As the Mars Reconnaissance Orbiter nears the end of its seven-month journey to the Red Planet, Nasa will attempt to restart the probe's main engine to slow it enough for orbit. 

The probe will need to get its main engine systems online and begin firing the main thrusters for about 27 minutes on Friday evening, cutting the probe's speed by a fifth to about 11,000 miles per hour.

"We have been preparing for years for the critical events the spacecraft must execute on Friday," said Jim Graf, project manager at the Jet Propulsion Laboratory in California.

"By all indications, we're in great shape to succeed, but Mars has taught us never to get overconfident. Two of the last four orbiters Nasa sent to the planet did not survive final approach."

Engineers must slow the spacecraft just enough for Mars' gravity to capture it in a very elongated elliptical orbit. For six months the craft will dive through the planet's atmosphere, using the friction to slow to a reasonable speed. The process is known as 'aerobraking'.

"Our primary science phase won't begin until November, but we'll actually be studying the changeable structure of Mars' atmosphere by sensing the density at different altitudes each time we fly through it during aerobraking," said Dr Richard Zurek, project scientist for the mission.

After braking begins Nasa will lose contact with the probe for a few hours as it passes around the planet but will then start bringing further systems online, like the cameras and radar. The probe is expected to function until December 2010.

Once in orbit it will send back more than 10 times as much data as current Mars missions, examining the surface, atmosphere and underground layers in great detail from a low orbit.

It can also act as a communications hub in the event of manned missions.