Lunar Base As a Precursor to Mars Exploration and Settlement

Lunar Base as a Precursor to Mars Exploration and Settlement

Wendell W. Mendell, Solar System Exploration Div. NASA Johnson Space Center Houston, TX 77058 USA

Abstract

Debates over strategy for human exploration of the solar system focus on whether the next interplanetary travelers ought to go to the Moon or to Mars. The Space Exploration Initiative explicitly calls for a lunar presence, but the "Mars Now" advocates argue for a token effort on the Moon with major emphasis on Mars. A strategy which moves directly to piloted Mars missions incurs serious programmatic risks from (1) our current lack of knowledge on human performance on long-duration deep space missions, (2) our lack of experience designing manned space systems which must remain trouble- free for years without refurbishment, (3) our lack of experience with complex on-orbit operations, and (4) the political instability of the funding process. A well planned program of human exploration of the Moon would provide a context within which to increase our capabilities and experience to levels required for Mars exploration.

Introduction istrator: (1) outpost on the Moon or (2) piloted missions to Mars. The more vocal advocates of In his remarks prepared for the Apollo 11 strategic planning for human space exploration 20th Anniversary Celebration at the National had argued forcefully for one or the other of Air and Space Museum, President Bush these options as the next "big" initiative in the declared: U.S. space program. At first glance, it would seem that the Pres- "I'm proposing a long-range continu- ident has settled the issue. Following the ing commitment: First, for the coming successful operation of Space Station Freedom, decade, for the 1990's, Space Station the nation will invest in a permanent lunar Freedom…; and next, for the new cen- presence followed by landings on Mars before tury, back to the moon, back to the fu- the end of the second decade of the 21st ture, and this time, back to stay. And Century. However, the less than spectacular then a journey to another planet, a initiation of the Space Exploration Initiative manned mission to Mars." has shifted the debate to changes in the emphasis of the objectives. For example, might By stating these objectives for the next century not an automated observatory on the Moon in space, the President defined a path by satisfy the lunar requirement while the which to implement one of the major compo- program emphasized expeditions to Mars? nents of President Reagan's space policy state- One cannot totally generalize the character ment of February, 1988: "Establishing a long- of the arguments on either side of the case. range goal to expand human presence and activ- However, it is true that "Moon First" propo- ity beyond Earth orbit into the Solar System". nents tend to emphasize the steady buildup of President Bush chose one of several paths infrastructure and capability in the Earth- for human exploration of space which had been Moon system, while "Mars Now" enthusiasts under study in NASA's Office of Exploration stress the excitement of new discoveries and the (OExp) since its formation in 1987. The OExp inspiration associated with reaching a planet work concentrated on two themes to human ex- where life might have arisen. Also implicit in ploration of the solar system as outlined by Dr. "Mars Now" is the belief that the nation (or Sally Ride1 in her report to the NASA Admin- the world) cannot do both. If a substantial Table 1. Moon/Mars Comparison3

Mission Target Typical Logistics Departures/ Annual Logistics Planet Surface (LEO) year tonnes/yr Buildup Rate tonnes per departure tonnes/yr Moon 110 6 660 80 Mars 1500 0.45 675 50 lunar program is undertaken, Mars may not be (d) inability of institutions to maintain reached within the lifetime of anyone now public support during long periods of extant. expensive hardware development The arguments are usually carried out on a when no accomplishments are plane that is more philosophical or propagan- apparent. distic than technical. Statements that, "The Lunar missions face these same difficulties. Moon is boring," are countered by, "Lunar However, the mission or program parameters in resources are useful for space development." the lunar case fall much closer to our current Nevertheless, all parties do agree that if operational and design experience. either program were to be undertaken and were to fail (i.e., be cancelled), then the future of Mission Parameters human spaceflight would be in severe jeopardy for the foreseeable future. Overview I will argue qualitatively in this short Several different approaches or "architect- paper that commencement now of a program of ures" have been proposed for crewed missions to human exploration of Mars incurs major techni- the Moon or to Mars. Therefore, it is difficult to cal risks that can be mitigated and probably do a thorough comparison without a rather eliminated within a program of human explo- lengthy analysis. I will try to take representa- ration of the Moon. I identify at least four tive scenarios for each program that illustrate major categories of mission risk which are the difference in scale of the respective mission currently beyond our understanding or our parameters. I will consider the architectures of capabilities: the type presented in the NASA 90-day Report (a) mission failure due to degradation of to the President2 because they have certain human performance either physiologi- common features that allow comparison. I will cally (including death) or psychologi- assume that the technology to build the trans- cally after long duration exposure to portation systems (propulsion, power, and life the space environment – both in freefall support) is equally well understood for either and in reduced gravity – and to the planetary mission. stress of isolation; In both cases, the interplanetary vehicles (b) mission failure due to equipment or will be launched from low Earth orbit (LEO) at software malfunction because of insuf- the 28.5˚ inclination of Space Station Freedom. ficient testing as an integrated system, In most lunar architectures a spaceport in LEO inability to guarantee sufficient mean is used as a transportation node for transship- time between failures by analysis, ping cargo and for maintenance of reusable inability to carry as many spare parts orbit-transfer vehicles. In most Mars mission as needed, and inability to predict all architectures, the interplanetary vehicles are contingencies before the mission; assembled in LEO. Reusability of these (c) critical cost overruns after a failure to vehicles is problematical, particularly if they meet a launch window when the Earth- use nuclear propulsion and return with a radio- to-orbit launch rate cannot be active reactor. I have not factored reusability maintained or when in-orbit assembly into the comparison between the two classes of and checkout proves to be more compli- missions because refurbishment of a complex cated than believed; or spacecraft on orbit has not been shown definitively to be cost effective.

2 Table 2. General Characteristics of Mars Mission Profiles3,4

Characteristic Conjunction Venus Opposition "Sprint" Nuclear Exotic Swingby Electric Propulsion Representative transit 30-35 20 months 15-16 1 year 1 year 60 days time (round trip) months months Representative stay 300-500 20-60 days 10-30 days 10-30 days 10-300 any de- time days days sired Representative ∆V's 8.3 - 8.7 10.4-13.0 13.3-23.1 14 in an 20 200-300 for transit mission "easy year" with aerocapture at (2003) Mars and at Earth (km/sec) Window duration 2-3 months 1-2 months short short many always months open Variability with op- slight modest high extreme usually slight portunity slight Table adapted from Woodcock (Ref. 3) with ∆V data taken from Hoffman (Ref. 4), who assumes direct entry on Earth return.

Woodcock3 has made a comparison of lunar (col. 2), the conjunction-class mission has a long mission modes and Mars mission modes. Tables round trip time and requires a very long stay on 1 through 3 are adapted from his paper. The the surface of Mars. values in the tables are intended to convey the The change in orbital velocity (∆V) re- relative magnitudes of important parameters quired for the spacecraft to leave LEO for Mars such as Earth to orbit launch rate, crew time in and to enter orbit around Mars is relatively con- the space environment, mission duration, stant from year to year for conjunction-class mis- schedule constraints, and complexity of in- sions. Slight variations are the result of the ec- space operations. For example, his two refer- centricities of the martian and terrestrial orbits ence models were scaled to produce approxi- about the Sun. The magnitude of the ∆V is di- mately the same annual launch rate of mass to rectly related to the amount of propellant re- LEO (Table 1, col. 3). As a result, in a 26-month quired onboard the interplanetary vehicle in period, one mission to Mars is launched, while LEO and therefore related to the mass which 13 missions could be flown to the Moon during must be launched from Earth for the mission. the same period. (For reference, the "aggres- The trip time to Mars can be reduced at the sive" option in the NASA 90-day Study2 expense of greater ∆V required at LEO. For a envisioned two launches to the Moon per year.) given chemical propulsion system, increases in The rate of buildup of infrastructure mass on the the ∆V requirement imply that a greater per- lunar surface is about 80% greater than the rate centage of spacecraft mass in LEO is propellant. on the martian surface. As a result, these two Secondly, the amount of the increase is very programs do not produce the same scale of sensitive to the alignment of the planets and planetary surface activities. can vary considerably form one opportunity to the next. There does exist a 15-year cycle in the Mars Mission Strategies alignments in which the patterns repeat. The least energy for a transfer between Opposition-class trajectories (Table 2, col. Earth and Mars is required when the spacecraft 4) take advantage of another form of alignment follows a Hohmann ellipse, tangent to the orbit between the two planets. On either the out- of the Earth and the orbit of Mars. The Earth bound or inbound leg of the mission, the trajec- and Mars align properly for this minimum en- tory passes inside of the orbit of the Earth, ergy transfer once every 26 months. In the par- crossing the orbit of Venus. The mission travel lance of Mars mission designers, the minimum time in space is not too much different from the energy Hohmann transfer is called a "conjunc- conjunction class, but the stay-time on the mar- tion-class mission". As can be seen from Table 2 tian surface is very much less, resulting in a

3 Table 3. Summary of Lunar Mission Modes3

Lunar Staging Point Parameter Lunar equatorial Lunar polar or- Moon-Earth L2 or Lunar surface orbit (100 km) bit (100 km) L1 halo orbit Lunar surface site access Near lunar equa- Any Any Any, assuming tor only approach via lunar orbit Transit time, Earth to stag- 3-5 days 3-5 days 5-8 days 3-5 days ing point Wait at staging point Not required 14-28 days Up to 7 days for Not required (usually 14) lowest ∆V Transit time, staging point ~2 hours ~2 hours 3 days Not applicable to lunar surface Departure window fre- ~9 days 55 days ~9 days ~9 days quency, SSF orbit ∆V (m/sec) Earth to staging point 4010 4010 3375 (LGA) 6110 Staging point to surface 2100/2000 2100/2000 2950 /2850 Not applicable Staging point to Earth 1100 1100 435 (LGA) 3050 LGA = Lunar Gravity Assist shorter mission duration. Note the wide varia- dezvous between the arriving crew and the pre- tion in ∆V requirements. One implication of the positioned equipment on the surface and/or in variation is that a spacecraft configuration Mars orbit. Depending on how much mission designed for an energetically favorable oppor- equipment is sent ahead, crew survival could tunity may not be able to launch at the next rest on the success of the rendezvous and on the alignment in case of a schedule slip. working condition of the unattended equipment. For some opposition-class trajectories, the The problems of large masses being assem- planet Venus is located so that the spacecraft bled in LEO and long flight times can be miti- can utilize a gravitational assist on either the gated (but not eliminated) with nuclear propul- outbound or inbound leg. For Venus swingbys, sion systems for the interplanetary vehicle. the ∆V requirements are less than for a straight However, development of the nuclear-depen- opposition-class mission, but the time spent in dent technology represents a departure from transit increases. The total mission duration current transportation system evolution and remains less than that for conjunction-class presents a technical and political risk to the missions. program. The "Split-Sprint" mission strategy was conceived to deal with the increased ∆V ( thus Lunar Mission Strategies increased propellant in LEO) requirements asso- In contrast, the trip time from LEO to the ciated with trajectories having decreased Moon is almost two orders of magnitude less flight times. An unmanned mission carrying than the trip time to Mars. The mass required cargo is sent to Mars on a low-energy trajectory. in LEO for a lunar mission is approximately one The cargo mission contains equipment for use on order of magnitude less. The frequency of mini- the martian surface. The "Sprint" component of mum energy launch opportunities is one order of the mission carries the crew on a fast trajectory. magnitude greater, reducing the criticality of The idea is to minimize the payload taken on meeting launch windows. The total duration of the piloted mission. In some versions the cargo mission is quite flexible. Apollo missions mission carries the crew lander, leaving it in lasted about two weeks. At the other end of the orbit around Mars, and in some proposals also spectrum, a crew could be supported on the lunar carries the fuel for the return to Earth. The surface indefinitely using currently available Split-Sprint strategies demand successful ren- technology.

4 On the other hand, it is important to note will fall below the threshold of human percep- that the scale of operations required to support tion if the spacecraft is rotated at a slow rate. a lunar surface installation, particularly if it is It is unknown whether simulation of full terres- permanently staffed, can be about the same as trial gravity is required or whether the small the scale to support of a sequence of piloted residual coriolis forces will cause some disori- Mars missions. In addition, a lunar transporta- entation in crew members. No data from a tion infrastructure could be used easily to simu- space-based facility exists, and the space life late the operations and time scales of human science research community is split over the va- exploration of Mars. lidity of the artificial gravity "solution". Spacecraft designers believe that the engi- Summary neering problems associated with a rotating Piloted missions to Mars will involve mis- manned vehicle are soluble with appropriate sion durations and scales of operations far be- testing on orbit. Everyone agrees that such a yond the current experience of the U.S. space spacecraft will be more massive than a nonro- program and only approached by the (now trou- tating craft, but not all agree on how much more bled) Soviet space program. Brute force techno- massive. Control of a rotating craft will be a logical attacks on these problems involve radi- challenge. cal (and therefore expensive) departure from Deconditioning is a critical issue for Mars the current directions of technology develop- missions because the crew will undergo high ment and operational philosophy. transient accelerations during descent to the A program incorporating a permanent lunar martian surface. Depending on the physiologi- surface base requires modest extrapolation of cal condition of the crew, these accelerations current technology but will provide an opportu- could be life-threatening. Once on the surface nity to develop an efficient operations infras- of Mars, the crew must recover without external tructure supporting permanent human presence medical support and must perform a series of in space. demanding tasks. The time required for recov- ery is particularly important if the surface stay Major Challenges for is short as in opposition-class missions. No one Human Exploration of Mars knows whether exposure to a gravity field lower than the Earth's will reverse the decon- Human Performance ditioning induced by space travel. Without Deconditioning in Weightlessness . Every- more research on these issues the performance one is familiar with the fact that the human of the crew on the surface of Mars cannot be body undergoes certain adaptations when ex- guaranteed. posed to weightlessness (i.e., zero-g). These Deconditioning under Low Gravity . No one changes are most debilitating when the space knows whether the types of deconditioning dis- traveler must readapt to gravity. The most cussed above will be manifested in low gravity serious known changes include cardiovascular as well as in the weightless condition. In other deconditioning, decrease in muscle tone, loss of words, if the crew arrives on Mars in good calcium from bone mass, and suppression of the shape, what will their condition be after immune system. spending a long time under martian gravity? A variety of countermeasures for these con- "Artificial gravity" cannot be provided on the ditions have been suggested, but none have been surface. The rates of change and the final lev- validated by thorough testing. The Soviets els of the effects observed on orbit may be linear have had some success with long periods of with level of gravity exposure or they may be daily exercise to maintain cardiovascular triggered only below some threshold of expo- capacity and muscle tone, but the monotonous sure. The Apollo missions to the Moon were too and time-consuming exercise regime affects the short to produce observable differences between efficiency and morale of the crew. the condition of the astronauts who went to the Artificial gravity is often put forward a surface and the condition of the astronauts who fallback solution. The entire spacecraft is stayed in orbit. rotated so that the crew experiences a constant Psychological Stress . Psychiatrists and "downward" acceleration simulating gravity. psychologists agree that piloted missions to It is generally assumed that coriolis effects Mars may well give rise to behavioral aberra-

5 tions among the crew seen on Earth in conditions whose magnitude would be similar to that of an of stress and isolation over long periods of time. actual mission. In addition, consider the moti- The probability of occurrence and the level of vation of a crew which would spend two or any such anomalous behavior will depend not three years in orbit pretending to go to Mars. only on the crew members individually but also Experience with the Hubble Space Tele- on the group dynamics among the crew and scope is illustrative of the problem. A full between the crew and mission control on Earth. quality assurance organization was in place but In general, the probability of behavior extreme was ineffective in preventing a fundamental er- enough to threaten the mission will decrease ror in the construction of the observatory in the with an increase in the size of the crew. How- absence of integrated testing. ever, the expense of sending large payloads to Another problem is inability of careful Mars limits the crew size to four or less in most analysis to predict every contingency. Once scenarios. At the present time, no known tech- again, we can point to the jitter in the solar niques for crew selection are adequate to guar- arrays of the Hubble when it passes from the antee psychological stability on a voyage to Earth's shadow into sunlight. The Galileo mis- Mars. Soviet experience suggests that a crew sion is threatened by a stuck antenna, caused by should train together for many years. degradation in a lubricant after delays in launch unanticipated by the original designers. Reliability and Lifetime of Complex Systems Consider the remarks from the German manu- The Mars mission interplanetary spacecraft facturer of tube amplifiers which failed aboard will be the most complex ever constructed, and the recent French TDF direct broadcast satel- the lives of the crew will depend on its reliable lite: performance for periods of a year to three years. The life support, propulsion, power, "It appears that tubes in orbit behave computer, and communication systems must worse than tubes on the ground despite perform without fail for a period of time longer vacuum testing and other simulation. We than that required from any previous manned have no explanation for this. It could the spacecraft. The mission duration will be two radiation environment of space, or in- orders of magnitude greater than current Space creased heat from the solar collectors. It Shuttle missions, and resupply will be shows we have no sure way of simulating impossible. the space environment."5 Although reliability of subsystems will be part of the design and testing philosophy, Yet, satellite communications is always treated redundancy through backup systems and spare as the most mature of space technologies. parts will also be important. However, the In a large, complex program, a manager constraints on mass launched from LEO will somewhere will take a shortcut under pressure place limitations on the degree of redundancy from budget and/or schedule. The consequences available to designers. Estimates of the mass of his/her action will not always be obvious to of necessary spare parts, extrapolated from program management. As a result, the reliabil- failure rates aboard the Shuttle, give numbers ity of the product will be overestimated. And which are prohibitively large. Therefore, management always expresses a very human testing and quality control will be paramount. tendency to believe good news. The net result of Particularly important will be testing of these phenomena can be illustrated by the the integrated flight system under conditions change in the official estimates of the similar to the actual mission for periods of time reliability of the Shuttle before and after the similar to (and preferably much greater than) Challenger tragedy. the actual mission. Integrated flight testing is In short, do not rely on a product that has truly critical if the flight system is the first of not been tested in its working environment, its kind. Unfortunately, if history is a guide, whether it is a new car, a complex piece of budget pressures will cause program manage- software, or a spaceship. ment to search for substitutions for full-up flight testing. After all, most of the expense of Reliability and Resilience of Earth-to-Orbit a mission to Mars resides in launch and opera- Launch Operations tions, two categories of expense for a flight test

6 In most Mars mission architectures, the in- If Space Station Freedom requires approxi- terplanetary vehicle is constructed in Earth or- mately ten years before launch of the first bit. The vehicle is generally more massive hardware element, one might ask what inter- than Space Station Freedom by a factor of five val of apparent inactivity would transpire or more. Most of the mass is propellant in the between approval and launch of the much more case of chemical propulsion systems. In most challenging piloted mission to Mars. Large cases, the vehicle is also larger than Space scale publicly funded programs are subject to Station Freedom. Therefore, we can look to the continuous critical scrutiny by technically difficulties being encountered in planning the unsophisticated observers who want simple assembly and operation of Freedom and quali- answers to simple (and often simplistic) tatively extrapolate to assess the magnitude of questions. Tangible performance is demanded the task of assembling the Mars vehicle. over time frames determined by political time No one proposing orbital assembly of an in- constants (two to four years in the U.S.). terplanetary vehicle believes it can be done If an institution wishes to be supported by without using a heavy lift launch vehicle. public funds for a project with a duration of New systems being proposed now for the U.S. many political time constants, then that insti- have payload capacities of 50 to 100 tonnes to tution must be sophisticated enough plan visi- LEO. The larger vehicles are preferable. In ble milestones, comprehensible by the public, at any case, the assembly operation will require intervals appropriate to the funding review at least 10 to 15 launches of cargo and crew over process. Historically, NASA has been reason- the 26-month interval between launch win- ably successful at maintaining funding of dows. Most of the cargo launches will occur decade-long programs in the face of an annual over a relatively brief interval just before the budget review. The vast majority of those pro- launch opportunity, carrying propellant to or- grams are understood by all to have a finite du- bit. ration. After a satellite has been launched and These rather general considerations make operated for a given period, it either fails or is it clear that the mission to Mars will require shut off. Neither NASA nor the Congress are levels of launch operations and of on-orbit yet comfortable with programs that are open- operations far beyond our current experience. ended, such as the Shuttle or Space Station or Please note that this is not a problem with human settlement of the solar system. hardware; the required number of launchers can In summary, the U.S. space program has not be designed and built. Rather, this is a problem yet adapted to the modern reality that ap- of meeting a constrained schedule with a man- proval of a goal by a President cannot guarantee agement system and a work force whose first job ten or twenty years of sustained funding. The is this demanding operation. The learning thirty-year time frames for human exploration curve cannot be steep enough. The launch to of Mars cannot be supported until the role of the Mars would challenge a team experienced in space program is well integrated into the na- tightly scheduled launch operations and tional agenda and the exploration of space is no complex in-space assembly. longer regarded as a subsidy of the aerospace industry. To accomplish this, the space Political Viability program must show concern for, and address, Going into the U.S. Fiscal Year 1992, the national needs ( visible contributions to Space Exploration Initiative has not been technology, science, environmental studies, funded by the Congress for even planning stud- education, inspiration of youth, etc.) while ies. I believe that there are several reasons for maintaining a thoughtful and challenging this, one of which is the poor performance (in agenda of human exploration of space in which the eyes of the Congress) of the Space Station the public can feel a partnership. Freedom program. Almost seven years after approval and after spending an average of almost a billion dollars per year, the fundamental purpose of the station is still being Human Exploration of the Moon debated and the design has undergone recent radical changes. The medical, technical, operational, and institutional challenges detailed in the previ-

7 ous section in the context of human exploration arises to involve many students in the explo- of Mars are also applicable to human exploration experience using robotics, telepresence, ration of the Moon. The difference is that in no and the national high speed data networks now case must our degree of knowledge or capability in place. Students in classes could accumulate be advanced to a much higher level in order to data from instruments on the Moon and even perform the exploration safely and scientifi- direct some of those instruments.6 An intelli- cally. In fact, lunar exploration provides an gently designed program could provide for real ideal context within which to advance our (if not direct) interaction between the scientists technology to the level required for planning of tomorrow and the lunar explorers of today. A Mars exploration. publicly visible participation in the explo- The Apollo missions demonstrate that no ration experience for people across the nation problem exists for adaptation to low gravity for would go far to keep the funding alive. short times. Modern lunar exploration would The question still remains whether a lunar extend stay time on the lunar surface to months program of exploration would preclude explo- and would monitor crew performance. Coupled ration of Mars. The formulation of such a ques- with long duration in weightlessness in Earth tion makes several assumptions. orbit, data could be efficiently accumulated to First it assumes that our technology and our predict the regimes of human performance on a institutions will not evolve and that space ex- Mars mission. Cumulative effects of galactic ploration will stay as relatively expensive as cosmic rays and solar particle events on the it is now. Such a prophecy will be self-fulfill- crews could be measured. Psychological issues ing if space utilization remains the low volume, raised by long duration in isolation could also elitist activity that it is today. Eventually be studied. society will produce a breakthrough technology The predecessors of interplanetary space- that will open the space frontier, but the status craft would accumulate operational time in an quo can be maintained for a long time as long as Earth-Moon transportation system. Data space exploration is the captive function of a would be accumulated on integrated system single institution. The more widely distributed reliability, maintenance rates, and degrada- the participation in space exploration, the tion of performance in the deep space environ- more quickly innovation and iconoclastic think- ment. Lunar surface life support systems could ing will invade the system. evolve into their martian counterparts. Opera- Secondly, the question assumes that the tional experience on a planetary surface would generation after ours will view exploration of be obtained. Power, transportation, communi- Mars with the same attitude that we do. It cation, construction, and resource utilization can assumes that exploring the Moon will so ex- all be elements of a lunar base. haust the nation that no one will have the A heavy lift launch vehicle is a natural energy to think about the planets beyond. Such element of a lunar program. Initial demands on thinking flies in the face of history. Opening a performance and launch rate are not as high as frontier does not induce inertia. On the con- in a Mars program. These conditions provide a trary, access to the frontier inspires creativity natural training ground for operations personnel and destroys old ways of thinking in the gener- and management and a chance to incorporate ation that is raised on its threshold. evolutionary improvements in the first model My answer is that human exploration of of the launch vehicle. Maintaining, refueling the Moon will accelerate human exploration of and refurbishing orbit-transfer vehicles on orbit Mars. provides the experience from which to build a professional and competent operations team for Conclusions future assembly of Mars spacecraft. Dealing with the institutional barriers to While an immediate program to land hu- human space exploration requires a different mans on Mars is technically feasible, the large kind of innovation which is not automatically advances in operational and technical capabil- supplied by lunar exploration. However, be- ity required present a significant risk for pro- cause the Moon is close to the Earth and because gram failure. An immediate commitment to it is possible to launch small payloads there piloted missions to Mars runs the risk of re- with relatively small rockets, the opportunity

8 visiting the fate of Apollo, where a crash program created by the political system was cancelled by the same system when the effort seemed no longer relevant. In the process of human exploration of the solar system, the establishment of a permanent presence on the Moon is not a diversion or an impediment. It is a necessary step in the steady progress of technology, operational experience, and the understanding of human capabilities in space. A lunar program provides an opportunity to build up space capability in an evolutionary and orderly way and to broaden the participation of the educational system in the excitement of space exploration.

References

1. Ride, Sally K. (1987) Leadership and America's Future in Space. A report to the Administrator of NASA, 63pp.

2. Cohen, Aaron (1989) Report of 90-Day Study on Human Exploration of the Moon and Mars. NASA Johnson Space Center, 143pp.

3. Woodcock, Gordon (1988) Modular Evolu- tionary System Architectures for Explo- ration Missions. Paper delivered at the Second Symposium on Lunar Bases and Space Activities of the 21st Century, Hous- ton, TX, April, 1988

4. Hoffman, Stephen J. (1991) Mission Design for Piloted Mars Missions. Lecture delivered at the International Space University, Toulouse, France, July, 1991. Based on work done in support of NASA/JSC Lunar and Mars Exploration Program Office.

5. de Selding, Peter (1991) Amplifiers Blamed for TDF-2 Failure. News Item in Space News , Vol. 2, No. 5, p. 9, February 18-24, 1991.

6. Mendell, W. W. (1991) A Proposal for an Early SEI Milestone: A Simple Automated Lunar Surface Telescope with an Emphasis on Educational Objectives. Paper accepted for Space92 Conference, Denver, Colorado, May 31 - June 4, 1992.