Scientists are struggling to find out what has gone wrong with the Mars rover Spirit after 24 hours of garbled communications and intermittent silence caused by an unknown fault.

Spirit: weak signal: Nasa reported that Spirit had radioed a signal indicating it was receiving Earth transmissions even though it had stopped sending data back. Despite the five-minute beep, it has failed to send back any data since Wednesday. The team made contact the following day using a radio frequency triggered when the rover switches to "fault" mode."In all exploration you are always going to have anxious moments," said Charles Elachi, director of Nasa's Jet Propulsion Laboratory in California. "The key thing that is important is that we stay calm and thoughtful. You can do harm by reacting too fast."

Spirit landed on Mars on Jan 3 on a planned three-month mission to explore its geologic history. Its problems will not affect the arrival of a second rover, the Opportunity, on the opposite side of the planet tomorrow.