Mars rovers seek to solve clues about planet's past

PASADENA, Calif. — The prospect of life on Mars has charged the public imagination for more than a century, ever since astronomers first spied what they thought were canals dug to irrigate the planet's ruddy surface.

But after spacecraft and Earth-based telescopes began taking a closer look at the planet, evidence of the canals — and the Martians who presumably created them — quickly vanished.

Instead, the scrutiny showed Mars to be a dusty, frigid world, shrouded by an atmosphere too thin to breathe, bombarded with radiation and largely dry beyond the ice that caps its poles. It seemed altogether hostile to life as we know it.

But ongoing scientific spadework continues to turn up evidence that suggests that long ago Mars was a wetter, if not warmer, world where rivers large enough to carve canyons the size of the United States flowed across its surface. Life, even if just tiny microbes, could have thrived in such a place.

Beginning late Christmas Eve, a small armada of exploratory spacecraft will reach the Red Planet, some attempting to enter orbit, others to land — a very risky business because of the engineering and physical challenges that await the robotic probes. Together, they represent one of the most ambitious efforts yet to resolve the contradictions that persist in alternately intriguing and beguiling scientists.

"There is no consensus and a lot of contradictions," said Michael Carr, a planetary geologist with the U.S. Geological Survey who has played a role in nearly every past mission to Mars.

A British spacecraft, the Beagle 2, is scheduled to land on Mars Dec. 24. That same day, Europe's Mars Express should enter orbit around the planet. Mars Express successfully released Beagle 2 on Friday, after carrying it piggyback most of the way to Mars.

Spirit, the first of NASA's identical robot explorers, is expected to land Jan. 3. Its sibling, Opportunity, is scheduled to settle on the opposite side of the planet Jan. 24.

The odds of all four spacecraft succeeding are slim.

Since 1960, roughly two-thirds of the three dozen spacecraft sent to Mars have failed, including two 1999 NASA missions, the Climate Orbiter and Polar Lander. Most have been lost on launch or arrival, the most perilous portions of any mission.

The most recent failure was the Japanese satellite, Nozomi, which failed to enter orbit around Mars earlier this month.