Phoenix takes off for Mars' north pole

The Delta II rocket carrying the 7-foot-tall lander lifted off at 5:26 a.m. on a scheduled 423million-mile journey that should deliver Phoenix to the Martian surface on May 25.

"Today's launch is the first step in the long journey to the surface of Mars," said Peter Smith, a University of Arizona astronomer who is lead scientist on the mission.

"We certainly are excited about launching, but we are still concerned about our actual landing, the most difficult step of this mission," Smith said.

NASA has a mixed record with Mars missions. Its twin rovers Spirit and Opportunity have been resounding successes, still going strong after 3 1/2 years on the surface.

But the last mission to the polar region, Mars Polar Lander, was lost on arrival in 1999.

If this mission unfolds as planned, the lander will parachute to the Red Planet's surface, using its descent engines to slow itself to about 5 mph. Once on the ground, it will unfurl its power-generating solar panels and extend its 7.7-foot robotic digging arm, the key component of the $420-million mission.

A scoop on the arm will dig down to a layer of water ice -- thought to lie within 3 feet of the surface -- that the Mars Odyssey spacecraft detected from orbit in 2002. A drill-like tool was added to the scoop after scientists realized that the ice on Mars could be much harder -- more like cement -- than ice on Earth.

Samples of soil and ice collected by the robotic arm will be transferred onboard for analysis. Phoenix carries eight tiny ovens that will heat the samples in search of organic compounds that could indicate past or present biological processes. Each oven will be used only once to avoid contamination.

NASA has tried to keep expectations low, asserting that Phoenix is not searching for life, merely trying to understand the water story. "Water is central to every type of study we will conduct on Mars," Smith said.

NASA learned to manage expectations with the Viking missions to Mars in 1976. The public, its appetite whetted by generations of science fiction writers envisioning Martians plying canals like Venice boatmen, waited excitedly for news of the discovery of life, only to have the spacecraft report a sterile, lifeless world.