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CRITICAL TECHNOLOGIES FOR THE DEVELOPMENT OF FUTURE SPACE ELEVATOR SYSTEMS

David V. Smitherman, Jr., Architect Technical Manager, Advanced Concepts Office NASA Marshall Space Flight Center, Huntsville, Alabama, USA E-mail: David.Smitherman@,nasa.gov

ABSTRACT:

A space elevator is a tether structure extending through geosynchronous earth orbit (GEO) to the surface of the earth. Its center of mass is in GEO such that it orbits the earth in sync with the earth’s rotation. In 2004 and 2005, the NASA Marshall Space Flight Center and the Institute for Scientific Research, Inc. worked under a cooperative agreement to research the feasibility of space elevator systems, and to advance the critical technologies required for the future development of space elevators for earth to orbit transportation. The discovery of carbon nanotubes in the early 1990’s was the first indication that it

might be possible to develop materials strong enough to make space elevator construction feasible. a This report presents an overview of some of the latest NASA sponsored research on space elevator design, and the systems and materials that will be required to make space elevator construction possible. In conclusion, the most critical technology for earth-based space elevators is the successful development of ultra high strength carbon nanotube reinforced composites for ribbon construction in the lOOGPa range. In addition, many intermediate technology goals and demonstration missions for the space elevator can provide significant advancements to other spaceflight and terrestrial applications.

INTRODUCTION length of about 100,OOOkm. Its center of mass would be in GEO such that it would orbit the During the past year, through a cooperative earth in sync with the earth’s rotation. Once in agreement, the Institute for Scientific Research, place, the structure would be used to deliver Inc., (ISR) in Fairmont, West Virginia, and the payloads from earth to orbit at a cost that could

u NASA Marshall Space Flight Center in be many times lower than conventional rocket Huntsville, Alabama, have been studying a launch systems, and could provide many other concept for space elevator construction and benefits by having a much larger capacity, and a operations first proposed by Dr. Bradley more benign launch environment. Edwards through a study grant from the NASA Institute for Advanced Concepts (NIAC) as The space elevator, being of interest to NASA, described in his book The Space Elevator’. The has been examined over the years to determine concept proposes deployment of a ribbon, from the technologies required, their current state of GEO to the Earth’s surface, that would be readiness, and overall feasibility. In 1984, several meters in width, thinner than a piece of Georg von Tiesenhausen referenced several paper, and because of the counter balance mass space elevator concepts in a NASA technical required, would extend beyond GEO to a total manual, “Tethers in Space: Birth and Growth of a New Avenue to Space Utilization’,” speculating what the future of tethers might be beyond the Tethered Satellite System missions that were under development at that time; see Figure 1.

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Figure 1: Early orbital tower and skyhook concepts’

In 1999, after many reports on the possible be developed, it appears that the other applications of carbon nanotubes, the NASA challenges have reasonable technological or Marshall Space Flight Center conducted a operational solutions that can be pursued to workshop3 to examine the feasibility of the make space elevators feasible. space elevator concept if a suitable carbon nanotube tether . could be developed, and identified many other technologies required and TECHNOLOGY READINESS some general design and operational considerations. In the 2000-2002 timefiame, NASA often uses a system for ranking NIAC fimded Dr. Bradley Edwards to further technologies called the Technology Readiness examine a new simpler concept for construction Level4 (TRL) to help managers make decisions of a space elevator utilizing a carbon nanotube on whether a particular technology of interest composite ribbon and robotic climbers, which should be fknded for further development, or is represents the baseline concept under study ready for inclusion in a flight program, Figure 2. today. And finally, following the NIAC study, There are 9 TRL levels, and in general they can NASA and the Institute for Scientific Research, be described as TRL 1-2, basic technology Inc., have conducted this study of the concept. research; TRL 3-5, technology development in In general, each of these examinations had laboratory experiments and prototypes, and TRL similar conclusions, in that the overall concept is 6-9, integrated systems that are ready for flight. quite large with many challenges, but if the Program managers consider the TRL ranking carbon nanotube composite ribbon material can when examining program risk because they .

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System Test. Laun Actual system “flight proven”through successful mission & Operations operations Actual system completed and “flightqualified” through test SystemEubsystem and demonstration (Ground or Flight) System prototype demonstration in a space environment Technology DemOnStration Systemlsubsystem model or prototype demonstration in a relevant environment (Ground or Space) Component andlor breadboard validation in relevant environment Component and/or breadboard validation in laboratory environment Research to Prwe FeaSarUty Analytical and experimental critical function and/or characteristic proof-of-concept Basic Tachnology Technology concept and/or application formulated

Basic principles observed and reported

Figure 2: Technology Readiness Level

know that any technology at TRZ. 5, and lower, importance to the development of future space brings with it the potential for cost increases and elevator systems. It is important to remember schedule delays if the technology is on the that many of these systems are either already critical path for their program. under development in other programs, or could be developed for other purposes that would have TRL Evaluation: broad applications to many space and terrestrial The space elevator is of interest to NASA as a systems in addition to future space elevator potential future system, but is not a funded developments. program or project because of its low ranking on the TRL scale. In particular, the ribbon material Carbon Nanotube (CNn Development: is perhaps the single most critical technology CNT research and development has been in development, which will need to be made of progress for over a decade at many research what we now refer to as an ultra high strength institutions around the world. Of particular carbon nanotube reinforced composite. Stresses interest is the development of single wall carbon in the ribbon have been calculated to be in the nanotubes because of their ultra high strength 60GPa to 80GPa range with an anticipated characteristics in the 15OGPa range. Lacking in strength requirement of at least 100GPa. The this field of research are controlled growth carbon nanotube is the primary candidate methods that will yield long, aligned, or material that might possibly provide composite continuous length tubes, testing and constructions in this range with a reported measurement standards for reporting of research strength above 15OGPa. There are many findings, and ultimate predictions on potential systems in the space elevator that have been production rates and cost. These developments examined and ranked on the TRL scale’. But, will have significant impact on what can or the lowest ranking and perhaps most critical cannot be accomplished in the development of systems that need developing are as follows in the ultra high strength carbon nanotube the next section. reinforced composite ribbon for the space elevator. Because of the successful laboratory Low TRL Systems (TRL 1-51: experiments to date, CNT technology can be The following systems have been examined and safely labeled at a TRL 4 rating. determined to be at a low TRL and of critical

3 CNT Composite Fiber Development: cross section and thereby help reduce wind Ultra high strength CNT reinforced composites loading. development consists of incorporating large 2) Tape Sandwich Construction: Several quantities of CNT into a suitable composite fiber ribbon fabrication methods have been that will provide overall strength to weight examined, each with particular advantages characteristics in the lOOGPa range. This will or disadvantages. The tapped sandwich require development of methods for CNT construction consists of a cross member that dispersion and alignment in the fiber, bonding keeps vertical fibers in the ribbon properly between the CNT and the fiber matrix, and a aligned and will transfer the load from a combination of these two that will yield high broken fiber to an adjacent fiber. CNT loading. In addition, the process must be Alternatives to this approach include woven scalable for mass production, be adaptable to methods that will transfer loads between surface coatings for protection from the space fibers as needed. environment, and be a fiber that is adaptable to 3) Environmental Coatings: The space fabrication techniques for ribbon construction, environment introduces atomic oxygen and splicing, damage repair, and maintenance of radiation that catl deteriorate fibers and the fibers and coatings. This technology is perhaps composite matrix that binds them together. the most important leap for future space elevator Coatings will likely be required at development. Because of the difficulties in appropriate altitudes to protect the ribbon development of the composite fibers, this from these effects. Some of these coatings, technology is still at a TRL 2, where theory and like nickel to protect carbon fibers from the experiments have been attempted without deteriorating effects of atomic oxygen, could success. The fiber for the ribbon, being the introduce significant additional mass. fundamental component of the space elevator, Detailed design, analysis, fabrications, and yields an overall system rating of TRL 2 for the testing will be required to find the best earth-based space elevator. It is important to engineering solution for these conditions. note here, and as will be mentioned later, that 4) lOOGPa Axial Strength: Developing space elevators at the moon and other locations suitable single wall CNT in the 15OGPa or do not require this high strength material, and in greater range is only the fvst challenge. many cases, are feasible with current materials Once developed, these nanotubes must be technology. incorporated into a fiber that can still yield lOOGPa strength even after fabrication and Ribbon Design: There are several issues application of all the design variations and to be examined in the ribbon design once a coatings along its length to accommodate suitable fiber has been developed. In particular, loading and environmental conditions. it should be noted that a single ribbon design of Here again like the fiber, a ribbon of suitable standard width and composition for the full strength has yet to be developed and yields a length of the space elevator is unlikely. The TU 2 rating for earth-based space elevator ribbon must be designed for the loading and applications. environments it will encounter along its length. Here are some examples of particular interest: Robotic Climber Operations: Robotic 1) Tapered Design: The space elevator ribbon climbers will be used to construct the ribbon, to will likely have an average width of about 1 conduct inspections and repairs as needed to meter. Additional width, perhaps 3 meters, maintain the structural integrity of the ribbon, will occur at GEO to accommodate and to deliver payloads from the surface of the maximum tensile loads, and at low earth earth to a geosynchronous altitude, or perhaps orbit (LEO) to withstand the high debris other orbital altitudes. The low TRL systems of environment. Minimal width, perhaps in the concern have to do with the amount of ribbon centimeters, is all that may be needed in the splicing, inspection, and potential repairs atmosphere due to minimal loads, and the required; and the speed with which they need to need to reduce the ribbons aerodynamic be accomplished.

4 1) Ribbon Splicing: Ribbon splicing is Ribbon Control Systems: Ribbon anticipated as part of the original control systems will be required to keep the construction to allow for either a) initial ribbon in its geosynchronous orbital slot, and deployment from GEO with additional control tension, induced waves, climber width added with climbers from the ground, dynamics, and atmospheric effects. Control b) deployment from GEO with end to end systems will likely include base maneuvering, splicing, or c) repair by splicing additional tension reels at the base station, counterbalance layers of ribbon over existing damaged thrust control, and possibly a thrust control ribbon sections. The splicing techniques spacecraft at the geosynchronous altitude. required for each of these scenarios is 1) Tension Control: Tension control will be different, in that example (a) requires edge required to help control overall dynamics of splicing, 0)requires end to end splicing, the ribbon, and to prevent the ribbon from and (c) requires an overlay splicing exceeding its maximum designed loading. technique. Speed is of the essence for the A properly designed ribbon would be edge splicing where additional ribbon will tapered with its maximum width or density be added full length, 100,OOOkm. The initial at GEO such that tensile loads can be conceptual plan was to develop climbers that maintained around 67GPa throughout the could do the splicing work at 200km per full length of the ribbon. It is anticipated hour, which would take each climber 21 that loads will vary from induced waves and days to travel the full length of the ribbon. climber effects, but can be controlled by The challenge here is that this splicing releasing and retracting ribbon at the base operation of nano-scale fiben represents an station. industrial process that has not been 2) Induced Wave Control: Induced wave demonstrated in the laboratory. This control will be required to counteract concern could be alleviated by a climber dynamics and object avoidance construction method using end-to-end maneuvers. When an induced wave is used splicing of full width ribbons. for object avoidance, a second wave may be 2) Ribbon Inspection: Inspecting the ribbon required to counteract the first. The effects for damage and deterioration should be on ribbon tension and wave tracking can straightforward. The LEO altitudes will become quite complex. require the greatest attention since it is 3) Climber Dynamics: Climbers will produce anticipated that there are many objects less dynamic effects on the ribbon as it climbs than lOcm in diameter that cannot be due primarily to the change in angular tracked which could impact and penetrate momentum imparted on the ribbon by the many ribbon fibers. climber. The mass and tension in the ribbon 3) Ribbon Repair Methods: Repairs could will have to absorb the changing angular include splicing fibers and ribbon sections to velocity of the climber as it climbs higher replace severed strands, and application of and the ribbon pulls it into higher orbital protective coatings. The challenge will be velocities. If the mass of the ribbon can not devising a system of repair that will not absorb this energy through increased tension continue to add mass to the ribbon and thus then thrusters on the climbers may be decrease its payload capacity. required, adding additional complexity to In general, robotic climber development spans the system. the range of TRL 3 to 5, because many of the 4) Atmospheric Effects: High wind speed at processes have been done in existing industrial lower altitudes can pull the ribbon and low operations, but simply have not been applied or altitude climbers down if the ribbon is not integrated into a robotic climber system. The properly designed for these aerodynamic splicing technique described in la above (edge effects and proper tension in the system is splicing) is probably at TRL 2, but it is worth not maintained. Control of the lower noting that there are other deployment methods altitude sections in the atmosphere may that could eliminate the need for this technology. require additional systems such as airships

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or balloons in the upper atmosphere to carry within lOkm of the International Space Station part of the load for the atmospheric sections, (ISS) is carefully tracked and analyzed to and assist in wave control and object determine the risk of impact and consideration avoidance on the overall ribbon. given to possible maneuvering to avoid the Ribbon control is only at an analytical stage of hazard. On the space elevator, a lOkm clear development, and is being studied through a zone would sweep through thousands of objects computer model program developed by NASA in LEO per year, making the maneuvering task for the Space Shuttle’s Tethered Satellite very difficult if not impossible. For this reason, Systems (TSS) missions. This activity using the a more precise tracking and prediction system General Tethered Object Simulation System will be required to reduce the clear zone down (GTOSS) tends to indicate that nibon control fiom kilometers to meters in diameter. Here, the systems are at about a TRL 2 to TRL 3 rating. need for more tracking stations yields a TRL 7 for tracking systems, where as the development Power Systems: There may be several of perturbation models to predict the orbital alternatives for power systems for the climbers track of oncoming objects is of unknown that have yet to be explored. The baseline complexity and yields perhaps a TRL 4 rating. concept uses laser power beamed fiom a ground station up to the climber to produce electricity for the climber’s electric motors. This CURRENT ACTIVlTIES technology has been demonstrated on the ground, but the development and demonstration Several activities have been in progress over the of a high-powered laser that can track a climber past year at NASA, ISR, and their support up to 100,OOOkm altitude has yet to be contractors on research and technology developed, implying that this technology is at a development to address the low TRL concerns TRL 4. Other alternatives could include nuclear described above. Of particular interest are the systems, solar arrays and batteries, or following: development of an electrical conductor in the ribbon for transmitting power directly ‘to the Materials Research & Development: climbers. All of these approaches may be The Center for Applied Energy Research possible, but no detailed analysis or (CAEiR), at the University of Kentucky, in experimental development has been carried out Lexington, Kentucky, has developed a process to determine the most practical means for the in-house to produce multi-wall CNT and climber power system. incorporate them into a pitch-based fiber. Research has focused on development of the Tracking Systems: Tracking systems, spin line process that will align the tubes in the and the models for predicting the orbits of fibers and produce continuous multiple fibers in tracked objects, have been in existence for a single run. Once in place the spin line will be several decades. The problem for the space used to incorporate other multi-wall and single- elevator is the degree of accuracy required. wall CNT from other sources to produce higher Current systems in place by the U.S. Air Force strength fibers for further research, analysis and Strategic Command were designed to track testing. The set-up will allow CNT researchers incoming nuclear missiles. The byproduct of to experiment with incorporation of their this system through incremental improvements products into a fiber, and will allow potential is the ability to track satellites and orbital debris. manufacturers of fibers to experiment with But, there are several vexing problems with the production techniques for full-scale current capabilities. First, the current system manufacturing.6 only tracks objects lOcm in diameter or larger; and second, the orbital perturbations are such Ribbon Design: that an object may come over the horizon Ribbon design and analysis is being conducted several kilometers off from where current at ISR to determine an optimal approach to a models predict. Any object predicted to come ribbon that can be both lightweight and transfer

6 load fiom severed fibers to adjacent fibers7. The current NASA / ISR cooperative agreement Although the actual fiber is not yet developed, will be active through the end of this calendar there is sufficient data available that can be year. Summaries of current findings from these applied to finite element analysis programs to activities are available in the papers referenced model how the ribbon will react given above, and frnal results will be available in a appropriate design elements, materials final report on the NASA / ISR cooperative properties, and constraints. In addition, a agreement to be released next year. competitive approach is also in progress that may yield results for ribbon development as described in the next section on robotic FUTURE NEEDS climbers. As indicated above, there are numerous Robotic Climber Development; technologies and systems that will need further Robotic climber development is important but research and technology development in order to was not a primary area of interest for the NASA move forward on a space elevator. A way to / ISR activity since this is an area where industry capture and integrate these technologies is already has significant related activities. Of through definition of ground and spaceflight interest though is the NASA Centennial demonstrations that could be done to prepare the Challenges grant that was recently awarded to way for space elevator development and the Spaceward Foundation to conduct tether complimentary missions. These are the types of strength and climber competition challenges. demonstration projects that are often laid out on This competitive program is anticipated to a roadmap, or milestone chart, showing produce ongoing interest in the space elevator progressively more , complex developments concept and develop new systems that will leading toward an ultimate goal. The following advance the technology for robotic climber section on Future Needs, are some preliminary systems, beamed power systems, ribbon design, thoughts on how such a technology development and space elevator development in general.’ roadmap for the space elevator might be formulated. Space Elevator Dynamics Modeling: In the 1980’s NASA developed the GTOSS to Ground Tests: model the tether dynamics of the tethered There are several developments that need to be satellite missions, TSS-1, and TSS-lR, flown accomplished in the laboratories and in ground from the space shuttle. This program has been systems tests before spaceflight tests will be updated to accommodate space elevator sized productive. These include Materials tethers to study the dynamics of the space Development, Materials Testing in a Relevant elevator and the various modes of operation such Environment, Vertical Treadmill Tests, and as ribbon deployment, climber dynamics, Tethered Balloon Tests. atmospheric dynamics, and induced waves for object avoidance. ’* lo Materials DeveloDment & Testing in a Relevant Environment: Materials development Tracking & Object Avoidance Simulations: has been discussed in previous sections noting Tracking and object avoidance activities have the need for further development of ultra high included use of Satellite Tool Kit (STK), the strength CNT reinforced composites. In STRATCOM satellite tracking system from the addition to that development, the composite US. Air Force Strategic Command, and model fibers must be incorporated into a ribbon data from the GTOSS program described above. designed to withstand the loads and These tools have made it possible to study an environmental conditions in which it will be approach to object avoidance by moving the subjected. This will include space environments base station and by inducing a wave in the tests in vacuum, and under simulated conditions ribbon timed to ropagate up the ribbon for for debris impacts, and exposure to radiation, object avoidance.IP atomic oxygen, and the full range of

7 atmospheric conditions at the lower end. These more impacts with larger objects up to lOcm tests will lead toward appropriate tether design in size. Here the climber would have to for each section which will likely include accommodate a wider cross-section for changes in width and density to accommodate inspections and repairs; and, although the loading, debris impacts, and atmospheric winds; repair work would be similar to the upper and changes in protective coatings for sections section described above, it could involve exposed to high radiation and atomic oxygen repairs to many more strands at each point environments. Once a suitable design is in place of impact. for each section of the ribbon then development 3) Radiation & Atomic Oxygen: The ribbon and testing of maintenance climbers can be done materials will have to survive the vacuum on vertical treadmill tests. and radiation environments of space along its entire length above the atmosphere, but Vertical Treadrm‘11 Test: The space of particular concern is the atomic oxygen in elevator can be divided up into four sections LEO. Metallic coatings may be required to based on the environments that each section will protect the composite fibers in the ribbon encounter. Some of these sections overlap based from deterioration. Here, the climbers will on the multiple environmental conditions in each need an inspection capability to examine the section. Here is a summary that should help coatings and make coating repairs as explain the ribbon design and. the climber needed. Also of concern is the wear on maintenance requirements that would be coatings from the climber traction wheels. developed and tested on a vertical treadmill test 4) Atmospheric Conditions: The last few for each section. hundred kilometers enters the earth’s Micrometeoroid Impacts: The longest atmosphere with a variety of effects from ribbon section would fall under this category wind, moisture, and lightning. Of particular of testing extending hmLEO up to the concern is the cross-sectional area of the counter weight at 100,OOOkm altitude. ribbon and its reaction to wind driven forces. Ribbon density would vary to accommodate It has been found that a, 1-meter wide ribbon maximum loads at GEO, but it is thought the in the atmosphere can cause a significant width could be a consistent 1-meter, and force that will elongate, or pull part of the survive the micrometeoroid impacts for 10 space elevator down into the atmosphere’. years without repairs. The task for the To avoid wind driven problems the cross- climber tests would be to make inspections sectional area could be reduced to something along this type ribbon and make appropriate more like a rope than a ribbon, and could be repairs to broken fiber strands to maintain supported by balloons or airships in the the ribbon’s structural integrity and thus upper atmosphere if needed. In either case, extend its life. When a strand is broken the the climbers will likely have a different role ribbon is designed with taped or woven that will require inspections of the rope like cross-links that transfer the load to adjacent section at the lower end and then splicing in strands. So, key to the repair operations will a replacement section when significant wear be devising a method to identify the broken or damage is found. strands, weave in a splice, and reload the repaired area. Tethered Balloon Tests: The idea LEO Objects and Debris: LEO objects have behind a tethered balloon test is to build been the subject of object avoidance to prototype sections of the space elevator ribbon avoid damage to the space elevator and and suspend them from a balloon or airship in active satellites. In addition, there is thought the upper atmosphere and do a full systems test to be many thousands of debris below lOcm of the climbers, their robotic functions, the in diameter that cannot be tracked with beamed energy system that powers the climbers current systems. An approach to this as the traverse the ribbon from the ground up to problem is to expand the width of the ribbon the balloon, and to test overall ribbon from 1-meter to about 3-meters to survive deployment and control systems. These tests

8 would likely include climbers for ribbon inspection, maintenance of coatings, repair to severed fibers, and splicing; and climbers with payload delivery systems.

Spaceflight Tests & Demonstrations: Several flight tests have been conducted on tether systems with varying degrees of success. As seen in the two tethd satellite missions there are unexpected events that can occur and result in deployment mechanisms locking up or static electricity discharge resulting in a severed tether. To avoid such events on a system as large a space elevator it be important to as will Figure 3: ISS concept for an attached tether conduct spaceflight tests to develop and using electrodynamics for propulsion demonstrate quality systems that will not fail. In this section we will examine progressively more Another system of interest that is quite large is complex spaceflight experiments in LEO, GEO, the concept for a LEO space elevator, Figure 4, and at the moon, to demonstrate space elevator sometimes called a “Bridge to Space” that uses a deployment and control leading up to the LEO tether several thousand kilometers in development and deployment of an earth based length to lift payloads into a GEO transfer orbit space elevator. (GTO). This concept would have to address orbital debris and object avoidance requirements LEO Deployment Demonstration: similar to those of a full-scale space elevator. There are perhaps many approaches to So, this type of system might be a good demonstration missions in LEO that could precursor for space elevator technology satisfy requirements of space elevator development. It operates by payloads delivered The interest here is to deployment testing. to the lower end fiom a suborbital vehicle, and demonstrate deployment of a tether down fiom a then released at the upper end into a GTO primary spacerraft with an end effecter trajectory. Such a system would significantly spacecraft in a gravity gradient stabilized orbit. reduce the requirements on both the launch This would seem like a simple demonstration, vehicle and the upper-stage. but it is important to remember that in the space shuttle missions, neither flight reached full TSS GEO Deployment & Traversing deployment of the tether. New deployment and Demonstration: The next major demonstration control tests could be done with a fiee flying mission should probably be done at GEO to spacecraft or as part of a payload fiom the space demonstrate tether deployment and control of station. Figure 3 shows an interesting advanced hundreds, or perhaps 1000’s of kilometers in concept for the space station using an length. In addition, a climber traversing this electrodynamics tether for propulsion. This type tether would be of interest as part of the of project would significantly advance the demonstration to observe control issues, conduct technology for tether deployment and control in inspections, and make repairs. Like the LEO space. The benefits for the ISS include a space elevator and ISS tether mentioned above, propellant-less propulsion system providing re- there are applications for this trpe of tether that boost through electrodynamics, and if desirable could be developed. They include: 1) a in the future, a system that can relocate the space geosynchronous communications satellite with a station to a lower inclination for use in future split power bus and transmitter; and, 2) a LEO to lunar mission architectures. The risks for this GEO space elevator. system include the possibility of a severed tether and its entanglement with other components on the ISS.

9 LEO to GEO Space Elevator: This elevator would be similar to the LEO space elevator described above and shown in Figure 4, except that it would be designed to hoist payloads directly from LEO to GEO, and could release payloads beyond GEO for delivery to the Moon, Mars, or other interplanetary destinations.

Full Scale Lunar Demonstration: Lunar space elevators, or lunar towers as they are called in Figure 5, are possible at the earth-moon L1 & L2 points. These structures are of interest as part of a transportation infktructure at the moon, and as a full-scale demonstration of space elevator construction, control, and operations. The length of a lunar space elevator would be about the same as an earth-based space elevator, but would not require material strengths as high due to the lower gravitational effects. Lunar space elevators have been studied before, and are being studied again under a NIAC grant.I3*l4 Figure 4: A LEO space elevator, or “Bridge to Figure 5 provides a summary of the space Space” concept by the Lockheed Martin Company.” elevator related operational systems that could be developed around the earth and the moon that 1) Geosynchronous Communications Satellite: could eventually lead to full-scale development A geosynchronous satellite with the power of an earth-based space elevator. All of the system bus at the upper end, and the tether concepts mentioned in this section, except transmitter suspended at the lower end in a for the earth-based space elevator, appear to be ME0 or LEO altitude could significantly possible to construct with current materials reduce the lag time for two-way technology. communications and thereby expand the utility of GEO satellites for telephone communications.

Figure 5: Concepts for space elevators around the Earth and Moon.

10 CONCLUSIONS for the Space Elevator, IAC-04- IAA.3.8.3.02, International Astronautical In conclusion, the most critical technology for Congress, Vancouver, CA,2004. earth-based space elevators is the successful 6. Weisenberger, M. C., R. Andrews, et al, development of ultra high strength carbon Multiwall Carbon Nanotubes in Pan-Based nanotube reinforced composites for ribbon Carbon Fiber, Space Exploration 2005, construction. Since this material is the critical Space Engineering and Science Institute item that will make space elevators at earth (SESI) Conference Series, Vol. 001,2005. possible, the space elevator along with the ultra 7. Spadaro, J. F., R. B. Martin, Space Elevator high strength carbon nanotube reinforced Tether Design: Analysis and Testing, Space composite material is at a TRL 2 rating. Once Exploration 2005, SESI Conference Series, ribbon material strengths above the 67GPa range Vol. 001,2005. are demonstrated to a higher TRL 6 level, then 8. NASA Centennial Challenges Program, practical plans for demonstration and accessed September 1, 2005, construction of an earth-based space elevator http://em>loration.nasa.gov/centennialchallen system will be possible. In the mean time, there ge/cc challenges.html are other demonstrations and practical projects 9. Lang, D. D., Approximating Aerodynamic using tethers in LEO, GEO and at the moon that Response of the Space Elevator to Lower could be investigated further for their own merit. Atmospheric Wind, Space Exploration 2005, In addition, many intermdate technology goals SESI Conference Series, Vol. 001,2005. and demonstration missions for the space 10. Lang, D. D., Space Elevator Dynamic elevator can provide significant advancements to Response to In-Transit Climbers, Space other space flight and terrestrial applications. Exploration 2005, SESI Conference Series, Vol. 001,2005. 11. Dzierski, D. C., A Case stud^^ and BIBLIOGRAPHY Simulation Depicting Orbital Debris Encountered by the Space Elevator, Space 1. Edwards, B. C., E. A. Westling, The Space Exploration 2005, SESI Conference Series, Elevator, Spageo Inc., San Francisco, CA, Vol. 001,2005. 2002. 12. Mottinger, T.A., L.S. Marshall, The Bridge 2. Von Tiesenhausen, G., Tethers in Space: to Space Launch System, Space Technology Birth and Growth of a New Avenue to Space and Applications International Forum, Utilization,NASA TM-82571, 1984. STAIF 200 1. 3. Smitherman, D.V., Space Elevators: An 13. Pearson, J., “Anchored Lunar Satellites for Advanced Earth-Space Infiastmcture for the Cislunar Transportation and New Millennium, NASA/CP-2000-2 10429, Communication.” J; Astro. Sciences, Vol. NASA, Marshall Space Flight Center, 26, NO. 1,39-62, 1979. Huntsville, Alabama, August 2000. 14. Pearson, J., Lunar Space Elevators for 4. Mankins, J.C., Technology Readiness Cislunar Space Development, NASA Levels, Internal white paper, Advanced Institute for Advanced Concepts study, Concepts Ofice, NASA, April 6,1995. accessed September 7, 2005, 5. Smitherman, D. V., Technology httP://www.niac.usra.eddstudies/studies.isp Development and Demonstration Concepts

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