A tardigrade can shrug off the vacuum of space and radiation that kills almost everything else

Among the most durable animals ever studied is one that most people would need a microscope to see. The tardigrade, a microscopic creature often nicknamed the water bear, has survived conditions that would instantly destroy nearly every other form of life, including exposure to the raw vacuum of space. Its ability to endure temperatures near absolute zero, crushing pressures, dehydration, and intense radiation has made it a favorite subject of biologists trying to understand the outer limits of what living tissue can withstand.

A survivor barely a millimeter long

Tardigrades are eight-legged animals that typically measure less than a millimeter in length, with plump, segmented bodies that give them their bear-like nickname. They are found almost everywhere on Earth, from the deep sea to Himalayan peaks, but they are most commonly encountered in the thin films of water that cling to mosses and lichens. There are more than a thousand known species, and they have been on the planet for hundreds of millions of years, outlasting multiple mass extinctions. The Smithsonian Institution’s overview of tardigrades describes them as one of the toughest known groups of animals.

What makes them remarkable is not their appearance but their response to stress. When their environment turns hostile, tardigrades do not simply die or flee. Instead, many species enter a state of suspended animation so profound that, by most measures, they appear to stop being alive at all.

The trick of drying out

The key to tardigrade survival is a process called cryptobiosis, and in particular a form of it triggered by dehydration. As water leaves their bodies, tardigrades pull in their legs and contract into a shriveled, barrel-shaped structure known as a tun. In this state their metabolism slows to a virtual halt, dropping to a tiny fraction of normal activity. A tun can persist for years in a kind of biological pause, then revive within hours once water returns.

To survive the loss of nearly all their internal water, tardigrades produce special protective molecules. Some species rely on a sugar that helps stabilize cell structures, while others use a group of proteins unique to the animals that form a glass-like matrix, holding delicate cellular machinery in place until rehydration. This protective shutdown is what allows the tun to endure environmental extremes that would tear apart an active, water-filled cell.

Sent into the vacuum of orbit

The tardigrade’s reputation for near-indestructibility was cemented by experiments that carried the animals beyond the atmosphere. In one European mission, dehydrated tardigrades were exposed directly to the vacuum of low Earth orbit, along with the ultraviolet radiation of unfiltered sunlight. A meaningful share of the animals survived the vacuum and even the radiation, and some later reproduced after returning to Earth. That result made tardigrades among the first animals known to endure open space and live.

Space is lethal for reasons that stack on top of one another. The vacuum causes explosive water loss and removes any external pressure, while cosmic and solar radiation shreds the molecular bonds that hold biological structures together. A tardigrade in its tun state sidesteps the first problem by already being dehydrated, and its protective molecules and efficient DNA-repair systems help it manage the second. Researchers have since identified genetic tools within tardigrades that appear to shield their DNA from radiation damage.

The limits of a nearly unkillable animal

For all their toughness, tardigrades are not truly immortal or invincible. Their extreme resilience is largely confined to the dormant tun state. An active, hydrated tardigrade going about its normal life is far more vulnerable and can be killed by ordinary hazards. The animals also have a ceiling for heat: sustained warmth, even at temperatures well below what many assume, can prove fatal, a finding that has prompted questions about how they might fare in a warming world.

Laboratory tests have nonetheless pushed the boundaries of their endurance. Tardigrades in the tun state have withstood temperatures approaching absolute zero, been heated well past the boiling point of water, and survived pressures many times greater than those at the deepest point of the ocean. They have also endured doses of radiation that would be lethal to a human many times over. The combination of these results is what earns the animal its status as one of the most extreme survivors in the animal kingdom.

Why scientists keep studying them

The practical interest in tardigrades goes well beyond curiosity. Understanding how their protective proteins stabilize cells could inform methods for preserving biological materials, from vaccines to transplant tissues, without the need for constant refrigeration. Their radiation-shielding genes are of interest to researchers thinking about how to protect human cells during long spaceflights. And their ability to essentially switch life on and off challenges basic assumptions about the boundary between living and non-living states.

The water bear, in other words, is a small animal with outsized implications. Its survival of the vacuum of space is not a stunt but a window into the chemistry that keeps cells intact under the harshest conditions imaginable, and into the surprising toughness that life can achieve at the microscopic scale.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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