Why planetary protection meant Cassini had to die


It’s not easy stop microscopic creatures hitching a ride into space. That’s where the Planetary Protection Office comes in, explains Katie Mack.


Artist’s impression of Cassini burning up in Saturn’s upper atmosphere.
Artist’s impression of Cassini burning up in Saturn’s upper atmosphere.
NASA/JPL-Caltech

You’d think it was the name of a superhero, but the Planetary Protection Officer doesn’t wear a cape or work in a secret underground lair. The most devious villains encountered by NASA’s Planetary Protection Office are microscopic creatures that live in pond water and vaguely resemble inflatable bears. Though the office is tasked with protecting Earth from alien threats (for instance, from samples brought home from outer space), its primary purpose is to save the rest of the solar system from us.

Life is messy. It gets everywhere. On Earth, every niche we’ve found where liquid water is even intermittently present is positively teeming with the stuff. Microbes swarm in drops of water, tiny arachnids burrow in your eyelash follicles (sorry), and super-hardy creatures known as extremophiles make their homes in seemingly hostile habitats such as frozen glaciers and deep-sea volcanic vents.

By flourishing so well where we’d least expect them on Earth, these extremophiles give us hope for finding life in the unfriendly conditions of the outer Solar System. But they also present a problem.

With our intrepid microbial menagerie always present, when we humans build and send out a spacecraft to look for life, we risk contaminating the very environment we’re trying to study. Aside from being generally impolite and perhaps setting off an extremely small-scale war of the worlds, contamination like this could make it hard to tell if any life we detect is really alien.

This is where planetary protection comes in. Article IX of the Outer Space Treaty, drawn up by the United Nations’ Office for Outer Space Affairs and to which 107 nations are party, asserts that space exploration should avoid “harmful contamination” of other space bodies. Whenever an agency such as NASA sends a probe out into the universe, non-contamination guidelines require sterilisation procedures appropriate to the probe’s ultimate destination. The rules are significantly stricter for a lander than an orbiter, and even more rigorous if the landing site is thought to have a high chance of hosting life. The Planetary Protection Officer oversees the process.

It can have frustrating consequences for exploration. Planetary protection is part of the reason for NASA plunging its Cassini spacecraft into Saturn’s atmosphere, vaporising it in a blaze of glory. Cassini was never meant to be a lander; but Saturn has moons with liquid water, such as Enceladus, that are considered some of the best bets for the possibility of alien life in the Solar System. If Cassini hadn’t burned up, it might at some point crash into one of those moons, potentially wrecking whatever biosphere that moon might contain.

Similar concerns have prevented anyone from sending landers to regions on the surface of Mars that appear to have intermittent seepage of liquid water, where chances for life might be high. No matter how hard we try to completely sterilise a spacecraft, there’s always some chance a tiny Earth denizen might be clinging on.

Some of those microscopic explorers have proven incredibly hard to get rid of. The tiny tardigrade, also known as a water bear, for example, is an eight-clawed aquatic animal that grows to a maximum length of about one millimetre and sort of looks like a cross between a monochrome panda and an air mattress.

Tardigrades have long been known to survive extreme temperatures, hard radiation and even the vacuum of space. When water isn’t present, they can shrivel up and wait it out in a dormant state for up to a decade, or perhaps – according to some researchers – even centuries.

While tardigrade habitats are usually far from spacecraft labs, it is conceivable one could stick to a shoe or blow around on the breeze; and there may be other tiny extremophiles we haven’t yet catalogued hitching rides into space.

When humans eventually walk on the surface of Mars, sterilising everything will be impossible. It may be that, at that point, all hope of avoiding contamination will be lost.

In the meantime, the Office of Planetary Protection is here to look out for our fellow citizens of the cosmos, even if it does mean we have to say goodbye to our favourite robotic explorers a little sooner than we’d hoped.

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Katie Mack is an astrophysicist at the University of Melbourne.
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