Spirit did not make radio contact with NASA's Mars Odyssey orbiter during a scheduled session two hours later or during another one Thursday morning. It also did not respond to the first two attempts Thursday to elicit an acknowledgment signal with direct communications between Earth and the rover, and it didn't send a signal at a preset time for doing so when its computer recognizes certain communication problems.
Possible causes include corruption of its software or computer memory. If the problems are with the software, NASA can fix it from Earth by beaming patches across more than 100 million miles of space or by rebooting the rover's computer. But if it's a hardware problem, the situation would be far worse, and perhaps beyond repair.
Meanwhile, NASA's next Martian rover, the Opportunity, is scheduled to land Sunday on the opposite side of the planet from the Spirit. The two rovers are designed to examine the planet's dry surface for evidence that it was once wetter and more hospitable to life.