'' Plague '' from Mars ? '' Not unimaginable. '' It is not a 0% chance, so scientists are working to establish safeguards.

When Carl Sagan imagined sending humans to Mars in his book * The Cosmic Connection, *  published in 1973, he posed a problem beyond such a mission's cost and complexity ; the possibility that life already existed on the red planet and that it might not play nice.

'' It is possible that on Mars there are pathogens,'' he wrote, ''organisms which, if transported to the terrestrial environment, might do enormous biological damage - a Martian plague.

Michael Crichton imagined a related scenario in his novel. ''The Andromeda Strain.''

Such situations, in which extraterrestrial samples contain dangerous tag-along organisms, are examples of backward contamination, or the risk of material from other worlds harming Earth's biosphere.

''The likelihood that such pathogens exist is probably small,'' Sagan wrote, ''but we cannot take even a small risk with a billion lives.

Scientists have long considered Sagan's warnings in mostly hypothetical terms. But over the approaching decade, they will start to act concretely on the risks of backward contamination.

NASA and the European Space Agency are gearing up for a shared mission called Mars Sample Return. A rover on the red planet is scooping up material that is to be collected by other spacecraft and eventually returned to Earth.

No one can say for sure that such material will not contain tiny Martians. If it does, no one can yet say for sure that they would not be harmful to Earthlings.

With such concerns in mind, NASA must act as if samples from Mars could spawn the next pandemic. '' Because it is not a 0 percent chance, we are doing our due diligence to make sure that there's no possibility of contamination,'' said Andrea Harrington, the Mars sample curator for NASA.

Thus, the agency plans to handle the returned samples similarly to the way the U.S. Centers for Disease Control and Prevention handles the Ebola virus : carefully.

''Carefully,'' in this case, means that once the Mars samples drop to Earth, they must be held initially held in a structure called the Sample Receiving Facility.

The mission's planners say the structure should meet a standard known as ''Biosafety Level 4,'' or BSL-4, which means it is capable of safely containing the most dangerous pathogens known to science.

But it also had to be pristine : functionally, a giant clean room that prevents substances on Earth from contaminating the samples from Mars.

The agency has little time to waste : If the samples return mission occurs on time - admittedly a big ''if'' Mars rocks could land on Earth by the mid 2030s.

It could take about that much time to build a facility that can safely contain the Martian materials, that is if it is built on schedule, without disruption from political or public challenges.

Because no existing lab was contained and clean enough for NASA, four scientists, including Dr. Harrington, went on a tour of some of the planet's most dangerous facilities.

In total, the team visited 18 facilities that handled biological horrors, maintain ultra-clean rooms or manufacture innovative equipment for either purpose.Members hoped to figure out what has worked at existing labs and what a NASA facility could appropriate or optimize to keep humanity safe.

To scientists like Dr. Harrington, the hurry and hurdles are worth it. ''This will be the first sample return mission from another planet,'' she said.

The first time another world has met humans, in other words, because humans introduced them.

The Publishing of Mars Essay, continues. The World Students Society thanks author Sarah Scoles.


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