Humans, chimpanzees and dogs can live in a space environment for but a few minutes before the air in their lungs expands, gas bubbles out of their blood and the saliva in their mouths begin to boil. But more fundamental organisms such as bacteria and lichen can tolerate the absence of pressure and searing cold. And now researchers have found that animals known as tardigrades, or water bears, can, too.

These microscopic animals—ranging in size from 0.06-inch (1.5-millimeter) adults to 0.002-inch (0.05-millimeter) larvae—are found in lichens or mosses, in soil, on mountaintops and in the sediment on the ocean floor at depths of 13,000 feet (4,000 meters). Given that their mossy homes can be prone to desiccation, some species of tardigrades can survive as long as a decade without moisture.

They also have an uncanny ability to resist damage that the sun's ultraviolet rays cause to humans and most other more complex animals.

So in an effort to test their space grit, ecologist Ingemar Jönsson of Kristianstad University and his colleagues put two different moss-dwelling species of tardigrades—Richtersius coronifer and Milnesium tardigradum—and their eggs on the European Space Agency's Foton M3 mission last September. While orbiting Earth at an altitude of more than 160 miles (258 kilometers), the tardigrades were exposed to the vacuum of space for 10 days. Although some of them were shielded from the sun's light, others were bathed in UV-A (long-wave) and UV-B (medium-wave) ultraviolet solar radiation.

"We found that both species of tardigrades survived exposure to space vacuum alone very well, with no significant difference in survival pattern compared to ground controls," says astrobiologist Petra Rettberg of the Institute of Aerospace Medicine in Köln–Porz, Germany, a member of the research team. "Samples exposed to the combined effect of vacuum and solar radiation had significantly reduced survival."

In fact, only 10 percent of the tardigrades exposed to both vacuum and radiation recovered when returned to a water environment—and none of the irradiated eggs hatched—but that "still represents the first animals that have survived simultaneous exposure," Jönsson notes. Rettberg speculates that their outer layer, known as the cuticle, may shield them from radiation.

Much like the microbe Deinococcus radiodurans, the tardigrades must also have some cellular mechanism that repairs radiation or desiccative damage. "There is no data on what is happening in the bodies of the tardigrades when exposed to radiation," Jönsson says. "So we don't know how damaged they get and we don't know to what extent they are able to repair the damage."

This proves that at least some animals can survive the rigors of space flight unprotected, a list that might also include the microscopic animals known as rotifers, nematodes (roundworms), drought-resistant insect larvae, and crustaceans like brine shrimp, according to the researchers—all of which share the tardigrades' ability to survive extreme dryness.

But lichens, which other species of tardigrades live on, show no harm from exposure to space. Perhaps such tiny animals and their plant homes are capable of spacefaring. "If sheltered from solar radiation, it is possible that they could survive for quite many years under space vacuum," Jönsson says of the water bears. "But the problems connected with ejection into space and reentry remains," such as the searing heat of friction as rock enters or leaves a planetary atmosphere.

Ultimately, such a trip between worlds might take millions of years. At least some of the tardigrades would be good for the first 10 days. The real problem would be finding another suitable home. "You have to end up in a less hostile place than space," Jönsson says, "in order to reproduce and establish a population."