History will note that the guy who discovered liquid water on Mars was an undergraduate at the University of Arizona, a 20-year-old who played guitar in a death-metal band and worked in a planetary science lab. One day, while comparing different satellite images of a single Martian crater taken at various times of year, he noticed something odd: a set of dark streaks in the soil that grew in the Martian summer and shrank in the winter. They seemed to flow down the crater’s slope, like a spill.

It took NASA a few years to gather more evidence after the student made his report, but finally, in September of 2015, the agency called a big press conference. It confirmed what the undergraduate had suspected almost right away: That was water in that crater.

Back in the 1970s, NASA scientists had informed everyone that the Red Planet was a dry, barren, dead place. Whoops. Now a new generation of NASA scientists were on a dais in Washington, DC, musing openly about what this new finding meant for the odds of discovering life on Mars. “When you look at Earth, everywhere we go where there’s liquid water,” said Jim Green, the agency’s director of planetary science, “we find life.” And the Martian water wasn’t confined to that one crater wall. Once scientists knew what to look for, they found similar dark streaks at more than a dozen other sites. The agency’s Curiosity rover was actually within striking distance of a few of these streaks. “We might be able to visit,” Green said. The announcement made headlines around the world. It also set off a bunch of quiet changes within the space agency itself.

About a month after the press conference, a NASA administrator named Cassie Conley was sitting in her office, staring into her computer screen at a crudely designed website called UFO Sightings Daily. She’d gotten a tip from someone at an astrobiology conference that she might want to check out a particular image posted there.

The site was a fairly typical UFO conspiracy operation, run by an amateur sleuth. There were posts that claimed to offer photographic evidence of miniature alien women, tiny star destroyers, and extraterrestrial squirrels. The item that Conley had come looking for was a photograph taken by the Curiosity rover and annotated by the website’s author. As purported evidence of intelligent life on Mars, the UFO buff had circled a pile of rocks and labeled them “building with a doorway”—which was just silly. But that streak in the soil descending from a crook between two rocks? The guy had labeled it “water.” And it really did look like water.

The closest dark streaks to Curiosity that NASA had previously found were about 2 miles away, up the steep slopes of Mount Sharp. These new ones were a few feet from the rover—Conley could see its tire tracks imprinted on the sand nearby. She picked up the phone.

She and the Curiosity team needed to talk.

These Are Exciting times for anyone who loves the thought of going to Mars. In just the next five years, the world’s space agencies aim to send five missions to the Red Planet, which will more than double the number of active robots and landers on its surface. In late April, SpaceX announced that it will send an unmanned Red Dragon capsule—the vehicle it eventually plans to use for a human round trip—to the planet as early as 2018. Between the star power of Elon Musk and the success of last year’s movie adaption of Andy Weir’s The Martian, American pop culture has space on its mind again. The notion of colonizing the Red Planet has become respectable, as if it were no more far-fetched (and maybe less) than favoring Marxism.

There’s just one problem: It’s looking more and more likely that Mars might already be inhabited—by Martians. Very tiny ones. Which concerns Cassie Conley.

Conley’s full-time job at NASA is to make sure that we don’t royally screw up our first encounter with aliens, however small. Her official title is US planetary protection officer. She’s a kind of interplanetary sheriff, whose main duty is to police the comings and goings of the tiniest Earthlings: microorganisms, which as it turns out are extraordinarily good at hitching rides on spacecraft. As NASA’s increasingly large and sophisticated fleet of robot explorers has spread through space over the past decade and sent thrilling findings home—water on Mars, huge geysers erupting from Saturn’s moon Enceladus, churning seas beneath Europa’s ice—Conley has logged 14-hour days making sure those robots don’t infect any heavenly bodies with germs from Earth. The more likely a destination is to support life, the more stringent she is.

The nurse pressed a phone to Conley’s ear. “Your worms survived the crash,” the voice said.

Conley’s tiny department at NASA, the Office of Planetary Protection (motto: “All of the planets, all of the time”), dates back to the Cold War, when the Soviets and Americans signed a treaty to prevent the contamination of space. One of the core rationales for the office’s existence is purely scientific. If alien life exists, researchers will of course want to study how it originated and evolved—to glimpse what planetary scientist Chris McKay calls a “second Genesis.” And to prevent false leads, humans have to be careful not to muddy space with our own trail of bacteria.

Another less official rationale for today’s planetary protection policy is ecological—you might even call it anticolonial. Essentially, Conley’s office serves to prevent NASA from doing to Martians what European explorers did to Native Americans with smallpox. Because Mars lacks Earth’s history of abundant life, it has that much more raw material for Earth’s bacterial stowaways to devour—should any of them, say, come into contact with water, find a niche they can survive in, and start to reproduce. “The whole planet is a dinner plate for these organisms,” she says. “They will eat Mars.” Conley wants to make sure we at least know whether Martian life exists before we introduce an invasive species that will wipe it out.

And the third main rationale for Conley’s office is, well, mildly apocalyptic. In the late 1960s, when the space age was brand-new, the specter of interplanetary contagion ran high in the public imagination. In 1969, Michael Crichton published The Andromeda Strain, his novel about an extraterrestrial microbe that drops to earth on a satellite and wipes out an Arizona town. Two months later, when the Apollo 11 crew returned from their triumphant moon landing, NASA spirited the astronauts from their ocean splashdown to a hermetically sealed facility in Houston, where they were isolated, poked, prodded, and scrubbed with bleach for two weeks before they could receive their full hero’s welcome. (A priceless photo shows Richard Nixon using a microphone to chat with the astronauts, who are locked, grinning, inside a sealed Airstream trailer.) In those days, planetary protection was primarily about us—about guarding this planet against potentially virulent alien life.

Those public fears gradually died down when the moon proved lifeless and the Viking rovers in the 1970s seemed to show Mars to be a barren place. But those findings turned out to be wrong—so now, who knows?

In short, when Conley picked up the phone after her visit to UFO Sightings Daily, it wasn’t to spur Curiosity’s team on toward that possible water. It was to keep the rover away from it.

In 2010, University of Arizona undergraduate Lujendra Ojha noticed dark streaks running down Martian hillsides in satellite images. NASA confirmed that the streaks were briny surface water in 2015. NASA / JPL-Caltech / Univ. of Arizona

Conley is a small person, slender and barely more than 5 feet tall. When we meet at NASA’s Goddard Space Flight Center in suburban Maryland, she all but disappears inside her heavy coat. But if anyone was born to the job of planetary protection officer, she is probably it. Her father was a mathematician who did work for NASA. Her mother was a biologist who studied fruit fly genetics. The family hamster was actually named J. B. S. Haldane, after a famous 20th-century British geneticist—whose theories about the origins of life helped inspire the idea of planetary protection.

Conley started out working for NASA as a research biologist, specializing in experiments that used nematodes, or roundworms—organisms the size of a speck of lint—as animal specimens. In January 2003, she managed to get one of her experiments sent up on space shuttle Columbia. It was designed to test how nematodes’ muscle development and metabolism would stand up to prolonged time in zero gravity, the kind of exposure humans would need to endure on a journey to Mars.

But that February, during its return descent into Earth’s atmosphere, the shuttle disintegrated at an altitude of 227,000 feet while traveling at 22 times the speed of sound. Seven astronauts died.

For Conley, one wreck was followed by another. Eleven weeks later, she was riding down a California highway with two friends when another driver collided with their car, killing one of her companions and disabling the other. Conley awoke from three days of unconsciousness with five cracked vertebrae and a morphine drip. Soon after, a nurse pressed a phone to her ear. The New York Times was calling. The reporter said, “Your worms survived the crash.”

The debris field from the shuttle disaster had stretched over hundreds of miles of Texas and Louisiana. Workers spent months searching through the wreckage, and they had just recovered five small aluminum canisters. Inside, most of Conley’s worms were still alive—dormant from months without food but otherwise none the worse for wear.

The twin crashes imbued Conley with a visceral sense of how hardy simple organisms can be. Life seems fragile to people, because people can live only in a very small range of conditions. No air, food, or water: We die. Too hot or too cold: We die. If our vehicle explodes at 227,000 feet—or just runs into another car on the road—we die. Keeping a small number of humans alive on a voyage into space is a fantastically complicated, expensive problem. Complex life is delicate and rare. But most life is simple, abundant, and incredibly strong.

Setting out to kill all the bugs that would tag along on a space mission is, it turns out, a pretty elaborate and costly proposition in itself. But that became Conley’s job in 2006, when she signed on as America’s planetary protector in chief. Day to day, her work involves calculating the odds that any given piece of equipment might encounter alien life (very low on a waterless body like our own moon; higher for spacecraft nearing Europa) and dialing in a regimen of cooking, sterilizing, and scouring in order to eradicate invasive Earth bugs accordingly. Every time scientists make new discoveries about Martian conditions, or even about the tolerances of extremophiles here on Earth, it alters the skein of probabilities that guide Conley’s planetary protection protocols.

Because it’s impossible to eliminate all of the microbes from a Mars rover, even the most stringent decontamination protocols are defined in ratios and probabilities. The most Conley can do is make sure there are no more than 0.03 microbes per square meter of spacecraft surface area. That was the standard applied to the Viking mission in 1976. The job requires a research scientist’s openness to strange possibility and a Vegas oddsmaker’s statistical discipline.

To find out what it looks like to eradicate all those bugs, I take a trip to the Goddard Space Flight Center and find myself dressed from head to toe in a bunny suit inside a gleaming white clean room. To even enter the room I wrestle with a series of hoods, boots, gloves, and garments using an intricate choreography designed to prevent any contact with unclean surfaces.

Inside, a guy sits for hours a day in his own bunny suit, hand-assembling a mass spectrometer that will be sent to the Red Planet on the European Space Agency’s ExoMars rover in 2020. A special hood constantly blows air and particles away from his work surface. Above it, mounted in the ceiling, is a device that emits microbe-killing ultraviolet-C light. Every day, a technician cleans the room with an alternating protocol of isopropyl alcohol and diluted hydrogen peroxide (to outflank microbes that grow resistant to one or the other), and then NASA microbiologist Erin Lalime swabs for microbial cultures and tests for levels of contamination. Once the mass spectrometer is assembled, it will get cooked for 60 hours at 110 degrees Celsius. And then, of course, it gets cast into space. But some microbes will still make it through.

To help me contemplate how this is even possible, Lalime describes how the strands of DNA inside a bacterial spore exposed to extreme conditions will huddle together into a small ball surrounded by a thick shell of protein. At that point, the spore is almost completely devoid of energy and water: “metabolically, almost dead,” she says. Dormant spores have been revived after thousands of years in airless isolation. To them, the “harsh vacuum of space”—Conley likes to put this phrase in air quotes—is a Sunday stroll.

Conley’s job is, by nature, a pretty lonely one. She’s a microbiologist in an agency dominated by physicists and engineers, a woman in a field dominated by men, and a sheriff (someone even gave her a joke badge) who stands a head shorter than many of her colleagues. Her work has made her an unpopular figure at times. To many engineers, geologists, and Mars enthusiasts, the whole project of planetary protection simply represents an expensive set of precautions taken in the name of extremely low-probability events. (To hear some critics talk, you’d think that Conley is to NASA what the unctuous EPA agent Walter Peck was to the Ghostbusters.)

But Conley is undeterred. History, she says, is littered with stories of human carelessness and environmental devastation, born not from malice but from cognitive limitation, a failure to contend with the murkiness of unknown unknowns. “We didn’t think we were going to make parts of Central America uninhabitable for hundreds of years by bringing malaria from Europe. We didn’t think we were going to cover the southern United States in kudzu. I never thought about whether nematodes could survive a shuttle crash. Then,” Conley says, “it crashed.”

The Indestructibles ——————-

Space: If it’s not boiling hot, it’s freezing cold, and the radiation is everywhere. But if it seems impossible that extraterrestrial life could ever thrive there, take a look at Earth’s own extremophiles. The more researchers learn about how these tiny organisms eke out an existence in our planet’s least hospitable niches, the more plausible it seems that life may have evolved on other planets. —Chelsea Leu

Tardigrades These rotund little animals can withstand extreme heat, cold, pressure, and radiation. In 2007, the European Space Agency sent more than 3,000 tardigrades into space for 12 days. Two thirds of them came back unscathed. Nematodes Also known as roundworms, some of these critters can freeze their metabolism when there’s no food or oxygen and enter a state called cryptobiosis. Nematodes were the only known survivors of the space shuttle Columbia disaster. Cave Bacteria In 2006, a group of microbiologists discovered a colony of bacteria in a deep fracture of a South African gold mine, more than a mile down. The colony had survived for millions of years without ever being exposed to the sun. Bacillus pumilus The spores of this bacterium display unusually high resistance to the treatment that NASA uses to sterilize rovers before they’re sent off to space. So it’s a decent bet that some of these are still stuck to the Curiosity rover, now on Mars.

The curiosity rover wasn’t supposed to be high on Conley’s list of worries. NASA had deliberately sent it to Gale Crater, thought to be among the least likely places to harbor life, because the rover was largely built for geological research. She’d helped pick the landing site herself. When Curiosity left Earth in 2011, it was subjected to milder decontamination controls: It was allowed 300 organisms per square meter.

Now, trundling around a possibly somewhat damp Gale Crater millions of miles away, Curiosity was very likely infested with tens of thousands of hardy Earth microbes that had survived the violent blastoff and months-long journey through the harsh vacuum of space. All they needed to reanimate and reproduce was the right combination of food, water, and heat.

At the meeting that eventually got called, Ashwin Vasavada, lead scientist in charge of Curiosity, said he was skeptical that the newly discovered streaks—the ones from UFO Sightings Daily—were actually water. His own research suggested that conditions in Gale Crater do occasionally produce briny surface water, but that these particular streaks were probably just tiny avalanches. Still, “probably” wasn’t really enough for Conley to go on. Vasavada agreed, because, as he later allowed, “we can’t be confident that we have a full understanding of these predictions.” Together, they decided that the Curiosity team would have to scrutinize each day’s new batch of photos for signs of water before moving ahead.

Vasavada knew he had gotten off fairly easy. “The people who are really having trouble with Planetary Protection,” he said later, “are the next mission.”

“There are huge battles going on inside NASA over planetary protection.”

The mission he was referring to is called Mars 2020, which will land another robot rover much like Curiosity. In fact, the 2020 rover is being assembled partly with equipment left over from Curiosity’s build. The new rover will be designed to search for signs of ancient microbial life on Mars and—even more ambitiously—to collect soil samples that can be retrieved by another spacecraft and sent back to Earth. The project is being developed at the Jet Propulsion Laboratory in Pasadena, California. And while the project’s leaders declined to talk to me, observers inside and outside of NASA say they’re in a heated battle with the Office of Planetary Protection over where they can land and what measures they need to take to sterilize their craft before it leaves for Mars. Because much of their equipment was designed or built well before the discovery of water, it wouldn’t withstand the strict protocols Conley would require today. And with the 2020 launch window rapidly approaching, the project has fallen behind schedule in getting its decontamination procedures approved by Conley’s office.

“There are huge, huge battles going on inside NASA,” says The Martian author Weir, who has become close to several people at the agency. “The planetary protection process is a huge burden to the design of these probes,” he says. “It makes every aspect more difficult.”

In 2013, a pair of astrobiologists published an article in Nature Geoscience arguing that planetary protection should be all but abandoned on the Red Planet. “If Earth microorganisms can thrive on Mars, they almost certainly do,” they wrote, reasoning that large meteor strikes had probably already sent organism-infected chunks of Earth crashing into Mars. And if Earth life can’t survive on Mars, they contend, we shouldn’t worry about sending more.

Conley has a response to this argument. Even if a microbe on Mars shares the same deep origins as us, a few thousand years down a separate evolutionary pathway would still make them pretty alien indeed. But ultimately, the best case for caution, and for planetary protection, may be simply to point to all the ways in which our knowledge of Mars has changed just in the past few years. Curiosity ended up almost on top of possible liquid water because we didn’t know what we didn’t know. In a way, Conley’s deepest arguments boil down to a quote from Haldane: “The universe is not only queerer than we suppose, but queerer than we can suppose.”

Sometimes, critics of Planetary Protection call the office a farce precisely because, well, we all know what’s coming: When humans arrive on Mars—with their coughs and sneezes and microbiomes—many of these sterilization protocols will go out the window. Conley understands that all too well. She knows that someday, probably before midcentury, men and women will walk on Mars. And to her, that means there is a small, precious window to learn as much as we can—and just maybe discover life that is truly alien to us—before they get there.

Kevin Carey (@kevincarey1) works at New America, a think tank in Washington, DC.

This article appears in the August 2016 issue.