Capa Canaveral (Florida): Nine months ago, Nasa's Phoenix probe blasted off for Mars with an unprecedented mission to sample water on another world. Before that can happen, however, the space agency faces a formidable challenge: landing. The odds are not great. Historically, 55% of all attempts to land on Mars have failed and the method being used for the touchdown of the Phoenix spacecraft on May 25 hasn't been attempted in 32 years. "This is no trip to grandma's for the weekend,' Ed Weiler, Nasa's associate administrator for space science, said during a news conference on Tuesday. After a nearly glitch-free ride, Phoenix is scheduled to settle near Mars' north pole at 7.36 pm EDT, but no one will know whether it succeeded until about 15 minutes later. That's how long it will take radio signals, travelling at light speed, to reach Earth, 171 million miles away. Rather than using airbags to cushion and bounce to a stop like the twin Mars exploration rovers Spirit and Opportunity, Phoenix is equipped with steering rockets to descend more precisely on target. A propulsive landing system also is better suited to the heavier spacecraft that Nasa would need to support eventual human expeditions on Mars. Nasa tried a rocket-powered descent on a probe called Mars Polar Lander in 1999. The mission came to an abrupt end during the final approach and landing. What went wrong remains a mystery. "They've done everything they can do to make this a success, but Mars has been known to cause trouble,' Weiler said. Nasa used a similar landing system for its twin Mars Viking probes in the 1970s. Phoenix will be the sixth lander US has sent to Mars, five of which touched down successfully. The statistic does little to allay the anxiety of Phoenix flight controllers, who will be stationed at Nasa's Jet Propulsion Laboratory in Pasadena, California, to await the news from Mars. The worst part will be what project manager Barry Goldstein calls "the seven minutes of terror'