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ELI5 Yahaaa

Half a millimetre long

The Animal That Was Left Outside a Spacecraft and Lived

In 2007 a batch of tardigrades was exposed to open space for ten days. Some of them came back and had young.

Awake Half a millimetre,eight legs, lives inmossDried into a tun Barrel-shaped, andeffectively not alive
The second state is the one that survives things.

  1. Step 1 of 5

    They are ordinary animals living in ordinary damp places.

    A clump of dampmoss Half a millimetrelong, eight legs,claws

    Tardigrades live in moss, lichen, leaf litter, soil, ponds and coastal mud. There are almost certainly some within a few metres of you. They walk on eight stubby legs with claws, eat plant cells and algae, and are visible under a cheap microscope.

  2. Step 2 of 5

    When the water goes, they pull themselves in and dry out.

    1 Waterstarts todisappearThe animaldetects itearly2 It retractsits legsand curlsReducing itssurface3 It expelsalmost allits waterNow a hardtun4 And itwaitsYears, ifnecessary

    Faced with drying, a tardigrade retracts its legs, curls up and loses almost all its body water. It becomes a hard barrel called a tun. Its metabolism drops to something immeasurably small — arguably to nothing at all.

  3. Step 3 of 5

    The trick is replacing the water with something that sets like glass.

    Water is replaced byprotective proteinsWhich set into aglass, not a liquidStructures are heldrigidly in positionSo there is nothingleft to go wrong

    Losing water normally destroys a cell — proteins unfold and membranes tear. Tardigrades flood themselves with special proteins that vitrify: they set into a glassy solid that holds every structure exactly in place until water returns.

    Nothing is metabolising in that state, which is why it costs nothing to stay in it. Waiting is free.

  4. Step 4 of 5

    In that state, almost nothing touches them.

    Times deep-trenchpressure,survivedHundreds of graysof radiation,survivedAnd near absolutezero, and aboveboiling

    Tuns have survived temperatures near absolute zero and above the boiling point of water. They have taken pressures far beyond the deepest ocean trench, and radiation doses hundreds of times what would kill a person. Not because they are tough while active — because there is very little happening to disrupt.

  5. Step 5 of 5

    Which is how they ended up bolted to the outside of a satellite.

    Ten days in open spaceVacuum: survived in large numbersUnfiltered solar UV: killed most of them
    Vacuum was the easy part. Sunlight was not.

    In 2007 the European FOTON-M3 mission carried dried tardigrades on the outside of the spacecraft, exposed to vacuum and unfiltered solar ultraviolet for ten days. Vacuum alone barely troubled them. Full ultraviolet did most of the killing — and some survived even that, rehydrated, and produced viable eggs.

    They are not immortal and they are not indestructible. Active, hydrated tardigrades are quite easy to kill. Everything remarkable here belongs to the tun.

The short version

A tardigrade survives by drying into a glassy tun in which nothing is happening, which is why it withstood ten days of open space in 2007 — with ultraviolet, not vacuum, doing most of the damage.

Try it yourself

Soak a pinch of dry moss from a wall in water for a few hours, then look at a drop under any microscope. Tardigrades are common, and rehydrating them is exactly how they are usually found.

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