The U.S. space agency, NASA, reports progress in repairing its crippled Mars rover as it guides a second rover toward landing later today (12:05 a.m. EST, Sunday).
Engineers at mission control in California are closer to determining why communications with the Spirit rover broke down Wednesday.
They believe its computer software is to blame. They suggest that part of the vehicle's computer memory is faulty, but predict that they can work around it to rely on another part of the memory.
"The rover has been upgraded from critical to serious," said project manager Peter Theisinger. He says his team does not yet know what caused the computer software problem, but adds that they have been able to command the malfunctioning rover to reset itself and shut down.
"This is very good news," he said. We've established an ability to communicate with the vehicle reliably. We've established that, in fact, we do have controllability of the vehicle."
If the engineers are correct, they will not be able to rely on the section of the computer memory that stores data for transmission. To get around this issue, they say they will have to finish sending large amounts of data everyday without storing it overnight while the rover is in a low power sleep mode.
Mr. Theisinger explains that tracking the origin of the rover's problem will significantly delay the return to its mission, in which scientists command it to inspect Martian soil and rocks for evidence that liquid water once existed on the planet.
"The mission consequences of this are uncertain at this time, but I think that we probably have more capability left in the vehicle than the worst case scenarios by quite a bit," he said. "To get back closer to routine operations, I think, we're probably three weeks away from driving, I'm guessing."
In the meantime, flight directors say an identical spacecraft named Opportunity is approaching Mars at nearly 11,000 kilometers per hour and is on course for landing on the other side of Mars later today.
Like the Spirit rover three weeks ago, Opportunity is programmed to automatically enter Mars' atmosphere, deploy a parachute and braking rockets to slow its descent, and then inflate protective airbags to cushion its touchdown.
According to the chief of the spacecraft navigation team, Louis D'Amario, it is likely to fall within an elliptical, or cigar-shaped target 74-by-five-kilometers.
"We're not going to land right at the center of the ellipse, but we're going to land very close to the center closer to the center than the boundaries," he said. "It looks like that we hit the bullseye again with Opportunity, just like for Spirit.
Opportunity is aiming for a large flat Martian plain where U.S. satellites have detected an iron compound called hematite that usually forms in the presence of water. In contrast, Spirit landed in a crater that displayed no such obvious minerals from above, but which looked like an ancient lake bed.