Mars mission reaches critical stage

Global Surveyor, the spacecraft which is set to revolutionise our knowledge of Mars, was last night ready for the critical rocket firings that will guide it into orbit.

Coming soon after the Pathfinder project, the spacecraft will perform the most comprehensive photographic survey of another planet ever undertaken, and provide scientists with better maps of our nearest planetary neighbour than existed of Earth itself, until recently.

In the early hours of this morning, ground control was due to attempt a risky 22-minute engine burn to slow the spacecraft to about 9,000mph and place it into an elliptical orbit. This will be followed by four months in which ground control will use a navigational technique called aerobraking, in which the vessel dives into the upper reaches of the Martian atmosphere to lose speed and height.

The craft, which is about the size of a large garden shed and weighs about one ton, will not land on Mars. Instead it will position itself in a low orbit, taking high-resolution photographs which will show objects as small as 1.5 metres across.

These pictures will be sharp enough to help scientists conduct detailed geological studies without needing to set foot on the planet. It will help to identify the likeliest sites where life might have taken hold, including areas where there was once water, and mineral remains of ancient hot springs.

The mission is the first in the second wave of a decade-long invasion of the red planet by Nasa, which will launch a probe to Mars every 26 months into the next decade.