The U.S. space agency, NASA, says its Spirit spacecraft is on track for a landing on Mars on Saturday at 11:35 p.m. The goal of the mission is to seek water.

Spirit has been on its way to Mars since June accompanied by a twin spacecraft named Opportunity.

If Spirit's fiery descent through Mars' atmosphere goes perfectly, it will deploy a six-wheeled rover that will prospect for minerals searching for evidence of past water.

Opportunity is to repeat the sequence three weeks later on the other side of the red planet.

Mission scientist Steve Squyres of Cornell University said any water evidence found will be used to guide future landers on a search for microbes.

"This is not a fossil hunt. What we're really doing is trying to find the places where evidence of life might be preserved so that when we can send much more sophisticated vehicles to return samples, we have the maximum chance of finding what we might be after," he explained.

NASA's last effort to land on Mars failed in 1999, and a British attempt last week to study Martian soil is in limbo because no signal has been received from its Beagle lander.