The U.S. space agency NASA is having trouble communicating with its robotic rover on Mars. Ground controllers are trying to get its attention.

Mission controllers at the Jet Propulsion Laboratory in California have heard only simple tones from the Spirit rover since early Wednesday.

The rover's tones are confirmations that it has received signals from mission controllers asking it to respond.

But Mission official Peter Theisinger says that the vehicle is not transmitting expected scientific or engineering data.

"We now know that we have had a very serious anomaly on the vehicle, and our ability to determine exactly what has happened has been limited by our inability to receive telemetry from the vehicle," he said.

NASA officials do not know the cause of the problem and are struggling to diagnose it.

Mr. Theisinger says it is unlikely that something in the Martian environment is to blame because the Spirit rover was built to endure the planet's tough radiation and weather conditions.

"Yes, something could break," he said. "Clearly, something could fail. That's a concern we seriously have. It's very serious. But there was not any obvious correlative event there's nothing like that."

Spirit arrived at Mars nearly three weeks ago to search for signs of water that would indicate the planet was once suitable for life. When the six-wheeled robot developed its communications problem Wednesday, it was preparing to scrape its first rock to look for chemical signatures of water. Apparently, it never received the commands to do so.

At first, the NASA team had believed that heavy thunderstorms around Canberra, Australia interfered with communication because one of the agency's big deep space dish antennas is located there. They have since discarded that notion.

Mission controllers say they will attempt to send more signals to the Spirit robot hoping it will reveal the source of its technical problem. In the meantime, they are also preparing for the Sunday arrival of a twin rover on the other side of Mars.