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?
Space Ethics
The basis for any evaluation must be a moral or philosophical
principle. The one introduced by Sagan and Coleman could be applied
to Europa with appropriate calculations. But their standard is
self-serving, in that it does not address the well-being of life on
another planet, except that it should survive long enough to satisfy
human curiosity.
At the opposite extreme would be another principle—that
preventing any interference with life on other planets should take
absolute priority. We call this concept the "prime
directive," borrowing this term from the television series
Star Trek, which was contemporaneous with Sagan and
Coleman's work and the COSPAR resolution. Actually, in Star
Trek the prime directive usually applied only to protecting
alien societies, and even then it was readily discarded as needed to
advance the plot line. Yet the phrase seems appropriate here because
it conveys a certain absolutism. The problem with this principle is
that, if rigorously applied, it would likely bring exploration of
some of the most interesting moons and planets to a halt.
Is there another moral principle that might provide a rational basis
for developing a standard of planetary protection, one more
objective than the principle of Sagan and Coleman and less
constraining than the absolute isolationism of the prime directive?
We recently proposed the following candidate principle, which is
objective but not absolute. It is based on the idea that there is
already a process of natural cross-contamination, something so far
mostly quantified in the context of the terrestrial planets, which
are thought to exchange chunks of crust from time to time after a
large meteorite or comet hits and sends ejecta off into space at
escape velocity. Living cells could conceivably survive such a
journey: After all, many kinds of delicate organic molecules
(including, perhaps, the very molecules that allowed life to develop
here in the first place) are regularly carried to Earth within meteorites.
As long as the probability of people infecting other planets with
terrestrial microbes is substantially smaller than the probability
that such contamination happens naturally, exploration activities
would, in our view, be doing no harm. We call this concept the
natural contamination standard.
On one hand, the natural contamination standard for Europa may seem
nearly as strict and confining as the prime directive, because the
natural transport of viable organisms from Earth may be so difficult
that it provides an impossibly stringent criterion. On the other
hand, it may be equally difficult for organisms from southern
California or Florida, where most planetary spacecraft are built and
launched, to survive the voyage and proliferate in the cold, icy
environments of Europa. One will not know until after careful
scientific study.
We believe the natural contamination standard has considerable
merit, but there may be other good candidates as well. The point is
that before anyone can establish rules for the sterilization of
planetary probes in a meaningful way, some fundamental principle,
based on ethical and philosophical considerations, is needed.
Tainting an Alien Ocean
Recognizing that Europa might be vulnerable to forward
contamination, NASA recently commissioned a study by the National
Research Council to evaluate standards of planetary protection for
upcoming missions. At the heart of the resulting report, issued just
last year, is a specific recommendation: “The probability of
contaminating a europan ocean with a viable terrestrial organism at
any time in the future should be less than 10–4 per
mission.” The rationale given for this number was citation of
the 1964 COSPAR resolution for Mars, a source that is neither
appropriate nor relevant.
In fact, the proposed Europa standard is flawed for a completely
different—and rather disturbing—reason. According to
several members of the committee that prepared the report (including
a statement by its chair), the recommended 10–4
value did not really come from the COSPAR resolution they cited;
rather it was simply a compromise among the subjective judgments of
the members of the group. The reference to COSPAR was added
afterward to lend an appearance of objectivity.
Before NASA proceeds too much further with planning for its Europa
campaign, the scientific community needs to reopen the discussion of
proper guidelines for preventing forward contamination. The starting
point might be Sagan and Coleman’s
preserve-it-until-we-are-done-with-it, the prime directive, the
natural contamination standard or some other principle that gains a
consensus. Once such a principle is in place, quantitative standards
for mission design, construction and operations can be developed
using the kinds of scientific information assembled in the National
Research Council report.
Another important reason for continuing discussion is that knowledge
of Europa has increased tremendously—even since that report
was issued—thanks to interpretation of data from the Galileo
mission. There is now a considerable body of evidence that the icy
crust has cracks and openings that may connect with the interior
ocean. So the Europan biosphere, if it exists, may well extend to
within centimeters of the surface. Any assessment of the probability
of forward contamination should include the latest understanding of
these conditions.
The new work makes Europa an even more inviting target for
exploration, because its ice-covered ocean appears more likely to be
able to support life, and organisms might be available for sampling
more easily. Of course, Europa would also be even more vulnerable to
forward contamination than most planetary astronomers had thought
possible just a year or two ago. Recognizing the potential
vulnerability, NASA should not continue with exploration of the
Jovian system—indeed, the agency should probably not go too
far even in planning—unless proper deliberations and
calculations are included to ensure that humanity will do no
inadvertent harm to any neighbors in space.