Tardigrades can survive boiling, freezing, and the vacuum of space, and we still are not sure how

Barely visible to the naked eye, the tardigrade looks almost comically ordinary: a plump, eight-legged micro-animal that lumbers through droplets of water like a bear in slow motion. Yet this creature, often called the water bear, routinely shrugs off conditions that would instantly kill nearly anything else on Earth, from being boiled and frozen to being flung, unprotected, into the vacuum of space.

That resilience has made tardigrades a favorite of biologists trying to understand the outer limits of life. Decades of experiments have identified some of the tricks behind their toughness, but the full picture of how a millimeter-long animal endures such punishment remains one of biology’s genuinely open questions.

The tun: shutting the body almost all the way down

The tardigrade’s signature survival strategy is a state called cryptobiosis. When conditions turn hostile, the animal expels almost all of the water from its body, retracts its legs and curls into a shriveled, dehydrated ball known as a tun. In this form, as detailed in the scientific overview of tardigrades, metabolism drops to a tiny fraction of its normal rate, effectively pausing the machinery of life. A creature in the tun state is not merely dormant; it has suspended the ordinary chemistry that makes it vulnerable to heat, cold and drying out. Researchers have revived tardigrades after long periods in this desiccated condition, and the tun is the key that unlocks most of the animal’s more spectacular feats of endurance.

Ten days in open space aboard a European capsule

The most famous demonstration of tardigrade toughness came not in a laboratory but in orbit. In 2007, an experiment known as TARDIS flew dried tardigrades on the exterior of a European Space Agency capsule and exposed them directly to the space environment. The results, reported in the study “Tardigrades survive exposure to space in low Earth orbit”, were striking: the animals endured 10 days of open exposure to the vacuum of space and cosmic radiation, and a portion even survived the additional assault of unfiltered solar ultraviolet radiation. Many were rehydrated back on Earth and resumed normal life, with some going on to reproduce. That made tardigrades the first animals shown to survive the combined vacuum and radiation of open space, a benchmark that continues to shape how scientists think about the durability of life beyond the planet.

Boiling, freezing and crushing pressure

Space is only part of the resume. In the tun state, tardigrades have withstood temperatures far below anything found in nature, approaching absolute zero, and brief exposure to heat above the boiling point of water. They can tolerate pressures many times greater than those at the bottom of the deepest ocean trench, and doses of radiation that would be lethal to a human hundreds of times over. A review of tardigrades in space research catalogs how these tolerances overlap: the same dehydrated, metabolically arrested body that survives the vacuum of orbit is also what allows the animal to ride out extreme temperature swings and desiccation on the ground, in habitats ranging from Himalayan peaks to Antarctic ice to the film of water on a patch of backyard moss. Notably, sustained high heat is one of the animal’s real weaknesses, a reminder that even the tardigrade’s armor has gaps.

Dsup and a molecular repair kit

Scientists have begun to identify the biochemistry underlying some of these abilities. Tardigrades produce a protein nicknamed Dsup, short for damage suppressor, which binds to the DNA-packaging structures inside cells and appears to physically shield genetic material from the reactive molecules that radiation and drying generate. In laboratory tests, introducing Dsup into human cells reduced X-ray-induced DNA damage substantially. Tardigrades also carry expanded sets of DNA-repair genes that let them mend damage quickly, and certain species rely on protective molecules, including specialized proteins that vitrify into a glass-like matrix as the animal dries, stabilizing cellular components that would otherwise fall apart. Together these mechanisms suggest the water bear does not have a single super-power so much as a layered toolkit for protecting and restoring the delicate contents of its cells.

Why the full explanation is still missing

For all that progress, the honest answer to how tardigrades pull it off is that no one fully knows. Different tardigrade species use different combinations of protective proteins and sugars, and the relative importance of each mechanism shifts depending on the stressor, whether it is radiation, vacuum, cold or dehydration. Some early claims about the animal’s genetics, such as the idea that a large share of its genome was borrowed from bacteria, were later attributed to contamination and revised, underscoring how much remains unsettled. What is clear is that the tardigrade’s endurance emerges from several overlapping strategies working in concert, and untangling exactly how they combine is an active area of research with practical stakes, from preserving vaccines and biological samples without refrigeration to protecting human cells from radiation. The water bear, in other words, is not just a curiosity but a working laboratory for the biology of survival, one that has not yet given up all of its secrets.

This article was produced with AI assistance and reviewed by the Morning Overview editorial team.

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