The recent losses could change NASA_s plans for riskier missions.

It was 3:35 a.m. ET on Dec. 7, Pearl Harbor Day, 1999. Sam Thurman, flight operations manager of the Mars Polar Lander, hesitated before answering an incoming call from Mike Malin, who operates the communications relay system aboard the Mars Global Surveyor. The last reasonable hope that the silent spacecraft might be raised, Thurman knew, depended on this message. He swallowed and looked up at the ceiling without speaking. Expectant faces crowded around him in the small mission operations center at NASA_s Jet Propulsion Laboratory. "Go ahead," Thurman said into his headset. "Sam, I_m sorry to report that all we have is HKTM. It looks like a nominal pass." With those words, the Mars Polar Lander was as good as dead. The mission had appeared perfectly on target as the lander entered the martian atmosphere Friday. It was to spend three months searching for water ice and studying the planet_s climate. It was accompanied by two small probes designed to test new technology, slamming into the soil at high speed. JPL Mars program manager Chris Jones announced Tuesday that both internal and external review boards would try to figure out what went wrong and what it might mean for the future of Mars exploration. While the Mars Global Surveyor will attempt to locate the lander's parachute - and thus determine if it made it through the atmosphere - most experts doubt that the exact fate of the mission will ever be known. Jones predicted that the recent losses could change NASA_s plans for riskier missions, such as the robotic return of martian rock samples, now scheduled for 2008.