Spacecraft expected to reach Mars on May 25

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Defying the odds, a Mars-bound spacecraft named Phoenix is headed toward a landing this month on the planet's icy north polar surface to search for evidence that liquid water and chemicals crucial for life existed there long, long ago.Equipped with a 7-foot robotic arm to dig beneath the surface, a dozen tiny ovens to cook soil and ice from permafrost samples, and a suite of instruments and microscopes to analyze it all, Phoenix is the most complex Martian laboratory since the two historic Viking landers failed to find anything alive on the planet more than 30 years ago.Mindful that five Russian Mars landing missions and two American ones -- more than half of all attempts -- have ended in failure, the scientists and engineers on the Phoenix team are anxiously counting the minutes to touch-down, now scheduled for 7:38 p.m. EST May 25. Confirmation is supposed to come by radio from the spacecraft at 7:53 p.m. "This is no trip to grandma's house," said Edward Weiler, NASA's science chief. "It's hard and risky."Phoenix was 12 million miles from Mars and 170 million miles from Earth this week when Weiler and leaders of the unmanned planetary expedition briefed reporters at a Washington news conference.Phoenix project manager Barry Goldstein said everyone will be nervous at the landing, when the spacecraft's heat shield, its parachute and its 12 thruster jets must all operate precisely within seconds in order to complete the 10-month journey from Earth.The three-month mission is perilously different from that of the two rovers, Spirit and Opportunity, which are still operating after four years on the surface of Mars. The rovers' bouncy landings were cushioned by giant air bags. By contrast, Phoenix must land on three slender legs, slowed in the final 18 seconds above the surface by downward-facing jets. If functioning properly, the jets will fire automatically.Signals from the $420 million mission will be relayed to mission control at the Jet Propulsion Laboratory in Pasadena, Calif., by NASA's Mars Reconnaissance Orbiter and Mars Odyssey, which are already flying above the planet.The two Viking spacecraft, which landed on Mars in 1976, were designed to look specifically for signs of life. They each carried three miniature laboratories containing chemical reagents that would signal if anything were alive, but they never found a single microbe -- nor anything that suggested signs of active organic chemistry.Phoenix and its delicate instruments, however, are not aimed at finding life itself."We're really doing chemistry experiments to see if the active processes of climate change are written into the soil over a history of billions of years, and if they once created a habitable zone," said the mission's chief scientist, Peter Smith of the University of Arizona's Lunar and Planetary Laboratory.Radar images from the orbiters flying high above reveal a landing site covered with "fairly loose" soil that must have been ejected long ago from a nearby impact crater, said Raymond Arvidson of Washington University in St. Louis. He leads the team that selected the Phoenix landing site from high-resolution images sent down by the three orbiters.The soil is about 5 or 6 inches deep, and below that lies dense ice as hard as a concrete sidewalk, Arvidson said.The soil is the ultimate target for the robotic scoop that Phoenix carries. Each sample of frigid soil it digs up must be delicately placed into the spacecraft's ovens and heated to 1,800 degrees Fahrenheit to vaporize its chemicals for analysis. The scoop holds a tiny finger-size tool that can scrape bits of ice lying beneath the soil to see what molecular changes may have occurred in that long-frozen water over the millennia.With average temperatures in the Martian far North running more than 115 degrees Fahrenheit below zero, water can't currently exist as a liquid, Smith noted. "But when the climate was warmer there it would have changed the chemistry of the soil and we can look for that evidence," he said."My greatest hope is that we're going to change the entire course of Mars exploration," Smith said. The little insulated lab aboard the spacecraft carries a tiny bit of sterile water, and for one experiment a soil sample will be impregnated with the water to see what chemical reactions take place when the soil is wet, he said.If Phoenix lands successfully on May 25, it will be the fourth American spacecraft to get to Mars.(E-mail David Perlman at dperlman(at)sfchronicle.com)(Distributed by Scripps Howard News Service, www.scrippsnews.com.)