After successfully touching down on a tiny asteroid 180 million miles away and collecting a sample from its surface, Japan's Hayabusa probe was at risk of being lost in space yesterday unless engineers can solve an engine problem quickly and get the spacecraft started on its journey home.

"The deadline is loose, but we assumed early December," project manager Junichiro Kawaguchi said in an e-mail. "Efforts to revive the spacecraft have begun, but we are not sure we can finish in time. The situation is not too optimistic."

Cosmic geometry requires Hayabusa to begin its 11/2-year return trip early this month or be trapped in space for eternity.

Hayabusa landed on the asteroid Itokawa on Saturday and fired two 10mm tantalum bullets into its surface, stirring a plume of dust that the spacecraft gathered in a horn-shaped collector. It was the first space mission to land on a celestial body other than the moon with the intent of picking up a sample and bringing it home.

But in withdrawing from the asteroid, Hayabusa sprang a leak in one of its sets of chemical thrusters, while a second set apparently froze. Kawaguchi said the two mishaps were apparently "distinct events," but "details are still under investigation."

Hayabusa's uncertain fate could turn out to be the final bitter irony for a spacecraft that has overcome remarkable hardships since the Japanese Aerospace Exploration Agency launched it in May 2003.

Solar flares damaged its electrical storage capacity, and after reaching Itokawa on Sept. 12, only one of Hayabusa's three variable-speed gyroscopes was functioning properly. The gyros are critical in maintaining the spacecraft's position in space.

An attempt to drop a tiny "hopping robot" called Minerva onto Itokawa went awry Nov. 4, with the lander ending up drifting away into space, and a Nov. 20 touchdown lasted 30 minutes but failed to collect a sample.

With time running out, engineers tried again last Saturday and confirmed success two days later. But the thruster problems arose four hours after liftoff, Kawaguchi said, and ground controllers lost radio contact with the probe until yesterday.

While Hayabusa struggled to complete its unprecedented journey, two sets of scientists announced results yesterday of European Space Agency missions to explore Saturn's largest moon, Titan, and to conduct the first subsurface radar probe of Mars.

In seven papers being published by the journal Nature, researchers detailed the 147-minute descent to Titan's surface in January by ESA's Huygens probe and the additional 69 minutes Huygens spent transmitting data before its mother ship, NASA's Cassini spacecraft, disappeared over the horizon.

"It's an extraordinary world which resembles the Earth in many respects," Huygens mission manager Jean-Pierre Lebreton said in a televised news conference from ESA's Paris headquarters. "There's a landscape with clear evidence of alluvial [river-related] activity."

Like Earth, Titan has a nitrogen-rich atmosphere, but there is no atmospheric oxygen, and its surface temperature is 291 degrees Fahrenheit below zero. Its weather is based on methane, which seeps from the moon's interior in some as yet unexplained way to cloak Titan in a blanket of smog.

"It is a humid atmosphere," said Francois Raulin of the University of Paris, in which "liquid methane is playing the role of water," rising as gas to create clouds, then falling as rain to etch rivers in Titan's icy ridges.

Scientists have not seen the methane "oceans" that were expected before Huygens landed, nor have large methane cloudbanks been observed in several Cassini flybys. Still, said Denis Bogan, NASA headquarters project scientist for Cassini/Huygens, "there is little doubt" of a methane-based meteorology. Cassini, he suggested, just has not seen it yet.

Huygens did, however, confirm that methane rising high in Titan's atmosphere breaks apart and recombines into more complex hydrocarbons that fall to the surface, depositing a layer of organic dust that can be as much as 0.6 miles thick.

Huygens landed in this "granular, unconsolidated material," which is "like clay or wet sand," said John Zarnecki of Britain's Open University. "I'm not saying that's the composition, but that's the consistency."

At Mars, meanwhile, the ESA's orbiting Mars Express spacecraft has been using its Mars Advanced Radar for Subsurface and Ionosphere Sounding to collect the first-ever profiles of what lies below the Red Planet's desert landscape.

In findings published yesterday in the journal Science, researchers reported that a layer of ice near the Martian north pole was exceptionally pure and about a mile deep. "There was no zone of melt," however, at the bottom of the ice layer, as there is with Earth's glaciers, said Jeffrey Plaut, of NASA's Jet Propulsion Laboratory.

Plaut, speaking at ESA's Paris news conference, said the radar had so far found "no convincing evidence of subsurface liquid water" on Mars but would begin a serious search for it next spring. "We can certainly say however, that we have observed significant amounts of water in the form of ice," he added.

In this artist's rendering, the Hayabusa probe collects surface samples after landing on an asteroid.Asteroid Itokawa is seen with the shadow of probe Hayabusa, center.