Safe space: the cosmic importance of planetary quarantine
“This, what you’re doing today, never happens,” Nasa’s David Seidel told us. “This is a rare chance,” agreed the director of the Jet Propulsion Laboratory, Michael Watkins, welcoming us into the lab’s spacecraft assembly facility, located in the hills outside Pasadena, California. The exceedingly unusual adventure awaiting us was a trip into the clean room where Perseverance, Nasa’s latest Mars rover, having been assembled under conditions of exacting sterility, sat awaiting shipment to Cape Canaveral.
Our visit, in December 2019, had been prefaced by a long email laying out extremely detailed rules: we were instructed not to wear any perfume, cologne, makeup or dangly earrings; flannel, wool or frayed clothing was not allowed; even our fingernails had to be smooth, rather than jagged. After a quick welcome, our phones and notebooks were confiscated, and a hi-tech doormat vacuum-brushed the soles of our shoes. In the gowning room, we were issued with face wipes, a sterile full-body “bunny suit”, plastic overshoes, hood, gloves and face mask, then offered a mirror in which to admire the final look. Finally, we were sent through the air shower – an elevator-sized chamber studded with nozzles that blasted us with pressurised air from all sides, in order to dust off any final stray particles – before stepping out into a whitefloored, white-walled room filled with white-suited engineers.
The rover itself – a white go-kart the size of an SUV – was cordoned off behind red stanchions. The obsessive attention to cleanliness required in order to enter the rover’s presence was, in part, to protect the machine’s sensitive optical equipment and electronics: volatile chemicals, loose fibres or even flakes of human skin could damage its delicate circuitry or settle on one of its 23 camera lenses. But the primary purpose was planetary quarantine: preventing the importation of Earth life to Mars. “I don’t know that we can say it’s the most sterile object that humans have ever created,” said one engineer. “But it’s extremely clean.”
It is a condition of space exploration that we cannot search for life on alien planets without bringing along very small amounts of very small Earth life. This process is known as forward contamination, and minimising it, if not preventing altogether, is the ultimate responsibility of Nasa’s planetary protection officer – “the second-best job title at Nasa,” according to Cassie Conley, who held it from 2006 until 2018. (The best job title, Conley told us, was director of the universe, but that position was sadly eliminated in an institutional reorganisation.)
The practice of planetary quarantine dates back to the 1950s, when it became clear that rocket technology was shortly going to put outer space within human reach for the first time. In an ideal universe, the robotic spacecraft that we send to explore the cosmos would be sterile. (Humans are, by definition, contaminants.) In reality, for technical and economic reasons, they are not. But the consequences of transferring biological material between celestial bodies are a fractal example of unknown unknowns: we don’t know what forms of Earth life might survive a space journey, which of them might then flourish in whatever extraterrestrial conditions await them, and whether life even exists elsewhere in the solar system, let alone how it might be harmed by Earth life – or vice versa.
Faced with such extreme uncertainty, but unwilling to stay at home, spacefarers, like so many before them, have turned to quarantine as the buffer that will allow them to explore space without endangering Earth or inadvertently polluting the cosmos. Quarantine as a practice – a period of time, traditionally 40 days, in which a suspicious person or object is watched for signs of disease until proven safe – was formalised and named during t e Black Death in the 14th century. Its rationale, and its implications, have become familiar to generations of humans during outbreaks over the subsequent centuries, most recently during the Covid-19 pandemic. On a daily basis, the complex calculus of quarantine is used to balance freedom of movement and risk in the global commodities trade – in cattle, for example, or citrus, or cacao plants. In a cosmic context, however, where the potential threat is almost entirely speculative but the stakes are existential, what role should quarantine play?
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The person in charge of protecting “all the planets, all the time” – as international planetary protection policy puts it – works out of a small office inside Nasa’s headquarters, a squat, undistinguished building in Washington DC. Just a couple of blocks north, in between the US Botanic Garden and the Air and Space Museum, lies a fairly recent addition to the Mall: the National Museum of the American Indian. It was established in response to the revelation that the National Museum of Natural History held the skeletons of nearly 20,000 Native Americans in its collections. Those remains, collected by force as the spoils of colonisation, are a reminder of the much larger toll incurred when two long-separated biospheres came into contact: “The greatest destruction of lives in human history,” according to the geographer W George Lovell. The concept of planetary quarantine arose at least partially in response to the catastrophic impact of that initial encounter.
It is impossible to know how many people lived in the Americas in 1491, before European explorers made first contact, but historians estimate that nine out of every 10 people in the New World died in the century or so that followed – most from infectious diseases. Before the conquistadors had even set foot in the major cities of South and Central America, their microbes had travelled ahead of them, passed from body to body, causing mass deaths. Without any previous exposure to smallpox, measles, influenza, typhus or diphtheria, the indigenous people of the Americas had no immunity to these common Old World diseases – and no concept of quarantine, having never had much need for it.
In 1957, as the Soviets successfully launched Sputnik and the cold war militarisation of space began to ramp up, some scientists began to worry that the encounter between Earth organisms and any lifeforms that might exist elsewhere in the solar system might also result in mutually assured destruction. Even prior to Nasa’s founding in 1958, the Stanford microbiologist Joshua Lederberg had begun making the case for an international agreement to prevent the contamination of extraterrestrial environments with Earth life, and vice versa. “We are in a better position than Columbus was to have our cake and eat it, too,” he wrote, arguing that planetary quarantine was essential to the “orderly, careful, and well-reasoned extension of the cosmic frontier”.
Lederberg seems to have been primarily motivated by concern for the scientific loss that would occur if Earth life wiped out alien life, rather than the ethical dimensions of such a tragedy. “The overgrowth of terrestrial bacteria on Mars would destroy an inestimably valuable opportunity of understanding our own living nature,” he argued.
Others felt humans had a moral responsibility to avoid causing harm elsewhere in the galaxy. CS Lewis, better known for chronicling the fantasy world of Narnia, also wrote a spacethemed trilogy in which he despaired at the idea that a flawed and sinful humanity, “having now sufficiently corrupted the planet on which it arose”, would overcome “the vast astronomical distances which are God’s quarantine regulations” and “seed itself over a larger area”. In the science community, one of Lederberg’s allies, the young astronomer Carl Sagan, later wrote that if there was life on Mars, humans must leave the planet alone. “Mars then belongs to the Martians, even if they are microbes,” he declared.
Largely as a result of Lederberg and Sagan’s campaign, the International Council of Scientific Unions, a non-governmental organisation dedicated to international cooperation in the advancement of science, formed Cospar, the Committee on Space Research, which still sets the ground rules for extraterrestrial exploration.
Cassie Conley, the former planetary protection officer, who often wears her hair in a long braid, is personally more aligned with Lewis. “I don’t particularly like humans,” she told us, as we sat in her office, the light outside fading. “I think we screwed up this planet well enough that we don’t deserve another one – but that’s just my personal bias and I’m very careful not to bring it into my job.”
Conley got that job when some of the tiny worms she had sent into orbit aboard the space shuttle Columbia in 2003, in order to study muscle atrophy in microgravity, were found to have survived the shuttle’s disastrous explosion. Her experiment provided an inadvertent demonstration that multicellular life might be able to survive a meteorite impact – and thus potentially spread between planets on meteors – and it caught the eye of then planetary protection officer John Rummel.
Rummel invited Conley to Washington on a year’s placement, and then, as he gradually eased his way toward the exit, left her to inherit the role. As a scientist, Conley is deeply curious about what we might find elsewhere in the universe. “I’m very interested in understanding the evolution of life,” she told us. But she is more invested in ensuring that we don’t do something that precludes the possibility of answering those questions before we even have the ability to ask them. “The best way to prevent forward contamination is simple: don’t go there,” she said. “But we’ve already decided we want to go there, so it’s a case of: in the absence of information, don’t do something that might reduce your ability to get information in the future.”
Back in the 1960s, as the scientific community tried to decide what form planetary protection should take, Nasa engineers were faced with two irreconcilable demands: internally, management insisted that anything the agency sent into space must be utterly sterile, while, on national television, John F Kennedy promised that the US would put a man – and his trillions of accompanying bacteria – on the moon by the end of the decade. In the absence of any absolute certainties, Cospar dithered, eventually deciding that planetary quarantine would have to operate based on a complex algebra of acceptable risk, in which the probability that a viable microbe would be brought to a planet on a landing craft would be divided by a guesstimate of how likely it was to survive there, in order to arrive at a global contamination allowance that could be divided between each spacefaring nation.
To fill in the parameters in that formula, Nasa began looking at the bacterial kill rates of different sterilisation techniques used in the foodprocessing industry, as well as in the army’s bioweapon laboratories at Fort Detrick in Maryland. Using a particularly hardy spore-forming bacteria as their model for a series of tests, Nasa scientists fumigated, irradiated and baked spacecraft components before smashing them to see how many bugs survived, lurking in cracks and in the threads of screws and bolts. They determined that it was possible to clean a spacecraft sufficiently well that only one in every 10,000 landings would transport a viable microorganism.
The likelihood that Earth life could survive on a particular solar system body was even harder to pin down. Somewhat arbitrarily, Cospar recommended that, for planets of biological interest, the total acceptable risk be kept to no more than a one in 1,000 chance of seeding another planet with terrestrial life in the course of exploring it. In the end, “acceptable” simply meant a figure that was the best engineers could achieve without breaking the budgets of member states’ space agencies. The total risk – a 0.1% chance of contamination – was then divided up among the spacefaring nations, with the US, as one of just two spacefaring superpowers, receiving nearly half of the total allocation.
Once astronauts get involved,