For humans, outer space has a host of unpleasant environmental issues that make a spacesuit a must: extreme cold, exposure to lethal doses of cosmic radiation and a complete lack of oxygen.

Swedish and German scientists have found at least one animal, however, that can survive in outer space: tiny invertebrates called tardigrades, commonly called "water-bears" that are found on wet lichens and mosses.

The creatures were flown aboard the European Space Agency's FOTON-M3 spacecraft in September 2007, according to a report on the experiment to be published in Tuesday's edition of Current Biology. The craft travelled in a low-Earth orbit.

"Our results... represent the first record of an animal surviving simultaneous exposure to space vacuum and solar/galactic radiation," the authors wrote.

Previously, only lichens and bacteria had been reported to survive the combined exposure to the vacuum of space and both solar and galactic radiation.

Tardigrades, which range in size from 0.1 to 1.5 mm, had already proved their resiliency in previous experiments, showing an ability to survive both an almost total loss of water and prolonged exposure to radiation.

Flying in outer space provided yet another chance to test their mettle. The tardigrades were all exposed to the vacuum of space and received different levels of exposure to solar radiation.

The vacuum of space had little impact on the tardigrades' survival and ability to reproduce, the scientists reported. Increasing exposure to solar radiation, on the other hand, led to a higher mortality rate.

But a few of one species of tardigrade, Milnesium tardigradum, did manage to survive both the vacuum and extreme levels of solar radiation.

This was surprising to the authors, given that solar radiation in outer space is more than 1,000 times higher than on the surface of the Earth, and that the high-energy particles that make up UV rays are capable of damaging a cell's genetic material.

"How these animals were capable of reviving their body... remains a mystery," the authors wrote, though they suggest the ability to survive both desiccation and radiation may be linked to a more efficient system of repairing DNA.