Astrobiological Benefits of Human Space Exploration

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.

1. Introduction

A strobiology is usually defined as the study of the origin, evolution, distribution, and future of life in the Universe. As such, it is intimately connected with space exploration, because it is only through exploration of the universe around us that we will be able to determine the existence, prevalence, and nature of life beyond our own planet. Moreover, regardless of whether life is common elsewhere in the Universe, space technology has the potential to provide a means by which to disseminate life outward from Earth and, as such, play a significant role in the future of life in the Universe. The question remains, however, as to the most appropriate form of space exploration from an astrobiological perspective, especially for the nearer-term objectives of assessing the extent and history of life in the Solar System. Currently, this is undertaken by robotic spacecraft, which, over the last five decades, have very successfully conducted an initial reconnaissance of most of the planets and larger moons of the Solar System and somewhat deeper studies of several of these objects. The question for future exploration is whether a continued reliance of robotic space probes will be sufficient to meet the core objectives of astrobiology or whether the search for life in the Universe would benefit from a renewed, and greatly expanded, program of human exploration beyond low Earth orbit (LEO).

Now is an appropriate time to consider this question because governments and space agencies are currently reviewing their plans for human space exploration. Some context for this activity is provided by the Global Exploration Strategy (GES), which was agreed upon by 14 of the world's space agencies in May 2007 and, to quote from its founding document, “elaborates a vision for globally coordinated space exploration focused on solar system destinations where humans will someday live and work” (GES 2007). The GES was itself largely stimulated by the US Vision for Space Exploration (NASA, 2004), the objectives and implementation of which have recently been reviewed by the Augustine Commission (2009). The situation regarding US human spaceflight policy is now somewhat uncertain, following the cancellation of NASA's Constellation program in the Administration's Fiscal Year 2011 budget request. In part, this reflects a less-than-ringing endorsement of the Constellation architecture by the Augustine Commission, although the Commission did largely endorse the overall objectives of the Vision for Space Exploration and confirmed that scientific and other benefits would result from human exploration beyond LEO. Thus, although the pace may be slower than originally intended, it still appears likely that US human spaceflight activities will be directed beyond LEO in the coming decades and that this will encourage wider international participation (as envisaged by the GES and specifically recommended by the Augustine Commission). In Europe, partly in response to these developments, efforts are underway to define a broad, predominantly science-driven, program for the robotic and human exploration of the Solar System (Worms et al., 2009), and the very existence of the GES indicates that other space-faring nations are also considering the extent to which they could contribute to such a program.

Given that the future objectives of human space exploration are currently uncertain, it is important that scientists take this opportunity to inform the policy debate by identifying those exploration scenarios that will yield the greatest scientific return. In this spirit, I here present the case that significant scientific, and specifically astrobiological, benefits will result from an enhanced international program of human exploration of the inner Solar System.

2. Benefits of Human Exploration

While some of the exploratory objectives of astrobiology can undoubtedly be met by suitably instrumented robotic probes, others would be greatly facilitated by a human presence, and some may be wholly impractical otherwise. Specifically, human planetary exploration would have the following scientific advantages over robotic missions:

It would enable rapid on-the-spot decision making and prioritization of exploration activities, including more intelligent and efficient collection of samples from a more diverse range of localities, and over wider geographic areas, than is likely to be practical with the use of robots alone (e.g., Spudis, 1992, 2001; Garvin, 2004; Snook et al., 2007). The Apollo experience demonstrated that astronauts, when suitably equipped with the means of surface mobility, are very efficient at this task (e.g., Heiken et al., 1991). Garvin (2004) presented a detailed comparison of the relevant skills and abilities of humans and robots as explorers of planetary surfaces and found that humans out-perform robots according to most of the criteria considered. This conclusion is corroborated by direct field comparisons of human and robotic exploration at planetary analog sites on Earth—reporting one such study, Snook et al. (2007 p 438) found that “humans could be 1–2 orders of magnitude more productive per unit time in exploration than future terrestrially controlled robots.”

Astronauts may be expected to make discoveries that would be overlooked by robots, owing to the uniquely human ability to recognize new observations or phenomena to be of importance even if not anticipated in advance (e.g., Cockell, 2004). The history of exploration on our own planet indicates that serendipitous discoveries are often among the most important, and the exploration of other planets is unlikely to be any different in this respect.

Perhaps most importantly, human missions to other planetary bodies will permit the return to Earth of a much larger, and more diverse, quantity of samples for detailed analysis in terrestrial laboratories than is likely to be achieved robotically. One of the major, but often unspoken, benefits of human planetary exploration is that, because the astronauts must return to Earth, a large quantity of geological samples can be returned with them. The Apollo haul alone was 382 kg, which comprised more than 2000 discrete samples (Heiken et al., 1991); nothing comparable has been, or is likely to be, achieved robotically.

Human missions will facilitate the landing, operation, and maintenance of more massive and complex scientific equipment than is likely to be feasible robotically. Because human missions, by their very nature, must land a significant amount of mass on planetary surfaces, the additional marginal cost of landing massive or bulky scientific equipment is relatively modest (as demonstrated by the range of equipment deployed by the Apollo missions; Heiken et al., 1991). Moreover, human beings are uniquely capable of maintaining and “troubleshooting” problems with complex equipment (of which the five successful repair and upgrade missions to the Hubble Space Telescope provide the best examples to date; NRC, 2005). A particular example relevant to future planetary exploration concerns drilling, which will have important astrobiological applications on both the Moon and Mars. In this context, Zacny et al. (2008) noted that “in the era of human exploration, sufficient mass and real-time supervision should be available to carry out truly deep penetration of the subsurface of extraterrestrial bodies.”

Last, but not least, the infrastructure developed to support human space exploration, especially the development of a heavy-lift launch capability, would have many other scientific applications. Examples of relevance to astrobiology include sophisticated robotic probes to the outer Solar System and the construction of large space-based telescopes for the study of extrasolar planetary systems (NRC, 2009).

In the following sections, these scientific benefits of human space exploration as applied to astrobiological objectives are expanded upon.

3. Low Earth Orbit

Currently, human space activities are concentrated on the completion and utilization of the International Space Station (ISS). Confined to LEO, this activity does not amount to exploration in the spatial or geographic sense. Nevertheless, the ISS, like other orbital facilities before it, does provide a platform for the scientific exploration of the unique microgravity and radiation environments of near-Earth space. The principal scientific rationales for research in this environment lie in the areas of materials science (reviewed by Ratke, 2006), fundamental physics (Dittus, 2006), and the life sciences (Gerzer et al., 2006). Aspects of particular relevance to astrobiology belong mainly to the latter area and may broadly be grouped into two main areas, as follows.

3.1. Biological studies of the adaptability and survivability of organisms in the space environment

Astrobiologically relevant experiments in LEO were reviewed by Gerzer et al. (2006) and Baglioni et al. (2007). Of particular interest are studies of the survival of terrestrial organisms to the radiation, vacuum, and temperature extremes of the space environment, because these studies help constrain models for the efficacy of lithopanspermia as a means by which life can transfer between planets (e.g., Mileikowsky et al., 2000; Horneck et al., 2002; Burchell, 2004). Space-based platforms also permit unadulterated access to the solar spectrum and thereby enable long-term studies of the biological consequences of exposure to solar UV radiation. This is important for understanding the possible role of UV radiation in the synthesis of prebiotic molecules (e.g., Bernstein et al., 2002), and the survival of life on planetary surfaces that lack atmospheric UV protection (e.g., Cockell, 2002). The European Space Agency's Expose facility on the ISS (Rabbow et al., 2009) will provide a powerful tool for the furtherance of these studies.

3.2. Studies of human physiology and medicine in preparation for exploration missions beyond LEO

Studies of the long-term physiological effects of the space environment will be required before humans can venture farther afield in the Solar System (e.g., Horneck et al., 2003). I argue below that significant scientific benefits will follow from the human exploration of the Moon, Mars, and near-Earth asteroids, but our understanding of the long-term effects of the space environment (especially the radiation, microgravity, and psychological aspects) is still not sufficient for us confidently to embark on the more ambitious of these ventures. Intensive research, which realistically can only be performed on the ISS, will be required to develop effective countermeasures to these effects (e.g., Freeman, 2000; Fong, 2001; White and Averner, 2001; Horneck et al., 2003). Similarly, life sciences research on the ISS will be required in order to design bioregenerative life-support systems (Horneck et al., 2003) that will also be required to support human exploration beyond LEO.

For these reasons, it is in the interests of astrobiology that the large international investment in the ISS continues to be utilized for scientific research, especially in the life sciences, even as we prepare for exploratory missions farther afield.

4. Lunar Exploration

Although the Moon has, almost certainly, never supported any indigenous life of its own, the lunar geological record nevertheless contains much that is of interest to astrobiology. For the last 4.5 billion years the Moon has been orbiting the only planet on which life is actually known to exist, and its ancient, relatively accessible, near-surface environment has preserved a record of the changing astronomical environment within which life appeared and evolved on Earth. Lunar exploration is also likely to play a major role in preparing for the exploration of Mars, which may also be expected to yield astrobiological benefits (discussed in Section 5 below) and provide insights into planetary protection issues. The astrobiological aspects of lunar exploration were reviewed in detail by Crawford (2006) and Gronstal et al. (2007), to which the interested reader is referred. Briefly, they consist of the following elements.

4.1. Characterization of the impact cratering rate in the Earth-Moon system

Although the Earth itself has lost any record of the impact flux to which it was subjected early in its history, this record is still preserved on the Moon. From the point of view of astrobiology, characterization of the impact rate in the Earth-Moon system between 4.5 and 3.5 billion years ago is especially important because it defines the impact regime under which life on Earth became established (e.g., Maher and Stevenson, 1988; Sleep et al., 1989; Ryder, 2003; for an illustration of this unresolved issue see Fig. 2.3 of NRC, 2007). Obtaining definitive knowledge of the cratering rate during the first billion years of Earth-Moon history will require the collection and dating of lunar samples from areas with a much wider range of ages than was achieved by the Apollo missions. In principle, this might be achieved with a number of robotic sample return missions sent to well-chosen localities, but it would be greatly facilitated by the enhanced sample return capabilities enabled by new human missions to the Moon.

4.2. Astrobiologically relevant records preserved in lunar regolith deposits

The lunar regolith efficiently retains exogenously implanted volatiles (e.g., D.S. McKay et al., 1991). These are mostly derived from the solar wind (e.g., Wieler et al., 1996; Levine et al., 2007) but also include a record of galactic cosmic rays and, possibly, Earth's atmosphere (e.g., Ozima et al., 2005, 2008). This is potentially a very rich record of considerable relevance to astrobiology. The strength of the solar wind in the first billion years of Solar System history is of particular interest, as it will provide an independent check of luminosity of the early Sun during the period when life became established on Earth (Whitmire et al., 1995). Furthermore, the archive of galactic cosmic ray fluxes potentially available in lunar soils may yield a record of high-energy galactic events that may have influenced life in the Solar System. For example, the hypothesis that cyclical changes in cosmic ray flux drive cycles of diversity of life on Earth (Medvedev and Melott, 2007) could be directly testable by examining the lunar record. It is also possible that passages of the Sun through dense interstellar clouds, and associated collapse of the heliosphere (which may also have had biological consequences; e.g., Smith and Scalo, 2009), may similarly be recorded in ancient lunar regoliths. Finally, if samples of Earth's early atmosphere can be isolated from lunar soils (e.g., Ozima et al., 2008), they would help constrain inferences about Earth's atmospheric evolution that are currently inferred rather uncertainly from geological evidence alone.

From the point of view of accessing ancient Solar System history, it will be desirable to find layers of ancient regoliths (paleoregoliths) that were formed and buried (and thus protected from more recent geological processes) billions of years ago (e.g., Spudis, 1996; Crawford et al., 2007; Fagents et al., 2010). This would be a very rich scientific record of clear astrobiological significance, but it will not be easy to access. Although robotic sampling missions might, in principle, be able to achieve these objectives at a limited number 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; Snook et al., 2007). Some of the operational requirements of a human lunar exploration architecture with the capacity to meet these objectives have been elaborated elsewhere (Crawford et al., 2007).

4.3. Preservation of crustal samples from early Earth

Armstrong et al. (2002) drew attention to the fact that the Moon may have collected meteorites blasted off early Earth and other terrestrial planets. The recovery of such material would provide an important window into the early history of the Solar System, including possible information on the nature and prevalence of early life. For example, such samples might be able to test the indications from phylogenetic analyses that life arose before 4.1 Gyr (e.g., Battistuzzi et al., 2004), or even to search for evidence of earlier independent origins of life on Earth that may have been destroyed during the heavy bombardment (e.g., Maher and Stevenson, 1988; Davies et al., 2009). In addition, finding terrestrial material on the Moon would help inform models of lithopanspermia (Mileikowsky et al., 2000; Burchell, 2004).

As discussed by Crawford et al. (2008), terrestrial samples on the lunar surface may have unique spectral properties that would cause them to stand out from the surrounding lunar regolith. Identifying them is likely to entail extensive geological fieldwork of the kind enabled by a human presence (e.g., Spudis, 1992, 2001; Garvin, 2004), and the desirability of having human explorers back on the Moon to locate and sample ancient terrestrial materials was explicitly recognized by Armstrong et al. (2002).

4.4. Polar ice deposits

There is growing evidence for ice-rich materials in permanently shadowed lunar polar craters (Feldman et al., 1998; Colaprete et al., 2010). If confirmed, much of this water is likely derived from the impacts of comets. While the original cometary volatiles will have been considerably reworked by post-impact processes, it remains possible that some information concerning the importance of comets in “seeding” the terrestrial planets with volatiles and prebiotic organic materials (e.g., Chyba and Sagan 1992; Pierazzo and Chyba 1999; Schulze-Makuch et al., 2005) may be preserved. Furthermore, as pointed out by Lucey (2000), lunar polar ice deposits will have been continuously subject to irradiation by cosmic rays and, as such, may have played host to organic synthesis reactions of the kind thought to occur in the outer Solar System and on interstellar dust grains. Although an initial examination of this material could be conducted by specially designed robotic landers (e.g., Schulze-Makuch et al., 2005), this is another area of lunar exploration that may benefit from the enhanced mobility and sample return capacity enabled by a human presence.

4.5. Life sciences on the Moon

In addition to the essentially planetary science investigations described above, human lunar exploration would permit a number of life sciences investigations of relevance to astrobiology. These have been reviewed in detail by Gronstal et al. (2007) and include

Study of the adaptation of terrestrial life to the lunar environment. This would extend life sciences research conducted in the microgravity environment of the ISS, and earlier orbiting laboratories, to the low (but nonzero) gravity and more severe radiation environment of the lunar surface. Such research would address questions of fundamental biology as well as human physiology and medicine relevant to planning for the long-term human habitation of the Moon and exploration missions elsewhere in the Solar System.

Use of the lunar environment for panspermia experiments and as a test bed for planetary protection protocols. This would be achieved by assessing the degree to which terrestrial microorganisms, and especially spore-forming organisms, can tolerate exposure to the lunar environment with varying degrees of protection. Of particular interest will be studies of the remaining bioload (if any) of spacecraft that have crashed or soft-landed on the Moon. These human artifacts on the lunar surface constitute a unique resource for astrobiology, a thorough analysis of which will result in major insights into the ability of microorganisms to survive the space environment, with clear implications for theories of panspermia and planetary protection issues (e.g., Glavin et al., 2004).

Use of the lunar environment as a test-bed for the development of bioregenerative life-support systems, for long-term use on the Moon and future long-duration deep space exploration missions.

Just as for the geological studies discussed previously, these life sciences investigations will be facilitated by a human presence on the Moon. Studies in human physiology, of course, will absolutely require humans to be present, but the other activities (e.g., the astrobiological investigation of crashed spacecraft) will also benefit from human mobility and flexibility.

4.6. Preparation for Mars

There are strong arguments (discussed below) that ultimately human exploration of Mars will be necessary if we are to arrive at a definitive answer to the question as to whether life exists, or has ever existed, on that planet. However, there is still much to learn about human physiological and psychological responses to long-term immersion in the space environment before we will be in a position safely to send people to Mars (e.g., Freeman, 2000; White and Averner, 2001). As recognized by the Augustine Commission (2009), human operations on the lunar surface, in conjunction with research on the ISS, will provide much of the knowledge required for later expeditions to Mars. Thus, insofar as human Mars exploration is expected to yield astrobiologically significant results, human exploration of the Moon can be seen as a necessary precursor activity, in addition to yielding unique astrobiological insights of its own.

5. Mars Exploration

Of all the locations in the Solar System, other than Earth, that may have once supported, or still support, indigenous life (of which there is now a growing list; see Shapiro and Schulze-Makuch, 2009), the planet Mars is by far the most accessible. It goes without saying that the discovery and, later, the biological and evolutionary characterization of martian life would be of enormous scientific significance. However, the astrobiological importance of searching for life on Mars transcends the intrinsic interest of any martian organisms that may be found. This is because environmental conditions on Mars during the first billion years of Solar System history appear to have been broadly similar to those that existed on Earth at about the same time (i.e., both were relatively warm, wet, rocky planets with dominantly CO₂ atmospheres; e.g., Kargel, 2004); a search for life on Mars will enable us to test the suggestion of some biochemists (e.g., de Duve, 1995; Russell and Hall, 1999) that the origin of life is almost inevitable under such conditions. As the Galaxy very likely contains billions of planets similar to Earth and Mars as they were 4 billion years ago, determining whether an independent origin of life occurred on Mars thus has profound implications for our understanding of the likely prevalence of life beyond the Solar System.

For all these reasons, Mars has a pivotal place in astrobiology research. The question for this paper concerns the extent to which the search for past or present life on Mars would be facilitated by human, as opposed to purely robotic, exploration of the planet. Strategies for searching for life on Mars can logically be broken into three subcategories, and I here address the added value of human exploration to each, as follows.

5.1. Searching for extant near-surface life

The surface of Mars is the only part of the planet to which we have ready access, and for this reason it has been the focus of robotic exploration to date. Unfortunately, the near-surface environment of Mars is extremely hostile to life as we know it (i.e., it is very cold, dry, oxidized, and exposed to solar UV radiation), and it is therefore perhaps not surprising that the Viking life-detection experiments were negative (or at best ambiguous; e.g., Klein, 1978). However, the small number of in situ experiments conducted to date are insufficient to get a definitive answer as to whether near-surface life currently exists on Mars. It is very important to get a robust answer to this question, because it will define the context for the development of future exploration strategies, including human missions to the planet.

Although, as for the lunar case, human missions to Mars would have many advantages over robotic missions in terms of flexibility and mobility, and these would undoubtedly aid surface exploration designed to detect life (e.g., Boston, 1999; Hiscox, 2001; Cockell, 2004; Garvin, 2004; Snook et al., 2007), there are also strong arguments for conducting the initial searches for life robotically. This is because of the greater risk of contaminating the martian environment with terrestrial life via human missions (e.g., Horneck, 2008). It will not be possible to sterilize a human mission to level IV of the Committee on Space Research (COSPAR) planetary protection guidelines (NRC, 2006).

If life presently exists near the surface of Mars, chemical signatures of active metabolism or the breakdown products of living organisms, or both, could be detected by suitably instrumented robotic spacecraft (e.g., Bada, 2001; Parnell et al., 2007). The Urey instrument, originally proposed for ESA's ExoMars rover (Bada et al., 2008), illustrates the kind of measurements that might be attempted. Should such experiments reveal evidence for near-surface life, then further robotic experiments could be designed to study it further, and the longer-term implications for future human exploration would have to be carefully assessed. However, given the harsh conditions currently prevailing at the surface, there must be a high probability that these initial robotic studies will, like Viking, not reveal convincing evidence of extant near-surface life (although we should be open to the possibility that dormant spores might be present in the soils; e.g., McKay, 2004).

5.2. Searching for extant subsurface life

The harsh conditions of the martian surface have led many to suggest that, if life does exist on Mars today, it is most likely to be found below the surface (e.g., Boston et al., 1992). Chemoautotrophic organisms can survive in deep crustal environments on Earth (Stevens and McKinley, 1995; Chapelle et al., 2002; Lin et al., 2005). Additionally, habitable subglacial environments might also exist within the martian polar caps (e.g., Cockell, 2006a), and deep (>100 m) subsurface permafrost layers may better preserve ancient organic molecules than possibly any other environments on the planet (Smith and McKay, 2005).

Discovering life, or evidence for past life, in such deep environments will not be readily amenable to the kind of small-scale robotic vehicles currently envisaged for the search for life on Mars. Given that an operation capable of drilling to depths of hundreds of meters to kilometers beneath the surface will be required, this is the kind of large-scale exploratory activity that would, at the very least, be greatly facilitated by a human presence (Hiscox, 2001; Garvin, 2004; Baxter et al., 2006; Zacny et al., 2008). A human presence will not only be desirable to “troubleshoot” technical problems (Zacny et al., 2008), but to handle and process the resulting drill cores and select which sections should be subjected to more detailed in situ analysis or dispatched to Earth. Moreover, the enhanced mobility implicit in human expeditions will provide opportunities to sample a wider diversity of locations than is likely to be feasible robotically (e.g., Garvin, 2004; McKay, 2004; Cockell, 2006b; Snook et al., 2007).

Finally, a serious search for, and subsequent study of, life in the martian subsurface (or even on the surface, for that matter) will require the analysis of such a large quantity of material, with such a wide range of different techniques, that it will soon outstrip the capabilities of either in situ robotic measurements or any plausible implementation of robotic sample return. The only viable long-term solution will be to have human specialists working on Mars with appropriate laboratory instrumentation (a preliminary list of scientific equipment likely to be required has been given by Cockell, 2006a). These in situ analyses would then enable investigators to identify and prioritize the very small subset of samples that could plausibly be transferred to Earth for more detailed analysis.

5.3. Searching for extinct life

Even if there is no life on Mars today, there are good grounds for believing that it may have existed 4.0–3.5 billion years ago when the surface was, as is currently held, both warmer and wetter (e.g., de Duve, 1995; Hiscox, 2001; Kargel, 2004). If such life is now extinct, as is perhaps the most likely case, the task will involve searching for fossil evidence, presumably fossilized microorganisms or their decay products. In this context, it is important to realize that the earliest microfossils found on Earth (e.g., as reviewed by Westall and Southam, 2006) have not been, and could not have been, identified by performing cursory geological inspections from small robotic vehicles. Rather, these discoveries have built on decades of careful geological fieldwork, followed by patient microscopic observations of carefully collected material by experienced human micro-paleontologists.

It is likely that the search for microfossils on Mars will have to proceed in a similar manner, which is not readily amenable to robotic exploration (Gould, 1994; Hiscox, 2001). Indeed, as discussed above in the context of studies of extant life, the search for fossil life will involve the analysis of such a large quantity of material, from so many different sites, that only in situ studies by human specialists may be practical. The recent controversies that have sprung up concerning the oldest terrestrial microfossils (e.g., Brasier et al., 2004) illustrate how difficult it will be to interpret data obtained robotically. Moreover, if evidence for past life is found on Mars, that will mark the beginning, not the end, of the new field of martian paleontology (Gould, 1994). The subsequent demand for additional samples, and supporting geological and environmental studies, will be considerable and may again outstrip the capabilities of purely robotic exploration or robotic sample return.

For all these reasons, the search for past or present life on Mars will benefit from the human exploration of the planet. That said, until the outstanding question of whether extant life exists in the near surface environment has been resolved, it appears sensible to proceed cautiously with the robotic exploration of Mars for the next several decades. This could be pursued in parallel with the development of a human space exploration capability focused on the Moon and other inner Solar System destinations for which important astrobiological objectives can also be identified but where planetary protection is less of a concern. Such a strategy would enable the eventual human exploration of Mars to be informed by the results of a thorough robotic search for life in the near-surface environment of the planet.

Mars is also important in considering the future of life in the Solar System. This is because, even if there is no life on the planet at present, there may be in the future as a result of human action. Human exploration efforts may accidentally introduce terrestrial organisms to Mars, hence the concern with planetary protection protocols (NRC, 2006). Moreover, at some future time, human civilization might choose to introduce life to Mars, either because an ethical judgment is made that a life-bearing planet is in some sense more “worthwhile” than a dead one or, more self-servingly (but not incompatibly), simply to make it more habitable for ourselves. This is not the place to review the practicalities of terraforming Mars (relevant discussions are given by C.P. McKay et al., 1991; Fogg, 1995; McKay and Marinova, 2001; Graham, 2004; and references cited by these authors). Here, I merely note that if, at some future time, a conscious decision is made to introduce terrestrial life to Mars, this would be facilitated by both the knowledge gained and the infrastructure developed by precursor human operations on the planet. Moreover, the ethics of doing so would depend very greatly on whether Mars already has a biosphere of its own (Fogg, 2000; McKay, 2009) and, as discussed above, human missions to Mars may ultimately be required to get to the bottom of that question.

6. Other Destinations

The only other inner Solar System objects likely to be the focus of human exploration in the next few decades are near-Earth objects (NEOs). The scientific benefits of human missions to NEOs were outlined by Jones et al. (1994) and Abell et al. (2009) and are derived from the same intrinsic characteristics of human missions that will also benefit lunar and martian exploration, namely, enhanced speed, versatility, and intelligence in pursuing exploration goals. As noted by Abell et al. (2009), “the biggest scientific asset of [a human mission] is its crew, which can adapt to specific situations and adjust experiments and operations with much more flexibility than a robotic spacecraft.” To which may be added the greatly enhanced sample collection and return capability inherent in human missions. The latter advantage was recognized by a recent National Research Council study (NRC, 2009 p 59) that found that “a crewed mission to a NEO is the best, and possibly the only, way to gather a sample within the context of its surroundings and return a significant amount of material to Earth.”

The principal scientific importance of NEOs results from the fact that they provide a relatively accessible sample of main-belt asteroids that have been gravitationally perturbed into the inner Solar System relatively recently (Jones et al., 1994). As the main asteroid belt is thought to be composed of planetesimals that were never incorporated into the planets, NEOs provide a record of early Solar System processes relevant to the time of formation of the terrestrial planets. Insofar as their study can better constrain theories of planetary system formation, the exploration of NEOs is clearly of astrobiological relevance. Moreover, a better understanding of NEOs is relevant to the future of life in the Solar System, both because they pose an impact threat to Earth, which increased knowledge of their physical properties may help mitigate (e.g., Ahrens and Harris, 1994), and because asteroidal raw materials may one day facilitate the expansion of human civilization into the Solar System (e.g., Lewis, 1996). Broadly considered, all of these are astrobiological considerations that would benefit from the human exploration of NEOs.

Although for the foreseeable future human exploration will, in practice, be limited to the Moon, Mars, and NEOs (and possibly the Earth-Moon and Sun-Earth Lagrange points for the purpose of servicing astronomical instruments), it should be noted that many of the scientific advantages of human space exploration will also apply to outer Solar System destinations. There is often an unspoken assumption that these will forever have to be explored robotically. However, as pointed out by Jones et al. (1994), such an assumption risks becoming a self-fulfilling prophecy, with scientific discovery losing out in the longer term. From the point of view of astrobiology, a human presence in the Jupiter and Saturn systems would greatly facilitate the exploration of, and search for life on, Europa, Titan, and Enceladus, all high on most short lists of habitability (e.g., Shapiro and Schulze-Makuch, 2009). Even if human beings are not able to land on these bodies, having humans in the locality with the capacity to operate telerobotic instruments on (or below) their surfaces would be a significant advantage over autonomous robotic operations. Moreover, as will 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 robotically. Prior experience of human operations in the inner Solar System (including advances in propulsion systems, radiation protection, mitigation of microgravity exposure, and development of reliable closed-loop life-support systems) may render such outer Solar System operations feasible in the longer term.

7. Benefits Arising from the Infrastructure of Human Spaceflight

Any ambitious human exploration program beyond LEO will require the development of a new generation of launch vehicles and other space infrastructure and operational capabilities, which will then be available for other scientific objectives. The large payload capacity of a new heavy-lift launch vehicle (e.g., ≥100 metric tons to LEO), which will have to be developed if humans are to return to the Moon or venture beyond, is especially enabling in this respect. In keeping with the terminology of the Augustine Commission (2009), I will here refer to such a launch vehicle as being of “Ares V-class” (while recognizing that the Constellation architecture itself is now unlikely to be implemented and that the nomenclature will change).

The wider scientific benefits of an Ares V-class launch vehicle have recently been considered by the National Research Council (NRC, 2009) and include the launching and servicing of large space telescopes and the sending of sophisticated robotic spacecraft to the outer Solar System. Mission concepts of specifically astrobiological interest that were identified by the committee (NRC, 2009) as benefiting from a new heavy-lift launch vehicle, intended primarily to support human exploration, include

Large space telescopes. All the concepts considered would have multiple astronomical applications, including studies of exoplanetary systems. The committee identified one of the concepts, the 8-Meter Monolithic Telescope (Stahl et al., 2009), as “the best [proposed] platform from which to observe Earth-like planets directly to search for signs of life” (NRC, 2009 p 32). In addition to being enabled by an Ares V-class launch vehicle, it was noted that large space telescopes would also benefit from human servicing missions (as demonstrated with the Hubble Space Telescope; NRC, 2005).

Lunar radio telescopes. The far side of the Moon is probably the best site for radio astronomy, especially low-frequency radio astronomy, anywhere in the Solar System (see review by Jester and Falcke, 2009). The committee considered a specific concept (the Dark Ages Lunar Interferometer; Lazio et al., 2009) that would be enabled by the heavy-lift capacity of an Ares V-class launcher. Other radio telescope concepts exist that would be deployed by astronauts in the context of a human return to the Moon. Lunar radio telescopes will be used for multiple astronomical purposes, of which the most important from an astrobiological perspective are their applications to the study of exoplanetary environments (e.g., Jester and Falcke, 2009) and to the Search for Extraterrestrial Intelligence (SETI) (e.g., Tarter and Rummel, 1990; Maccone, 2005).

Robotic missions to the outer Solar System. The committee noted that Ares V-class launchers have significant potential for exploring the outer Solar System. Titan missions are perhaps of most interest to astrobiology, and the committee went out of its way to note that “Ares V has great potential for Titan missions” (NRC, 2009 p 3). Moreover, the committee drew attention to “the possibility that Ares V could enable sample return missions to the outer Solar System” (NRC, 2009 p 15), the astrobiological benefits of which could be very significant.

In the longer term, other scientific benefits from human space exploration can be identified, especially in the construction and maintenance of very large space-based telescopes. For example, to resolve surface features on a planet orbiting a nearby star (say at a distance of 5 parsecs) with a spatial resolution of 800 km (comparable to the pre–space age telescopic resolution of the surface of Mars) would require an optical interferometer array with a baseline of order 100 km. Moreover, to obtain a sufficient signal-to-noise ratio, the array would require a significant light-collecting area (probably corresponding to several tens of 4-meter-class telescopes; Labeyrie, 1996). Such an array might be constructed on the lunar surface (e.g., Burke, 1985; Labeyrie, 1994) or in free space (e.g., Lester et al., 2004); but, in either case, it seems unlikely that it will be possible to construct or maintain instruments of this size and complexity without access to the heavy-lift launch capacity and in-space construction capability that could be provided by a well-developed human spaceflight infrastructure.

In the even more distant future, we should be open to the possibility that a human spaceflight infrastructure may one day enable the construction of interstellar space probes for the exploration of planetary systems around nearby stars. We already know that spacecraft are required for the in-depth study of planets in our own Solar System, so it seems clear that we will eventually require spacecraft to make in situ studies of other planetary systems also. This may prove to be especially important for astrobiology. For example, in the event that a space-based interferometer one day identifies bone fide spectral evidence for life on an apparently Earth-like planet orbiting a nearby star (e.g., Cockell et al., 2009), what would our next steps be? There is a limit to how much we can hope to learn about this new biosphere using astronomical techniques alone. To fully understand the diversity of an alien ecosystem, including its biochemistry and its evolutionary history, will require transporting sophisticated scientific instruments across interstellar space. This is not the place to discuss the technical challenges and proposed solutions of interstellar spaceflight; the interested reader will find reviews given by Crawford (1990) and Matloff (2000). What is clear is that, while the first interstellar probes will certainly themselves be robotic, the scale of the undertaking, and the highly energetic energy sources that will have to be employed, will require that their construction take place in space. The potential long-term scientific benefits of interstellar space probes are incalculable (and are discussed in more detail by Crawford, 2010), but it is important to realize that a significant human spaceflight infrastructure in our own solar system will almost certainly be a prerequisite for developing this long-term capability.

8. Policy Implications

There are a number of implications for the development of human spaceflight policy, as it relates to astrobiology, that follow from this discussion. First, it is important to realize that a space program of this scale will not be conducted for scientific purposes alone. Both the Global Exploration Strategy (GES, 2007) and the Augustine Commission (2009) identified a range of industrial, economic, societal, and geopolitical factors that are at least as important as scientific considerations as drivers for human space exploration. Some of these nonscientific drivers, such as the stimulus for economic growth and the encouragement of international cooperation, are desirable in themselves even in the absence of any scientific benefits. Thus, although I have argued here that science in general, and astrobiology in particular, will benefit from an ambitious program of human space exploration, scientists should not fall into the trap of believing that such a program will be pursued solely, or even mainly, for their benefit. The totality of the case for investing in human spaceflight is a complex mixture of scientific and societal factors (Crawford, 2004; Fong, 2004; GES, 2007), and any responsibly formulated public space policy must take a holistic view of the scientific and nonscientific benefits together.

That said, if governments do decide to invest in human space exploration because of multiple perceived benefits in a number of different policy areas, it is up to the scientific community to advocate an implementation from which science stands to gain the greatest benefit. From the point of view of astrobiology, the above discussion points to a preferred program with the following elements:

The development of launch vehicles and other infrastructure sufficient to return humans to multiple sites on the lunar surface for extended periods for the purposes of conducting geological and biological field activities (this will include the development of capabilities in surface mobility, subsurface drilling, and in situ sample analysis, in addition to a significant sample return capacity);

The development of vehicles, infrastructure, and operational experience (including mitigation of the physiological effects of enhanced radiation and low-gravity environments), which will permit human missions to the vicinity of Mars and NEOs, as well as the tending of future astronomical instruments in deep-space locations;

The development of these human infrastructure elements in parallel with continued robotic missions to Mars, such that once these have completed an initial reconnaissance of the near-surface environment of the planet (including a thorough search for extant near-surface life), human expeditions can safely and efficiently be mounted in order to conduct more detailed exploration; and

The assurance that the infrastructure developed to pursue these objectives is also compatible with the launching (or in-space construction) of the next generation of large space-based telescopes and advanced robotic missions (including sample return missions) to the outer Solar System.

Either the “Moon First” or “Flexible Path” options considered by the Augustine Commission (2009) would go a long way toward meeting these requirements, provided the latter option includes an early “off ramp” to the lunar surface. It is also clear that a program along these lines would benefit from being an international endeavor, as envisaged by the Global Exploration Strategy (GES, 2007).

9. Conclusions

A number of ways in which a rigorous program of human space exploration will benefit the study of astrobiology have been identified. These include the exploitation of the lunar geological record to elucidate conditions on the early Earth, the detailed study of NEOs for clues relating to the formation of the Solar System, the search for evidence of past or present life on Mars, and the development of sophisticated space-based astronomical tools for the study of extrasolar planetary systems. In all these areas, I have argued that a human presence in space, and especially on planetary surfaces, will yield a net scientific benefit over what can plausibly be achieved by relying on autonomous robotic systems alone. Astrobiologists therefore have a strong interest in the implementation, over the coming decades, of an ambitious international program of human space exploration, such as that envisaged by the Global Exploration Strategy (GES, 2007).

Footnotes

Acknowledgments

I thank the referee, Dr. Chris McKay, for comments which have improved the quality of the manuscript.

Author Disclosure Statement

No competing financial interests exist for the author of this manuscript.

Abbreviations

GES, Global Exploration Strategy; ISS, International Space Station; LEO, low Earth orbit; NEO, near-Earth object.

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