MACROSCOPE
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.
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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