NASA's planetary protection policy and its implementation demonstrate to the public that NASA is safeguarding the planets, including our own, during space exploration. The U.S. is signatory to a 1967 international agreement, monitored by COSPAR, which establishes the requirement to avoid forward and back contamination of planetary bodies during exploration. To help implement requirements, NASA established a Planetary Protection Office and has issued a document, NHB 8020.12, that delineates planetary-protection requirements for all NASA robotic extraterrestrial missions. This document is specifically directed to: 1) the control of terrestrial microbial contamination associated with robotic space vehicles intended to land, orbit, flyby, or otherwise be in the vicinity of extraterrestrial solar-system bodies, and 2) the control of contamination of the Earth and Moon by extraterrestrial solar-system material collected and returned by such missions.

The increasing interest in Mars exploration and the long time elapsed since consideration of the scientific rationale for such exploration, have prompted a new look at the planetary protection requirements for forward contamination. In 1992, the Space Studies Board of the U.S. National Academy of Sciences recommended changes in the requirements for Mars landers that significantly alleviated the burden of planetary protection implementation for these missions. The recommend-ations were published in "Biological Contamination of Mars: Issues and Recommendations" and pre-sented at the 1992 29th COSPAR Assembly in Washington DC. In 1994, a resolution addressing these recom-mendations was adopted by COSPAR at the 30th Assembly and has been incorporated into NASA's planetary protection policy. As we learn more about Mars, the requirements may change again to reflect current scientific knowledge.

The academy's recommendations, sub-sequently adopted by COSPAR, recognize the very low probability of growth of micro- organisms on the martian surface. With this in mind, the policy shifts from probability of growth considerations to a more direct and determinable assessment of the number of micro-organisms with any landing event. For landers that do not have life-detection instrumentation, the level of biological cleanliness required is that of Viking prior to heat sterilization, which can be accomplished by class 100,000 clean-room assembly and component cleaning. This is a conservative approach that minimizes the chance of compromising future exploration. Landers with life detection would be required to meet Viking post-sterilization levels or levels dictated by the experiment. It is recognized that the sensitivity of a "life-detection" instrument may impose the more severe constraint on the mission.

Included in the changes to the COSPAR policy is the option that an orbiter is not required to remain in orbit for an extended time if it can meet the standards of a lander without life-detection experiments. Also, the probability of inadvertent early entry has been relaxed compared to previous requirements.

The policy for samples returned to Earth is directed toward containing potentially hazardous martian material. Concerns have included a difficult-to-control pathogen capable of directly infecting human hosts (extremely unlikely) or of a life form capable of upsetting the current natural balance of Earth's ecosystem. It is of paramount importance to address the potential public perception that might attribute an epidemic, personal illness, or unusual event to an introduced martian contaminant.

For a Mars sample return mission, all samples would be enclosed in a hermetically sealed container; the contact chain between the return vehicle and the Mars surface must be broken in order to prevent the transfer of uncontaminated surface material via the spacecraft exterior; and the sample would be subjected to a comprehensive quarantine protocol to investigate whether or not harmful constituents are present. It should be noted that even if the sample return mission has no exobiological goals, the mission would still be required to meet the planetary protection sample return procedures and the life-detection protocols for forward con-tamination, not only to mitigate concern of potential contamination but also to prevent a hardy terrestrial hitchhiker from masquerading as a martian life form. In today's environment, public concern and legal requirements (in multiple jurisdictions) would be significant drivers in mission planning and planetary protection implementation. It may be worthwhile to consider maximizing those experiments that could be done on the martian surface, thereby extending the time before a sample return and perhaps relaxing fears of back contamination, leading to delayed, and possibly reduced, cost.

Because of the high level of public concern over the possibility that a sample returned from Mars might contain components harmful to our health or to the Earth's biosphere, NASA must strive for public education as well as informed public and legal consent well in advance of a sample return mission. In part, the planetary protection office provides a visible regulatory function which might mitigate concerns of forward and back contamination.

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