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Infecting Other Worlds

B. Randall Tufts, Richard Greenberg

When isolated ecological communities come into contact, the result can be opportunity or catastrophe, depending on one’s point of view. Some organisms find new places to colonize; others are destroyed. The changes are dramatic and can last forever.

Because extraterrestrial life may exist, planetary exploration could bring trouble if people are not careful enough. This danger was recognized decades ago, when astronauts ventured to the Moon. When the crews returned, they were quarantined to prevent "back contamination," the hazard that some infectious extraterrestrial germ might be riding with them. The safety procedures were largely symbolic: After all, who knew the incubation period for some hypothetical other-worldly microbe? Whether the hardware and samples returned needed sterilization was also largely a matter of speculation. Subsequent planetary exploration has not involved astronauts, nor have samples or hardware been returned, so back contamination has not been an issue. But forward contamination—that is, the infection of alien ecosystems by terrestrial organisms hitchhiking on a spacecraft—is a distinct possibility.

During the 1960s, considerations of forward contamination focused on Mars, the Moon being considered inhospitable to terrestrial microbes. It came as a great surprise when Apollo 12 astronauts retrieved, along with many rocks, the camera from a robotic probe sent to the Moon three years earlier, and subsequent investigation of that unit found bacteria inside that could still be cultured. With Jupiter’s moon Europa and its ice-covered ocean now a prime candidate in the search for extraterrestrial life, the possibility that similarly hardy microbial stowaways might be carried there creates concern.

By definition, forward contamination does not affect the Earth, so why care? To a large extent this question is one of ethics: Is it morally right to endanger life elsewhere? There are practical dimensions as well. One is the far-out possibility that we might antagonize potentially proactive enemies. That hazard seems remote, especially given that people have damaged so much life on Earth without having provoked conscious retaliation. A more plausible prospect is that a campaign of exploration would contaminate another planet before fully characterizing life there. If space probes destroyed or modified extraterrestrial life before finding out about it, they would fail to achieve one of the key goals of planetary exploration.

With just this concern in mind, the late Carl Sagan and Sidney Coleman derived in the early 1960s a quantitative requirement for the sterilization of spacecraft to be used in an anticipated program for the exploration of Mars. The probability that these missions would characterize Martian life before significantly contaminating that planet needed to be nearly 100 percent. Sagan and Coleman set, as an arbitrary value, the figure at 99.9 percent. That is, the chance of messing things up should be less than one in a thousand. Then, assuming conditions relevant to Mars and making educated guesses about the strategy that might be used to detect life, they showed how to compute the demands for sterilization. This work established the maximum acceptable probability of a single viable organism remaining aboard any vehicle intended for planetary landing: The number was 10–4, or 1 in 10,000.

Figure 1. Europa, one of the four Galilean . . .Click to Enlarge Image

That evaluation was adopted by the Committee on Space Research ("COSPAR") of the International Council of Scientific Unions in 1964, because in principle and according to treaty, establishment of standards and monitoring of compliance regarding planetary protection is an international effort. Although the result was given this impressive imprimatur, it had a rather shaky foundation. For one, it derived from a limited notion of what planetary protection is for (preventing destruction of alien life until after its study). It selected the level of acceptable risk arbitrarily. Also, it relied on a pre-Space Age understanding of the planet Mars and some rather crude assumptions about how well a terrestrial organism might survive the trip and colonize the new setting. Finally, the prescription was based on a 1964 guess about the nature of a future campaign for exploring the Red Planet.

Whether the COSPAR policy was ever appropriate for Mars is therefore highly questionable. And the prescription is certainly not appropriate for Europa, which has a completely different environment. How then should one go about setting proper criteria for planetary protection?

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