A U.S. spacecraft named Spirit is approaching Mars for a landing on the cold, barren, rocky planet. After it arrives, it is to deploy a six-wheeled robotic rover laden with instruments and cameras on a 90-day hunt for water and signs of life. Three weeks later, the process is to repeat itself on the other side of the red planet with an identical lander and rover.

If all goes as planned, the speeding Spirit spacecraft will enter Mars' thin atmosphere faster than 19,000 kilometers an hour, using friction with its heat shields to slow it somewhat before deploying a parachute two minutes before landing. Seconds before impact, braking rockets are to further slow the descent, and air bags are to inflate to cushion the impact as the craft bounces and rolls up to three kilometers.

It is a landing strategy that succeeded with the U.S. Pathfinder craft in 1997, the last time NASA touched down on Mars. A 1999 effort that was to have used only braking rockets to land failed, although no one is blaming it on this technology. An investigation found the cause to be management failures, too low a budget, and overworked scientists and engineers.

Despite the similarity in landing techniques to the 1997 project, the current Mars mission is far more ambitious.

"This is the most sophisticated science package that will ever have been used to really explore the surface of another planet," said Stephen Squyres.

Stephen Squyres of Cornell University is the lead investigator for the Spirit rover and its twin, named Opportunity, which is to land later in January. He points out that they both can roam up to 50 meters a day the distance it took the much smaller Pathfinder rover weeks to cover. They are equipped with panoramic cameras on masts to help scientists identify interesting terrain features to inspect, and tools to grab and break into rocks for viewing by a microscope.

"We have, for the first time, with this vehicle [the] ability to move across the surface, to look off into the distance to find a target that may tell us something fundamental, to move to it, and then to reach out and touch it, to scrape below the surface to see what lies underneath," he said.

The new Mars rovers are also smarter than the 1997 version. With their powerful computing capability, they can navigate autonomously to a destination, once ordered there by ground controllers, and maneuver around obstacles to reach it.

NASA's chief of space science, Ed Weiler, says the rovers will look for signs of water at two landing sites where U.S. satellite overflights have revealed there may be evidence of it.

"On Earth, water is the key to life," said Ed Weiler. "But water has to persevere for a length of time for life to have a chance to come into being. That is what these rovers do uniquely. They will look for clues of the perseverance of water in the past. They will answer questions about [whether] the surface was once suitable for life?"

The landing site for the Spirit spacecraft is a crater near the Martian equator that scientists believe is the location of an ancient lake. Opportunity is to touch half way around Mars on a smooth plane. Both places show evidence of a mineral called hematite, which on Earth usually forms in association with water.

Stephen Squyres says hematite can form in any water, be it a deep lake or ocean, an underground spring, or merely a wet rock. How much of the mineral is found can provide clues to the water history of a site and its potential for having harbored a microbial life form.

"A tiny coating of moisture on the outside of a rock, it might have been there for a brief period of time," he said. "Long-standing liquid water body - forming massive hematite deposits - a totally different story in terms of what it means for life."

The results of the U.S. rover missions will help determine the course of NASA's future research on Mars. Ed Weiler says the findings may make more likely an attempt to return a sample of Martian soil and rocks to Earth, a robotic mission perhaps a decade away.

"If successful, Spirit and Opportunity will help humans take a giant leap forward in their understanding of Mars' potential as a site for past or current life," he said.