A computer glitch has crippled the Mars Global Surveyor, a satellite that has been mapping the surface of the Red Planet for the past six years. 

Problems started on 30 July when the main computer unexpectedly switched to the back-up system and rebooted.

The back-up computer switched back to the primary computer on 26 August, but it had gone into 'safe' mode which shuts down key systems. 

The glitch occurred just as the satellite was due to take detailed photographs of the Mars Polar Explorer probe, which crashed onto the surface of Mars in 1999.

The images are needed to see whether last minute changes will be required to the Phoenix probe due for launch next year, which will attempt to land on the poles of Mars like its predecessor.

Engineers are now attempting to restart the main computer with all functions intact by beaming commands to the satellite. The results will be known by the end of the week.

Mars Global Surveyor was only expected to last six years in orbit and so far has lasted nearly three times that, making 28,792 orbits that have taken photos of most of the planet's surface.