The prevention of forward and backward contamination is the goal of planetary protection as stated in the NASA policy,
which focuses on the protection of science and the Earth. The prevention of backward contamination has been of practical concern only once during the history of the U.S. space program
during the initial Apollo missions to the Moon. At that time, the implementation of steps to avoid back contamination was handled by the manned-spaceflight organization, separately from the activities of the nascent Planetary Quarantine Office, which was concerned chiefly with robotic missions. A recent review of this activity is given by Allton et al. (19). Although many scientists at the time had come to the conclusion that the Moon was a very unlikely place to encounter extraterrestrial life, NASA determined to be cautious and to provide for a quarantine of the returning samples and astronauts. One of the most obvious lessons of this activity, however, was the difficulty of ensuring the protection of Earth from an unknown and low-probability threat while ensuring the safety of three very real and at-risk astronauts during the process. This juxtaposition inevitably led to compromises that were considered by some to have reduced the effectiveness of the lunar quarantine. Nonetheless, the astronauts from both Apollo 11 and 12 were quarantined for 30 days after their return to Earth, and the samples were subjected to an extensive life-detection and biohazard protocol (20). These analyses, however, detected nothing alive in materials returned by the early Apollo missions (19), and the quarantine was not continued for Apollos 14-17 (Apollo 13 did not land on the lunar surface). Under current policy, the Moon is considered to be effectively a part of the Earth.
Given the pervasive nature of life on Earth, it has been easier to envision the tradeoffs inherent in implementing forward contamination controls, and the arguments against these controls have been judged within a less-charged (if not always certain) framework. Under NASA's planetary-protection policy, the prevention of forward contamination has been practiced on all outgoing spacecraft but has been most notable when applied to spacecraft traveling to solar-system bodies of interest to the study of chemical evolution and the origin of life and where Earth life might survive. To date, Mars has been the only such body on which the United States has landed spacecraft. The first landings on Mars by the Viking missions, mentioned earlier, involved extensive design and implementation procedures intended to reduce greatly the biological load carried by the two Viking landers. In what was a heroic effort, each aspect of spacecraft assembly and test was focused on allowing the most stringent precautions to be used. In the process, each of the Viking landers were cleaned thoroughly and then heat treatedbaked in an oven for 30 h after the coldest contaminated point reached a temperature of at least 110°Cboth to protect Mars and to safeguard the spacecraft's biology package from contamination by Earth organisms. Results from Viking have indicated that most of the surface of Mars is less likely to support Earth life than once was thought (21). With the Space Studies Board's recommendation and COSPAR affirmation, these results have allowed for the deletion of the heat-treatment step for subsequent Mars landers (such as 1996's Pathfinder mission) that do not seek to detect life on Mars. Nonetheless, the Viking cleaning procedures still are considered to be the standard preparation for landings on Mars (Fig. 1), whereas missions seeking to detect life (e.g., by cultivation techniques) are subject still to full heat-treatment procedures or their equivalent.
Fig. 1. Before the mission's launch to Mars in 1996, microbiological assays are conducted on the Pathfinder lander spacecraft and the Sojourner rover at NASA's Kennedy Space Center. Although conditions on most of the Martian surface are no longer thought to warrant heat sterilization for the prevention of forward contamination, the prelaunch cleanliness requirements are strictly monitored nonetheless. Photo by Robert C. Koukol, Jet Propulsion Laboratory.
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