Evidence is growing, albeit slowly, that the lost Mars Polar Lander is the source of the mystery signal that NASA began investigating last week, and the space agency plans to step up efforts to talk to the craft in hopes that it is working on the red planet.
Radio astronomers in England, Italy and the Netherlands are adding dish antennas to the hunt, but the signal is exceptionally weak _ about as strong as a cell-phone signal _ and even at the speed of light would take 17 minutes to travel the 190-million miles from Mars to Earth.
Despite the new hope, experts played down the chance that they could resume communication with the wayward $165-million craft and warned that even if communication with the lander could be restored, its scientific mission would probably be impossible to pursue or complete.
Still, if the craft is functioning, it may mean the mission failed only because it had the bad luck to land in inhospitable terrain. In that case, NASA's approach to exploring Mars would look better than some critics had suggested after the mission failed. The lander vanished Dec. 3, and NASA announced Jan. 17 that the craft was probably dead.
But last week the National Aeronautics and Space Administration raced to re-establish contact with the vanished lander after scientists at Stanford University reported that an analysis of data gathered by their 150-foot dish antenna on Dec. 18 and Jan. 4 showed signs of an extremely weak signal from Mars.
In Pasadena, Calif., NASA experts at the Jet Propulsion Laboratory, which runs the Mars mission, warned that the signals might be earthly interference, but this week scientists said a new round of tests conducted at Stanford last Wednesday and Thursday and analyzed over the weekend detected other signals that were consistent with the tantalizing ones gathered earlier.
"There's a handful of candidates," Ivan R. Linscott, the Stanford scientist in charge of the hunt, said in a telephone interview Sunday, adding that further analysis might determine whether the putative signals were actually from the Mars craft.
Although NASA has said it would need time to confirm that the signals were from the lander, the Stanford team's analysis, based on frequency shifts induced by planetary motions, is said to strongly lessen the likelihood that the signal is terrestrial interference.
Bruce Murray, a former director of the Jet Propulsion Laboratory, said the scientists there "are taking this quite seriously."
"It's not a fluke," he said.