What tardigrades know about surviving space

Tardigrades, also called water bears or moss piglets, are microscopic animals with eight stubby legs, each tipped with claws. More than 1,300 species have been described, living in ocean sediments, hot springs, Antarctic lakes and, most commonly, in the thin film of water on mosses and lichens. Most are less than half a millimeter long.

They are famous for surviving conditions that would kill almost anything else. But the reputation needs a correction. An active tardigrade, swimming and eating, is a fairly delicate creature. Their extreme toughness appears mostly when they enter a state called cryptobiosis, in which metabolism slows to an undetectable level. It is less a superpower than a very good way of not being home when trouble arrives.

An evolutionary answer to drying out

It helps to ask what tardigrades are actually adapted to. No tardigrade evolved to survive the vacuum of space or a dose of gamma rays, because neither exists in moss. What does exist in moss is drought. A clump on a wall may be wet in the morning and bone dry by afternoon, and an animal living in its water film has to either cope or die, many times over.

That offers a simple explanation for an otherwise strange list of talents. Losing nearly all your water is itself catastrophic: membranes crack, proteins unfold, and DNA breaks. Anything that protects a cell against drying will tend to protect it against other stresses that do similar molecular damage, including radiation and extreme cold. Space resistance is, on this view, a side effect. The animals are overqualified by accident.

The tun

When its environment dries out, a tardigrade pulls in its legs and head, loses almost all of its body water and curls into a compact barrel called a tun. In this state it can survive for years. Tuns have withstood temperatures close to absolute zero and brief heating well above the boiling point of water. Researchers in Japan revived and bred tardigrades from an Antarctic moss sample that had been kept frozen for more than 30 years.

Several strategies seem to work at once. Tardigrade-specific proteins that have no fixed shape in water take on structure as the cell dries, and a 2017 study argued that they help turn the cell interior into something like glass. Later work has suggested that the picture is more complicated, with some of these proteins forming gels. Either way, molecules are held in place rather than tearing apart, and can be released intact when water returns.

An active tardigrade, swimming and eating, is a fairly delicate creature.

Ten days in orbit

In 2007, researchers sent dried tardigrades into low Earth orbit aboard the European FOTON-M3 capsule and exposed them to open space for ten days. Many survived the vacuum and cosmic radiation, and some went on to reproduce. Those exposed to the Sun’s full ultraviolet light fared far worse, though a few still recovered. It was the first demonstration that an animal could survive direct exposure to space.

In 2019, an Israeli lunar lander called Beresheet crashed on the Moon carrying a small payload of dried tardigrades. Whether any could have survived the impact is doubtful: later experiments firing tardigrades from a gas gun found they did not survive collisions above about 900 meters per second. Either way, without water they could never revive. The persistent claim that the Moon now hosts living tardigrades is true only in the sense that dead or dormant animals may be lying on it.

A protein for shielding DNA

In 2016, researchers sequencing the genome of the tardigrade Ramazzottius varieornatus found a protein that binds to DNA and protects it. They named it Dsup, for damage suppressor. When human cells in culture were engineered to produce it, they suffered about 40 percent less DNA damage from X-rays.

The radiation numbers are startling. A dose of a few grays can kill a person, while tardigrades can tolerate doses of thousands of grays, something like a thousand times more. Shielding is only part of the story: a 2024 study found that tardigrades exposed to radiation sharply increase their production of DNA-repair proteins, and identified a further tardigrade-specific protein that responds to the damage. They appear to take the hit, then fix it.

Borrowing the tricks

In 2025, researchers from MIT and the University of Iowa reported in Nature Biomedical Engineering that they had delivered messenger RNA for Dsup to mice in nanoparticles. The treated tissue made the protein for a short time, and irradiated tissue showed less DNA damage than controls. The effect was confined to the injection site, and the work is at an early stage, but the idea has an obvious appeal: temporary protection for healthy tissue during cancer radiotherapy. Others hope to use glass-forming proteins to stabilize vaccines without refrigeration.

For astrobiologists, tardigrades offer a more general lesson. It is tempting to rank environments by how harsh they look, as though the limits of life were a matter of toughness. The tardigrade suggests something different. An organism does not have to thrive somewhere to persist there. It only needs a way to wait, and water to come back to. That is worth remembering when judging whether Mars once harbored life, or whether it could have held on as the planet dried.

Sources

• Jönsson et al., “Tardigrades survive exposure to space in low Earth orbit”, Current Biology, 2008
• Hashimoto et al., “Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells”, Nature Communications, 2016
• Boothby et al., “Tardigrades Use Intrinsically Disordered Proteins to Survive Desiccation”, Molecular Cell, 2017
• Tsujimoto et al., “Recovery and reproduction of an Antarctic tardigrade retrieved from a moss sample frozen for over 30 years”, Cryobiology, 2016
• Traspas and Burchell, “Tardigrade Survival Limits in High-Speed Impacts”, Astrobiology, 2021
• “Comparative transcriptomics reveal a novel tardigrade-specific DNA-binding protein induced in response to ionizing radiation”, eLife, 2024