The U-S space agency NASA says it is hearing signals from its probe on Mars, but that the rover has stopped sending home intelligible data.
The agency says on its web site that the "Spirit" space vehicle sent out a beep early Thursday, confirming it had received a transmission from Earth. Scientists also say that Spirit is transmitting random radio noise.
But NASA says the probe has not returned any useful data from the red planet since early Wednesday.
It says scientists are working to determine the cause of the problem. NASA officials say possibilities include a software crash or a fault with the probe's power supply.
Project manager Pete Theisinger downplayed the possibility of the Martian environment somehow harming the vehicle, saying that Spirit was built to endure the planet's tough radiation and weather conditions.
NASA scientists first reported problems communicating with the rover on Wednesday. They originally blamed the problem on thunderstorms around Canberra, Australia, where one of the agency's deep space dish antennas is located. But they have since discounted that theory.
"Spirit's" twin robot, "Opportunity," is heading for a landing in the next few days, on the opposite side of Mars.