PASADENA, California: The first chance for a signal from the Mars Polar Lander passed in silence, heightening fears that the US space agency's US$165 million mission may have ended in failure - just like another Mars expedition three months ago.

A two-hour window of opportunity for contact with the still-silent spacecraft opened at 04:30 GMT, and nothing was immediately received.

During the next opportunity, the spacecraft is programmed to automatically send signals using a different antenna.

If the second antenna works, the transmission would first be detected by the Mars Global Surveyor satellite and relayed to Earth.

Controllers said they have not exhausted all the simple explanations for the silence, like a mispointed antenna or a computer reset.

The spacecraft was in good shape and on course before communications ended on Friday as expected before descent.

Critics, however, said the lander may have been in trouble even before it left the launch pad 11 months ago.

Weeks before the craft began its descent towards Mars, questions were being raised about the mission and whether it might be doomed by tight budgets and understaffing.

If the Mars lander is lost forever, the momentum to study the planet may be diminished, said Robert Zubrin, president of the Mars Society. (Xinhua)