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Homely but hardy. Tiny tardigrades can outsurvive just about every other animal on the planet.

Goldstein Lab/UNC Chapel Hill

"Water Bears" Survive Earth Orbit

No air. No water. And lethal doses of radiation. Tardigrades seem to be able to survive these conditions without much trouble, researchers have discovered. The pinhead-sized animals known as water bears shrugged off 10 days of exposed low-Earth orbit as though it were just an uncomfortable weekend in a seedy hotel. Exactly how the creatures can survive remains a mystery, but scientists say uncovering that mechanism could help them devise better protection for human astronauts on long missions.

Talk about hardy pioneer stock. Tardigrades--a phylum that includes thousands of species from about 1.5 millimeters to about 0.1 millimeter long--just seem to dig in and set up housekeeping wherever they land. Biologists have found them happily residing under rocks above 6000 meters high in the Himalayas and doing likewise in ocean trenches more than 4000 meters deep. They've even caught tardigrades basking in hot springs with temperatures well above water's boiling point. Laboratory tests have revealed even more surprises: Tardigrades can live indefinitely in a vacuum and withstand some of the harshest sunlight.

But can they reside so easily in space? A team led by ecologist K. Ingemar Jönsson of Kristianstad University in Sweden decided to find out. In September 2007, the researchers sent two species, Richtersius coronifer and Milnesium tardigradum, on a European Space Agency science mission aboard a crewless Russian spacecraft. They divided 120 tardigrades from each species into four groups. When the spacecraft reached orbit, it opened the experiment's door and exposed one group of water bears to the vacuum of space, while shielding them from all radiation. Two groups were exposed to the vacuum and either ultraviolet-A or ultraviolet-B radiation. The final group was exposed to both types of radiation, plus the vacuum.

After 10 days of space exposure, almost all of the tardigrades had dried out. But a quick dip back inside the craft returned them to their former vigor. The UV damage was not quite as reversible. Although most of the tardigrades exposed to UV-A radiation survived and went on to reproduce as well as tardigrades not exposed to radiation, UV-B and combined radiation took a toll. Survival rates beyond a few days plummeted to 10% to 15% among the UV-B group and dropped to zero in the combined-radiation group. However, the short-term survivors gave birth to normal offspring, the team reports in tomorrow's issue of Current Biology.

If the tardigrades' extraordinary survival mechanisms can be uncovered, Jönsson says, it will be "of importance for understanding how living organisms, including humans, may be protected against the conditions of space." One key step, he says, will be discovering why the tardigrades' DNA apparently was not damaged by the desiccation and radiation. Comparative biologist James Clegg of the University of California, Davis, speculates that tardigrades have evolved some sort of DNA rapid-repair mechanism.

Understanding the risks of radiation and the other extreme environments of space flight "is critical to provide safe passage for human exploration to the moon and Mars," says microbiologist Cheryl Nickerson of Arizona State University in Tempe. She says the research has the potential to expand understanding of cellular responses to the space environment to improve astronaut health, safety and performance.