Tardigrades pulled from Antarctic moss can survive a decade without water, temperatures near absolute zero, and a 10-day exposure to the open vacuum of space in 2007, curling into a dehydrated husk called a tun until conditions improve. This extreme survival strategy relies on molecular vitrification, transforming their internal cellular architecture into a glass-like state.
The Mechanics of the Tun and Cryptobiosis
Often called water bears, tardigrades are microscopic animals measuring less than a millimetre in length. They navigate their environments on eight stubby, clawed legs, feeding on plant cells and algae by piercing them with a stylet. However, their active metabolic life—including feeding, moving, and reproducing—strictly depends on a surrounding film of liquid water.
When desiccation occurs, the tardigrade initiates a radical transformation. It pulls its limbs inward, contracts its body into a barrel roughly half its normal length, and expels almost all internal water. This triggers cryptobiosis, a state of hidden life where metabolic activity drops to an almost unmeasurable whisper. The resulting protective barrel is known as a tun.
Inside this structure, the organism is neither fully alive nor dead. It is entirely paused.
Vitrification and the Role of TDPs
To prevent cellular catastrophe during extreme dehydration, tardigrades utilize specialized molecules called Tardigrade Intrinsically Disordered Proteins (TDPs). As water evacuates the cell, these proteins reorganize into a stable, glass-like matrix that permeates the interior.
Cellular components—including DNA, ribosomes, membranes, and enzymes—are locked firmly in place, suspended as if encased in clear resin. This process, known as vitrification, halts the chemical degradation of proteins. Furthermore, because virtually no free water remains to expand into jagged ice crystals, the cell membranes are spared the punctures that typically destroy ordinary biological tissues.
When hydration returns, the glassy matrix dissolves instantly. The tardigrade unfolds its legs and resumes normal biological activity.
The 2007 Space Exposure Mission
The limits of this biological resilience were tested in 2007 aboard the European Space Agency’s FOTON-M3 capsule. Researchers exposed dried tardigrade tuns to the harsh conditions of low Earth orbit for ten days. Mounted on an exterior tray, the samples faced a near-zero pressure vacuum, unfiltered solar ultraviolet radiation, and cosmic rays.
According to mission findings, the vacuum alone had almost no negative impact, resulting in near-total survival rates. While unfiltered solar UV—particularly the volatile UV-B and UV-C wavelengths typically blocked by Earth’s atmosphere—proved lethal to a large portion of the sample, a fraction of the exposed tardigrades survived. Upon returning to ground laboratories, they rehydrated, walked, and produced viable offspring.
Subsequent missions continued testing these organisms, including a 2011 flight aboard NASA’s space shuttle Endeavour and a high-profile payload spill during the 2019 crash of Israel’s Beresheet lunar lander. While microbiologists emphasize that these space-faring tardigrades cannot actively colonize celestial bodies without liquid water and oxygen, their cellular mechanisms continue to provide data on extreme biological endurance.
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