America's hopes of a successful return to the red planet were put on frustrating hold when the Mars Polar Lander and its two micro-probes stubbornly refused to call home.
Repeated attempts to make contact with the three spacecraft following their presumed successful landing on Friday at the Mars south polar region met with no success.
But scientists at NASA's Jet Propulsion laboratory in Pasadena said that it was too early to panic and added they would keep trying to contact the lander and the two microprobes.
Lander project manager Richard Cook said there was a possibility that the lander had failed to separate from the cruise-craft that took it to Mars.
He said one possible scenario for the failure of the three craft, which each carry their own antennas, was the cruise stage "not separating from the lander. We do not know that it did separate."
Such a failure would have sent the joined craft plummeting to the Martian surface.
But a more likely scenario, Cook added, was that the lander was either asleep or lost. The former would occur if one of the lander's many instruments had failed or was playing up. Once the onboard computer detected that something was amiss it would automatically send the craft into a "safe" mode to correct it.
"If it was asleep, which in essence is what happens in a safe mode, then the lander would not be able to hear us when we sent instructions," he said. It would not come out of its safe mode until this evening, when fresh attempts would be made to contact it, Cook added.
The craft would in effect be lost if there was a problem with its gyro compass. It would not know its location on Mars and therefore would not be able to find Earth and point its main antenna in that direction.
Cook said his mood was still upbeat despite the early setbacks. "We have a long way to go before we get concerned," he said, adding that his team would keep trying to contact the lander for the next several days and perhaps longer.
"I am very confident that the lander survived the descent. We are a long way from being concerned, let alone being panicked. We have plenty of additional opportunities [for communications] and we have a carefully worked out contingency plan. We haven't even thought about [the possibility of] losing the craft," he added.
The two grapefruit-sized microprobes -- known as the Deep Space 2 experiment -- were supposed to separate from the lander shortly after the lander had separated from the cruise stage and slam into the Martian soil at 644kph.
Deep Space 2 project manager Sarah Gavit said two attempts by the Mars Global Surveyor satellite orbiting the planet to contact the microprobes following their presumed impact with Mars had failed, but said that she was still confident contact would be made and more attempts would be made throughout the night.
However, the microprobes have tiny batteries that are not rechargable. After 29 hours on the surface the microprobes would go into automatic transmit mode, transmitting for one minute every three hours, which would cause the batteries to run out after two or three days.
Deep Space 2 project scientist Sue Smrekar said she believed the microprobes may have landed in or near a crater and that the sides of the crater were at a 10 degree slope. "It would be of concern because at that angle they could skip off the surface rather than penetrate it. We might have preferred to avoid this crater."
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