NEW YORK - When astronomer 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."
Author 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, but we cannot take even a small risk with a billion lives," Professor Sagan wrote.
Scientists have long considered Prof Sagan's warnings in mostly hypothetical terms. But over the approaching decade, they will start to act concretely on backward contamination risks. US space agency Nasa and the European Space Agency are gearing up for a shared mission called Mars Sample Return. A rover on the red planet is currently scooping up material that will 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 they are not 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 zero per cent chance, we are doing our due diligence to make sure that there's no possibility of contamination," said Dr Andrea Harrington, the Mars sample curator for Nasa. Thus, the agency plans to handle the returned samples similarly to how the Centres for Disease Control and Prevention (CDC) handles the Ebola virus: carefully.
"Carefully", in this case, means that once the Mars samples drop to Earth, they must be 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 has 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 sample return mission is on time - admittedly a big "if" - Mars rocks could land on Earth by the mid-2030s. It could take about as much time to build a facility that can safely contain the Martian materials, and that is if it is built on schedule, without disruption from political or public challenges.
Because no existing lab was both contained and clean enough for Nasa, Dr Harrington went on a tour of some of the planet's most dangerous facilities. She was joined by three colleagues, and they called themselves "Nasa Tiger Team Rama". While this moniker sounds like the name of a military scouting party, it is an acronym out of the first names of the team members - Mr Richard Mattingly of Nasa's jet propulsion laboratory; Dr Andrea Harrington; Mr Michael Calaway, a contractor for the Johnson Space Centre; and Dr Alvin Smith, also of the jet propulsion laboratory.
The group visited hot spots like the National Emerging Infectious Diseases Laboratories in Boston, the US Army Medical Research Institute of Infectious Diseases at Fort Detrick in Maryland, and the CDC's ominously and vaguely named Building 18 in Atlanta.
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 optimise 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.
Materials from around the solar system have come to Earth for study before: Moon rocks and dust from American, Soviet and Chinese missions; samples from two asteroids collected by Japanese probes; and particles from solar wind and a comet gathered by spacecraft.
But Mars presents what Nasa deems "significant" backward contamination risk, so samples from the red planet fall under a legal category called "restricted Earth-return". Dr Nick Benardini, the planetary protection officer at Nasa, said: "We have to treat those samples as if they contain hazardous biological materials."
Dr Benardini oversees policies and programmes that try to prevent Earth's microbes from contaminating planets or moons in our solar system and extraterrestrial material from hurting Earth.
Dr John Rummel, who served two stints in the office between 1987 and 2008, thinks it is right for the space agency to take the risks seriously, even if they are slim and seem like science fiction.
"There are significant unknowns with respect to the biological potential," he said. "A place like Mars is a planet. We don't know how it works."
Part of the point of Mars Sample Return is, of course, to figure out how the planet works - something that cannot be done properly on site because scientists and their myriad instruments cannot travel there yet.
The mission has already begun, however. Nasa's Perseverance spacecraft, which arrived on Mars in 2021, is gathering and caching samples for future pickup. The samples will then be shuttled by the same rover or a robotic helicopter to a lander with a rocket. The rocket will then shoot them to Mars orbit, where a European-built spacecraft will catch the material and fly it back towards Earth.
Once the spacecraft approaches this pale blue dot, optimistically in 2033, the samples will fall to the desert of the expansive Utah Test and Training Range, Earth's own Martian landscape. Then, scientists can study the samples with the heavy instrumentation that Earth labs allow. NYTIMES