Please Sterilize Your Spacecraft

Almost 43 years after launching from Cape Canaveral in September 1977, NASA’s space probe Voyager 1 has traveled 13.9 billion miles from Earth—nearly 150 times the distance from our planet to the sun. With six of its 10 science instruments offline as the spacecraft conserves its dwindling power, Voyager 1 is cruising through space at more than 38,000 miles per hour with respect to the sun, which from that outlying location would look more than 22,000 times dimmer than it does to us. Voyager 1 is the farthest human-made object from Earth. On a cosmic scale, though, the probe has barely left home.

Although Voyager 1 has technically entered interstellar space, it’s still in our sun’s backyard: The probe won’t even enter the Oort Cloud, the most remote region of our solar system, for hundreds of years, and it will take tens of thousands of years to travel through it. Signals currently take almost 21 hours to travel to or from Voyager 1 at light speed, which means it’s less than a light-day away. The closest planet outside of our solar system, the potentially habitable Proxima b, is more than 4.2 light-years away. Were Voyager 1—which is moving at more than 10 miles per second, fast enough to circumnavigate Earth in under 40 minutes—pointed in the right direction, it would still take about 75,000 years to reach Proxima b, our solar system’s nearest neighbor. Proxima b’s star, Proxima Centauri, is one of hundreds of billions of stars in our Milky Way galaxy, which probably harbors billions of Earth-like planets. And the Milky Way is only one of 100 billion to 200 billion galaxies in the universe. There’s a mind-blowing amount of matter out there. And all but a few precious specks of it are inconceivably far away.

Much of our species has spent the past several months social distancing to slow the spread of the novel coronavirus, which has led to a lot of isolated and lonely lives. Even when we’re staying six feet (or farther) apart, though, we’re right on top of each other relative to the rest of the universe. Life—unintelligent life, at least—could be common on other planets; as 19th-century English thinker Thomas Carlyle said, “If they be not inhabited, what a waste of space.” But we can’t currently reach whatever would-be friends, foes, or microbes exist outside of our solar system. No human has flown as fast as Voyager 1 (although other NASA space probes have attained higher speeds), and we must go faster if we want to see other suns in person. While we have ideas about how to do that, those sci-fi solutions are still speculative. At present, the vast expanse of space is the ultimate order to shelter in place. For its whole history and its foreseeable future, humanity has been and will continue to be quarantined from all but a handful of the potential locations of extraterrestrial life.

At irregular intervals, however, we break local quarantine and send emissaries to other worlds in our sun’s vicinity—such as Voyager 1, which passed by and snapped pictures of Jupiter, Saturn, and Saturn’s largest moon, Titan, before peacing out of the solar system. And whenever we send a spacecraft somewhere that could be hospitable to life, we try to ensure that our spacecraft aren’t carrying microscopic stowaways. Just as we wear masks and use hand sanitizer to avoid spreading disease during a pandemic, we sterilize our spacecraft to keep contamination to a minimum, a practice known as planetary protection. “Planetary protection is such an important part of any of these missions that go and explore environments where conditions are suitable for Earth life as we know it,” says Morgan Cable, a research scientist and group supervisor in the Astrobiology and Ocean Worlds Group at the NASA Jet Propulsion Laboratory. “Because what would be worse than finally discovering life for the first time somewhere else and then realizing that we put it there?”

Brian Shirey is one of JPL’s planetary protection engineers—“guardians of the galaxy,” he jokes. It’s his job to prevent the kind of contamination Cable is talking about: organisms from Earth hitching an unticketed ride to another world, where their presence could confound scientists who are searching for extraterrestrial life. But that’s only one component of planetary protection. “Part of that is protecting the planets that we visit, but also protecting the Earth from anything that we could possibly bring back from another planet as well,” Shirey says. “So we have forward planetary protection and backward planetary protection that we’re both responsible for.”

Shirey and his colleagues are about to be tested again. On Thursday morning, NASA’s next Mars rover mission, Mars 2020, launched from Cape Canaveral, close on the heels of a Chinese Mars rover that launched last week. If all goes as planned, the Perseverance rover will touch down in the red planet’s once-wet Jezero Crater next February. In addition to deploying a helicopter named Ingenuity and generating oxygen from Mars’s thin, 95 percent carbon-dioxide atmosphere—a useful proof of concept for future crewed missions to Mars—Perseverance will spend the next couple of Earth years searching for biosignatures, or signs of ancient life that thrived before most of Mars’s atmosphere was lost to space and its liquid water boiled away. A decade after the rover starts rolling, NASA hopes, another spacecraft will carry rock and soil samples collected by Perseverance back to Earth for further analysis.

Missions such as this one present a dilemma for inquisitive but conscientious spacefarers. “Humanity is very curious and we can’t not go,” Cable says. “So there’s this balance between wanting to explore these worlds, but not wanting to irreparably change them. And that’s something that NASA and a lot of other space agencies struggle with. We can do our best to be sure that our spacecraft are cleaned and limit where they end up going in some of these environments to reduce the risk as much as possible. It’s never going to be zero.” Traditionally, a one in 10,000 chance of contamination per mission has been deemed an acceptable risk.

Even Voyager required a planetary protection protocol, because of its close approach to potentially habitable worlds. (Jupiter’s moon Europa and Saturn’s moon Enceladus, whose icy crusts cover global subsurface oceans, are two of our solar system’s likeliest footholds for life.) But Mars 2020 has a higher bar to clear. NASA’s space protection policy classifies each mission under one of five categories pertaining to the threat of contamination it poses, ranging from Category I on the low end to Category V on the high end. A mission’s classification depends on both its destination and its type: A lander or rover faces stricter standards than an orbiter or a probe designed to fly by a world without stopping. Voyager I, which couldn’t contaminate a moon unless it inadvertently crashed into one, was classified as Category II.

“We’re a Category V mission, which is the highest, because we’re taking samples that possibly we will return one day,” says Shirey, who serves as the deputy lead for planetary protection on Mars 2020 and the planetary protection lead for Ingenuity. “And so that gives us the most stringent requirements that we’ve had to date.”

Those requirements are measured in terms of the maximum allowable number of spores, the metabolically dormant forms of some bacteria that can survive for hundreds or thousands of years in the harsh conditions of space or the surfaces of worlds with thin atmospheres. (Life finds a way.) “The entire flight system has an allocation of no more than 500,000 spores,” says Kristina Stott, another JPL planetary protection engineer who is working on Mars 2020 as well as Europa Clipper, a Category III mission that is scheduled to launch in 2024 and orbit the Jovian moon. “Sounds like a lot, but it’s actually not a lot. It’s almost equivalent to less than what you would find on your camera lens on your smartphone.” The parts of the spacecraft that are intended to land on Mars—including the rover, the parachute, and the descent stage that delivers the rover to the surface—can’t exceed 300,000 spores, and the rover itself is restricted to 41,000 spores. Many microorganisms don’t form spores but can still survive extreme conditions, so the spore count is a proxy for the total “bioburden” of unwanted passengers.

The Mars 2020 spore police belong to a larger Office of Planetary Protection that comprises 23 people, who are split between Mars 2020, Europa Clipper, and other research. They report to planetary protection officer Lisa Pratt, who was appointed in 2018 and also oversees partnerships with outside entities such as the SETI Institute. NASA’s planetary protection policies stem from the 1967 Outer Space Treaty, which has been signed by more than 130 countries. The treaty is best known for prohibiting the placing of nuclear weapons in space, limiting the use of the moon and other worlds to peaceful purposes, and establishing that no nation can claim sovereignty of space or any space objects, which may create conflicts with the United States Space Force. But Article IX of the treaty also binds its signatories to “conduct exploration of [the moon and other celestial bodies] so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter.”

Conversations about potential planetary protection started at the seventh session of the International Astronautical Congress in September 1956, predating not only NASA but spaceflight itself. “Concerns about avoiding biological contamination of other planets and also this one were first discussed the year before the launch of Sputnik,” says NASA astrobiologist Catharine Conley, the longest-serving former planetary protection officer, who preceded Pratt and occupied the office from 2006 to 2018. “I’m pretty sure this is the first time in history that humans as a species decided we would avoid doing things that could have consequences we didn’t understand, before we were technologically capable of doing them.”

Our technological capabilities have increased significantly since then. As we’ve developed the capacity to send spacecraft to potentially habitable worlds and operate them remotely on the surface, the pressures on the planetary protection group have increased. “As each mission progresses, we get more and more complex, and this rover is more complex than the last one,” Shirey says. “So with that increasing complexity, we also have to increase the complexity of all of our implementation strategies and plans.”

Planetary protection engineers rely on three main techniques to keep spacecraft as free of spores as possible. The first is assembling the rover and its interplanetary ride in “clean rooms,” where air is filtered and surfaces and floors are frequently treated to kill living microbes. (Some extremophiles can survive the clean rooms, although the frigid, irradiated vacuum of space is intimidating too.) The second precaution is issuing protective garments to act as barriers between engineers and the spacecraft. “We are, as humans, the biggest contributor to contamination,” Stott says—hence the “bunny suits” that are worn at all times in the clean room and include hoods, gloves, and masks. Third, engineers and technicians sterilize the spacecraft, in part by manually cleaning with polyester wipes doused with 70 percent isopropyl alcohol.

To assess the effectiveness of those measures, planetary protection officials regularly swab assorted pieces of the spacecraft using a long wooden stick with a Q-tip–type extremity. Then they process the samples using bioassay tests that tell them how many microbes were present at the time of the swab. “We have a database where we kind of roll up all of those numbers from all of the components that make up this spacecraft and say, ‘Are we under that 500,000-spore requirement that we have as a requirement for this mission?’” Stott says. “And I can tell you that the answer is yes. And that’s why we’ve been given the go-ahead to launch from a planetary protection perspective.”

At a certain point, the planetary protection team has to stop swabbing: Mars 2020’s planetary protectors took their last set of samples toward the end of last week, which left a week without testing prior to launch. “Getting a spacecraft clean is only part of the challenge,” Shirey says. “After you get it clean, you have to keep it clean.” He and his colleagues mitigate the risk of spore build-up before launch by “breaking that chain of contact from the outside environment to the spacecraft.” The rover is transported to the launchpad sealed inside the fairing of the Atlas V rocket that lifts it into orbit, so it isn’t exposed to any unclean conditions. NASA also employs a technique called “trajectory biasing,” in which the rocket is initially aimed on a trajectory that would miss Mars, so that if communications are lost en route to the red planet, the craft won’t crash into the surface (like the Mars Polar Lander did in 1999) and risk releasing microbes that may be embedded inside.

So far this all sounds simple enough (by NASA’s rocket-science standards). Of course, there’s a catch: Just as some hand sanitizers can be toxic to the humans they’re supposed to protect, some sterilization methods can be toxic to spacecraft. Certain surfaces and components are incompatible with isopropanol. And although the mid-1970s Viking landers were sterilized thoroughly by baking them inside a ceramic sheath at 233 degrees Fahrenheit for 30 hours, Shirey says that isn’t an option now. “Some of these components and equipment are very sensitive,” he says. “And so they can’t withstand a lot of these bake-out temperatures, real high temperatures. So we have to look at other ways to clean the spacecraft.” One new way that has helped exterminate spores on Mars 2020 is the use of vaporized hydrogen peroxide, which allows the planetary protectors to sterilize the spacecraft without subjecting delicate electronic equipment to high heat.

Mars 2020’s mission is to detect remnants of long-ago life, not microbes that could still be surviving in some less lethal alcove. (Some scientists still believe that the Viking landers detected life on Mars in 1976.) Thus, as a final safeguard, Perseverance will avoid designated “special regions” on Mars where water and weather are less hostile to life. Nonetheless, it’s crucial to keep samples as pristine as possible, although if any DNA is detected in a substance Perseverance collects, NASA can compare it to a genetic inventory of potential contaminants that may allow scientists to establish whether the specimen is Martian or originated on Earth (assuming that life on Earth didn’t come from Mars in the first place, via meteorite).

Shirey and Stott emphasize how closely they communicate and collaborate with the engineers who work on other aspects of Mars 2020. As Conley notes, though, on some missions, “There is very much an oppositional environment when you’re dealing with engineers who haven’t thought about planetary protection as part of their design constraints.” Engineers relish a challenge, she says, but when planetary protection measures are viewed as an add-on instead of a core requirement—first you told us to design a functioning rover that would work on Mars, and now you’re telling us we have to keep it clean?—they may be viewed as an imposition. “Viking is the only mission where planetary protection was actually integrated from the beginning,” Conley continues. “That’s how to do it right. If you try to add in heat sterilization after the fact, you’re simply going to discover that half the components that you picked are not tolerant to heat. In the JPL engineering design constraints for Mars missions, there is no guideline, there is no requirement to be heat tolerant.”

Conley is concerned about what she perceives to be a loss of outside input into planetary protection procedures. She cites the recent silence of the Planetary Protection Subcommittee, which she describes as a body composed of “external, non-NASA funded, disinterested people from a wide range of scientific fields.” NASA, she says, “shouldn’t be making these decisions because it’s a conflict of interest to both the regulator and the implementer.” But according to Conley, the subcommittee “was not allowed to meet since 2016 because they were advising NASA things about the Mars 2020 mission that NASA didn’t want to hear.” Others have echoed the call for an independent process, and NASA continues to update its policies. Ensuring the continued protection of other planets is, perhaps, of particular importance under a presidential administration that evinces scant concern for protecting this planet, let alone others. “I would say that the attitude of an incoming administration where the priority was on doing something, and not ensuring that whatever was being done was safe, is definitely a concern that was raised by the [Planetary Protection Subcommittee] before they got suspended,” Conley says.

Compared to Conley, Pratt has been receptive to loosening restrictions on exploring Mars, which some scientists support, given that humans may soon be bringing their microbes to Mars anyway. Conley acknowledges that attitudes about planetary protection vary based on backgrounds and expertise. “What the scientific community thinks of planetary protection is dependent on what their background is, and what their interests are, and really what their prejudices are, in terms of whether humans are good at stuff or not very good at stuff,” she says. “And if you are very confident that you know how to deal with anything that comes at you, you might think that planetary protection is less important.” Conley points out that the pandemic is a reminder of how hard it is to control terrestrial life, to say nothing of potential extraterrestrial life that we know nothing about. “We’re now dealing with something that a lot of people thought we would be competent to take care of as it was coming at us,” she says. “And obviously, we’re not.”

Although the pandemic hasn’t affected the priorities of the planetary protection group, the parallels between the tasks it performs and the preventative measures implemented on Earth amid the pandemic are inescapable. During the Apollo era, the Office of Planetary Protection was called the Planetary Quarantine Program. And some of the skill sets overlap: Before joining JPL in 2016, Shirey worked at the Centers for Disease Control and Prevention, where among other tasks he developed and tested protocols to aid federal, state, and local public health labs conduct outbreak investigations. Now he’s working to protect our planet and others from a different kind of contamination.

Cable asked what would be worse than encountering “extraterrestrial” life only to realize that it was a terrestrial false positive. Two even more tragic outcomes could occur. First, an Earth organism could do damage to an extraterrestrial one, either by attacking it directly or by outcompeting it and driving it into extinction. Mars isn’t an environment where life can easily circulate, and it’s unlikely that a Mars mission to a single spot on the surface would happen to destroy the last living Martian microbe. On a world where conditions are less desolate, though, an Earth invader could spread. “One of the major concerns about Europa is that it’s not just one location that gets contaminated, it’s the entire global ocean,” Conley says. If there are creatures of some kind in the pitch-black ocean of Europa, they would probably depend on geochemical or geothermal energy, like the life that clusters close to vents in the depths of Earth’s oceans. Those energy sources tend to be less rich than sunlight, which might limit those organisms’ abundance, size, or sophistication. But if life subsists in liquid water there, Cable says, “it’s going to potentially use similar building blocks to what we use,” unlike the more exotic life that could lurk in Titan’s methane lakes.

The other possibility is a real-life equivalent of the fictional fates laid out in Michael Crichton’s The Andromeda Strain, H.P. Lovecraft’s “The Colour Out of Space,” and Stephen King’s short story “Weeds,” in which an extraterrestrial organism proves toxic to life on Earth. It’s improbable (if not impossible) that a pathogen that arose independently from life on Earth could hijack our cellular apparatus to replicate itself, as a terrestrial virus does. But that doesn’t mean it wouldn’t do us harm. It’s more likely, Conley says, for there to be “a scenario where they’re not actually using our biological machinery, they’re just perceiving us as a bunch of minerals that are tasty, carbon being a mineral. And in that kind of scenario, then of course our immune system, which is tuned to recognize things from Earth, would be less likely to be able to fight that off effectively.” It’s also conceivable that to an extraterrestrial organism, Antarctica could be a paradise in which it would proliferate rapidly, melting the ice and accelerating sea-level rise as algae has in Greenland.

However far-fetched those doomsday scenarios seem, they illustrate why earthlings and E.T. alike need transparent and powerful planetary protection. “If the entire global community is willing to accept a level of risk, that’s a very different scenario than if a few people are willing to accept a certain level of risk and then don’t tell the rest of us what they’re doing,” Conley says.

If our robot ambassadors’ samples stay spore-free, we may soon confirm that Earth life isn’t alone. That would answer one of the burning questions that keeps Cable awake. But she isn’t worried about running out of topics to research if our data set of inhabited worlds expands from one to two. “A lot of the stuff that I work on, I’m going to pass down to the next generation of scientists, and they’ll pass down, and we may not find the answer for a hundred years,” she says. “But I am part of that story. And I love that. It’s a way of leaving a legacy.” Provided, that is, that an earthbound bug doesn’t destroy us in the interim—and that we don’t all catch an incurable cold from Mars.