerful new survey instrument for studying supernovae, galactic nucleosynthesis, gamma-ray bursts (GRBs), compact objects, and the laws of physics (see Figure B.1).

National Research Council, Science Opportunities Enabled by NASA’s Constellation System: Interim Report, The National Academies Press, Washington, D.C., 2008. Information describing the scientific objectives and current development status of the characteristics of each mission concept evaluated here is derived from the materials provided to the committee by the teams that developed the respective proposed mission concepts. The first two subsections in this section are based on S.E. Boggs, University of California, Berkeley, for the ACT Study Team, “ACT, Advanced Compton Telescope: Witness to the Fires of Creation,” NASA Vision Mission Concept Study Report, arXiv:astro-ph/0608532v1, December 2005; and J. Kurfess, U.S. Naval Research Laboratory and Praxis, Inc., “The Advanced Compton Telescope Mission,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 21, 2008. See footnote 2 above. The original Compton Gamma Ray Observatory (CGRO) was the second of NASA’s “Great Observatory” telescopes (after the Hubble Space Telescope) and operated in low Earth orbit from 1991 to 2000. At the time of its launch, it was the heaviest scientific instrument placed in orbit.

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Instrument 01.5m antenna (1.42 x 1.42 x 0.75 envelope)

Solar Array 34 m² (shown)

Bus Structure and sub-system layout similar to GLAST bus design

Thermal radiator End panels deploy (22 m² shown) Propulsion Tank Battery Box Cryo-cooler (not visible) 0.58m x 1.02m (green) Reserve envelope included in Delta IV 4m fairing 309 kg capacity 2 each 2 each design 01.0m x 0.6m

FIGURE B.1 Artist’s illustration of the Advanced Compton Telescope both in mission configuration and stowed within a Delta IV Heavy Launch Vehicle payload shroud. NOTE: GLAST, Gamma-ray Large Area Space Telescope. SOURCE: Courtesy of S. Boggs, NASA. Figure A.1.eps Includes low resolution bitmap images

Since the gravitational collapse of matter into stars and galaxies a few hundred thousand years after the big bang, much of the visible matter in the universe has been processed through the slow but spectacular life cycle of matter—stellar formation and evolution ending in novae or supernovae, with the ejection of heavy nuclei back into the galaxy to seed a new generation of stars. Nuclear gamma-ray astrophysics is the study of emission from radioactive nuclei as tracers of this cycle of creation. In particular, ACT would map our galaxy in a broad range of nuclear line emission from radioactive decays, nuclear de-excitations, and matter-antimatter annihilations. It would measure the radioactive gamma-ray and positron emitters among the particles propagating from supernovae, novae, and stellar winds populating our galaxy. Additionally, gamma rays from accretion of matter onto galactic compact objects and massive black holes in active galactic nuclei (AGN) are used to test accretion disk and jet models and to probe relativistic plasmas. Gamma-ray polarization can be used to study the emission processes in GRBs, pulsars, AGN, and solar flares. The origins of the diffuse cosmic MeV background can also be identified. The ACT telescope would increase detection efficiency by up to two orders of magnitude over the COMPTEL instrument (the Compton Telescope on the Compton Gamma Ray Observatory [CGRO]). The ACT instrument design is driven by its primary science goal—spectroscopy of the 56Co (0.847 MeV) line from type Ia supernovae (SNe Ia), which is expected to be Doppler-broadened to ∼3 percent. ACT would allow hundreds of SNe Ia detections over its primary 5-year survey lifetime. In the process, ACT becomes an all-sky observatory for all classes of gamma-ray observations, Table B.1 provides estimates of potential ACT observations compared with those of the COMPTEL. The ACT science would be complementary to the Gamma-ray Large Area Space Telescope (GLAST) science. GLAST addresses the high-energy gamma rays from ∼20 MeV to 300 GeV, and ACT would cover the region from 0.2 MeV to 10 MeV. GLAST also includes a 10-keV to 30-MeV GRB detector.

GLAST (now called the Fermi Gamma-ray Space Telescope) was successfully launched in June 2008.

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TABLE B.1 Estimates of Potential Observations and/or Detections of Various High-Energy Events by the Advanced Compton Telescope (ACT) over a 5-Year Survey Period, Compared with Those by the COMPTEL

Number of Observations and/or Detections Sources Actual by COMPTEL Potential by ACT Supernovae 1 100-200 Active galactic nuclei and blazars 15 200-500 Galactic 23 300-500 Gamma-ray bursts 31 1,000-1,500 Novae 0 25-50 NOTE: COMPTEL, Compton Telescope on the Compton Gamma Ray Observatory. SOURCE: Adapted from ACT, Advanced Compton Telescope: Witness to the Fires of Creation, NASA Vision Mission Concept Study Report, December 2005, available at http://arxiv.org/ftp/astro-ph/papers/0608/0608532.pdf, p. 3.

The ACT mission involves a single instrument composed of a large array of multichannel gamma-ray detectors, surrounded by anticoincidence (ACD) shields on all sides, mounted on a zenith-pointing spacecraft. ACT can be launched from the Kennedy Space Center on a Delta IV vehicle into a 550-km circular orbit with 8 degrees inclination for a 5-year minimum (10-year desired) lifetime. The ACT total mass is ∼4,000 kg, power is 3,340 W, and the data rate is 69 megabits per second (Mbps). The ACT concept fits within the Delta IV’s 4-m fairing. The ACT mission is not specifically identified in any decadal survey of the National Research Council (NRC). However, advanced gamma-ray mission concepts have been under consideration by NASA over the past decade, and such missions were considered as part of NASA’s roadmap activities, including what is called the Universe Strategic Roadmap. NASA’s Gamma Ray Astrophysics Program Working Group named ACT as its highest- priority major mission in reports published in 1997 and 1999. The committee believes that this mission offers a significant advance in its scientific field. However, this mission would require priority endorsement in the next NRC astronomy and astrophysics decadal survey before initiation of development.

Characteristics of the Mission Concept as Developed to Date The ACT concept has matured to the point that flight detector technology development and selection appear to constitute the major open item. Several detector alternatives are available, all entailing considerable and challenging development efforts: (1) Si-Ge baseline D2 germanium detectors, (2) thick Si detectors, (3) liquid Xe detectors, or (4) silicon controlled-drift detectors. These detector alternatives, combined with the significant improvement in collecting area over prior instruments, appear to provide reasonable risk-mitigation paths and alternatives to satisfy the ACT requirements. There are also major challenges to be met for these technologies related to power and cryogenics system needs. Consequently, the highest priority is to focus on detector development and final selection.

Relative Technical Feasibility of the Mission Concept New technology development required for ACT is relatively modest, with the most important technology development needed for the detectors. There are no significant mission design issues. The ACT mission is comparable in size, mass, and complexity to GLAST and has similar spacecraft and mission operations requirements (GLAST’s mass is 4,627 kg). The mission mass (∼4,000 kg) and orbit (550 km low Earth Orbit, <10°) are within the capabilities of the

NASA, Universe Exploration: From the Big Bang to Life: A Strategic Roadmap of Universe Exploration to Understand Its Origin, Struc- ture, Evolution and Destiny, Washington, D.C., May 20, 2005.

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Delta IV and Ares I launch vehicles. The European Space Agency’s (ESA’s) Ariane V could provide a low- altitude, near-equatorial-orbit minimizing background, therefore making this a potentially attractive mission for international cooperation.

General Cost Category in Which This Mission Concept Is Likely to Fall The ACT study team estimates an ACT mission cost of ∼$760 million in fiscal year (FY) 2004 dollars. Com- paring ACT with the comparable mission, GLAST, and considering the later time frame for development, ACT is likely a ∼$1-billion-class mission.

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The Ares V launch vehicle’s capabilities are not required for this mission concept, since ACT can easily be packaged to fit on a Delta IV. Although the Ares V would allow for significantly greater collecting area of detectors, the science provided by the Delta IV ACT concept is a significant order-of-magnitude improvement in gamma-ray astrophysics. A gamma-ray mission of the Ares V class would logically follow only after an ACT Delta IV-class mission had been implemented.

Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The committee determined that the ACT mission concept does not deserve further study as a Constella- tion-enabled science mission. The primary reason is that the spacecraft size does not exceed current Evolved Expendable Launch Vehicle (EELV) capabilities. The committee determined that it is worthy of further study as a Delta IV-class mission. ACT provides significant science that deserves to be addressed by the next astronomy and astrophysics decadal survey.

KILOMETER-BASELINE FAR-INFRARED/SUBMILLIMETER INTERFEROMETER Scientific Objectives of the Mission Concept The long-term goal of far-infrared astronomy has been a combination of large collecting area with high angular resolution. This combination is provided by the Submillimeter Probe of the Evolution of Cosmic Structure (SPECS), a Michelson interferometer consisting of two 4-m telescopes separated by up to 1 km (Figure B.2) to provide angular resolutions of a few tens of milliseconds of arc, similar to the Hubble Space Telescope (HST), James Webb Space Telescope (JWST), and the Atacama Large Millimeter Array (ALMA) at far-infrared and submillimeter wavelengths. The spacecraft would be located at the Sun-Earth L2 point. Far-infrared (>40 µm) observations of distant galaxies, the intergalactic and interstellar medium, and circumstellar winds and disks are considered necessary to an understanding of the formation of stars and galaxies and the energy balance in the universe. Two big problems have prevented far infrared astronomy from becoming as important as optical astronomy is as an information channel for studying the universe—relatively low sensitivity and poor angular resolution owing to the small diameters of those telescopes, and the strong, variable far infrared (FIR) background from the zodiacal cloud, galactic cirrus, and extragalactic light from unresolved galaxies. The poor angular resolution is especially

The first two subsections in this section are based on M. Harwit, Cornell University, and the Interferometer Vision Mission Team, “Kilo- meter-Baseline Far-Infrared/Submillimeter Interferometer,” Vision Mission Final Report, May 2005; and M. Harwit, Cornell University, and D. Leisawitz, NASA Goddard Space Flight Center, “A Kilometer-Baseline Far-Infrared/Submillimeter Interferometer in Space: Submillimeter Probe of the Evolution of Cosmic Structure, SPECS,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constel- lation System, February 20, 2008. See footnote 2 above.

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FIGURE B.2 Artist’s illustration of the Kilometer-Baseline Far-Infrared/Submillimeter Interferometer. SOURCE: Courtesy of the SPECS Consortium, prepared as part of a NASA Vision Missions study.

problematic compared with the typical size of distant galaxies, ∼0.1 arcsec or less at high redshifts, and it will limit the sensitivity of even large infrared telescopes, such as the SAFIR Telescope. If it works as planned, the SPECS will advance many areas of astrophysics by substantial factors. For example, typical galaxies at redshifts beyond 1 are on the order of 0.1 arcsec or less in size. Resolving these galaxies in the far infrared will be essential to study ongoing star formation in the same manner as is done now for local galaxies by comparing optical, infrared, and radio images. Most regions of active star formation have significant obscura- tion from dust penetrated only in the far infrared. Very high resolution is also required to see placental stars under assembly. Circumstellar disks in the act of creating new planets will produce gaps a few tens of milliseconds of arc in size around the nearest young stars, and the temperatures of the disk material will be cool enough, ∼50 K, that most of the energy will be emitted at FIR wavelengths. Line radiation at these wavelengths will provide even more diagnostic information about galaxies, disks, and the interstellar medium than the continuum light. These are only a few of the important observations that a SPECS could do. Sensitive FIR observations at the resolution provided by a kilometer-baseline interferometer will be both essential to and unique for solving a wide suite of astrophysical problems. It is for this reason that the 2000 astronomy and astrophysics decadal survey ranked far-infrared space telescopes very highly, and the scientific case for these capabilities will remain strong and uncompromised for the foreseeable future. There is little doubt that if the goals of the currently proposed SPECS can be achieved, it will represent a major advance in observational astronomy that is unlikely to be obtained any other way. The committee believes that this mission does offer a significant advance in its scientific field. An ESA study of a similar telescope is currently examining both a free-flying option and a connected spacecraft. This mission could be attractive for future international cooperation.

National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001.

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Characteristics of the Mission Concept as Developed to Date This mission pushes far-infrared technology to new limits. No large FIR space telescope has been operated in space, and no interferometer of the scale proposed by this mission concept has ever been tested. The challenges of beam combination and aperture synthesis, particularly with the current generation of FIR detectors, are daunting. Several levels of technology development would be needed to bring this mission to maturity, including the following:

1. Large, cooled telescopes operating in space (these might derive from JWST heritage); 2. Controlled tethered telescopes or formation flying to maintain baseline control; 3. Advanced far-infrared array detectors and heterodyne detectors working efficiently at the quantum limit; and 4. Large, rapid, cooled delay lines operating in space.

Additionally, aperture synthesis routines for a rapidly rotating two-telescope interferometer with a constantly changing baseline would need to accommodate complex engineering data sets (metrology of the baselines) with complex data streams from the detectors to assemble useful astronomical data. By most traditional measures, this mission is immature and would require several technical developments, probably as heritage from precursor missions, to reach flight readiness.

Relative Technical Feasibility of the Mission Concept Every approach to far-infrared space astronomy considered by the mission team is challenging. The approach adopted in this Vision Mission concept is one reasonable choice among several. It appears technically feasible only after considerable investment to develop space hardware as outlined above.

General Cost Category in Which This Mission Concept Is Likely to Fall At present, this mission appears to be considerably more expensive than the SAFIR Telescope owing to the amount of technical development yet to be done, meaning a cost of more than $5 billion. Future developments in tethered flight, FIR detectors, cooled telescopes, and fast delay lines might decrease these costs substantially, but the upfront investment at this time appears large.

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The current project as proposed does not require the Ares V capabilities because it meets the mass and volume limits of the EELV. However, the technical goals are ambitious, and it appears likely that many of the packaging problems of fitting an interferometer into a small fairing and deploying it in space might be alleviated by using the Ares V. Complete analysis of the fuel budget needed for moving the array may show the advantages of using an Ares V to increase the lifetime of the mission. But the committee determined that the fundamental technological challenges of the mission are substantial and would not benefit from the capabilities of the Ares V or other aspects of the Constellation System.

Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The committee concluded that the Kilometer Baseline Far-Infrared/Submillimeter Interferometer mission concept does not deserve further study as a Constellation-enabled science mission. This mission concept currently appears to be within the capabilities of the EELV family. A FIR interferometer is a long-term goal for this important wavelength range, and that goal is unlikely to be diminished by developments in the next two decades. It is not

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FIGURE B.3 Artist’s illustration of the Single Aperture Far Infrared Telescope. SOURCE: Courtesy of John Frassanito and Associates and Northrop Grumman Space Technology.

known how difficult it will be to develop the contributing technologies necessary to make this mission viable, but each of the elements has plausible development paths with a modicum of investment over the next decade.

SINGLE APERTURE FAR INFRARED (SAFIR) TELESCOPE Scientific Objectives of the Mission Concept The Single Aperture Far Infrared Telescope mission is a 10-m telescope cooled to a few degrees above absolute zero (goal of 4 K) for observations between the thermal infrared (∼20 µm) and the submillimeter (Figure B.3). It would operate at the Sun-Earth L2 point. As designed, it will improve the sensitivity of far-infrared observations by two to four orders of magnitude and the angular resolution by more than a factor of 10 compared with the sensitivity and resolution of previous missions. More importantly, the combination of sensitivity and resolution will allow the SAFIR Telescope to have a major impact on almost every major subfield of astronomy at distances stretching from the closest stars to the most distant observable galaxies. It achieves the threshold angular resolution of 0.5 arcsec in the thermal infrared needed to isolate galaxies at very high redshifts (z > 2), making it uniquely capable of addressing problems involving observations of the early universe at wavelengths longer than 20 µm. The SAFIR Telescope is also a steppingstone to the long-range goal of very high angular resolution for far infrared astronomy. The instrument suite will also include low-resolution spectrographs. The thermal and FIR wavelength range is most sensitive to cool regions in the interstellar and intergalactic medium at temperatures from a few to a few hundred kelvin. In galaxies, interstellar dust and gas typically absorb and reradiate much of the light from stars and active galactic nuclei, assumed to be black holes, meaning that the FIR radiation often contains the majority of the energy budget. Far infrared lines provide the best way to study

The first two subsections in this section are based on D. Lester et al., “Science Promise and Conceptual Mission Design Study for SAFIR—The Single Aperture Far Infrared Observatory,” June 2005; and D. Lester, University of Texas, “Constellation Architecture and the Single Aperture Far Infrared Telescope (SAFIR),” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 20, 2008. See footnote 2 above.

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the gas free from the obscuring effects of dust at ultraviolet (UV) and optical wavelengths. These observations provide a unique complement to optical, x-ray, and radio observations, and it is safe to say that researchers cannot understand the early evolution of stars, the creation of metals, and the creation of dust without FIR observations at angular resolutions at least as high as those proposed for the SAFIR Telescope. The extragalactic case for a facility such as the SAFIR Telescope is unique and is unlikely to be eroded by other observatories in the next two decades. Far infrared observations of galactic clouds of gas and dust are also essential to an understanding of the physics of star formation. Circumstellar disks around young and old stars alike, indicative of planet formation in the former case, emit the bulk of their luminosity at thermal and FIR wavelengths, making the SAFIR Telescope an important facility to study planet formation and making it an essential complement to the ALMA, as well as the JWST and the Stratospheric Observatory for Infrared Astronomy (SOFIA). There are a variety of novel uses of FIR observations for studying the interstellar medium and other circumstellar regions that highlight the importance of this wavelength region to an understanding of how the interstellar medium behaves and takes part in the life cycle of new stars and planets. The key science objectives as outlined in the proposal are the following:

• Probe the earliest epochs of metal enrichment and see the galaxy-forming universe before metals are created. Understand the origin of dust grains in the universe. • Resolve the far-infrared cosmic background—trace the formation and evolution of star-forming and active galaxies since the dawn of the universe, and measure the history of star formation. • Explore the connection between embedded nuclear black holes and their host galaxies. Understand the relationship of active nuclei to galaxy formation. • Track the chemistry of life. Follow prebiotic molecules, ices, and minerals from clouds to nascent solar systems. • Identify young solar systems from debris disk structure and map the birth of planetary systems from deep within obscuring envelopes. Assess the degree of bombardment that they face, and the degree of habitability.

A fundamental difficulty with observations at wavelengths longer than about 50 µm is that the relatively low angular resolution combined with zodiacal light and the light from many distant galaxies creates the primary noise source; that is, the sensitivity is limited by foreground emission and confusion. However, because the SAFIR Telescope will still gain orders of magnitude in observing capability over all existing and planned facilities in this important wavelength range, its science case is very strong. The committee believes that this mission does offer a significant advance in its scientific field.

Characteristics of the Mission Concept as Developed to Date The SAFIR Telescope would build on the technological improvements developed for the James Webb Space Telescope and would appear to be a logical follow-on to that observatory (subject to evaluation by the decadal survey). It is 1.5 times larger than JWST but with far less stringent requirements on surface accuracy. It has much more demanding requirements for instrument cooling (4 K for SAFIR against 40 K for JWST). Because the longest wavelengths are confusion-limited by distant galaxies, even modest gains in cooling over JWST—to 20 K, for example—will satisfy many of the science goals, allowing some margin in this challenging technological development. The SAFIR Telescope would need improved FIR detectors (by about an order of magnitude), requiring unknown cost, but it seems likely that great improvements could be made with a modest investment. In all other respects, the concept is relatively mature.

Relative Technical Feasibility of the Mission Concept The current SAFIR Telescope concept arises from the heritage of JWST, and therefore it appears technically feasible with the proviso that the SAFIR Telescope would have to achieve lower temperatures under passive cooling

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than those of any telescope to date. Of the different design concepts considered by the mission team, the current one most closely derives from already-developed technology. The SAFIR Telescope does not propose enormous technological advances over JWST, although the cooling to 4 K is likely to be a major challenge.

General Cost Category in Which This Mission Concept Is Likely to Fall The SAFIR Telescope mission is likely to be similar in cost to JWST ($4.5 billion). It uses similar technology, although it will have a 50 percent larger mirror with more demanding thermal requirements but less demanding surface accuracy, wavefront control, and pointing. It is not known how much investment is needed to improve FIR detectors to take full advantage of the SAFIR Telescope. The full execution of this mission concept will likely cost more than $5 billion.

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The Ares V capabilities are not required for the SAFIR Telescope mission concept as proposed. The size of the telescope in the Vision Mission study was chosen as one that can fit in an EELV using at least some JWST heritage, and at this size it would permit significant scientific advances to be achieved. Assuming that JWST is successfully developed, SAFIR should not be inherently more difficult to design and build. However, if JWST proves to pose major design challenges, then large telescopes such as SAFIR could potentially benefit from having a larger shroud on a more capable launch vehicle, easing packaging and integration of the telescope.

Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The committee concluded that the SAFIR Telescope mission concept does not deserve further study as a Constellation-enabled science mission. The SAFIR Telescope is likely to be very attractive as a Delta IV-class mission and does not require the increased capabilities of the Ares V launch vehicle. The SAFIR Telescope could be considered as a candidate stand-alone Ares V mission, but the heavy launch vehicle is not required. The SAFIR Telescope received a high rank in the 2001 astronomy and astrophysics decadal survey,10 and the more developed concept presented in the Vision Mission study should make it equally attractive in the next decadal survey.

SOLAR SYSTEM EXPLORATION/ASTROBIOLOGY VISION MISSION: PALMER QUEST11 Scientific Objectives of the Mission Concept Palmer Quest, a proposal for a mission to the North Polar Cap of Mars, would include a nuclear reactor- powered “cryobot” to drill down through the polar ice to its bottom and determine if life once existed on Mars (Figure B.4). The main science goals of Palmer Quest are to assess the possibility of the presence of life on Mars and to evaluate the habitability of the basal domain of the martian polar caps. Science questions for Palmer Quest are as follows:

• Does life currently exist on Mars? • Did life ever exist there? • How hospitable was and is Mars to life?

10 National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001. 11 The first two subsections in this section are based on F.D. Carsey, Jet Propulsion Laboratory, and the Palmer Quest Team, “Palmer Quest, the Search for Life at the Bed of the Mars Polar Cap,” July 2005; and L. Beegle, Jet Propulsion Laboratory, “Palmer Quest: A Mission to the Martian Polar Cap,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 21, 2008. See footnote 2, above.

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Surface Station FRAM Rover Unique, small surface- Long-range inflatable, atmosphere powered by RPS observation platform

Cryobot Nuclear fission powered thermal probe

FIGURE B.4 Illustration of the Palmer Quest spacecraft elements, including a cryobot powered by a nuclear reactor, and the surface station and rover. NOTE: FRAM, Far Ranging Arctic Mission; RPS, radioisotope power source. SOURCE: Courtesy of NASA, Jet Propulsion Laboratory, California InstituteFigure of Technology. A.4.eps Includes low resolution bitmap image

The Palmer Quest objectives include looking for the presence of microbial life, amino acids, nutrients, and geochemical heterogeneity in the ice sheet; quantifying and characterizing the provenance of the amino acids in Mars’s ice; assessing the stratification of outcropped units for indications of habitable zones; and determining the accumulation of ice, mineralogical material, and amino acids in Mars ice caps over the present epoch. The Palmer Quest mission would also address several objectives from decadal reports of the planetary community, including developing an understanding of the current state and evolution of the atmosphere, surface, and interior of Mars; determining if life exists or has ever existed on Mars; and developing an understanding of Mars in support of possible future human exploration. The science to be addressed by Palmer Quest has the potential for paradigm- altering discoveries related to life on Mars. The mission would launch in August 2022 on a Delta 4050 heavy launch vehicle and a probe would descend through the ice sheet of the Mars North Polar Cap to search for life at its bed. It could be extended to quantify surface fluxes of biochemicals, nutrients, and water ice over the annual cycle and to examine outcropped ice cap strata and basal units from a surface rover. The mission would land a surface package that included a thermal drill (the cryobot, powered by a nuclear reactor), a mobile vehicle (the Far Ranging Arctic Mission [FRAM], powered by a nuclear thermal source), and a surface observation system (the surface station, powered by the nuclear reactor). The drill would pass through approximately 2 km of dusty ice while simultaneously acquiring data. There

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would also be data recorded with regard to the outcropping at the contact of the ice cap and the bed in addition to the sedimentary record stored in the polar layered deposits. The probe would also observe the seasonal cycle

of the accumulation of water and CO2 ice, dust, and organics on the polar cap surface. The development of the thermal drill may be useful in future missions involving drilling through the ice surface layer of Jupiter’s moon Europa and Saturn’s moon Enceladus. The spacecraft mass (wet) is approximately 4,900 kg. The committee believes that this mission does offer a significant advance in its scientific field. However, the mission has not been endorsed by any recent NRC reports and does not appear in the 2003 solar system exploration decadal survey.12

Characteristics of the Mission Concept as Developed to Date The maturity of the Palmer Quest mission concept is low. Some of the scientific instruments follow in the footsteps of current and planned Mars missions such as Phoenix Lander and the Mars Science Laboratory. How- ever, the use of a thermal drill, FRAM rovers, and a substation all powered by nuclear fission are unique to this mission. The challenges and technical feasibility associated with these components, illustrating the low maturity of the mission, are described below.

Relative Technical Feasibility of the Mission Concept The Palmer Quest mission concept is based on past successful Mars missions while incorporating novel technologies yet to be developed and that are not yet launch-ready. The technological complexity of the mission concepts is high. Three areas of technology development required by this mission and described in the presentation to the committee are thermal issues, radiation protection, and instrumentation capabilities. Data provided as part of this presentation include a listing of the high-level risks associated with each of the components.

1. The cryobot is a tool designed to melt its way through the “ice” and provide a variety of measurements related to water and biological-related chemical components. The experimenters have listed the risks associated with the work of the cryobot as (a) breaking of the tether in the ice, (b) clogging of the filter system, (c) failure of pumps and valves that supply samples to the instruments, and (d) low technology readiness level (TRL) of instruments for life detection. 2. The rover, the workhorse of the experiments that moves equipment and carries out exploration traverses of some 300 km, has listed as possible risks (a) failure of the inflatable wheels, (b) failure to successfully deploy the rover, (c) difficulties during towing of the surface station, and (d) survivability over the martian northern winter. 3. The surface station, whose role is in situ determination of rates of accumulation of gases and dust, the estimation of annual net gases and dust accumulation, and the determination of some properties of fine-scale morphology and structure related to current polar climate, has listed as risks (a) getting stuck on the main lander body during deployment; (b) the assumption of a solid, flat surface for the deployment; and (c) physically altering the environment especially during and after winter. 4. Mass is limited by the aeroshell. There is little room for mass growth in this mission without a major new aeroshell development; therefore, Ares V offers no improvement without a complete redesign of the spacecraft and mission concept.

The biological assay involved in the mission is not described in enough detail to determine its technical feasibility. The use of nuclear fission and the complexity of this mission place it in a high-risk category.

12 National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003.

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General Cost Category in Which This Mission Concept Is Likely to Fall Although the mission costs were not directly calculated in the written report presented to the committee, a rough cost in the $3 billion to $5 billion range was mentioned in the oral presentation to the committee. This cost did not include the technology development of the more expensive mission elements, including the nuclear reactor, Mach 3 parachute, and biology-focused instrumentation. The development, required safety elements, and use of a nuclear-powered device are major cost contributors in this mission. Additional costs would be required for the new technology developments needed by this mission. Therefore, the committee places the cost estimate for the Palmer Quest mission at greater than $5 billion.

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The Constellation System is not required for this mission because the entry mass at Mars limits the mission size. The payload volume and weight requirements are met by the Delta IV Heavy launch vehicle. Although the Ares V could launch far more payload to Mars than any of its predecessor launch vehicles could, there remains a severe limitation on how much mass can be delivered to the Mars surface. Increasing that mass delivery capability would require an entirely new aeroshell design and associated technology development and testing. However, the Constellation System could possibly provide increased safety for the nuclear components during launch.

Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The committee concluded that the Palmer Quest mission concept does not deserve further study as a Constellation-enabled science mission. This is an expensive, complex, and high-risk mission that does not require the Constellation System and could use an existing launch vehicle such as the Delta IV. The Palmer Quest mission has not appeared in any previous NRC reports, including the solar system exploration decadal survey13 and the recent report An Astrobiology Strategy for the Exploration of Mars.14 In addition, the technical maturity of this mission is low.

SUPER-EUSO MISSION15 Scientific Objectives of the Mission Concept The purpose of the Super-EUSO (Extreme Universe Space Observatory) mission is to open a new window into high-energy particle astronomy through the study of Ultra High Energy Cosmic Particles (UHECP). These are charged particles, photons, neutrinos, or yet-undiscovered particles with energies in excess of E ≈ 1019 eV, which enter Earth’s atmosphere, creating an extensive air shower (EAS). The flux is very low (less than one particle per square kilometer per century, at energies E ≥1020 eV). To detect such rare events, an extremely large detector volume is required (Figure B.5). Each EAS event is the result of a cascade of nuclear interactions in the atmosphere initiated by a single cosmic- ray particle. An EAS consists of hundreds of billions of electrons, positrons, and photons, all traveling together at nearly the speed of light. This flux of particles excites nitrogen atoms in the atmosphere which de-excite either by collisions with other atoms or by emitting light. This fluorescent light is emitted isotropically, and because all of the particles in the EAS are exceeding the speed of light in air, the EAS also produces Cherenkov radiation that

13 National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003. 14 National Research Council, An Astrobiology Strategy for the Exploration of Mars, The National Academies Press, Washington, D.C., 2007. 15 The first two subsections in this section are based on J.H. Adams, Jr., NASA Marshall Space Flight Center, “Super EUSO Mission,” in response to request for information by the Committee on Science Opportunities Enabled by NASA’s Constellation System, May 2008. See footnote 2 above.

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FIGURE B.5 Illustration of the layout for the overall Super-EUSO (Extreme Universe Space Observatory) spacecraft. SOURCE: Courtesy of NASA and the Orbiting Wide-angle Light-collector (OWL) Collaboration. Figure A.5.eps Bitmap image - Low resolution is strongly forward-beamed. The fluorescent light appears primarily in three emission lines, at 337, 357, and 391 nm. The Cherenkov light is continuous, and its intensity decreases with wavelength, λ, as 1/λ2. Plans for the proposed Super-EUSO would be for it to observe EASs from space. It would accurately measure the nature, energy, and arrival direction of the primary particles using a target volume of air far greater than is possible from the ground (Figure B.6). The primary science goals are to determine the origin of UHE particles, learn about the propagation environment from the source to Earth, and study particle physics at energies well beyond those achievable in human-made accelerators. The proposed investigation here is intended to address the goals identified by the Science Program for NASA’s Astronomy and Physics Division (2006) and the need for research on high-energy cosmic rays to be detected by space instruments that look back down on Earth and prove more effective than ground-based observatories.

Characteristics of the Mission Concept as Developed to Date The mission concept presented for Super-EUSO is based on earlier concepts such as the Orbiting Wide-angle Light-collector (OWL), selected for a NASA Cross-Enterprise Technology Development Program (CETDP) grant and recommended as high priority for the 2008-2013 time frame; and the Extreme Universe Space Observatory (EUSO), first selected by ESA for a concept study before the Columbia accident, which prevented it from being transported to the International Space Station (ISS). Super-EUSO was proposed to ESA’s Cosmic Vision solicita-

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FIGURE B.6 Time-history of the light signal from an extensive air shower seen from a 400-km orbit. SOURCE: Courtesy of NASA. Figure A.6.eps Bitmap image - Low resolution tion and selected for technology development, but the review committee believed that the mission was not technically mature enough to proceed to flight at the time. The mission proposed here is an enlarged and improved version of the OWL and EUSO mission concepts, improving on the performance by exploiting novel technologies. The design of the experimental apparatus exploits the results of the detailed studies carried on during the phase A of EUSO mission in order to redesign the apparatus in such a way as to fully satisfy the scientific requirements. To investigate and identify the sources of (ultrahigh energy) cosmic rays in the nearby universe and enable extreme energy neutrino astronomy will require very large collecting areas such as that proposed by Super-EUSO. The current scenario calls for an experiment able to detect events with an energy threshold of 1019 eV and the ability to view very large volumes of Earth’s atmosphere. Additionally, the experiment needs to have a long enough operating lifetime to collect many samples of the rare ultrahigh events and to detect weakly interacting particles.

Relative Technical Feasibility of the Mission Concept ESA’s reviewers classified the readiness level of the current design optics for the Super-EUSO, critical for the mission, as TRL 2 and their design to be very preliminary. The reviewers also thought that deployability of a large folded Schmidt telescope corrector plate (proposed so

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that Super-EUSO could fit in a smaller fairing) was very challenging as well (Figure B.7). Thermal design appeared to be at an early stage but is likely a solvable problem. A rigid (one-piece) corrector plate could be launched using Ares V. Feasibility, manufacturing, and launch of the large, single element optics remains a concept only. No one has attempted such a project at even half the size as proposed by Super-EUSO. Gap detectors for the focal surface were also classified with a TRL 2 because the response in the UV is not yet acceptable or demonstrated. The focal plane is large and complex but technically feasible. The required data- handling system with 1 × 106 of channels is considered immature and was classified with a TRL 3; the power budget (5 kW) was also regarded as challenging.

General Cost Category in Which This Mission Concept Is Likely to Fall The Super-EUSO proposal was subjected to a detailed cost and technology readiness review by ESA. ESA concluded that the mission would cost $1.7 billion. This estimate is no doubt very low, as nearly all the technical challenges required to be solved for Super-EUSO are in early stages of development. The committee judges that the cost of Super-EUSO as described would fall into the $1 billion to $5 billion range.

Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The proposed Super-EUSO experiment requires observation of a large volume (owing to the rarity of ultrahigh cosmic-ray events) as well as a large collecting area owing to the low flux levels expected. These requirements drive the project to need a very large aperture, wide-angle telescope to be placed in orbit. High optical throughput requires the telescope to have a fast focal ratio. The baseline telescope at present is an 11-m primary Schmidt design with a 6.7-m corrector lens. The size and mass of such a telescope (with a deployable primary mirror) would require an Ares V launch vehicle, as it offers a sufficiently large fairing to launch this experiment.

Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The current mission design is very early in the planning stages for all the critical components. The technology readiness level is quite low (TRL-2 to TRL-3), according to the proposers. The optics are a particular concern,

FIGURE B.7 Illustration of potential methods for folding the optics for the Super-EUSO mission. SOURCES: Left: Courtesy of Carlo Gavazzi Space SpA; middle and right: Courtesy of NASA and the Orbiting Wide-angle Light-collector (OWL) Col- laboration. Figure A.7.eps

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FIGURE B.8 Artist’s conception of the Super-EUSO spacecraft from the revised version of the concept. SOURCE: Courtesy of NASA.

as there seems to be no technical solution for their manufacturing or launch or a technical path to such development. Other, more feasible mission concepts, such as multiple smaller telescopes or different wide-angle telescope designs that can be manufactured, as well as ground-based expansions of systems such as the Pierre Auger Observatory, should be considered as a more cost-effective means of reaching the scientific goals. The committee determined that the Super-EUSO mission concept, as presented to the committee, does not deserve further study as a Constellation-enabled science mission. Additional note: The proposers submitted a revised and updated version of the Super-EUSO concept to the committee several months after making their initial proposal and presentation. The revised proposal appears to answer some of the concerns about fitting the optics into the Ares V shroud without the need of folding and deployment. (See Figure B.8.) It also appears to address concerns raised by the original proposal about the large corrective optics required. However, the data provided were very limited, and insufficient for the committee to assess the relative TRL for the technology solutions. Furthermore, the revised proposal still does not provide a clear rationale explaining why an Ares V-sized mission is necessary compared with smaller and more affordable solutions, including ground-based solutions.

Request for Information

Date: March 6, 2008

To: Members of the Space Science Community

From: George Paulikas, Chair, and Kathryn Thornton, Vice Chair NRC Committee on Science Opportunities Enabled by NASA’s Constellation System

The Space Studies Board and the Aeronautics and Space Engineering Board of the National Research Council (NRC) have begun a study of science opportunities enabled by NASA’s Constellation System of launch vehicles and spacecraft. The Committee on Science Opportunities Enabled by NASA’s Constellation System will first analyze a set of “Vision Mission” concepts provided by NASA. The results of this analysis will be included in an interim report to be completed by the end of April 2008. The mission concepts that the committee is analyzing for its interim report are listed on the committee’s website:

http://www7.nationalacademies.org/ssb/constellation2008.html

In order to obtain the greatest possible input of ideas from the community about potential mission concepts addressing space science research, we are soliciting input from the broad community concerning ideas for missions or programs that are uniquely enabled by NASA’s Constellation System. The capabilities of the Constellation System, some or all of which should be used in this input, are also available at the committee’s website. These missions or programs can include (but are not limited to): Earth sciences, solar system exploration, heliophysics, astronomy and astrophysics.

We invite you to write a concept paper for a new space-based mission or program, from existing or new vantage points, that promises to advance an existing or new scientific objective. Proposals that are selected by the NRC’s Committee on Science Enabled by NASA’s Constellation System will be asked to make a formal presentation at the committee’s third meeting June 9-11 in Boulder, Colorado.

The committee will analyze the following information for each mission concept:

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1. Scientific objectives of the mission concept; 2. A description of the mission concept; 3. The relative technical feasibility of the mission concepts compared to each other; 4. The general cost category into which each mission concept is likely to fall; 5. Benefits of using the Constellation System’s unique capabilities relative to alternative implementation approaches.

The committee will identify the mission concepts most deserving of future study. Identification of promising mission concepts by the committee does not imply future study funding by NASA. The time horizon for the launch of possible missions should extend from 2020 to approximately 2035. These may include science missions benefitting from the unique capabilities of the Constellation System, or from human spaceflight enabled by Constellation missions in lunar orbit, other orbits, or missions to planetary objects. In addition, constellations of spacecraft or spacecraft that fly in formation with existing, planned, or future spacecraft may also be considered. The committee will use two criteria for evaluating the concepts:

1. Does the concept offer a significant advance in a scientific field (“significant” is defined as providing an order of magnitude or more improvement over existing or planned missions)? 2. Does the concept have a unique requirement for Constellation System capabilities, e.g., —Does use of the Constellation System’s elements make a previously impossible mission technically feasible? —Does use of the Constellation System’s elements reduce mission risk or enhance mission success for a previously complicated mission? —Does use of the Constellation System capabilities offer a significant cost reduction (i.e., 50 percent or more) in the cost of accomplishing the mission?

All responses will be considered non-proprietary public information for distribution with attribution. Those submitting responses must also fill out the relevant (i.e., government or non-government) NRC copyright form provided on the committee’s website.

The concept papers should be no longer than ten pages in length and provide the following items (by numbered sections), if possible:1

1. A summary of the mission concept, including how it is uniquely enabled by the Constellation System. 2. A summary of the science goals, including a description of how the proposed mission will help advance science. 3. In addition to the two criteria listed above, other factors pertaining to the mission concepts may be used to evaluate and prioritize the candidate proposals:

a. Whether the mission has been identified as a high priority or requirement in previous studies, for example NRC reports; b. How the mission contributes to important scientific questions facing space sciences today (scientific merit, discovery, exploration); c. How the mission complements other space science systems; d. NASA has asked the committee to analyze “the general cost category into which each mission concept is likely to fall.” We recognize the lack of accuracy of cost estimates for space missions in the early conceptual stages of development. You may consider using the NASA Advanced Missions Cost Model located at http://cost.jsc.nasa.gov/AMCM.html to determine approximate costs.

Ten-page limit is a rough guideline, not an absolute limit, and refers to single-spaced text excluding references and front matter.

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e. Technology development required by the proposed mission; f. Risk mitigation provided by use of the Constellation System.

Please submit the concept papers to the NRC by May 5, 2008. Papers should be submitted to constellationrfi@ nas.edu.

Definitions for Technology Readiness Levels

Technology readiness levels (TRLs) are a systematic metric or measurement system that supports assessments of the maturity of a particular technology and the consistent comparison of maturity between different types of technology. TRLs, first introduced by NASA in the 1980s, initially included seven levels. The system was later expanded to include nine levels, summarized in Table D.1. Table D.1, “Technology Readiness Levels (TRLs),” is reprinted from Appendix J of NPR [NASA Procedural Requirements] 7120.8, “NASA Research and Technology Program and Project Management Requirements.” (That document is still in draft form, but it will supersede the previous TRL definitions.)

TABLE D.1 Technology Readiness Levels (TRLs) TRL Definition Hardware Description Software Description Exit Criteria 1 Basic principles Scientific knowledge generated Scientific knowledge generated underpinning Peer reviewed observed and underpinning hardware basic properties of software architecture and publication of reported. technology concepts/applications. mathematical formulation. research underlying the proposed concept/application. 2 Technology Invention begins, practical Practical application is identified but is Documented concept and/or application is identified but is speculative, no experimental proof or detailed description of the application speculative, no experimental analysis is available to support the conjecture. application/concept formulated. proof or detailed analysis Basic properties of algorithms, representations that addresses is available to support the and concepts defined. Basic principles coded. feasibility and conjecture. Experiments performed with synthetic data. benefit. 3 Analytical and Analytical studies place the Development of limited functionality to Documented experimental technology in an appropriate validate critical properties and predictions analytical/ critical context and laboratory using non-integrated software components. experimental function and/or demonstrations, modeling and results validating characteristic simulation validate analytical predictions of key proof of concept. prediction. parameters.

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TRL Definition Hardware Description Software Description Exit Criteria 4 Component and/ A low fidelity system/component Key, functionally critical, software Documented or breadboard breadboard is built and operated components are integrated, and functionally test performance validation in to demonstrate basic functionality validated, to establish interoperability and demonstrating laboratory and critical test environments, begin architecture development. Relevant agreement environment. and associated performance environments defined and performance in this with analytical predictions are defined relative to environment predicted. predictions. the final operating environment. Documented definition of relevant environment. 5 Component and/ A medium fidelity system/ End-to-end software elements implemented Documented or breadboard component brassboard is built and and interfaced with existing systems/ test performance validation operated to demonstrate overall simulations conforming to target environment. demonstrating in relevant performance in a simulated End-to-end software system, tested in relevant agreement environment. operational environment with environment, meeting predicted performance. with analytical realistic support elements that Operational environment performance predictions. demonstrates overall performance predicted. Prototype implementations Documented in critical areas. Performance developed. definition of scaling predictions are made for requirements. subsequent development phases. 6 System/ A high-fidelity system/component Prototype implementations of the software Documented subsystem model prototype that adequately demonstrated on full-scale realistic problems. test performance or prototype addresses all critical scaling Partially integrate with existing hardware/ demonstrating demonstration issues is built and operated software systems. Limited documentation agreement in an operation in a relevant environment to available. Engineering feasibility fully with analytical environment. demonstrate operations under demonstrated. predictions. critical environmental conditions. 7 System A high fidelity engineering Prototype software exists having all key Documented prototype unit that adequately addresses functionality available for demonstration test performance demonstration in all critical scaling issues is and test. Well integrated with operational demonstrating an operational built and operated in a relevant hardware/software systems demonstrating agreement environment. environment to demonstrate operational feasibility. Most software bugs with analytical performance in the actual removed. Limited documentation available. predictions. operational environment and platform (ground, airborne, or space). 8 Actual system The final product in its final All software has been thoroughly debugged Documented completed and configuration is successfully and fully integrated with all operational test performance “flight qualified” demonstrated through test hardware and software systems. All user verifying analytical through test and and analysis for its intended documentation, training documentation, and predictions. demonstration. operational environment and maintenance documentation completed. All platform (ground, airborne, or functionality successfully demonstrated in space). simulated operational scenarios. Verification and Validation (V&V) completed. 9 Actual system The final product is successfully All software has been thoroughly debugged Documented flight proven operated in an actual mission. and fully integrated with all operational mission operational through hardware/software systems. All documentation results. successful has been completed. Sustaining software mission engineering support is in place. System has operations. been successfully operated in the operational environment. NOTE: Generic TRL descriptions are found in NASA, NASA Systems Engineering Processes and Requirements, NPR 7123.1, Table G-19. SOURCE: Reprinted from Appendix J of NPR [NASA Procedural Requirements] 7120.8, “NASA Research and Technology Program and Project Management Requirements.”

Committee and Staff Biographical Information

Committee Members

GEORGE A. PAULIKAS, Chair, has been at the forefront of advances in space science and space systems, and he has made many technical contributions to the development of national security space systems. He retired after 37 years at the Aerospace Corporation, having joined Aerospace in 1961 as a member of the technical staff and later becoming department head, laboratory director, vice president, and senior vice president. He became executive vice president in 1992. His contributions to space science and the development of national security space systems have been recognized by the Aerospace Corporation, the U.S. Air Force, and the National Reconnaissance Office. Dr. Paulikas is a past vice-chair of the National Research Council’s (NRC’s) Space Studies Board (SSB). He has also served on a number of NRC study committees, including the Committee on the Scientific Context for Exploration of the Moon (chair), the Committee on an Assessment of Balance in NASA’s Science Programs (vice chair), the Committee on the Scientific Context for Space Exploration, the Committee on Systems Integration for Project Constellation, the Workshop Committee on Issues and Opportunities Regarding the Future of the U.S. Space Program, and the Committee to Review the NASA Earth Science Enterprise Strategic Plan.

KATHRYN C. THORNTON, Vice Chair, is a professor in the Department of Science, Technology and Society and in the Department of Mechanical and Aerospace Engineering at the University of Virginia. She is also associate dean for graduate programs in the School of Engineering and Applied Science. Dr. Thornton has extensive human spaceflight experience and served for 12 years as a NASA astronaut, flying on four space shuttle missions and performing extravehicular activities (i.e., spacewalks) on two of them. Dr. Thornton served on the NRC Aeronautics and Space Engineering Board, the Committee for Technological Literacy, and the Committee on Meeting the Workforce Needs for the National Vision for Space Exploration, and served as co-chair of the Stanford University/ Planetary Society Workshop on Examining the Vision: Balancing Science and Exploration.

CLAUDIA ALEXANDER is a member of the technical staff at the Jet Propulsion Laboratory and the project manager and project scientist for NASA for the European Space Agency’s Rosetta mission to study comet 67P/ Churyumov-Gerasimenko. Her research focuses on the evolution and interior physics of comets, Jupiter and its moons, magnetospheres, plate tectonics, space plasma, the discontinuities and expansion of solar wind, and the planet Venus. Previously, she was a science representative on the Galileo mission to Jupiter. She is a member of

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the American Geophysical Union (AGU) and the Association for Women Geoscientists, and she was awarded the 2003 Emerald Honor for Women of Color in Research and Engineering by the Career Communications Group. Dr. Alexander has served on several NRC committees, including the Committee on Solar and Space Physics and the Committee on Distributed Small Arrays of Small Instruments for Research and Monitoring in Solar-Terrestrial Physics: A Workshop.

STEVEN V.W. BECKWITH is the vice president for research and graduate studies for the University of California System and professor of astronomy at the University of California, Berkeley. He is a former professor of physics and astronomy at Johns Hopkins University and the former director of the Space Telescope Science Institute. Previously, he was managing director of the Max-Planck-Institut für Astronomie. His principal research interests are the formation and early evolution of planets, including those outside the solar system, and the birth of galaxies in the early universe. Dr. Beckwith served as chair of the NRC Panel on Ultraviolet, Optical, and Infrared Astronomy from Space.

MARK A. BROSMER is general manager of the Launch and Satellite Control Division at the Aerospace Corpo- ration, where he is responsible for Aerospace’s support to the Air Force Satellite Control Network and Spacelift Range. He is responsible for launch operations at Cape Canaveral Air Force Station and the Vandenberg Air Force Base. He joined the Aerospace Corporation in 1985 as a member of the technical staff in the Thermal Control Department of the Engineering and Technology Group. He transferred to the Fluid Mechanics Department in 1987. He has since held several positions, including manager of the Launch Vehicle Thermal Department, Engineer- ing and Technology Group, and project engineer for the system integration and launch readiness of the Titan IV Solid Rocket Motor Upgrade. He joined the Evolved Expendable Launch Vehicle (EELV) Program in 1996 as a senior project engineer, serving as Aerospace’s integrated product team lead for systems engineering and integration for the Delta IV launch system. In 1998 he was promoted to systems director for Delta IV development, and in 2001 he was promoted to principal director of Delta IV. While supporting the EELV Program, he provided technical leadership from the early-development phase and source-selection process through the eight inaugural launches of the Delta IV, including the first operational launches of the medium-, intermediate-, and heavy-lift configurations.

JOSEPH BURNS is the Irving Porter Church Professor of Engineering, professor of astronomy, and vice provost for physical sciences and engineering at Cornell University. He is heavily involved with the imaging team on the Cassini mission around Saturn. Dr. Burns’s current research concerns planetary rings and the small bodies of the solar system (dust, satellites, comets, and asteroids). He is the president of the International Astronomical Union’s (IAU’s) Commission on Celestial Mechanics and Dynamical Astronomy. Dr. Burns is a fellow of the AGU and the American Association for the Advancement of Science, a member of the International Academy of Astronautics, and a foreign member of the Russian Academy of Sciences. In 1994 he was awarded the Masursky Prize by the Division of Planetary Sciences of the American Astronautical Society (AAS). Dr. Burns previously served as a member of the NRC Committee on a New Science Strategy for Solar System Exploration.

CYNTHIA CATTELL is a professor of physics in the School of Physics and Astronomy at the University of Min- nesota. She is a fellow of the AGU. She is an author on more than 130 refereed journal articles and a contributing author in Auroral Plasma Physics (in the Space Sciences Series of the International Space Science Institute). She is a co-investigator on Polar, Cluster, FAST (Fast Auroral Snapshot), STEREO (Solar-Terrestrial Relations Observatory), and RBSP (Radiation Belt Storm Probes) and a principal investigator on the AMPS (Auroral Multi- Probe Satellite) mission study. Dr. Cattell has been a member of various advisory committees, including the NRC Committee on Solar Terrestrial Research, the NRC Plasma Sciences Committee, the NASA Sun-Earth-Connection Advisory Subcommittee, and the Sun-Solar System Connections SSSC Roadmap Committee. She was chair of the 2003 NASA Plasma Sails Working Group and a member of the Advisory Committee to the Basic Plasma Science Facility at the University of California, Los Angeles (UCLA). She has also served on the science definition teams for a number of missions, including the Mercury Dual Orbiter and the Grand Tour Cluster. She is a member of

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the Physics Force, a team performing large-scale “physics circus” shows for K-12 schools and the general public throughout Minnesota and the upper Midwest.

ALAN DELAMERE is a retired senior engineer and program manager at Ball Aerospace and Technology Corpo- ration. He is currently involved as co-investigator on the Mars Reconnaissance Orbiter High Resolution Imaging Science Instrument and on the Deep Impact mission to Comet Tempel 1. Mr. Delamere has been involved in the Mars program since the 1980s. His expertise focuses on instrument building and mission design. He was a member of the NRC Committee on Preventing the Forward Contamination of Mars and the Committee on New Opportunities in Solar System Exploration.

MARGARET FINARELLI is a senior fellow in the Center for Aerospace Policy Research at George Mason University. Ms. Finarelli’s earlier career with NASA and other U.S. government agencies focused on strategy development and negotiations in the fields of domestic space policy and international relations in science and technology. At NASA, she served as associate administrator for policy coordination and international relations. She was responsible for developing the international partnerships in the International Space Station program, and she led the U.S. team conducting the international negotiations that resulted in the agreements governing NASA’s cooperation with Europe, Japan, and Canada. As the International Space University’s vice president for North American Operations, she was responsible for strategic partnerships and business development in the United States for the international university in Strasbourg, France.

TODD GARY is the director of the Institute for Understanding Biological Systems at Tennessee State University, where he leads research efforts in astrobiology. He is also the codirector of the Minority Institute Astrobiology Collaborative, the principal investigator on the NASA Astrobiology Institute Minority Institution Research Sup- port (MIRS) program, a member of the NASA Astrobiology Institute Astrovirology focus group, and part of the NASA Goddard Space Flight Center education and public outreach team for the Mars Science Laboratory. He was the first candidate chosen for a faculty fellowship in astrobiology by the MIRS program and completed his fellowship in astrobiology at UCLA. Part of his work centers on the integration of research on viruses in extreme environments and on extrasolar planet detection into educational settings. Dr. Gary is the principal investigator on several National Science Foundation (NSF) and NASA programs developing national astrobiology research and education opportunities within Native American, African American, and Hispanic communities. He received his Ph.D. in molecular biology from Vanderbilt University, where he published one of the first studies on how viruses evolve at the deoxyribonucleic acid (DNA) level, and he completed a 2-year research fellowship within the Center for Space Medicine at Vanderbilt University Medical Center.

STEVEN HOWELL is an associate astronomer and assistant director of the WIYN Observatory at the National Optical Astronomy Observatory in Tucson. Arizona. He has held previous jobs as a faculty member, NASA center scientist, and physics researcher. Dr. Howell has more than 20 years of teaching and research experience, with research interests in interacting binaries, extrasolar planets, wide-field photometric surveys, and two-dimensional digital detector instrumentation. Dr. Howell’s space mission experience includes the building and use of astrophysics experiments flown on the space shuttle, service as a member of the Galileo spacecraft solid-state imager team, and participation as a guest observer on essentially every NASA astrophysics mission flown in the past 25 years. He was an adviser on the design and operation of the imagers flown on the Cassini and Deep Impact missions, and he chaired the science advisory committee for NASA’s Extreme-Ultraviolet Explorer spacecraft. Dr. Howell regularly serves on NASA and NSF oversight and review panels, referees journal articles for numerous publications, and is currently a science team member on the NASA Kepler Discovery mission slated for launch in 2009.

ARLO LANDOLT is the Ball Family Professor Emeritus of Physics and Astronomy in the Department of Physics and Astronomy at Louisiana State University and Agricultural and Mechanical College, where he has taught since 1962. He served as the director of the Louisiana State University Observatory from 1970 to 1988, and also served as a program director in the astronomy section of the NSF. He served as the secretary of the AAS. Dr. Landolt has

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served on several NRC committees and on the U.S. National Committee for the International Astronomical Union of the National Academies; the U.S. delegation to the 25th General Assembly of the International Astronomical Union; and as a delegate to the 23rd General Assembly of the International Astronomical Union in Kyoto, Japan.

FRANK MARTIN has worked for 4-D Systems since October 2002; a major focus of his work has been on improving the performance of NASA teams. He is also the president of Martin Consulting, Inc., providing services to aerospace projects. Previously, Dr. Martin was the program director for space systems and engineering at Lockheed Martin Space Systems, where he directed programs in space and Earth sciences, life sciences, and space exploration. Dr. Martin also has extensive NASA experience, including assignments as the assistant administrator for the Office of Space Exploration at NASA Headquarters and as the director of Space and Earth Sciences at Goddard Space Flight Center. He resigned from NASA in 1990 and retired from Lockheed Martin in 2001. He received the NASA Outstanding Leadership Medal; the Exceptional Service Medal; and the presidential ranks of Distinguished Executive and Meritorious Executive. He also served on the NRC Committee on Advanced Concepts.

SPENCER R. TITLEY is a professor in the Department of Geosciences at the University of Arizona. He previously worked on NASA’s Lunar Orbiter program and was also a member of the Apollo Field Geology Investigation Team, serving on Apollo missions 16 and 17. His current research involves the study of the origin of mineral deposits and the distribution and location of mineral and mineral fuel resources. His research has also included the study of chemical baselines of trace elements in rocks and ores for environmental purposes. Dr. Titley is a member of the National Academy of Engineering. He previously served on the NRC Committee on the Assess- ment of Solar System Exploration.

CARL WUNSCH is the Cecil and Ida Green Professor of Physical Oceanography at the Massachusetts Institute of Technology. His research focuses on ocean-observing technologies and on the general circulation of the ocean and its implications for climate change. Dr. Wunsch has chaired a number of ocean science advisory groups, such as the NRC Ocean Studies Board and the International Steering Group for the World Ocean Circulation Experiment. He is a member of the National Academy of Sciences, a foreign member of the Royal Society, a recipient of the AGU’s Macelwane Award and Bowie Medal and of the American Meteorological Society’s Henry Stommel Medal.

STAFF

DWAYNE A. DAY, Study Director, has a Ph.D. in political science from the George Washington University and has previously served as an investigator for the Columbia Accident Investigation Board. He was on the staff of the Congressional Budget Office and also worked for the Space Policy Institute at the George Washington University. He has held Guggenheim and Verville fellowships and is an associate editor of the German spaceflight magazine Raumfahrt Concret, in addition to writing for such publications as Novosti Kosmonavtiki (Russia), Spaceflight, and Space Chronicle (United Kingdom). He has served as study director for several NRC reports, including Space Radiation Hazards and the Vision for Space Exploration (2006), Grading NASA’s Solar System Exploration Program: A Midterm Review (2008), and Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity (2008).

VICTORIA SWISHER joined the Space Studies Board in December 2006 as a research associate. She recently received a B.A. in astronomy from Swarthmore College. She presented the results of her research at the 2005 and 2006 AAS meetings and at various Keck Northeast Astronomy Consortium undergraduate research conferences. Her most recent research focused on laboratory astrophysics and involved studying the x-rays of plasma, culminat- ing in a senior thesis entitled “Modeling UV and X-ray Spectra from the Swarthmore Spheromak Experiment.”

CATHERINE A. GRUBER is an assistant editor with the Space Studies Board. She joined SSB as a senior program assistant in 1995. Ms. Gruber first came to the NRC in 1988. She was a research assistant (chemist) in the

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National Institute of Mental Health’s Laboratory of Cell Biology for 2 years. She has a B.A. in natural science from St. Mary’s College of Maryland.

RODNEY N. HOWARD joined the Space Studies Board as a senior project assistant in 2002. Before he joined SSB, most of his vocational life was spent in the health profession—as a pharmacy technologist at Doctor’s Hospital in Lanham, Maryland, and as an interim center administrator at the Concentra Medical Center in Jessup, Maryland. During that time, he participated in a number of Quality Circle Initiatives that were designed to improve relations between management and staff. Mr. Howard obtained his B.A. in communications from the University of Baltimore County in 1983. He plans to begin coursework next year for his master’s degree in business administration.

LEWIS GROSWALD is the Autumn 2008 Lloyd V. Berkner Space Policy Intern with the Space Studies Board. Mr. Groswald is a first-year graduate student pursuing his master’s degree in international science and technology policy at the George Washington University (GW). A recent graduate of GW, he studied international affairs with a double concentration in conflict and security and Europe and Eurasia as an undergraduate. Mr. Groswald has expressed an interest in space since childhood, but it was not until he had the opportunity to work with the National Space Society during his senior year at GW that he decided to pursue a career in space policy, educating the public on space issues, and formulating policy.