The U.S. space agency NASA says its Spirit spacecraft is on a perfect course for a planned landing on Mars Sunday at 4:35 UTC. The most dangerous part of the nearly 500 million kilometer journey is still ahead.

Spirit is approaching Mars at more than 10,000 kilometers per hour on a mission to prospect minerals for water to determine if conditions conducive to life ever existed there.

Navigators at NASA's Jet Propulsion Laboratory say the trajectory is so accurate that they were able to eliminate the last two of five course corrections. According to navigation team member Louis D'Amario, Spirit has almost a 100 percent chance of hitting its target near the center of a cigar-shaped area 67 by five kilometers in a Martian crater.

"When you take into account how far we've traveled and what our target at Mars is, it's kind of like playing a par five hole in golf, where you tee off in Paris and the hole is in Tokyo," he explained.

But to get to the crater, Spirit first must go through a fiery six minute, 130 kilometer descent through the Martian atmosphere, automatically executing a series of precision maneuvers perfectly. These include jettisoning its heat shield, deploying a parachute and firing braking rockets to slow it down, cutting the parachute loose, and inflating airbags to cushion its landing.

Mission officials warn that an unexpected gust of wind or landing on sharp rocks could scuttle the mission. NASA's chief of space science, Ed Weiler, recalls that the United States has failed to land on Mars once in two previous tries and Britain's effort to communicate with its Beagle craft has been unsuccessful since its apparent touchdown on Christmas. "It's an incredibly difficult place to land. Some have called it the "death planet" for good reason," he said.

If the U.S. Spirit lander survives, ground controllers will be listening for its first signals. They say direct communications is probably not possible since Earth will be below Mars' horizon at landing time. So they have scheduled overflights of U.S. satellites, which they hope will relay a signal. But this depends on the angle of the landing and the direction the antennas are pointing, so mission controllers may not hear anything at first because the lander could be busy putting itself in an upright right position.

Mission scientist Steve Squyres points out that the team might have to wait for a signal when direct communications is possible later Sunday after Earth rises back into view. "It could be a long wait. You know, it could be a long night," he said.

If successful, the Spirit rover will spend a week or more completing a series of engineering and scientific tasks before moving off the lander in search of rocks.

Mr. Squyres says it will crack open and analyze rocks for traces of ancient water to determine whether Mars was once habitable for simple microbes. "We see dried up river beds on Mars. We see dried-up lake beds," he said. "We see these intriguing hints that Mars may have been a very different world in the past. Now, if you want to find out whether or not those hints are really telling us that Mars was once a place that would have been suitable for life, then what you really need to do is go there and read the story in the rocks."

The Spirit mission is to last 90 days and is to be duplicated in three weeks when an identical lander arrives to examine geological features for water on the other side of Mars.