After a curving journey of 480 million kilometers to a planet 170.2 million kilometers from Earth, one might think the hardest part of the trip was over. But now the airbags must be removed, the rover must raise its mast and train its stereoscopic cameras around its environment to hunt for promising rocks, and Spirit's solar-powered batteries must operate flawlessly.
As landing time neared, Louis D'Amario, chief space navigator for the mission, said: "It's been like teeing off in Paris for a golf game and hitting a hole in Tokyo with a breeze and a little water hole the size of an ocean in between.
"But as Spirit's course looks right now, it's been perfect navigation."
The spacecraft's landing site, Gusev, is a giant crater just north of the Martian equator. Images from space show it may have been a huge lakebed billions of years ago, filled with water from streams flowing in torrents down from the southern highlands of Mars.
Spirit had been scheduled to wait nine days or so after its scheduled landing while its high-resolution stereo camera surveys nearby rocks to pick out the most likely ones that show signs of holding sediments from Gusev's possibly ancient flowing water.
The craft's spectrometers are designed to detect evidence of ancient sediments from within the lake and its grinding tool will scrape away the surfaces of sedimentary rocks.
In another 21 days, Spirit's counterpart, the spacecraft named Opportunity, will land on a site called Meridiani Planum, halfway on the other side of the planet from the Gusev crater. And if water were ever there, it could have created rocks called grey hematite, an iron oxide frequently formed within iron-rich water. The rover's instruments are specially equipped to identify that glittering silvery rock.
Said Ed Weiler, NASA's director of space science: "This mission is not about rocks, it's not about pictures; it's part of our first steps in answering a great human question: Are we alone?"
Two Viking spacecraft landed there in 1976 seeking organic chemicals and possible micro-organisms beneath the surface but found only exotic chemistry.
The mission called Pathfinder carried a tiny rover called Sojourner that analyzed a variety of Martian rocks in 1997 and found no conclusive evidence of ancient water. Another effort to land on a Martian polar ice cap in search of liquid water crashed four years ago.
Updated NASA information on the Mars Rover Exploration is on the Web at http://marsrover.jpl.nasa.gov/home/index/html.
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