Nasa scientists in the United States are working to restore contact with their robotic probe on Mars, which is no longer responding.
Nasa scientists are trying to work out what went wrong
The Mars rover Spirit has stopped sending data back in what the agency calls a "serious" breakdown.
Experts say it may have been caused by a problem with the computer software controlling transmissions with earth.
The next attempt to contact Spirit are due to be made via satellites orbiting around Mars at 0310 GMT on Friday.
"This is an extremely serious anomaly," said Pete Theisinger, the Spirit project manager.
At around 0302 GMT on Thursday, the Mars Global Surveyor spacecraft in orbit around the planet received a signal from the rover suggesting that its UHF radio was on.
But Spirit was only transmitting "pseudo-noise", a random series of zeroes and ones in binary code and not anything the scientists could decipher.
TIMELINE OF EVENTS
Wed 1430 GMT - Weak signal cuts short commands
Wed 1513 GMT - No high gain antenna contact made
Wed 1747 GMT - Mars Odyssey pass, but no data received
Thu 0302 GMT - Mars Global Surveyor receives noise
Thu 0536 GMT - Mars Odyssey pass receives no data
Thu 0800 GMT - No "direct-to-Earth" link established
Deputy project manager Richard Cook said: "Effectively, what it means is the radio was on but the computer wasn't sending information over to it."
The US space agency will now try to send commands to the rover to try to get it to respond.
The next big opportunity is at around 0310 GMT on Friday as Mars Global Surveyor listens for a signal from the rover.
It is hoped that Spirit will move itself into a fault mode in which it can initiate communication with Earth.
The scientists had reported a bad communications link on Wednesday when they tried to send commands to the rover. It appears only some of the commands reached the vehicle.
The difficulties were initially put down to bad weather over a radio telescope in Canberra, Australia, which was sending the signals to the Red Planet.
It now appears the problems could have had some other cause.
While some sort of signal - even noise - continues to be returned from Spirit, scientists can be confident that the vehicle has not completely lost power - providing the mission team with hope that full contact can be re-established.
Spirit's twin rover Opportunity is due to land on the surface of Mars on 25 January at 0505 GMT.
Some staff working on Spirit had been preparing to start working on the Opportunity landing. How staff should divide their time between the projects will now be reviewed.
Nasa scientists will need to decide whether to stand the Spirit operation down until Opportunity lands.