Since the time of Viking, the solar system appears to have become more rather than less interesting as a potential abode for extraterrestrial life, at least of the microbial sort. We also have a much more extensive appreciation of the widespread distribution and hardiness of Earth microbes, whether they are challenged by the extremes of heat, cold, desiccation, or radiation. The practice of planetary protection has become correspondingly more challenging as a result.
With respect to forward-contamination control, issues include the effective characterization and/or control of the load of Earth organisms carried by spacecraft and how to accomplish these tasks in the face of increasingly complex computerized systems and sensors. In facing the decontamination of complex electronics and machinery, however, NASA is not alone, and it is thought that many of the contamination-control solutions being developed for the bioengineering world will be adaptable to spaceflight missions. More esoteric questions involve the potential for survival and transport of organisms deposited on another worldwhether it be a place like Mars, with blowing winds and dust but little apparent surface turnover, or a place like the ice-covered moon Europa, where the specific processes that reshape its surface and allow surface communication and mixing with the subsurface material are not well understood. Both the likely liquid-water ocean under the Europan surface and the deep subsurface of Mars (or any near-surface aquifers that still may exist) seem potentially to be conducive environments for some Earth microbes. Practices and procedures to avoid the contamination of these environments during upcoming missions are under development. Additionally, there is an ongoing debate about the ethical considerations associated with the risks involved in solar-system exploration (cf. refs. 22 and 23).
Currently announced plans for sample-return missions and their planned return dates include Genesis (2003), Stardust (2006), the Japanese mission MUSES-C (2006), and the first Mars Sample Return mission (2011-2013). On the basis of the expectation for life to exist on the other solar-system bodies to be sampled, before launch such missions are examined for their potential for back contamination (24) and their potential to present a hazard to the Earth's biosphere. Of the currently planned missions, only the Mars Sample Return mission is thought to have any potential to introduce biological contamination, although even in the case of Mars the prospects for extraterrestrial life to be encountered on the surface are considered to be small (25). Nonetheless, the probability that a mission returning samples from Mars will return a living entity is considered to be nonzero, and the potential for such an entity to cause damage to the Earth's biosphere cannot be discounted, because even organisms from other terrestrial continents may be the cause of major ecological disturbances (cf. ref. 26).
Balancing the benefits of a sample-return mission against its potential risks is not strictly a task for planetary protection, but it is clear that avoiding the risks from such a mission carries no ethical quandary of the sort that accompanies forward contamination considerationsrather it is a question of simple prudence. To that end, the Space Studies Board (25) has provided a series of recommendations to NASA on how to approach such a mission (Table 1). NASA is proceeding to plan a sample return from Mars with those considerations in mind.
Currently, the analyses that will be used to determine that a Mars sample does not contain a biological hazard are under development, with a wide variety of participants and expertise being represented. Questions to be addressed in designing these analyses are listed in Table 2.
Additional considerations for a Mars sample-return mission include the need to reduce and/or characterize spacecraft bioload to accomplish forward-contamination goals and minimize the potential for Earth organisms to make the round trip and be misidentified as Mars organisms. Work such as that of Gladman et al. (27) and the evidence that the Earth is the target of a natural influx of material from Mars (e.g., ref. 28) suggests that Earth organisms may have been transported to Mars in the course of the last 4 billion years or so, and some of them may have survived there. Conversely, organisms that may have originated on Mars may have come to Earth in the past. One goal of the exobiological study of Mars will be to examine this issue, and round-trip contamination certainly would obscure the ability to address these questions. Other, more-mundane considerations include the selection of a safe landing site, the location and capabilities of a sample-receiving facility to accomplish the required planetary-protection analyses, and the means of moving a returned sample from the landing site to the receiving facility.
A far more interesting question, of course, will address the means for proceeding if life is ever detected in a Mars sample or in a sample returned from Europa or some other solar-system location.