In Mars exploration, of course, it's the Red Planet itself that gets top billing. But there are some good reasons to keep tabs on Phobos, the innermost and larger of Mars' two diminutive moons, which the Russian space agency plans to study with a probe set for launch next month on board a Zenit rocket. Called Phobos Grunt, the three-year mission is to land a spacecraft on the distant moon, scoop up soil samples for analysis and launch one of them back to Earth for further inspection. The probe's name means "Phobos's soil" in Russian.

If successful, Phobos Grunt would represent something of a coup for the Russian Federal Space Agency (Roscosmos). Russia has long struggled to reach Mars and has not carried out a successful interplanetary mission in decades. The nation launched Mars 96 in 1996 to explore the Red Planet, but a rocket failure occurred in Earth orbit. Phobos Grunt would also mark the first sample returned from a planetary surface since the U.S.S.R.'s Luna 24 in 1976.

As moons go, Phobos is strange. Shaped like a potato, it measures only 27 kilometers across at its widest point and orbits Mars at a distance of only about 6,000 kilometers. Researchers think Phobos and its even smaller companion Deimos started out as asteroids that were gravitationally pulled into Mars's orbit. Space agencies would like to know whether the moons' porous interiors harbor water that could be exploited by future visitors to Mars.

Russia attempted to reach Phobos before. Computer glitches sidelined the Phobos 1 and 2 craft, both launched in July 1988, before they could complete their missions. Now, for the past several months, Phobos Grunt has been subject to rumors that Roscosmos would likely have to delay the launch until 2011 at the earliest, when the flight window opens again.

Such a delay could give researchers time to get their heads around a tantalizing add-on to the probe's payload. The Planetary Society, a California-based space exploration and advocacy group, working with the Space Research Institute of the Russian Academy of Sciences, has arranged for Phobos Grunt to carry with it the Living Interplanetary Flight Experiment (LIFE), a group of samples of Earth life that will fly to Phobos and back in the Russian craft.

The goal, says Bruce Betts, the manager of the experiment for the Planetary Society, is to seek evidence for transpermia, the idea that life could have arisen on one body in the solar system and been transported via meteorites to seed life on another. For example, he says, "Could life have evolved first on Mars, been ejected off Mars, and then come to Earth?"

Although LIFE cannot test such a detailed scenario, Betts says the results will build on those of similar experiments flown on spacecraft. Bacteria have survived in low Earth orbit for up to six years but have only rarely, and briefly, left the protection of Earth's magnetosphere, which deflects the damaging cosmic rays that penetrate deep space. (When Apollo 12 returned to Earth in 1969 with the camera from the unmanned Surveyor 3 lander, which had spent more than two years on the lunar surface, scientists found Streptococcus mitis, a possible stowaway from before the probe was launched. But the bacterium's provenance is unclear, and some suspect the camera was contaminated after its return to Earth.)

The LIFE organisms were chosen with this danger in mind. Among the four bacteria to make the trip will be radiation-resistant Deinococcus radiodurans. Tardigrades, microscopic, eight-limbed invertebrates also known as water bears, were selected for their ability to repair DNA damage. Rounding out the group are three species of archaea—sometimes called "extremophiles" for their ability to thrive in conditions too harsh for other Earth life—along with yeast, plant seeds, and a soil sample collected from Israel's Negev Desert. Most of the samples will be freeze-dried and inert for the trip, to better resist the cold of space.

The LIFE menagerie will be packed in individual vials loaded into a titanium disc the width of a credit card. Betts says the packaging has been tested for acceleration forces up to 4,000 g's, or 100 times the expected acceleration during sample return.

Still, some space scientists are scratching their heads why anyone would risk putting Earthly microorganisms anywhere near Mars in the first place, especially given the relatively modest scientific payoff. "Meteorites are millions of years old—a three-year trip is really not relevant," says Daniel Glavin, an astrobiologist at the NASA Goddard Space Flight Center in Greenbelt, Md.

Strange as it may sound, the experiment satisfies international prohibitions against contaminating Mars or other bodies that might support life, says Catharine Conley, NASA's planetary protection officer. Unlike its neighboring planet, airless Phobos is surely sterile, so as long as Roscosmos provides detailed flight information confirming that Phobos Grunt is expected to reach its target moon—a safe distance from Mars—the mission should be in the clear.

"It is an uncomfortable thing," Conley says, "but we have policy. We have good policy."