ASTROBIOLOGY Volume 10, Number 6, 2010 Forum Article ª Mary Ann Liebert, Inc

ASTROBIOLOGY Volume 10, Number 6, 2010 Forum Article ª Mary Ann Liebert, Inc. DOI: 10.1089/ast.2010.0476

Astrobiological Beneﬁts of Human Space Exploration

Ian A. Crawford

Abstract

An ambitious program of human space exploration, such as that envisaged in the Global Exploration Strategy and considered in the Augustine Commission report, will help advance the core aims of astrobiology in multiple ways. In particular, a human exploration program will confer significant benefits in the following areas: (i) the exploitation of the lunar geological record to elucidate conditions on early Earth; (ii) the detailed study of near-Earth objects for clues relating to the formation of the Solar System; (iii) the search for evidence of past or present life on Mars; (iv) the provision of a heavy-lift launch capacity that will facilitate exploration of the outer Solar System; and (v) the construction and maintenance of sophisticated space-based astronomical tools for the study of extrasolar planetary systems. In all these areas a human presence in space, and especially on planetary surfaces, will yield a net scientific benefit over what can plausibly be achieved by autonomous robotic systems. A number of policy implications follow from these conclusions, which are also briefly considered. Key Words: Astrobiology—Space exploration—Human spaceflight. Astrobiology 10, 577–587.

An astrobiology that truly meets the challenge of its name cannot remain Earthbound. It requires exploration, and the entire universe beckons —Andrew H. Knoll (2003)

1. Introduction renewed, and greatly expanded, program of human exploration beyond low Earth orbit (LEO). strobiology is usually defined as the study of the Now is an appropriate time to consider this question be- Aorigin, evolution, distribution, and future of life in the cause governments and space agencies are currently re- Universe. As such, it is intimately connected with space ex- viewing their plans for human space exploration. Some ploration, because it is only through exploration of the uni- context for this activity is provided by the Global Exploration verse around us that we will be able to determine the Strategy (GES), which was agreed upon by 14 of the world’s existence, prevalence, and nature of life beyond our own space agencies in May 2007 and, to quote from its founding planet. Moreover, regardless of whether life is common else- document, ‘‘elaborates a vision for globally coordinated where in the Universe, space technology has the potential to space exploration focused on solar system destinations provide a means by which to disseminate life outward from where humans will someday live and work’’ (GES 2007). The Earth and, as such, play a significant role in the future of life in GES was itself largely stimulated by the US Vision for Space the Universe. The question remains, however, as to the most Exploration (NASA, 2004), the objectives and implementation appropriate form of space exploration from an astrobiological of which have recently been reviewed by the Augustine perspective, especially for the nearer-term objectives of as- Commission (2009). The situation regarding US human sessing the extent and history of life in the Solar System. spaceflight policy is now somewhat uncertain, following the Currently, this is undertaken by robotic spacecraft, which, cancellation of NASA’s Constellation program in the Ad- over the last five decades, have very successfully conducted ministration’s Fiscal Year 2011 budget request. In part, this an initial reconnaissance of most of the planets and larger reflects a less-than-ringing endorsement of the Constellation moons of the Solar System and somewhat deeper studies of architecture by the Augustine Commission, although the several of these objects. The question for future exploration is Commission did largely endorse the overall objectives of the whether a continued reliance of robotic space probes will be Vision for Space Exploration and confirmed that scientific and sufficient to meet the core objectives of astrobiology or whe- other benefits would result from human exploration beyond ther the search for life in the Universe would benefit from a LEO. Thus, although the pace may be slower than originally

Department of Earth and Planetary Sciences, Birkbeck College, University of London, London, United Kingdom.

577 578 CRAWFORD intended, it still appears likely that US human spaceflight that, because the astronauts must return to Earth, a large activities will be directed beyond LEO in the coming decades quantity of geological samples can be returned with and that this will encourage wider international participation them. The Apollo haul alone was 382 kg, which com- (as envisaged by the GES and specifically recommended by prised more than 2000 discrete samples (Heiken et al., the Augustine Commission). In Europe, partly in response to 1991); nothing comparable has been, or is likely to be, these developments, efforts are underway to define a broad, achieved robotically. predominantly science-driven, program for the robotic and Human missions will facilitate the landing, operation, human exploration of the Solar System (Worms et al., 2009), and maintenance of more massive and complex scien- and the very existence of the GES indicates that other space- tific equipment than is likely to be feasible robotically. faring nations are also considering the extent to which they Because human missions, by their very nature, must could contribute to such a program. land a significant amount of mass on planetary surfaces, Given that the future objectives of human space explora- the additional marginal cost of landing massive or bulky tion are currently uncertain, it is important that scientists scientific equipment is relatively modest (as demon- take this opportunity to inform the policy debate by identi- strated by the range of equipment deployed by the fying those exploration scenarios that will yield the greatest Apollo missions; Heiken et al., 1991). Moreover, human scientific return. In this spirit, I here present the case that beings are uniquely capable of maintaining and ‘‘trou- significant scientific, and specifically astrobiological, benefits bleshooting’’ problems with complex equipment (of will result from an enhanced international program of hu- which the five successful repair and upgrade missions to man exploration of the inner Solar System. the Hubble Space Telescope provide the best examples to date; NRC, 2005). A particular example relevant to 2. Benefits of Human Exploration future planetary exploration concerns drilling, which will have important astrobiological applications on both While some of the exploratory objectives of astrobiology the Moon and Mars. In this context, Zacny et al. (2008) can undoubtedly be met by suitably instrumented robotic noted that ‘‘in the era of human exploration, sufficient probes, others would be greatly facilitated by a human mass and real-time supervision should be available to presence, and some may be wholly impractical otherwise. carry out truly deep penetration of the subsurface of Specifically, human planetary exploration would have the extraterrestrial bodies.’’ following scientific advantages over robotic missions: Last, but not least, the infrastructure developed to sup- It would enable rapid on-the-spot decision making and port human space exploration, especially the develop- prioritization of exploration activities, including more ment of a heavy-lift launch capability, would have many

intelligent and efficient collection of samples from a other scientific applications. Examples of relevance to more diverse range of localities, and over wider geo- astrobiology include sophisticated robotic probes to the graphic areas, than is likely to be practical with the use outer Solar System and the construction of large space- of robots alone (e.g., Spudis, 1992, 2001; Garvin, 2004; based telescopes for the study of extrasolar planetary Snook et al., 2007). The Apollo experience demonstrated systems (NRC, 2009). that astronauts, when suitably equipped with the means In the following sections, these scientific benefits of human of surface mobility, are very efficient at this task (e.g., space exploration as applied to astrobiological objectives are Heiken et al., 1991). Garvin (2004) presented a detailed expanded upon. comparison of the relevant skills and abilities of humans and robots as explorers of planetary surfaces and found 3. Low Earth Orbit that humans out-perform robots according to most of the criteria considered. This conclusion is corroborated Currently, human space activities are concentrated on the by direct field comparisons of human and robotic ex- completion and utilization of the International Space Station ploration at planetary analog sites on Earth—reporting (ISS). Confined to LEO, this activity does not amount to one such study, Snook et al. (2007 p 438) found that exploration in the spatial or geographic sense. Nevertheless, ‘‘humans could be 1–2 orders of magnitude more pro- the ISS, like other orbital facilities before it, does provide a ductive per unit time in exploration than future terres- platform for the scientific exploration of the unique micro- trially controlled robots.’’ gravity and radiation environments of near-Earth space. The Astronauts may be expected to make discoveries that principal scientific rationales for research in this environment would be overlooked by robots, owing to the uniquely lie in the areas of materials science (reviewed by Ratke, 2006), human ability to recognize new observations or phenom- fundamental physics (Dittus, 2006), and the life sciences ena to be of importance even if not anticipated in advance (Gerzer et al., 2006). Aspects of particular relevance to as- (e.g., Cockell, 2004). The history of exploration on our own trobiology belong mainly to the latter area and may broadly planet indicates that serendipitous discoveries are often be grouped into two main areas, as follows. among the most important, and the exploration of other 3.1. Biological studies of the adaptability planets is unlikely to be any different in this respect. and survivability of organisms in the space environment Perhaps most importantly, human missions to other planetary bodies will permit the return to Earth of a Astrobiologically relevant experiments in LEO were re- much larger, and more diverse, quantity of samples for viewed by Gerzer et al. (2006) and Baglioni et al. (2007). Of detailed analysis in terrestrial laboratories than is likely particular interest are studies of the survival of terrestrial to be achieved robotically. One of the major, but often organisms to the radiation, vacuum, and temperature ex- unspoken, benefits of human planetary exploration is tremes of the space environment, because these studies help ASTROBIOLOGY AND HUMAN SPACE EXPLORATION 579 constrain models for the efficacy of lithopanspermia as a is still preserved on the Moon. From the point of view of means by which life can transfer between planets (e.g., astrobiology, characterization of the impact rate in the Earth- Mileikowsky et al., 2000; Horneck et al., 2002; Burchell, 2004). Moon system between 4.5 and 3.5 billion years ago is espe- Space-based platforms also permit unadulterated access to cially important because it defines the impact regime under the solar spectrum and thereby enable long-term studies of which life on Earth became established (e.g., Maher and Ste- the biological consequences of exposure to solar UV radia- venson, 1988; Sleep et al., 1989; Ryder, 2003; for an illustration tion. This is important for understanding the possible role of of this unresolved issue see Fig. 2.3 of NRC, 2007). Obtaining UV radiation in the synthesis of prebiotic molecules (e.g., definitive knowledge of the cratering rate during the first Bernstein et al., 2002), and the survival of life on planetary billion years of Earth-Moon history will require the collection surfaces that lack atmospheric UV protection (e.g., Cockell, and dating of lunar samples from areas with a much wider 2002). The European Space Agency’s Expose facility on the range of ages than was achieved by the Apollo missions. In ISS (Rabbow et al., 2009) will provide a powerful tool for the principle, this might be achieved with a number of robotic furtherance of these studies. sample return missions sent to well-chosen localities, but it would be greatly facilitated by the enhanced sample return 3.2. Studies of human physiology and medicine capabilities enabled by new human missions to the Moon. in preparation for exploration missions beyond LEO Studies of the long-term physiological effects of the space 4.2. Astrobiologically relevant records preserved environment will be required before humans can venture in lunar regolith deposits farther afield in the Solar System (e.g., Horneck et al., 2003). I The lunar regolith efficiently retains exogenously im- argue below that significant scientific benefits will follow planted volatiles (e.g., D.S. McKay et al., 1991). These are from the human exploration of the Moon, Mars, and near- mostly derived from the solar wind (e.g., Wieler et al., 1996; Earth asteroids, but our understanding of the long-term ef- Levine et al., 2007) but also include a record of galactic cos- fects of the space environment (especially the radiation, mic rays and, possibly, Earth’s atmosphere (e.g., Ozima et al., microgravity, and psychological aspects) is still not sufficient 2005, 2008). This is potentially a very rich record of consid- for us confidently to embark on the more ambitious of these erable relevance to astrobiology. The strength of the solar ventures. Intensive research, which realistically can only be wind in the first billion years of Solar System history is of performed on the ISS, will be required to develop effective particular interest, as it will provide an independent check of countermeasures to these effects (e.g., Freeman, 2000; Fong, luminosity of the early Sun during the period when life be- 2001; White and Averner, 2001; Horneck et al., 2003). Simi- came established on Earth (Whitmire et al., 1995). Further- larly, life sciences research on the ISS will be required in

more, the archive of galactic cosmic ray fluxes potentially order to design bioregenerative life-support systems (Hor- available in lunar soils may yield a record of high-energy neck et al., 2003) that will also be required to support human galactic events that may have influenced life in the Solar exploration beyond LEO. System. For example, the hypothesis that cyclical changes in For these reasons, it is in the interests of astrobiology that cosmic ray flux drive cycles of diversity of life on Earth the large international investment in the ISS continues to be (Medvedev and Melott, 2007) could be directly testable by utilized for scientific research, especially in the life sciences, examining the lunar record. It is also possible that passages even as we prepare for exploratory missions farther afield. of the Sun through dense interstellar clouds, and associated collapse of the heliosphere (which may also have had bio- 4. Lunar Exploration logical consequences; e.g., Smith and Scalo, 2009), may sim- Although the Moon has, almost certainly, never supported ilarly be recorded in ancient lunar regoliths. Finally, if any indigenous life of its own, the lunar geological record samples of Earth’s early atmosphere can be isolated from nevertheless contains much that is of interest to astrobiology. lunar soils (e.g., Ozima et al., 2008), they would help con- For the last 4.5 billion years the Moon has been orbiting the strain inferences about Earth’s atmospheric evolution that only planet on which life is actually known to exist, and its are currently inferred rather uncertainly from geological ancient, relatively accessible, near-surface environment has evidence alone. preserved a record of the changing astronomical environ- From the point of view of accessing ancient Solar System ment within which life appeared and evolved on Earth. history, it will be desirable to find layers of ancient regoliths Lunar exploration is also likely to play a major role in pre- ( paleoregoliths) that were formed and buried (and thus pro- paring for the exploration of Mars, which may also be ex- tected from more recent geological processes) billions of pected to yield astrobiological benefits (discussed in Section years ago (e.g., Spudis, 1996; Crawford et al., 2007; Fagents 5 below) and provide insights into planetary protection is- et al., 2010). This would be a very rich scientific record of sues. The astrobiological aspects of lunar exploration were clear astrobiological significance, but it will not be easy to reviewed in detail by Crawford (2006) and Gronstal et al. access. Although robotic sampling missions might, in prin- (2007), to which the interested reader is referred. Briefly, they ciple, be able to achieve these objectives at a limited number consist of the following elements. of favorable sites, fully sampling the lunar paleoregolith record will benefit from the enhanced mobility, flexibility, and sample return capacity of human missions (e.g., Garvin, 2004; 4.1. Characterization of the impact cratering rate Snook et al., 2007). Some of the operational requirements of a in the Earth-Moon system human lunar exploration architecture with the capacity to Although the Earth itself has lost any record of the impact meet these objectives have been elaborated elsewhere flux to which it was subjected early in its history, this record (Crawford et al., 2007). 580 CRAWFORD

4.3. Preservation of crustal samples from early Earth ology and medicine relevant to planning for the long- term human habitation of the Moon and exploration Armstrong et al. (2002) drew attention to the fact that the missions elsewhere in the Solar System. Moon may have collected meteorites blasted off early Earth Use of the lunar environment for panspermia experi- and other terrestrial planets. The recovery of such material ments and as a test bed for planetary protection proto- would provide an important window into the early history cols. This would be achieved by assessing the degree to of the Solar System, including possible information on the which terrestrial microorganisms, and especially spore- nature and prevalence of early life. For example, such sam- forming organisms, can tolerate exposure to the lunar ples might be able to test the indications from phylogenetic environment with varying degrees of protection. Of analyses that life arose before 4.1 Gyr (e.g., Battistuzzi et al., particular interest will be studies of the remaining bio- 2004), or even to search for evidence of earlier independent load (if any) of spacecraft that have crashed or soft- origins of life on Earth that may have been destroyed during landed on the Moon. These human artifacts on the lunar the heavy bombardment (e.g., Maher and Stevenson, 1988; surface constitute a unique resource for astrobiology, a Davies et al., 2009). In addition, finding terrestrial material on thorough analysis of which will result in major insights the Moon would help inform models of lithopanspermia into the ability of microorganisms to survive the space (Mileikowsky et al., 2000; Burchell, 2004). environment, with clear implications for theories of As discussed by Crawford et al. (2008), terrestrial samples panspermia and planetary protection issues (e.g., Glavin on the lunar surface may have unique spectral properties et al., 2004). that would cause them to stand out from the surrounding Use of the lunar environment as a test-bed for the de- lunar regolith. Identifying them is likely to entail extensive velopment of bioregenerative life-support systems, for geological fieldwork of the kind enabled by a human pres- long-term use on the Moon and future long-duration ence (e.g., Spudis, 1992, 2001; Garvin, 2004), and the desir- deep space exploration missions. ability of having human explorers back on the Moon to locate and sample ancient terrestrial materials was explicitly Just as for the geological studies discussed previously, recognized by Armstrong et al. (2002). these life sciences investigations will be facilitated by a human presence on the Moon. Studies in human physiology, of 4.4. Polar ice deposits course, will absolutely require humans to be present, but the other activities (e.g., the astrobiological investigation of cra- There is growing evidence for ice-rich materials in per- shed spacecraft) will also benefit from human mobility and manently shadowed lunar polar craters (Feldman et al., 1998; flexibility. Colaprete et al., 2010). If confirmed, much of this water is likely derived from the impacts of comets. While the original 4.6. Preparation for Mars cometary volatiles will have been considerably reworked by post-impact processes, it remains possible that some infor- There are strong arguments (discussed below) that ulti- mation concerning the importance of comets in ‘‘seeding’’ the mately human exploration of Mars will be necessary if we terrestrial planets with volatiles and prebiotic organic ma- are to arrive at a definitive answer to the question as to terials (e.g., Chyba and Sagan 1992; Pierazzo and Chyba whether life exists, or has ever existed, on that planet. 1999; Schulze-Makuch et al., 2005) may be preserved. Fur- However, there is still much to learn about human physio- thermore, as pointed out by Lucey (2000), lunar polar ice logical and psychological responses to long-term immersion deposits will have been continuously subject to irradiation in the space environment before we will be in a position by cosmic rays and, as such, may have played host to organic safely to send people to Mars (e.g., Freeman, 2000; White and synthesis reactions of the kind thought to occur in the outer Averner, 2001). As recognized by the Augustine Commission Solar System and on interstellar dust grains. Although an (2009), human operations on the lunar surface, in conjunction initial examination of this material could be conducted by with research on the ISS, will provide much of the knowl- specially designed robotic landers (e.g., Schulze-Makuch et al., edge required for later expeditions to Mars. Thus, insofar as 2005), this is another area of lunar exploration that may human Mars exploration is expected to yield astro- benefit from the enhanced mobility and sample return ca- biologically significant results, human exploration of the pacity enabled by a human presence. Moon can be seen as a necessary precursor activity, in addition to yielding unique astrobiological insights of its own. 4.5. Life sciences on the Moon 5. Mars Exploration In addition to the essentially planetary science investiga- Of all the locations in the Solar System, other than Earth, tions described above, human lunar exploration would per- that may have once supported, or still support, indigenous mit a number of life sciences investigations of relevance to life (of which there is now a growing list; see Shapiro and astrobiology. These have been reviewed in detail by Gronstal Schulze-Makuch, 2009), the planet Mars is by far the most et al. (2007) and include accessible. It goes without saying that the discovery and, Study of the adaptation of terrestrial life to the lunar later, the biological and evolutionary characterization of environment. This would extend life sciences research martian life would be of enormous scientific significance. conducted in the microgravity environment of the ISS, However, the astrobiological importance of searching for life and earlier orbiting laboratories, to the low (but non- on Mars transcends the intrinsic interest of any martian or- zero) gravity and more severe radiation environment of ganisms that may be found. This is because environmental the lunar surface. Such research would address ques- conditions on Mars during the first billion years of Solar tions of fundamental biology as well as human physi- System history appear to have been broadly similar to those ASTROBIOLOGY AND HUMAN SPACE EXPLORATION 581 that existed on Earth at about the same time (i.e., both were near-surface life (although we should be open to the possi- relatively warm, wet, rocky planets with dominantly CO2 bility that dormant spores might be present in the soils; e.g., atmospheres; e.g., Kargel, 2004); a search for life on Mars will McKay, 2004). enable us to test the suggestion of some biochemists (e.g., de Duve, 1995; Russell and Hall, 1999) that the origin of life is 5.2. Searching for extant subsurface life almost inevitable under such conditions. As the Galaxy very The harsh conditions of the martian surface have led many likely contains billions of planets similar to Earth and Mars to suggest that, if life does exist on Mars today, it is most as they were 4 billion years ago, determining whether an likely to be found below the surface (e.g., Boston et al., 1992). independent origin of life occurred on Mars thus has profound Chemoautotrophic organisms can survive in deep crustal implications for our understanding of the likely prevalence environments on Earth (Stevens and McKinley, 1995; Cha- of life beyond the Solar System. pelle et al., 2002; Lin et al., 2005). Additionally, habitable For all these reasons, Mars has a pivotal place in astrobi- subglacial environments might also exist within the martian ology research. The question for this paper concerns the ex- polar caps (e.g., Cockell, 2006a), and deep (>100 m) subsur- tent to which the search for past or present life on Mars face permafrost layers may better preserve ancient organic would be facilitated by human, as opposed to purely robotic, molecules than possibly any other environments on the exploration of the planet. Strategies for searching for life on planet (Smith and McKay, 2005). Mars can logically be broken into three subcategories, and I Discovering life, or evidence for past life, in such deep en- here address the added value of human exploration to each, vironments will not be readily amenable to the kind of small- as follows. scale robotic vehicles currently envisaged for the search for life on Mars. Given that an operation capable of drilling to 5.1. Searching for extant near-surface life depths of hundreds of meters to kilometers beneath the surface will be required, this is the kind of large-scale exploratory The surface of Mars is the only part of the planet to which activity that would, at the very least, be greatly facilitated by a we have ready access, and for this reason it has been the human presence (Hiscox, 2001; Garvin, 2004; Baxter et al., focus of robotic exploration to date. Unfortunately, the near- 2006; Zacny et al., 2008). A human presence will not only be surface environment of Mars is extremely hostile to life as we desirable to ‘‘troubleshoot’’ technical problems (Zacny et al., know it (i.e., it is very cold, dry, oxidized, and exposed to 2008), but to handle and process the resulting drill cores and solar UV radiation), and it is therefore perhaps not surprising select which sections should be subjected to more detailed that the Viking life-detection experiments were negative (or in situ analysis or dispatched to Earth. Moreover, the en- at best ambiguous; e.g., Klein, 1978). However, the small hanced mobility implicit in human expeditions will provide number of in situ experiments conducted to date are insuf- opportunities to sample a wider diversity of locations than is ficient to get a definitive answer as to whether near-surface likely to be feasible robotically (e.g., Garvin, 2004; McKay, life currently exists on Mars. It is very important to get a 2004; Cockell, 2006b; Snook et al., 2007). robust answer to this question, because it will define the Finally, a serious search for, and subsequent study of, life in context for the development of future exploration strategies, the martian subsurface (or even on the surface, for that matter) including human missions to the planet. will require the analysis of such a large quantity of material, Although, as for the lunar case, human missions to Mars with such a wide range of different techniques, that it will soon would have many advantages over robotic missions in terms outstrip the capabilities of either in situ robotic measurements of flexibility and mobility, and these would undoubtedly aid or any plausible implementation of robotic sample return. The surface exploration designed to detect life (e.g., Boston, 1999; only viable long-term solution will be to have human spe- Hiscox, 2001; Cockell, 2004; Garvin, 2004; Snook et al., 2007), cialists working on Mars with appropriate laboratory instru- there are also strong arguments for conducting the initial mentation (a preliminary list of scientific equipment likely to searches for life robotically. This is because of the greater risk be required has been given by Cockell, 2006a). These in situ of contaminating the martian environment with terrestrial analyses would then enable investigators to identify and pri- life via human missions (e.g., Horneck, 2008). It will not be oritize the very small subset of samples that could plausibly be possible to sterilize a human mission to level IV of the transferred to Earth for more detailed analysis. Committee on Space Research (COSPAR) planetary protection guidelines (NRC, 2006). 5.3. Searching for extinct life If life presently exists near the surface of Mars, chemical signatures of active metabolism or the breakdown products Even if there is no life on Mars today, there are good of living organisms, or both, could be detected by suitably grounds for believing that it may have existed 4.0–3.5 billion instrumented robotic spacecraft (e.g., Bada, 2001; Parnell et al., years ago when the surface was, as is currently held, both 2007). The Urey instrument, originally proposed for ESA’s warmer and wetter (e.g., de Duve, 1995; Hiscox, 2001; Kargel, ExoMars rover (Bada et al., 2008), illustrates the kind of 2004). If such life is now extinct, as is perhaps the most likely measurements that might be attempted. Should such exper- case, the task will involve searching for fossil evidence, iments reveal evidence for near-surface life, then further ro- presumably fossilized microorganisms or their decay prod- botic experiments could be designed to study it further, and ucts. In this context, it is important to realize that the earliest the longer-term implications for future human exploration microfossils found on Earth (e.g., as reviewed by Westall and would have to be carefully assessed. However, given the Southam, 2006) have not been, and could not have been, harsh conditions currently prevailing at the surface, there identified by performing cursory geological inspections from must be a high probability that these initial robotic studies small robotic vehicles. Rather, these discoveries have built on will, like Viking, not reveal convincing evidence of extant decades of careful geological fieldwork, followed by patient 582 CRAWFORD microscopic observations of carefully collected material by sions to NEOs were outlined by Jones et al. (1994) and Abell experienced human micro-paleontologists. et al. (2009) and are derived from the same intrinsic charac- It is likely that the search for microfossils on Mars will teristics of human missions that will also benefit lunar and have to proceed in a similar manner, which is not readily martian exploration, namely, enhanced speed, versatility, amenable to robotic exploration (Gould, 1994; Hiscox, 2001). and intelligence in pursuing exploration goals. As noted by Indeed, as discussed above in the context of studies of extant Abell et al. (2009), ‘‘the biggest scientific asset of [a human life, the search for fossil life will involve the analysis of such a mission] is its crew, which can adapt to specific situations large quantity of material, from so many different sites, that and adjust experiments and operations with much more only in situ studies by human specialists may be practical. flexibility than a robotic spacecraft.’’ To which may be added The recent controversies that have sprung up concerning the the greatly enhanced sample collection and return capability oldest terrestrial microfossils (e.g., Brasier et al., 2004) illus- inherent in human missions. The latter advantage was rec- trate how difficult it will be to interpret data obtained ro- ognized by a recent National Research Council study (NRC, botically. Moreover, if evidence for past life is found on 2009 p 59) that found that ‘‘a crewed mission to a NEO is the Mars, that will mark the beginning, not the end, of the new best, and possibly the only, way to gather a sample within field of martian paleontology (Gould, 1994). The subsequent the context of its surroundings and return a significant demand for additional samples, and supporting geological amount of material to Earth.’’ and environmental studies, will be considerable and may The principal scientific importance of NEOs results from again outstrip the capabilities of purely robotic exploration the fact that they provide a relatively accessible sample of or robotic sample return. main-belt asteroids that have been gravitationally perturbed For all these reasons, the search for past or present life on into the inner Solar System relatively recently (Jones et al., Mars will benefit from the human exploration of the planet. 1994). As the main asteroid belt is thought to be composed of That said, until the outstanding question of whether extant planetesimals that were never incorporated into the planets, life exists in the near surface environment has been resolved, NEOs provide a record of early Solar System processes rel- it appears sensible to proceed cautiously with the robotic evant to the time of formation of the terrestrial planets. In- exploration of Mars for the next several decades. This could sofar as their study can better constrain theories of planetary be pursued in parallel with the development of a human system formation, the exploration of NEOs is clearly of as- space exploration capability focused on the Moon and other trobiological relevance. Moreover, a better understanding of inner Solar System destinations for which important astro- NEOs is relevant to the future of life in the Solar System, both biological objectives can also be identified but where plane- because they pose an impact threat to Earth, which increased tary protection is less of a concern. Such a strategy would knowledge of their physical properties may help mitigate

enable the eventual human exploration of Mars to be in- (e.g., Ahrens and Harris, 1994), and because asteroidal raw formed by the results of a thorough robotic search for life in materials may one day facilitate the expansion of human the near-surface environment of the planet. civilization into the Solar System (e.g., Lewis, 1996). Broadly Mars is also important in considering the future of life in the considered, all of these are astrobiological considerations Solar System. This is because, even if there is no life on the that would benefit from the human exploration of NEOs. planet at present, there may be in the future as a result of Although for the foreseeable future human exploration human action. Human exploration efforts may accidentally will, in practice, be limited to the Moon, Mars, and NEOs introduce terrestrial organisms to Mars, hence the concern (and possibly the Earth-Moon and Sun-Earth Lagrange with planetary protection protocols (NRC, 2006). Moreover, points for the purpose of servicing astronomical instru- at some future time, human civilization might choose to in- ments), it should be noted that many of the scientific ad- troduce life to Mars, either because an ethical judgment is vantages of human space exploration will also apply to outer made that a life-bearing planet is in some sense more Solar System destinations. There is often an unspoken as- ‘‘worthwhile’’ than a dead one or, more self-servingly (but not sumption that these will forever have to be explored robot- incompatibly), simply to make it more habitable for ourselves. ically. However, as pointed out by Jones et al. (1994), such an This is not the place to review the practicalities of terraforming assumption risks becoming a self-fulfilling prophecy, with Mars (relevant discussions are given by C.P. McKay et al., scientific discovery losing out in the longer term. From the 1991; Fogg, 1995; McKay and Marinova, 2001; Graham, 2004; point of view of astrobiology, a human presence in the Ju- and references cited by these authors). Here, I merely note that piter and Saturn systems would greatly facilitate the explo- if, at some future time, a conscious decision is made to in- ration of, and search for life on, Europa, Titan, and troduce terrestrial life to Mars, this would be facilitated by Enceladus, all high on most short lists of habitability (e.g., both the knowledge gained and the infrastructure developed Shapiro and Schulze-Makuch, 2009). Even if human beings by precursor human operations on the planet. Moreover, the are not able to land on these bodies, having humans in the ethics of doing so would depend very greatly on whether Mars locality with the capacity to operate telerobotic instruments already has a biosphere of its own (Fogg, 2000; McKay, 2009) on (or below) their surfaces would be a significant advantage and, as discussed above, human missions to Mars may ulti- over autonomous robotic operations. Moreover, as will mately be required to get to the bottom of that question. generally be the case for human missions, the capability for on-the-spot decision making, in situ sample analysis, and sample return will all be greater than could be achieved ro- 6. Other Destinations botically. Prior experience of human operations in the inner The only other inner Solar System objects likely to be the Solar System (including advances in propulsion systems, focus of human exploration in the next few decades are near- radiation protection, mitigation of microgravity exposure, Earth objects (NEOs). The scientific benefits of human mis- and development of reliable closed-loop life-support sys- ASTROBIOLOGY AND HUMAN SPACE EXPLORATION 583 tems) may render such outer Solar System operations feasi- bility that Ares V could enable sample return missions ble in the longer term. to the outer Solar System’’ (NRC, 2009 p 15), the astrobiological benefits of which could be very significant. 7. Benefits Arising from the Infrastructure of Human Spaceflight In the longer term, other scientific benefits from human space exploration can be identified, especially in the con- Any ambitious human exploration program beyond LEO struction and maintenance of very large space-based tele- will require the development of a new generation of launch scopes. For example, to resolve surface features on a planet vehicles and other space infrastructure and operational orbiting a nearby star (say at a distance of 5 parsecs) with a capabilities, which will then be available for other scientific spatial resolution of 800 km (comparable to the pre–space objectives. The large payload capacity of a new heavy-lift age telescopic resolution of the surface of Mars) would re- launch vehicle (e.g., 100 metric tons to LEO), which will quire an optical interferometer array with a baseline of order have to be developed if humans are to return to the Moon or 100 km. Moreover, to obtain a sufficient signal-to-noise ratio, venture beyond, is especially enabling in this respect. In the array would require a significant light-collecting area keeping with the terminology of the Augustine Commission (probably corresponding to several tens of 4-meter-class (2009), I will here refer to such a launch vehicle as being of telescopes; Labeyrie, 1996). Such an array might be con- ‘‘Ares V-class’’ (while recognizing that the Constellation ar- structed on the lunar surface (e.g., Burke, 1985; Labeyrie, chitecture itself is now unlikely to be implemented and that 1994) or in free space (e.g., Lester et al., 2004); but, in either the nomenclature will change). case, it seems unlikely that it will be possible to construct The wider scientific benefits of an Ares V-class launch or maintain instruments of this size and complexity with- vehicle have recently been considered by the National Re- out access to the heavy-lift launch capacity and in-space search Council (NRC, 2009) and include the launching and construction capability that could be provided by a well- servicing of large space telescopes and the sending of so- developed human spaceflight infrastructure. phisticated robotic spacecraft to the outer Solar System. In the even more distant future, we should be open to the Mission concepts of specifically astrobiological interest that possibility that a human spaceflight infrastructure may one were identified by the committee (NRC, 2009) as benefiting day enable the construction of interstellar space probes for from a new heavy-lift launch vehicle, intended primarily to the exploration of planetary systems around nearby stars. support human exploration, include We already know that spacecraft are required for the in- depth study of planets in our own Solar System, so it seems Large space telescopes. All the concepts considered clear that we will eventually require spacecraft to make in situ would have multiple astronomical applications, includ- studies of other planetary systems also. This may prove to be

ing studies of exoplanetary systems. The committee especially important for astrobiology. For example, in the identified one of the concepts, the 8-Meter Monolithic event that a space-based interferometer one day identifies Telescope (Stahl et al., 2009), as ‘‘the best [proposed] bone fide spectral evidence for life on an apparently Earth-like platform from which to observe Earth-like planets di- planet orbiting a nearby star (e.g., Cockell et al., 2009), what rectly to search for signs of life’’ (NRC, 2009 p 32). In would our next steps be? There is a limit to how much we addition to being enabled by an Ares V-class launch can hope to learn about this new biosphere using astro- vehicle, it was noted that large space telescopes would nomical techniques alone. To fully understand the diversity also benefit from human servicing missions (as dem- of an alien ecosystem, including its biochemistry and its onstrated with the Hubble Space Telescope; NRC, 2005). evolutionary history, will require transporting sophisticated Lunar radio telescopes. The far side of the Moon is scientific instruments across interstellar space. This is not the probably the best site for radio astronomy, especially place to discuss the technical challenges and proposed so- low-frequency radio astronomy, anywhere in the Solar lutions of interstellar spaceflight; the interested reader will System (see review by Jester and Falcke, 2009). The find reviews given by Crawford (1990) and Matloff (2000). committee considered a specific concept (the Dark Ages What is clear is that, while the first interstellar probes will Lunar Interferometer; Lazio et al., 2009) that would be certainly themselves be robotic, the scale of the undertaking, enabled by the heavy-lift capacity of an Ares V-class and the highly energetic energy sources that will have to be launcher. Other radio telescope concepts exist that employed, will require that their construction take place in would be deployed by astronauts in the context of a space. The potential long-term scientific benefits of inter- human return to the Moon. Lunar radio telescopes will stellar space probes are incalculable (and are discussed in be used for multiple astronomical purposes, of which more detail by Crawford, 2010), but it is important to realize the most important from an astrobiological perspective that a significant human spaceflight infrastructure in our are their applications to the study of exoplanetary en- own solar system will almost certainly be a prerequisite for vironments (e.g., Jester and Falcke, 2009) and to the developing this long-term capability. Search for Extraterrestrial Intelligence (SETI) (e.g., Tarter and Rummel, 1990; Maccone, 2005). 8. Policy Implications Robotic missions to the outer Solar System. The committee noted that Ares V-class launchers have significant There are a number of implications for the development of potential for exploring the outer Solar System. Titan human spaceflight policy, as it relates to astrobiology, that missions are perhaps of most interest to astrobiology, follow from this discussion. First, it is important to realize and the committee went out of its way to note that ‘‘Ares that a space program of this scale will not be conducted for V has great potential for Titan missions’’ (NRC, 2009 p 3). scientific purposes alone. Both the Global Exploration Strat- Moreover, the committee drew attention to ‘‘the possi- egy (GES, 2007) and the Augustine Commission (2009) 584 CRAWFORD identified a range of industrial, economic, societal, and been identified. These include the exploitation of the lunar geopolitical factors that are at least as important as scientific geological record to elucidate conditions on the early Earth, considerations as drivers for human space exploration. Some the detailed study of NEOs for clues relating to the formation of these nonscientific drivers, such as the stimulus for eco- of the Solar System, the search for evidence of past or present nomic growth and the encouragement of international co- life on Mars, and the development of sophisticated space- operation, are desirable in themselves even in the absence of based astronomical tools for the study of extrasolar planetary any scientific benefits. Thus, although I have argued here systems. In all these areas, I have argued that a human that science in general, and astrobiology in particular, will presence in space, and especially on planetary surfaces, will benefit from an ambitious program of human space explo- yield a net scientific benefit over what can plausibly be ration, scientists should not fall into the trap of believing that achieved by relying on autonomous robotic systems alone. such a program will be pursued solely, or even mainly, for Astrobiologists therefore have a strong interest in the im- their benefit. The totality of the case for investing in human plementation, over the coming decades, of an ambitious in- spaceflight is a complex mixture of scientific and societal ternational program of human space exploration, such as factors (Crawford, 2004; Fong, 2004; GES, 2007), and any that envisaged by the Global Exploration Strategy (GES, responsibly formulated public space policy must take a ho- 2007). listic view of the scientific and nonscientific benefits together. That said, if governments do decide to invest in human Acknowledgments space exploration because of multiple perceived benefits in a number of different policy areas, it is up to the scientific I thank the referee, Dr. Chris McKay, for comments which community to advocate an implementation from which sci- have improved the quality of the manuscript. ence stands to gain the greatest benefit. From the point of view of astrobiology, the above discussion points to a pre- Author Disclosure Statement ferred program with the following elements: No competing financial interests exist for the author of this The development of launch vehicles and other infra- manuscript. structure sufficient to return humans to multiple sites on the lunar surface for extended periods for the purposes of Abbreviations conducting geological and biological field activities (this will include the development of capabilities in surface GES, Global Exploration Strategy; ISS, International Space mobility, subsurface drilling, and in situ sample analysis, Station; LEO, low Earth orbit; NEO, near-Earth object. in addition to a significant sample return capacity); The development of vehicles, infrastructure, and oper- References ational experience (including mitigation of the physiological effects of enhanced radiation and low-gravity Abell, P.A., Adamo, D., Jones, T., Korsmeyer, D., and Landis, R. environments), which will permit human missions to (2009) Scientific investigation of near-earth objects via the the vicinity of Mars and NEOs, as well as the tending of Orion crew exploration vehicle, white paper submitted to the future astronomical instruments in deep-space locations; 2009–2011 NRC Planetary Science Decadal Survey. 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