A possible signal from Polar Lander.

Did the Mars Polar Lander phone home last month, and will it ever do so again?

Since last Dec. 3, when the spacecraft vanished shortly before it was scheduled to touch down on the Red Planet (SN: 12/11/99, p. 373), scientists have been searching for a signal that the lander might still be alive. In January, just as NASA announced it had given up efforts to make contact with the craft, a faint ray of hope emerged. An extremely weak radio signal recorded Jan. 4 from a radio dish at Stanford University may have come from the lander, scientists say.

The signal, about as powerful as a cell phone broadcasting from Mars, has several characteristics suggesting that it came from the Red Planet and that the lander sent it, notes John Callas of NASXs Jet Propulsion Laboratory (JPL) in Pasadena, Calif. It arrived shortly after one of NASXs many attempts to make radio contact with the craft, and the signal's frequency, 401.5 megahertz, falls in the range that the lander could broadcast.

Moreover, the frequency rose, then fell, like the radiowave equivalent of a whistle. Part of that change in pitch could be due to the Doppler effect caused by the rotation of Mars relative to Earth. The rest could represent the change in frequency that would be expected as the craft's transmitter warms up.

Scientists at Stanford and JPL independently found the signal in the Jan. 4 data, but as of mid-February, they hadn't detected it again. In contrast, they could still record the far weaker signal of the orbiting Mars Global Surveyor spacecraft.

On Feb. 4, two other telescopes--Jodrell Bank in Cheshire, England, and Westerbork in Schattenberg, the Netherlands--joined the Stanford dish in searching for a signal. Preliminary analyses showed no sign of one. Another Stanford search, conducted Feb. 8, has also failed to find a signal.

If the Jan. 4 signal proves real and engineers can eventually recontact the craft, scientists could learn how it has fared--for example, whether it properly deployed its parachute and if two experimental probes that piggybacked on the lander separated from it as planned. Such data could prove vital to revamping NASA's troubled Mars program, which also lost the Mars Climate Orbiter last fall. There's little chance that the lander could ever conduct scientific observations, says Callas.

In the meantime, Mars Global Surveyor has visually scanned the region near the south pole where the lander was expected to touch down. The search is difficult because Surveyor's camera barely has the resolution to image the lander. The camera does have a somewhat better chance of detecting the lander's parachute, if it opened, because it's larger. As of early February, the last time Surveyor looked, the craft had found neither.