Microscopic animals called tardigrades are among the few lifeforms thought capable of surviving the intense radiation, extreme temperatures and life-sucking vacuum of outer space.

Even their eggs can survive space-like conditions, hinting at the possibility of successful hatches on other planets.

"[I]f we are to assess the ability of tardigrades to survive transfer among planets or to thrive in extreme environments, they must be able to reproduce," wrote astrobiologists who tested tardigrades in a study published April 10 in Astrobiology.

Adult tardigrades, also known as water bears, thrive in wet conditions and eat algae, bacteria or single-celled animals. If their puddles dry, they don't die, but enter a state of total metabolic shutdown called anhydrobiosis. There they can remain for up to a decade, then spring back to life when it's wet again.

Researchers in 2007 launched anhydrobiotic adults into orbit above Earth to see if they would survive. Those animals endured naked exposure to space for 10 days, and a few even made it through an excessive dose of ultraviolet radiation while back on Earth.

Other laboratory experiments show that adult tardigrades can survive cold near absolute zero (-459 degrees Fahrenheit), heat exceeding 300 degrees Fahrenheit, pressures dozens of times greater than at the bottom of the Marianas Trench, and intense blasts of radiation.

But what of tardigrade eggs? Some flew on the 2007 mission, but they weren't exposed to the extreme temperatures and radiation found outside Earth's protective magnetic shield.

To learn how the eggs would fare, NASA and astrobiologists in Japan devised three extreme stress tests for the eggs of a tardigrade species called Ramazzottius varieornatus.

In one set of tests, more than 70 percent of anhydrobiotic eggs survived temperatures as low as -320 degrees Fahrenheit and as high as 122 degrees Fahrenheit. Eggs exposed to vacuum-like conditions hatched just as well as normal eggs. Finally, more than half of anhydrobiotic eggs endured 1,690 Grays of radiation. A human would die in days if exposed to one percent of that dose.

Fully hydrated eggs, however, barely survived any of the tests.

It's not known how tardigrade eggs survive such punishment. Whatever the mechanisms, the study's authors think their results are good news for dried-out tardigrade families ejected into space, perhaps by an asteroid strike.

And if a few should somehow end up on, say, Mars, and be fortunate enough to find liquid water at Earth-room temperature, they might even hatch there.

"This study provides support for the possibility of successful transfer in the vacuum of space by showing that indeed anhydrobiotic eggs were substantially more resistant to radiation than hydrated eggs," the authors wrote.

*Images: 1) An adult tardigrade less than 1 millimeter in length. (ESA/Ralph O. Schill) 2) Anhydrobiotic tardigrade eggs (left) and hydrated eggs (right). (D.D. Horikawa et al./Astrobiology)

*

Citation: "Tolerance of Anhydrobiotic Eggs of the Tardigrade Ramazzottius varieornatus to Extreme Environments." By Daiki D. Horikawa et al. Astrobiology, published online April 10, 2012. DOI: 10.1089/ast.2011.0669