Spirit, the first of the probes launched, this week made its third flight path correction, in a maneuver that involved igniting its engines for just over two minutes, according to the Jet Propulsion Laboratory (JPL) in Pasadena, California.

Another three flight path corrections are necessary ahead of Spirit's Mars landing, scheduled for January 4, said Louis D'Amario, head of probe navigation at the JPL.

A Rover robot on Spirit will be sent out to explore the planet for three months.

The second probe, Opportunity, is carrying an identical robot, and will touch down at the opposite side of Mars to Spirit on January 25.

The two probes started up their onboard computer in the past two weeks, to prevent any corruption of the information files that might have been provoked by unusual levels of radiation during solar storms in late October and early November.

"We had no evidence of memory problems, but we considered it prudent to reboot both spacecraft to assure memory integrity, using the sleep-wake cycle that we plan to do each night after the rovers are on the surface of Mars," said Peter Theisinger, project manager for the Mars Exploration Rover Project at the JPL.

On Wednesday, Spirit was 91.5 million kilometers from Mars. The probe had traveled about 397 million kilometers (246 million miles) since its June 10 launch.

Opportunity, since it July 7 launch, had traveled 326 million kilometers (202 million miles) and is now around 130 million kilometers (81 million miles) from Mars.

SPACE.WIRE