For over half a century, humanity has been exploring our Solar System, whether in person or by proxy, and returning astronauts and spacecraft to Earth. For even longer, we have been imagining still more far-ranging explorations. But any such exploration carries the risk of contamination - a risk we may or may not wish to take. Planetary protection considerations fall into two broad categories: the dangers of organisms from outside the Earth’s biosphere contaminating our world, and the risk that Earth organisms may be carried into hitherto pristine environments elsewhere.

Earth under attack

Some of the early science fiction to consider biocontamination issues focused on the risk to Earth of life from space. This narrative can be traced back to the invasive red weed brought by the Martians in Wells’ The War of the Worlds, and is closely related to alien invasion stories and post-apocalyptic tales involving Earth-native disease or invasive plants and animals. However as our understanding of microorganisms and the nature of our Solar System developed, the texts became more detailed and scientific in their approach.

A quite surprisingly early example of a detailed discussion of biocontamination from space in the form of plant life can be found in the 1938 novella “Who Goes There” by John W Campbell (writing as Don A Stuart in Astounding SF). In this classic science fiction horror, an expedition to the arctic locates a crashed vehicle which appears to be a flying saucer. Finding a body buried in the ice nearby, the scientists of the arctic outpost discuss concerns which read as ahead of their time:

“There must have been microscopic life associated with this creature. There is with every living thing we know. And Norris is afraid that we may release a plague — some germ disease unknown to Earth — if we thaw those microscopic things that have been frozen there for twenty million years”

and later:
“Blair’s answer is that there may be such still-living germs, but that Norris has the case reversed. They are utterly non-immune to man”

before concluding:

“The thing is not Earthly. It does not seem likely that it can have a life-chemistry sufficiently like ours to make cross-infection remotely possible. I would say that there is no danger.”

It’s a conclusion they come to regret as they find themselves hunted by a newly revived shape-changing alien. The story was loosely adapted as the film The Thing from Another World (1951, dir. Nyby), with the alien in this case represented as a plant-person who proves to be alive, but dependent on mammalian blood to survive on Earth. Interestingly the discussion of possible cross-contamination of micro-organisms, and the threat they post to Earth is retained in the film, appearing around 28 minutes into the programme.

The story and successful film undoubtedly influenced other writers. When The Quatermass Experiment (BBC TV, 1953, written by Nigel Kneale) was broadcast, humanity was still almost a decade away from reaching and returning from space [1]. In the television serial, the experimental launch of the first three men into space goes disastrously wrong. On its return, Bernard Quatermass, director of the British Experimental Rocket Group, finds two men apparently missing and one unconscious. It gradually becomes clear that the rocket experienced an encounter in orbit which infected the surviving crew member, Victor Carroon, with a plant- or fungus-like organism that gradually subsumes his body. On microscopic study of samples, Quatermass establishes that the organism is likely to release spores and threatens not only the safety of the public, and the future of the rocket group, but potentially all life on Earth. As might be expected, Quatermass is eventually able to defeat the menace, destroying the organism with argument and fire.

Written in 1962, Hugh Walters' juvenile novel Expedition Venus begins with news that a grey mould is spreading across some regions with frightening rapidity. This novel is one of the Chris Godfrey of Unexa series of early rocket adventures, in which the main characters frequently visit space, land on other planets, and return without obvious signs of precautions against cross-contamination. Despite that, this book hints that at least some such precautions exist:

"You know the routine when a space probe returns," Benson went on. "It is subject to careful disinfection by radiation to prevent the introduction to Earth of any alien spores or viruses which might be harmful to us. Well - one of the probes wasn't disinfected carefully enough. It brought back from Venus a spore which escaped destruction."

Perhaps inevitably, Godfrey and his friends from the UN's Universal Exploration Agency are sent to Venus to find a solution.

The alien-organism comes to Earth trope - this time with a highly-intelligent, albeit monstrous, alien - also appears in a 1965 episode of Voyage to Bottom of the Sea: "The Monster from Outer Space" [2]. This imagines a 1978 probe return from Saturn. The majority of the story is a simple alien-mind-control-uncontrolled-growth narrative, but the story starts with some interesting (albeit flawed) ideas about planetary protection precautions. When life is detected on the approaching probe, efforts are made to destroy it before it reaches the surface. As the mission director says:

"We've got to kill it. We can't risk allowing a hostile lifeform to reach the Earth. It could be dangerous to all of us. Let me have your attention please: We've picked up an exobiological form on the space probe. I'm going to forego the planned reentry orbits and bring the craft into a penetration reentry. That will give us maximum burn-off. We may lose the instrumentation capsule, but we must kill whatever that is clinging to it."

When the probe lands after this procedure, contaminated regardless, it is sprayed with acid for further decontamination, irradiated with ultraviolet lights and then quarantined. Unfortunately these efforts prove ineffective. It's noteworthy that the organism considered a threat here is a macroscopic lifeform - there is no apparent concern for the bacterial or microorganism load that this and other space probes in the series might carry as well.

By contrast, The Andromeda Strain (1971, dir. Wise, based on a 1969 novel of the same name by Michael Crichton) was made during the era of human moon landings and returns to Earth. It imagines a satellite crashing to Earth, carrying an airborne pathogen that proves highly infectious and highly deadly. The story focuses on scientific efforts to contain, identify and destroy the pathogen, in the midst of an atmosphere of tense anxiety and paranoia. When the disease threatens to escape the Wildfire isolation laboratory, difficult decisions must be made. The film (and novel) ends on an ominous note, warning of what might come next and the likelihood of future incidents.

The idea of an alien biocontamination capable of devastating Earth can be found as a narrative driver in many other science fiction films and novels, beyond the examples above, including more recent cases.

Written in 2015, the short story “Riding the White Bull” by Caitlín R Kiernan is a dark story about an agency fighting a semi-secret war against an alien infection that is devastating humanity in a near-future 2069. As gradually becomes clear, the intelligent life form loose on Earth was returned from the ice moon Europa (one of Jupiter’s satellites) after a scientific probe mission penetrated its sub-surface oceans. Acting like a form of virus, it corrupts human forms and infiltrates their minds at the same time.

A more extended and unconventional take on the concept of an Earth biosphere contaminated by extraterrestrial life can be found in the Wormwood Trilogy by Tade Thompson (Rosewater, 2017; The Rosewater Insurrection, 2019 and The Rosewater Redemption, 2019). This describes the arrival on Earth of an entity with biology similar in some aspects to a plant, and in others to a terrestrial fungus. Travelling underground, it settles in a Nigerian city, creating what at first seems to be a utopia of medical miracles. However the development of human telepathy is a sign of the infiltration of neural tissue by “xenoforms” (a form of fungal mycelium). This is fundamentally changing the biosphere of Earth in ways which may not be entirely in humanity’s interests. Here the biocontaminant arrived on Earth through its own devices, but the inability of humanity to contain it, and particularly its all-pervasive xenoforms, speaks to the challenge of planetary protection issues.

Our Vulnerable Neighbours

While Earth might be vulnerable to contamination by returning spacecraft, other fiction has observed that our planetary neighbours are equally vulnerable to contamination passing in the other direction. Indeed, there is strong evidence that the Moon, at the very least, has already been contaminated by microorganisms through human action. Ninety-six bags of human waste were left behind on the Moon by human astronauts during the Apollo era. Still more worrying is the fate of the Israeli probe Beresheet. It was designed to carry a closed capsule containing tardigrades - a microscopic life-form that can tolerate extreme low temperatures and hard vacuum in a dormant state - in order to monitor their condition. However the probe crashed on the Moon, potentially scattering tardigrades over a large area (although it is now considered unlikely they survived the crash). While any survivors are expected to remain dormant under Moon conditions, they remain an example of a potentially-viable Earth life form that has been transported from our planet to one of our vulnerable neighbours.

In “Before Eden” (short story, Amazing stories, 1961), Arthur C. Clarke suggested that the first explorers to Venus might find extremophile life at high elevation in the (relatively) cool polar mountains [Warning: Spoiler for this story ahead]. This is a black patch of what might be vegetation or some form of colony life-form:

“He still thought of the thing as a carpet – a deep-pile one, ravelled into tassels at the edges. It varied in thickness as it moved; in some parts it was a mere film; in others, it heaped up to a depth of a foot or more. As it came closer and he could see its texture, Jerry was reminded of black velvet. He wondered what it felt like to the touch, then remembered that it would burn his fingers even if it did nothing else to them”

The story focuses on the wonder of the two explorers as they identify the life form and study it in situ for an all-too brief period before being forced to return to base. We’re told that “life called to life, across the gulfs of space.” […] “So Graham Hutchins, the happiest biologist in the solar system, told himself as he helped Garfield collect their refuse and seal it into a plastic disposal bag.”

This disposal, while more careful than many, is fundamentally flawed:

“Nothing that it did was beyond the capacity of any plant or tree on Earth – but it moved a thousand times more quickly, requiring only minutes to reach its goal and pierce through the plastic film. And then it feasted on food more concentrated than any it had ever known. It absorbed the carbohydrates and the proteins and the phosphates, the nicotine from the cigarette ends, the cellulose from the paper cups and spoons. All these it broke down and assimilated into its strange body.”

However the end sequence of this narrative is shocking in its stark conclusion. While the resources might be richer than any the Venusian life had hitherto consumed, they also carry a devastating load of micro-organisms.

The possibility of life on Venus, particularly in the cloud layer, remains. However the majority of the planet’s surface is at temperatures and pressures well beyond the survival threshold of any known Earth life. By contrast, Mars (despite its hostility to humans) may be considerably more vulnerable to bio-contamination.

Clarke returned to the question of biosphere contamination in his late novels, jointly written with Stephen Baxter, of the Time Odyssey trilogy. In this narrative, early explorers on Mars take extreme measures to ensure that their environments (including wastes) remain isolated from any potential biosphere on the Martian surface. Indeed, when Martian life is indeed discovered in the second book of the trilogy, Sunstorm, its dying discoverer can do little but ensure her hermetically sealed suit is secure and watch the first flowering of Martian plant growth (triggered by a solar flare event) in millennia as she dies. Part of the more routine operation of planetary protection protocols for Martian settlers is described further in the third book Firstborn (2008):

“today was PPP day, planetary protection, when she had to make her regular formal check to ensure the environment was properly sampled in a band kilometers wide around the station, thus monitoring the slow seepage of their human presence into the skin of Mars. There was even a bit of paper she had to sign, for ultimate presentation to an agency on Earth.”

Of course other fictions are far less concerned by issues of Solar System planetary protection, weighting it low in priority as compared to human needs. An example is here is The Expanse series (novels, S A Corey; TV) which is set in a future which has seen extensive human settlement throughout the Solar System. It is apparent that no life has been found beyond Earth in this future scenario, and no notable discussion of protecting pristine environments occurs. The lack of quarantine and planetary protection procedures becomes significant when an alien bio-engineered proto-molecule infiltrates the human-occupied asteroids and worlds.

Another, curiously ambivalent, example on this topic can be found in the animated television series Thunderbirds Are Go (2015-2020, based on the older television series Thunderbirds). As I've discussed before, Thunderbirds Are Go contains a surprisingly well realised vision of near-future space-flight, including private and commercial operators. In the episode “Deep Search” (2016), a married couple of social media influencers, the Prendergasts, decide to drill into the thick ice cover of Jupiter’s moon Europa and take a private submersible into the ocean beneath. This moon has long been considered one of the most likely sites for extraterrestrial life in the Solar System, given the inferred existence of a water ocean, which is heated by thermal vents driven by tidal forces from Jupiter. Neither the Prendergasts, nor the International Rescue team despatched to rescue them after the inevitable disaster they encounter, give serious thought to protecting the environment of Europa from Earth microbes carried on the outer hull of their craft or their diving suits (although to be fair, there *might* be some unseen and never-discussed decontamination process). It’s interesting to note that by contrast, after having identified jellyfish-like Europan life, the crew of Thunderbird Three are warned that they will have to spend at least a month in quarantine on their return, due to Earth’s own planetary protection protocols (probably wise, given the possible alternative already mentioned in “Riding the White Bull”!).

The Vulnerability of Astronauts

The front-line status of astronauts in planetary protection narratives has been further discussed in a range of fictions in which isolated groups of individuals rather than Earth itself prove vulnerable to alien organisms.

“The Sickness” by William Tenn (1955) is a short story in which a joint Indian-US-Russian Mars expedition finds its members falling sick and face the possibility that it will never be able to return to Earth. In fact, the sickness and coma proves to be the result of a symbiotic bacterium that permeates the brains of its victims. A similar theme can be found in “Martian Fever”, a short story included in the anthology Strangest of All (2020) released by the European Astrobiology Institute, and written by its editor, author and space scientist Julie Nováková. This story describes a settlement of 100 humans established by an idealistic billionaire, and comprising both dedicated settlers and scientists seconded from a range of space research agencies. When one of them falls ill, experiencing a massive immune cascade and falling into a coma, the medics and scientists in the settlement, as well as the colonists, are forced to accept that they will never be permitted to return to Earth. Interestingly, the story discusses the nature of the Martian life and the way it might use different bases in its DNA, incorporate different elements which are rare on Earth, and make use of chemical processes in its biology. Written in the context of professional astrobiology research, the short story stresses how low the probabilities of microbial life capable of infecting humans are, and how unlikely it is to have an extinction-level impact on any life on Earth is, but notes that even the very small probabilities of cross-contamination are too great a risk to take.

Still involving Mars and looking into a future of permanent settlement, the 2009 Doctor Who special episode “Waters of Mars” centres on the first human colony on the red planet. Comprising just eight people, the crew of Bowie Base One uses biofilters on the ice melt-water used in their agricultural dome, but a failure in the filter allows a microscopic life form known as the Flood to infiltrate the base and the bodies of its crew. The alien, and humans possessed by it, proves to have a rather physically-unlikely ability to generate and control liquid water, but there’s relentless tension as the uncontaminated humans try to escape, with the Doctor’s assistance. From the start of the story, however, it is clear that the base and its crew will meet an unfortunate end, and this occurs when their rocket pilot, Ed Gold, destroys their contaminated spacecraft on its pad and the base commander, Adelaide Brooke, orders the annihilation of the base by an atomic explosion. The sacrifice of the astronauts in the interests of planetary protection proves to be an important motivation for further human expansion into space.

While Mars might be the most likely source of biocontamination to astronauts, it’s not the only one. After the first moon landings by Apollo 11, 12 and 14, their crews were placed in bio-isolation garments on return to Earth and isolated for twenty-one days, while their craft and gear were decontaminated. The procedure was eventually abandoned after it became clear that there was no life to be found at the Apollo landing sites. The Apollo missions nonetheless highlighted the difficulties of protecting astronauts: despite the firm intention of isolating the astronauts from their environment, the clinging dust of the lunar regolith proved to be all-pervasive, getting into the cabins and causing respiratory inflammation. 

The risk to astronauts of exposure to irritants, as well as infections, is exacerbated by the still little-understood effect of prolonged space travel on the human body. The isolation, radiation exposure and physical degeneration associated with low gravity could all negatively impact the immune system. An interesting glimpse into the possible effects - while not specifically about planetary protection - can be found in the short story “Homesick” by Lyn Venable (Galaxy science fiction, 1952, text, audio). This describes a crew returning to Earth after a 30 year journey - only to discover that they are vulnerable to infection in the all-pervasive Earth biosphere and allergic to everything on the planet of their birth. While any feasible journey is unlikely to last so long, similar allergic reactions, as well as immune reactions, are a risk all astronauts will face when exposed to alien environments.

A Universe of Contaminants

Of course, much science fiction extends well beyond the limits of our own solar system, and questions of biocontamination remain.

This was a major theme, for example, in Becky Chambers’ short novel To be Taught if Fortunate (2020). This follows the crew of Merian, an interstellar spaceship launched by a crowd-funding organisation known as OCA - Open Cluster Astronautics - as they explore the four potentially-habitable worlds of an extrasolar planetary system. While witnessing and describing a wide range of life, the crew’s account is also careful to explain the careful decontamination procedures that protect them from the alien biosphere, and protects that biosphere from them. As Chambers’ narrative character notes:

“Even after washing, human skin is laden with bacteria that are, to us, good and necessary, but would wreak havoc in a new ecosystem. We exhale bacteria, too, in micro-droplets of moisture that travel through our airways. Symbiotic microbes aside, there’s no telling what human contaminants could do to an environment. Is our skin oil toxic to the life there? Do we shed allergens? Are we passively poisonous? There’s no way of knowing. Plus, we could get sick too, and that’d be the end of the mission (and likely ourselves). Hence, suits.” (pg 26, Hodder paperback, 2020).

In Chambers’ narrative the objective of the mission is ethical scientific exploration, minimising human impact, but even here some cross-contamination is recognised as inevitable.

By contrast. many of the long-running science fiction television series appear to occupy a universe in which planetary protection is given a low priority. The Stargate, Star Trek and Doctor Who universes, for example, all feature regular landings on alien planets with little to no apparent protection from their biospheres. In Doctor Who, the Doctor’s space-time ship the TARDIS is perhaps sufficiently advanced to provide a Clarke’s Third Law [3] form of decontamination. Similarly Star Trek’s matter transporter technology supposedly includes biocontamination filters which operate between dematerialisation and reintegration of its subjects, while “quarantine fields” are used for medical isolation. However these technologies ignore the fundamental fact that microorganisms are integral to the human organism, including our digestive health, and would be present in every lungful of air breathed in an alien biosphere. Indeed, disease does continue to exist in these universes (see, for example, Star Trek: The Animated Series episodes “Albatross”, “The Pirates of Orion”, ST:TNG “Unnatural Selection, ST:DS9 “Babel”) but there is much more emphasis placed on preventing cultural contamination than on planetary protection.

The Stargate franchise presents a slightly different narrative. Here the eponymous stargate is a found alien technology, and the setting is contemporary or near-future, meaning that most technologies are little more advanced than our own (although this changes as more and more alien technology is recovered and retroengineered). Unprotected teams are routinely despatched to, and return from, alien planets - many of which have already been settled by humans transported there in the distant past. Perhaps inevitably, bio-contamination incidents do occur in this universe. The television series Stargate: SG-1 experiences disease outbreaks in the episodes “The Fourth Horseman parts 1 and 2” (in which a disease is deliberately spread by an alien culture) and in “Frozen” (in which a viral disease is accidentally released when a long-frozen individual is defrosted).

The issue is taken still more seriously in the spin-off series Stargate: Atlantis. Set in a technologically-advanced alien city in another galaxy, the premises somewhat reduces the chances of contamination of Earth. However Atlantis experiences an outbreak of Kirsan Fever in “Tabula Rasa” and experiences a total lockdown in “Quarantine”. This later occurs when the city’s disease quarantine protocols are triggered, and includes a more detailed discussion of the threat than appears elsewhere in this universe.

Indeed much of the science fiction of interstellar exploration and settlement takes a similar approach. Planetary protection against microbial cross-contamination in the interests of the alien biosphere or in the interests of Earth organisms is largely neglected, while medical planetary quarantine procedures exist in the event of infectious disease amongst humans. The latter can be seen for example in Pariah Planet (aka This World is Taboo, 1961) and other stories in Murray Leinster’s Med Service series, in Anne McCaffrey’s The Ship Who Sang (1969), and in Ambulance Ship (1979) and others in James White’s Sector General series.

Possibilities of Panspermia

While the need to protect against toxic life from distant exoplanets may be dim and distant, the careful sterilisation of human space-probes sent to any of our neighbour worlds with even a slight potential capacity for supporting life has become routine, and most space agencies have a clear planetary protection policy. These have evolved a significant way from the early efforts of the 1960s that helped inspire the SF examples mentioned above, and which also motivated Article IX of the Outer Space Treaty (1967):

States Parties to the Treaty shall pursue studies of outer space, including the Moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter and, where necessary, shall adopt appropriate measures for this purpose.

Modern space probes undergo rigorous, exhaustive and expensive decontamination procedures and are kept in clean-room conditions before being sealed into their rocket housings. However there are those who argue that this may be both unjustified and unnecessary. In the former category are those who argue that the needs and interest of humanity in exploring our solar system take precedence over those of native microbial or primitive life forms.

The second category of arguments are more subtle and fall into two main groups. Planetary protection may be unnecessary because any alien biosphere is very unlikely to be based on the same nucleic bases or genetic pattern as Earth life. As such it is equally unlikely to be able to infect Earth organisms, or be infected by them in turn. The two biospheres might coexist without negatively impacting one another. Of course, such arguments ignore the competition between different forms for the same resources, the possibility that the metabolic byproducts of alien life may be toxic even if not infectious, and the chance of allergic reactions which require no direct infection.

Alternatively, planetary protection, some argue, is unnecessary as biological cross-contamination between neighbouring worlds such as Earth and Mars has likely occurred numerous times through natural processes. The earliest interplanetary probes were not sterilised. More importantly, over a dozen meteorites recovered on Earth to date have proved to have a Martian origin, suggesting that many more remain undiscovered. While Earth asteroids may be less common on Mars (due to our thicker atmosphere and higher gravitational potential) they are certainly possible. These asteroids have passed through a violent ejection process, interplanetary space and atmospheric reentry. It nonetheless remains possible that any extant primitive life might remain deeply embedded within the rock. On the whole asteroids from Earth (where life is abundant) are perhaps slightly more likely to be contaminated than asteroids from elsewhere (where life would likely struggle and be confined to limited habitats).

Such transfer of organic and living material between planets on rocky bodies or comets is known as panspermia, and was popularised by Hoyle and Wickramasinghe in the 1970s and 80s. While infeasible in its most extreme proposed forms, a number of studies have shown that limited panspermia may indeed be possible within a solar system, and contemporary astrobiology research considers it a real possibility. This is reflected in discussions between characters in Clarke & Baxter’s Firstborn, for example, which describes evidence of asteroid ejecta in ice cores drilled at the Martian poles:

Yuri said, “You can even find traces of Earth in this Martian ice, meteorites blasted off the home world, just as Mars meteorites find their way to the Earth.” He grinned. “I’m still looking for traces of the dinosaur killer.” (chapter 21)

However Novakova’s “Martian Fever” points out that a history of genetic exchange between planets could actually present a still greater danger:

“Earth microorganisms may have already gotten to Mars many times. If they survived there, they could have evolved very differently from their Earth relatives. But because of their relatedness, potentially dangerous backward contamination would be very feasible, and a very considerable risk, because the potential ‘Martians’ might be able to interact directly with life on Earth and disrupt our ecosystems.”

The narrative here is fictional, but the possibility, and its potential threat, is very real.

Mars, the atmosphere of Venus, the hydrocarbon-rich Saturnian moon Titan and the ice moons of Europa (Jupiter) and Enceladus (Saturn) have all been proposed as possible abodes of life within our Solar System. All, together with other bodies including comets, are subject to strict bioprotection protocols and great care is taken in their exploration. However with landers extant or planned to several of these locations, and given the impossibility of completely reliable sterilisation, cross contamination may be inevitable. This could confuse our ability to determine whether any life that is found is native, naturalised or simply carried by our own craft. If (as might be more likely) no life is discovered, we risk contaminating pristine environments. Long before microorganism-riddled humanity visits these worlds, we need to give thought to how to preserve them, for the sake of our own descendants… or theirs.