The long-lived Mars rover Spirit, already suffering from a gimpy wheel, has suffered a new round of aging problems in recent days, including a bout of what one NASA official called "amnesia."
John Callas, the rover program manager at the Jet Propulsion Laboratory in La Canada Flintridge, said Spirit reported last Sunday that it had received its driving directions for the day. But it had not moved.
This was not a complete surprise, NASA said. There are a number of reasons the rover might not be able to move. It could temporarily be stuck in sand or on a hard-to-negotiate slope. It also might not be properly oriented to the sun, which supplies power to the rover's wing-shaped solar arrays.
In the past, rover drivers have been able to figure out a solution to each of those problems. After the latest difficulty, troubleshooters at JPL guessed the problem was bad positioning, so they ordered Spirit to locate the sun with its camera.
The rover reported back that it could not find the sun. That was a surprise to rover scientists because they knew the sun was up. Later, on Tuesday, Spirit reported that it had finally found the sun, but not in its expected location.
"We're investigating whether it's the gyros or a camera problem," Callas said.
On Friday, JPL received the latest diagnostic data from Spirit's Inertial Measurement Unit, a combination of gyroscopes and accelerometers that helps the vehicle know where it is.
According to Callas, the gyroscopes appear to be working fine. The accelerometers, however, showed an "offset," which he compared to a compass that is off true north by a few degrees.
Though that's a problem, operators can correct for it.
The other trouble was even more of a surprise. It turned out that Spirit had failed to record last Sunday's activities in its nonvolatile memory, which remains active even when other systems are shut down. Callas referred to the problem as a 90-minute bout with amnesia.
He said the various problems did not appear to be linked.
Sharon Laubach, chief of the team that writes and checks commands for Spirit and its twin, Opportunity, was also unsure what's going on with the rover.
"We don't have a good explanation yet for the way Spirit has been acting for the past few days," she said. "Our next steps will be diagnostic."
Among the range of possible explanations is that a well-aimed cosmic ray hit the electronics, giving a kind of shock to the system. Despite the name, cosmic rays are high-energy particles, usually protons, that are accelerated to high speeds by the sun, as well as cataclysms in deep space.
Whatever the cause, Spirit later recovered its memory and was back on the job. "Right now, Spirit is under normal sequence control, reporting good health and responsive to commands from the ground," Callas said.
When Spirit and Opportunity landed on Mars in 2004, scientists were hoping to get 90 days of use from them. Instead, they've lasted five years, challenging the imagination of rover scientists to find new jobs for them.
"Is this an indicator the systems are wearing out?" Callas said, unwilling to speculate. "We can't say."