Scientific Opportunities in the Human Exploration of Space (1994)

Given the scientific goals of space science and the relative capabilities of robots and humans, CHEX has identified two areas in which human presence can enhance important scientific opportunities: (1) field studies of planetary surfaces and (2) the construction and maintenance of large and/or complex scientific instruments. Both of these areas can benefit from human cognitive abilities and from the flexibility provided by in situ or proximate human presence. Additional scientific opportunities arise in the study of the physiological response of living organisms to microgravity and fractional gravity environments and in studies of human behavior during protracted sequestration and other stressful situations. Moreover, technology developed for a human exploration program may enable unrelated robotic space science missions.

Field work, a collection of activities in which processes and materials are studied in their natural setting, is intrinsic to several natural sciences, especially geology and biology. Humans bring unique capabilities to field studies: discovery and response accommodate the unexpected and allow the opportunity to redesign an approach. Human presence allows real-time testing of hypotheses using techniques ranging from simple manipulation to conducting a well-designed in situ experiment. Initiative and inductive and deductive thinking are uniquely human capabilities. People innovate and

anticipate; their thought processes allow them to distinguish the trivial from the important. Humans are capable of intuitive leaps based on incomplete information. Such an ability enables us to sort out logical from illogical or contradictory information. Humans experienced in field studies can synthesize diverse and disparate field observations, thereby expanding the opportunity for further discovery.

The value of human presence in conducting field work will depend on the inclusion in crews of experienced scientists with relevant scientific judgment and intuition. Their participation is, however, insufficient if they are not given the opportunity to perform as scientists. For example, the plans, procedures, and schedules of geological traverses must be sufficiently flexible to allow scientist-astronauts to modify sampling procedures, time on site, traverse routes, and so on, on the basis of their real-time assessment of in situ observations. To restrict this flexibility is to relegate the scientist-astronaut to the role of a human robot controlled from Earth.

The discussion of the advantages of human presence in planetary exploration is not theoretical: it has been demonstrated on the Apollo lunar missions. ¹ Twelve astronauts, in six missions of increasing complexity, conducted tasks ranging from surface sample collection, with associated observations and photographic documentation of the geological context, to drilling and coring of the regolith, to emplacement of geophysical instruments. Photographic documentation of the sample sites proved invaluable in the interpretation of analyses of the returned samples. The astronauts, despite being encumbered by the spacesuits, proved adept at dealing with unforeseen problems such as repairing their roving vehicle and wrestling stuck drill bits and core tubes out of the ground. The geological training of the crews and the (relayed) interaction with the science teams in the Houston “back room” were sufficiently good to prove that excellent science can be accomplished in human exploration. Although the last Apollo mission included a scientist, many of the potential advantages of his presence were negated by the short duration of the mission and its rigid timeline.

As illustrative examples of human exploration activities, four diverse applications are examined that are particularly enhanced by the techniques of field investigation. In no particular order, these are the study of the lunar regolith as a probe of solar history, the search for martian fossil and extant life, determination of the meteorite bombardment history of the inner solar system, and the study of martian climate history. It can obviously be argued that, in theory, any of the discussed field activities could be accomplished robotically given sufficient advances in robotics and an adequate budget. That possibility is not examined here; CHEX's sole purpose is to look at the more useful activities that human explorers might conduct given their presence on the Moon or Mars for reasons other than science.

The committee hastens to note that it does not expect that a few mis-

sions or so will provide sufficient data to yield final, definitive answers to the scientific problems addressed by the examples of field activities mentioned below. Field experience on Earth relevant to determining climate history and to the origin of life and Apollo experience pertaining to solar emission history and to deciphering cratering flux demonstrate the complexity as well as the potential of the challenge.

Knowledge of long-term variations in the properties of the solar wind and solar energetic particles could provide important clues about the evolution of the Sun and the role of the solar wind in the formation and early development of the solar system.² Because solar wind particles impinge on and are implanted in the Moon's regolith, it may be possible to measure these variations by analysis of carefully selected lunar samples with a known geological context. This selection entails establishing the age of a given subunit. We must understand the early growth, formational dynamics, and continued evolution of the regolith through time. Thus, this activity is a field study problem in both geology and solar physics.

Study of the early growth and formation of the regolith is best accomplished by a two-pronged approach. First, excavations into the regolith should be studied to provide detailed geological information on its three-dimensional structure. At mare sites, it should be possible to excavate (in trenches or pits) and/or core down to the local lava flow bedrock (at depths of 5 to 8 meters). In such a manner, researchers could study regolith-bedrock contacts and learn about the earliest stages of regolith growth, an area that is poorly understood.

Second, study of the incipient growth of regolith on fresh bedrock surfaces on the Moon (for example, melt sheets of large fresh craters) would provide data for making inferences about stages of early growth exposed in regolith-bedrock contacts elsewhere on the Moon. ³, ⁴

Both of these studies require detailed field work, not only to collect samples intelligently, but also to make the observations and synthesize the visual clues needed to understand regolith growth dynamics. Outcrops of bedrock, such as those discovered on the wall of Hadley Rille by the Apollo 15 astronauts, are logical sites to begin such explorations.

To obtain “snapshots” of the solar particle output in ancient times, we need to find ancient regoliths on the Moon. Such fossil regoliths might be found sandwiched between lava flows of radiometrically determinable age. Locating such deposits and selecting unaltered or minimally altered samples for laboratory analysis (to measure the chemical and isotopic properties of these precisely controlled samples) are complex tasks requiring field study. Data from a variety of sites will constitute a set of solar wind “index fos-

sils,” that is, detailed measurements of the chemical and isotopic properties of the Sun at precisely defined intervals in the geological past. These can then be used to interpret and understand the solar record preserved in the regolith all over the Moon. Such knowledge will enable scientists to better interpret the solar record at regolith trenches and pits that may be excavated at other sites on the Moon, for example, during the construction of an underground habitat or the emplacement of instruments.

The search for potential fossil and extant life on Mars, however low the probability for its existence is thought to be, continues to be a substantial goal of Mars exploration. ⁵, ⁶ Detailed field studies will be required for this search, using robots initially but with increasing proximate human participation as the capability develops. Indeed, the robotic search for evidence of life on Mars began with the Viking landers in 1976.

The identification of sites to be analyzed for traces of life will require both extensive and intensive studies. These will include preliminary sampling by machines and, probably, robotic sample return to Earth. Even on Earth, however, the environments occupied by organisms are diverse and not necessarily obvious: there are organisms that thrive or survive within rock surfaces, in association with thermal vents and hot springs, at ice-water interfaces, and in liquid inclusions in salt deposits.⁷ Proper site selection therefore may be motivated to a considerable extent by subtle idiosyncrasies: a crust within a sediment bed, a discoloration on ice or rock, or a boundary film between permafrost and regolith. Site selection will require subjective decisions based on astute observations of the specific locale, probably requiring a trained field observer.

Additionally, access to important sites may require the versatility of human workers. For instance, complex maneuvers will be required to reach sites in the polar ice caps or in the canyons of Valles Marineris, and coring or drilling may be required to reach ice-regolith interfaces or geothermal zones.

Evidence for past or present life on Mars will probably be sought in at least three ways: macroscopic and microscopic imaging, isotopic and chemical analysis, and culturing suspected life forms. Imaging procedures are capable of detecting macroscopic remains (such as stromatolites) and microscopic fossils. Because of the unique character of biomolecules, chemical methods are by far the most sensitive methods available to identify life, past or present. Isotopic analysis of carbon-bearing (e.g., organics, carbonate) or inorganic (e.g., sulfur) deposits can provide evidence for life because biochemical reactions create distinct isotopic fractionations.

Each of these analytical methods requires highly sophisticated sample

processing and instrumentation. Imaging to search for microfossils will require sample preparation and electron microscopy. Chemical analyses will require chromatographic separations and mass spectrometry. Isotopic analyses will rely on chemical processing and high-resolution mass spectrometry. Initially, samples should be returned to Earth for analysis. However, the subsequent search for life will probably require iterative field study and in situ analysis because of the need for rapid feedback between analysis and further sampling.

The sophistication of the analytical methods and the variability of sample types that must be anticipated weigh against full automation of such analyses in the foreseeable future; human field workers/laboratory technicians will be required. On the other hand, this required analytical sophistication and complexity could argue for continued sample return. CHEX anticipates that trade-offs between in situ analysis and sample return will have to be made on the basis of further experience with martian materials and development of microanalytical techniques.

Through the study of impact history, the geological time scale for the formation of the surface units of the terrestrial planets can be reconstructed.⁸ This process involves understanding the flux history of impacting bodies and then using such knowledge to convert relative ages determined by the density of impact craters into the estimates of absolute age required to address such topics as geological evolution and biological history.⁹

Determining the history of the cratering bombardment flux for the planets is, in practice, difficult. It involves obtaining samples appropriate for isotopic age dating from a variety of geological settings and locations; one must be able to unambiguously relate such samples to geological features of known relative age. For the latest stages of planetary evolution on both the Moon and Mars, there exists a variety of volcanic plains, from which “grab” samples are likely to yield lava crystallization ages appropriate to interpret as extrusion ages for the flows. Thus absolute ages for large tracts of planetary surfaces can be determined rather directly. Selecting a variety of grab samples is easily accomplished through robotic means and was, in fact, accomplished on the Moon in the 1970s by the Luna 16, 20, and 24 missions of the former Soviet Union.

In the earliest phases of planetary history, most geological units consist of crater deposits. In contrast to determining the absolute ages of lava plains, the dating of impact features is rather difficult. The only samples appropriate for dating large impact craters are relatively clast-free samples of impact melt, which typically constitute a few percent of the ejecta in cratering events. Although traces of a crater impact melt sheet can be

recognized remotely and robotic missions can retrieve samples from such locales, it is not certain that such samples will be appropriate for radiometric dating. Even if such samples yield analytically good ages, their interpretation and relation to the age of the impact crater remain problematical. The careful collection of geologically controlled samples for dating impact craters is a difficult and complex problem and can be aided by human decisions and interactions.

Extensive channel systems on Mars suggest a warmer and wetter climate in the past. Layered deposits visible in the polar ice caps may have preserved a unique record of climate swings that occurred over the last few hundred million years.¹⁰ Portions of this record may be recovered by drilling into the “sediments” and ice and analyzing the core samples. The two major causes of climate variations are thought to be martian orbital effects and temporal changes in solar irradiance. Because orbital effects have periods in the range of 10⁵ to 10⁶ years, their signal might be determined by studying a statistically significant number of cycles. After extraction of the signal due to orbital effects, the remaining variations might reveal the solar effects. Comparison with similar terrestrial data may verify a common external forcing function for global climatic changes in planetary atmospheres. The martian atmosphere is in many ways a simpler system than the terrestrial atmosphere because of the absence of a biosphere and massive oceans. Sorting out orbital from solar effects on climate may therefore be done more easily for martian samples than for terrestrial ones.

Human participation in these experiments would have two advantages: human judgment is needed to locate the best sites for drilling, and the number of samples would probably be so large that it would be best to conduct at least some of the chemical, isotopic, and mineralogical analyses in situ rather than after return to Earth.

The use of the Moon as a platform for continuing studies of the planets, the Sun, other astronomical objects, and cosmic rays is an intriguing possibility. Although many instruments could be emplaced robotically, improved results could come from human interaction through more accurate positioning and troubleshooting capability. In addition, larger and more complex instruments conceivably could be constructed with human intervention. To some extent, having humans nearby could expedite maintenance and repair of broken equipment.

For probing the properties and environments of the Moon and Mars, instruments such as seismometers and meteorological stations will be necessary. While rudimentary facilities can be deployed globally by robotic probes, careful emplacement and attendance of advanced instruments at a few sites by humans may enable more sophisticated measurements with greater accuracy and precision. For example, placing a seismometer squarely on bedrock provides good coupling to the planet and improves the quality of the data dramatically over its emplacement on loose rubble. In fact, humans have significant experience emplacing seismometers, including on the Moon and, via robot surrogates, on the ocean floor and Mars. With the establishment of martian meteorology stations, significantly advanced instrumentation could be emplaced by humans, including tall towers or active sounders such as lidars, which could profile the atmosphere in considerable detail.

The surface of the Moon represents, potentially, an excellent platform for selected astronomical studies.¹¹, ¹² The lack of any appreciable atmosphere allows distortion-free images and complete spectral coverage. Sites shielded from direct sunlight can use passively cooled infrared detectors, obviating the need for expendable cryogens. Early missions to the Moon could carry small telescopes, which could be emplaced robotically. However, studies have indicated that fully assembled telescopes with apertures of the order of 1 to 2 m are the largest that could be deployed on the Moon in the initial phases of a lunar exploration program.¹³ Larger telescopes would require assembly in place, most likely with on-site human assistance.

Several examples of the types of astronomical observations CHEX believes to be appropriate for a lunar observatory are noted below. However, the committee cautions that there has not yet been an independent, systematic analysis of how one should plan for astronomical or space physics observations in conjunction with a program of human exploration. Indeed, studies sponsored by proponents look at the Moon essentially in isolation from alternative ways (for example, in Earth orbit or ground-based) of conducting the desired observations. ¹⁴, ¹⁵ The report of the Synthesis Group, for example, discusses the possibility of establishing a magnetospheric observatory on the Moon.¹⁶ However, spacecraft in other orbits around Earth might be far superior platforms for studies that use remote sensing techniques to study the global properties of the magnetosphere. Others have suggested that astronauts on the Moon set up and maintain an observatory for monitoring variations in the composition of the solar wind. Although the lunar surface is a good place to study the solar wind's long-term, integrated composition, experience from the Apollo program shows that local magnetic fields complicate and invalidate the study of any short-term variations from the lunar surface. Although much was learned about the solar wind from analysis of samples collected in aluminum foils deployed by the

Apollo astronauts, future studies of the solar wind's composition using collection techniques would be better performed from a free-flying spacecraft that can face the Sun at all times. ¹⁷

The Astronomy and Astrophysics Survey Committee addressed lunar-based astronomy in its chapter, “Astronomy and the Space Exploration Initiative. ”¹⁸ It recognized the potential to conduct some first-rate astronomy from the Moon, at the same time pointing out potential disadvantages as well as unknowns about the lunar environment that must be ascertained before one can properly evaluate the possibilities. As is true with planetary science, any program of lunar-based astronomy must be constructed in the context of a vigorous and comprehensive astronomy program with Earth-based and free-flying components.

The European Space Agency's recent Phase-1 study of science on and from the Moon also found specific opportunities for astronomical observations, especially interferometry. However, it too urged a conservative approach and recommended a set of further studies. ¹⁹

CHEX endorses the findings of the Astronomy and Astrophysics Survey Committee report on the next decade in astronomy,²⁰ which called for an evolutionary approach to lunar astronomy, one that complements the Earth-orbiting and ground-based astronomy program. It urged that such a step-by-step approach incorporate a comparative analysis of different opportunities, assessment of the lunar environment, initiation of advanced technology and instrument development (both, as has already been mentioned, considerably underfunded in current NASA programs), and progressive use of certain new techniques first on Earth, then in Earth orbit, and finally on the Moon. The Survey Committee advocated early initiation of a suitable small automated lunar astronomy mission as a reasonable way to start.²¹

A major objective that can be addressed from the Moon is the detection and characterization of planetary systems around other stars.²² This goal was endorsed by the Astronomy and Astrophysics Survey Committee²³ and in a recent National Aeronautics and Space Administration report. ²⁴ A particularly powerful tool for such a search is a large optical or infrared interferometer. One approach is to use an array of five 1.5-m passively cooled telescopes that could be individually soft-landed on the Moon and put into operation with limited human intervention for observations in the 0.2- to 5-micron range.²⁵

The Moon is potentially superior to Earth orbit for such a device because its gravity and solid surface (free from seismic disturbances) can stabilize interferometer baselines without the complex metrology and continuous station-keeping needed with free-flying telescopes. Proposals to

use humans to construct and align such a large interferometer recognize the difficulty in trying to do so robotically.

The energy spectrum of galactic cosmic rays is known to have a change in slope, or a knee, between 10¹⁵ and 10¹⁶ electron volts (eV).²⁶ Possible explanations for the knee include a decrease in the effectiveness of acceleration of particles by shocks or an increase in the leakage of the more energetic particles out of the galaxy. To distinguish between these and other possibilities, researchers need to know the variation of elemental abundances of the cosmic-ray particles both above and below the knee.²⁷ There are, however, no direct composition measurements near the knee, and estimates of the composition range from pure hydrogen to pure iron. A lunar site would be highly suitable for an experiment designed to make such measurements, the so-called High Energy Abundance Project (HEAP).²⁸

The Moon is ideal because it has no atmosphere and the heaviest part of HEAP, more than 150 metric tons of inert absorbing material, could consist of lunar soil. These measurements are not possible from Earth because of the atmosphere, nor are they practical in Earth orbit because of the cost of transporting that necessary amount of material into space. The 4-m cube of layered detectors and soil is perhaps most easily constructed by robotically assisted humans rather than robots, and humans would probably need to perform occasional maintenance.

The study of high-energy processes both in the Sun and in cosmic sources requires subarc second imaging in corresponding high-energy emissions such as hard x rays and gamma rays. ²⁹ At such energies, imaging by conventional techniques (such as mirrors and lenses) is not possible. The emissions can, however, be imaged using “pinhole-camera methods” such as coded aperture masks and pairs of parallel-slit grids, which produce a Moiré fringe pattern in the detector plane.³⁰ The requirement for a sufficiently large field of view sets lower limits on the characteristic dimension of the apertures (be they pinholes or slits), and in turn the angular resolution requirement sets a lower limit to the separation of the grid pairs or masks.

Instrumentation of this type with modest collecting area and angular resolution down to a few arc seconds has been considered for use in Earth orbit around the turn of the century. Advanced, second-generation (subarc second) instruments of this genre would require accurate and stable positioning of apertures some hundreds of meters apart, an apparent impracti-

cality for orbiting structures. Such a goal might, however, be met by a large lunar-based structure, one that would be extremely stable to both translational and torsional deformation. On-site engineers might be required to construct such a structure to the necessary tolerances and to conduct maintenance operations such as realignment of apertures.

One of the more important physical features that influenced the evolution of life on Earth, and which places constraints on the development and functioning of all living organisms, is gravity. Once the factor of gravity is removed from the environment, living systems are altered, and the study of such alterations may lead to new insights into life processes.

The space life sciences are still in their infancy, and there have been few opportunities to carry out well-controlled experiments on living organisms in space. Thus it is not yet possible to predict how prolonged exposure to near-zero or fractional gravity will alter living systems. However, sufficient information is available to know that the absence of normal gravity profoundly alters living systems; thus exploration missions to the Moon and Mars will offer additional opportunity beyond Earth-orbiting space stations, to investigate the fundamental biological processes by which gravity affects living organisms.³¹

Missions to the Moon and Mars will also provide an opportunity for behavioral studies on crews under highly stressful conditions as well as over prolonged periods of time in close confinement. Such research would build on more than three decades of experience of human behavior and performance gathered from overwintering personnel at polar research stations. However, behavioral studies of the crews at a lunar outpost or on a Mars mission will provide new insights into human behavior because no polar base or even space station environment can duplicate all the conditions astronauts would experience on extended mission in deep space. ³² In the case of Mars, additional stress will result from the absence of any ready means of escape.

Both the gravitational biology and the behavioral studies are truly opportunistic; they are not now currently of high scientific priority in the life sciences community absent a program of human space exploration.

The technology developments needed for successful exploration of the Moon and Mars are numerous and are spread throughout many disciplines. For example, a recent study identified 14 relevant areas of technology de-

velopment.³³ Some of the general benefits to scientific investigations of two of these areas—spacesuits and telerobotics—are discussed above.

Some technology developments could enable robotic space-science missions unrelated to Moon/Mars exploration. For example, nuclear electric propulsion could enable several high-priority missions in heliospheric physics. Principal among these is the so-called interstellar probe. ³⁴ This mission would penetrate a significant distance beyond the heliopause to provide the first comprehensive in situ studies of the plasma, energetic particles, cosmic rays, magnetic fields, gas, and dust in interstellar space. An advanced propulsion system is required to send a spacecraft 250 astronomical units from the Sun in significantly less than the 25 years or more required by conventional propulsion aided by gravity assists. Once such an advanced propulsion system is available, it could also be used for other high-energy missions, such as to propel instruments to large distances above the solar poles or into a short-period, circular solar polar orbit, and, perhaps, even a short-period eccentric orbit that skims through the solar corona at altitudes as low as three solar radii.³⁵

CHEX has given considered thought to how space science might benefit from the existence of a program of human exploration of the Moon and Mars, undertaken primarily for reasons other than science. History tells us that no matter when such a program is undertaken, a major activity will be scientific research. Indeed, CHEX concludes that there will be opportunities offering the potential for significantly enhancing our understanding of the Moon and Mars and for using them selectively as observation platforms. CHEX thus foresees a productive scientific role for human explorers as well as for continuing and enhanced robotic missions. The obvious conclusion is that scientists must participate in any eventual program of human exploration, although the question of how best to involve them must still be answered.

Scientists' past experiences with piloted spaceflight have been both good and bad. We can learn much from those (particularly the Apollo program) in terms of how NASA should approach science management and the involvement of scientists in a program of human exploration. That topic is under study and will be the subject of the third CHEX report. It is already clear to the committee, however, that scientists must be intimately involved in every stage of the endeavor and contribute to success by assuring that quality science is accomplished, that the science supported takes the best advantage of human presence, and that the resources available to the whole of space science are competitively allocated.

1. William David Compton, Where No Man Has Gone Before, A History of the Apollo Lunar Exploration Missions, The NASA History Series, NASA SP-4214, NASA, Washington, D.C., 1989.

2. NASA, A Planetary Science Strategy for the Moon, JSC-25920, Lunar Exploration Science Working Group, Johnson Space Center, Houston, Texas, July 1992.

3. NASA, Geosciences and a Lunar Base: A Comprehensive Plan for Lunar Exploration , NASA Conference Publication 3070, NASA, Washington, D.C., 1990.

4. NASA, A Planetary Science Strategy for the Moon, JSC-25920, Lunar Exploration Science Working Group, Johnson Space Center, Houston, Texas, July 1992, page 8.

5. Space Studies Board, The Search for Life's Origins: Progress and Future Directions in Planetary Biology and Chemical Evolution, National Academy Press, Washington, D.C., 1990, page 8.

6. Space Studies Board, 1990 Update to Strategy for the Exploration of the Inner Planets, National Academy Press, Washington, D.C., 1990, page 24.

7. Space Studies Board, Biological Contamination of Mars: Issues and Recommendations, National Academy Press, Washington, D.C., 1992, Chapter 4.

8. Space Studies Board, Strategy for Exploration of the Inner Planets: 1977-1987, National Academy of Sciences, , Washington, D.C., 1978, page 71.

9. NASA, A Planetary Science Strategy for the Moon, JSC-25920, Lunar Exploration Science Working Group, Johnson Space Center, Houston, Texas, July 1992, page 6.

10. Space Studies Board, Space Science in the Twenty-First Century: Imperatives for the Decades 1995 to 2015—Planetary and Lunar Exploration, National Academy Press, Washington, D.C., 1988, page 101.

11. Y. Kondo(ed.), Observatories in Earth Orbit and Beyond, Proceedings of the 123rd Colloquium of the International Astronomical Union, Greenbelt, Maryland, April 24-27, 1990, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1990.

12. Astronomy and Astrophysics Survey Committee, The Decade of Discovery in Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1991, Chapter 6.

13. Synthesis Group, America at the Threshold, Report of the Synthesis Group on America's Space Exploration Initiative, U.S. Government Printing Office, Washington, D.C., 1991, page A-24.

14. NASA, Future Astronomical Observatories on the Moon, NASA Conference Publication 2489, NASA, Washington, D.C., 1988.

15. Michael J. Mumma and Harlan J. Smith(eds.), Astrophysics from the Moon, AIP Conference Proceedings 207, American Institute of Physics, New York, 1990.

16. Synthesis Group, America at the Threshold, Report of the Synthesis Group on America's Space Exploration Initiative, U.S. Government Printing Office, Washington, D.C., 1991, page A-26.

17. Space Studies Board, A Strategy for the Explorer Program for Solar and Space Physics. National Academy Press, Washington, D.C., 1984, pages 29-30.

18. Astronomy and Astrophysics Survey Committee, The Decade of Discovery in Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1991, Chapter 6.

19. European Space Agency, Mission to the Moon: Europe's Priorities for the Scientific Exploration and Utilization of the Moon, Report of the Lunar Study Steering Group, ESA SP-1150, European Space Agency, Noordwijk, The Netherlands, June 1992.

20. Astronomy and Astrophysics Survey Committee, The Decade of Discovery in Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1991.

21. Astronomy and Astrophysics Survey Committee, The Decade of Discovery in Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1991, page 108.

22. Bernard F. Burke, “Astrophysics from the Moon,”, Science, 250, December 7, 1990, page 1365.

23. Astronomy and Astrophysics Survey Committee, The Decade of Discovery in Astronomy and Astrophysics, National Academy Press , Washington, D.C., 1991, page 104.

24. NASA, TOPS: Toward Other Planetary Systems, A report by the Solar System Exploration Division, NASA, Washington, D.C., 1992.

25. Astronomy and Astrophysics Survey Committee, The Decade of Discovery in Astronomy and Astrophysics, National Academy Press , Washington, D.C., 1991, page 104.

26. Space Studies Board, Space Science in the Twenty-First Century: Imperatives for the Decades 1995-2015—Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1988, page 31.

27. Space Studies Board, Assessment of Programs in Solar and Space Physics 1991, National Academy Press, Washington, D.C., 1991, page 14.

28. Michael L. Cherry, “Particle Astrophysics and Cosmic Ray Studies from a Lunar Base,” , Astrophysics from the Moon, Michael J. Mumma and Harlan J. Smith(eds.), AIP Conference Proceedings 207, American Institute of Physics, New York, 1990, page 593.

29. Laurence E. Peterson, “High Energy Astrophysics from the Moon,” , Astrophysics from the Moon, Michael J. Mumma and Harlan J. Smith(eds.) AIP Conference Proceedings 207, American Institute of Physics, New York, 1990, page 345.

30. Paul Gorenstein, “High-Energy Astronomy from a Lunar Base,” , Future Astronomical Observatories on the Moon, NASA Conference Publication 2489, NASA, Washington, D.C., 1988, page 45.

31. Space Studies Board, Assessment of Programs in Space Biology and Medicine 1991, National Academy Press, Washington, D.C., 1991.

32. Space Studies Board, Assessment of Programs in Space Biology and Medicine 1991, National Academy Press, Washington, D.C., 1991, Chapter 4.

33. Synthesis Group, America at the Threshold, Report of the Synthesis Group on America's Space Exploration Initiative, U.S. Government Printing Office, Washington, D.C., 1991, page 83.

34. Space Studies Board, Space Science in the Twenty-First Century: Imperatives for the Decades 1995 to 2015—Solar and Space Physics, National Academy Press, Washington, D.C., 1988.

35. Space Studies Board, Assessment of Programs in Space Biology and Medicine 1991, National Academy Press, Washington, D.C., 1991, Chapter 4.