The use oftardigrades in space, first proposed in 1964 because oftheir extreme tolerance to radiation, began in 2007 with theFOTON-M3 mission inlow Earth orbit, where they were exposed to space's vacuum for 10 days, and reanimated, just by rehydration, back on Earth. In 2011,tardigrades were on board theInternational Space Station onSTS-134. In 2019, a capsule containing tardigrades was on board the Israelilunar landerBeresheet which crashed on the Moon.

Tardigrades are smallarthropods able to tolerate extreme environments. Many live in tufts of moss, such as on rooftops, where they get repeatedly dried out and rewetted. Others live in the Arctic or atop mountains, where they are exposed to cold. When dried, they go into acryptobiotic 'tun' state in which metabolism is suspended.^[1][2] They have been described as thetoughest animals on Earth.^[2] Their DNA is protected from damage, such as by radiation, byDsup proteins.^[3]

In 1964, R.M. May and colleagues proposed that the tardigradeMacrobiotus areolatus would be a suitablemodel organism for space experiments because of its exceptional radiation tolerance.^[2][4]

In 2001, R. Bertolani and colleagues proposed tardigrades as a model for a study of animal survival in space.^[2][5] As terrestrial experiments on tardigrades proceeded, knowledge oftheir survival abilities grew, enabling K.I. Jönsson in 2007,^[6] and then other researchers such as Daiki Horikawa in 2008^[7] and Roberto Guidetti in 2012,^[8] to present evidence that they would resist desiccation, radiation, heat, and cold, suiting them forastrobiological studies.^[2]

In 2008, F. Ono and colleagues suggested that tardigrades might be able to survive a journey through space on a meteorite, enablingpanspermia, the transfer of life from one planet to another.^[9]

Tardigrades have survived exposure to space. In 2007, dehydrated tardigrades were taken intolow Earth orbit on theFOTON-M3 mission carrying theBIOPAN astrobiologypayload. For 10 days, in the "Tardigrade Resistance to Space Effects" (TARSE) experiment, groups ofParamacrobiotus richtersi tardigrades, some of them previously dehydrated, some of them not, were exposed to thehard vacuum of space, or vacuum and solarultraviolet radiation.^[10] Back on Earth, more than 68% of the subjects protected from solar ultraviolet radiation were reanimated within 30 minutes following rehydration; although subsequent mortality was high, many produced viable embryos.^[2][10]

In contrast, in the "Tardigrades in Space" (TARDIS) experiment, hydrated samples exposed to the combined effect of vacuum and full solar ultraviolet radiation had significantly reduced survival, with only three subjects ofMilnesium tardigradum surviving.^[10] The space vacuum did not much affect egg-laying in eitherRichtersius coronifer orM. tardigradum, whereas UV radiation did reduce egg-laying inM. tardigradum.^[2][10]

The third FOTON-M3 experiment, "Rotifers, Tardigrades and Radiation" (RoTaRad) focused mainly on radiation survival.^[2]

In 2011, Angela Maria Rizzo and colleagues sent tardigrades on board theInternational Space Station Endeavour along with extremophiles onSTS-134, in the "Tardigrades in Space" (TARDIKISS) experiment.^[2][11] They concluded thatmicrogravity andcosmic radiation "did not significantly affect survival of tardigrades in flight" and that tardigrades were useful in space research,^[12][13] with implications forastrobiology, where they should be suitablemodel organisms.^[14][8][15]

The mission was a prototype for the "Living Interplanetary Flight Experiment" (LIFE)^[17] which was to have travelled to the Martian moonPhobos on the RussianFobos-Grunt spacecraft.^[18][19] The spacecraft however failed to leave Earth orbit and was destroyed.^[20][21]

In 2019, a capsule containing tardigrades in acryptobiotic state was on board the Israelilunar landerBeresheet which crashed on the Moon. They were described as unlikely to have survived the impact because the shock pressure of the crash would have been well above the 1.14 GPa that they have been measured as surviving.^[16][22] Despite tardigrades' ability to survive in space, they would still need food, lacking on the moon, to be able to grow and reproduce.^[23] The possibility that tardigrades survived the crash attracted concern aboutcontamination of the Moon with biological material.^[24] However, even supposing they had survived the crash, they are unlikely to become rehydrated because of the lack of liquidwater on the Moon.^[25]

Spilling tardigrades across the Moon is legal.^[26][27] TheOuter Space Treaty only explicitly bans weapons and experiments or tools that could interfere with other missions.^[28] Large space agencies typically follow guidelines for sterilizing mission equipment, but there is no single entity to enforce these rules globally.^[29]

• Tardigrades
• Space exposure experiments

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