NASA mission managers said the first of twin Mars rovers streaked down to what they believed was a "bull's-eye" landing in Gusev Crater on Saturday night — and received a momentary indication that the spacecraft was bouncing over the Martian surface.
If the landing was successful, it would mark the beginning of NASA's first exploration of the Martian surface since 1997's wildly successful Mars Pathfinder mission — and mark another step in the search for traces of ancient water and perhaps ancient life on Mars. But if the landing went wrong, the Spirit rover would take its place among the many other failures at Mars.
Plenty could have gone wrong during the final stage of the rover's seven-month, 300-million-mile (480-million-kilometer) trip to the Red Planet — a descent through the atmosphere that NASA's associate administrator for space science, Ed Weiler, called "six minutes from hell."
Controllers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., reported that everything was on track for Spirit's landing at 8:35 p.m. PT (11:35 p.m. ET) Saturday. In the final days leading up to the descent, they decided there was no need to use the last three opportunities to change the spacecraft's trajectory.
"That's just a testament to how good of a job this navigation team has done," Wayne Lee, landing system engineer for the rover team, said Saturday.
The mission plan called for the spacecraft's heat shield to weather more than 2,600 degrees Fahrenheit (1,450 degrees Celsius) during the plunge through Mars' thin atmosphere. That friction slowed down Spirit, wrapped in its landing shell, from a speed of 12,000 mph to 1,000 mph (19,200 to 1,600 kilometers per hour). Retro rockets and a parachute slowed the descent even more. About 50 feet (15 meters) above the Martian surface, airbags inflated around the rover and its shell to cushion the impact.
Protected by the airbags, the spacecraft could have bounced as high as a four-story building, rolling to a stop as much as a half-mile (1 kilometer) from the initial impact. But even if everything up to that point worked perfectly, strong cross winds and a particularly sharp rock could have blown a gaping hole in the plan.
“All it will take in the last few minutes is a strong gust of wind, more than we expected, and the mission will be over,” Weiler said during a briefing earlier this month.
Spirit was programmed to transmit radio tones that could be received on Earth or relayed via NASA's Mars Odyssey orbiter. Controllers could hear confirmation of a safe landing as little as 10 minutes after the landing — or it could take hours.
"Just because we don't hear from it tonight does not mean that it didn't survive the landing," Lee said.
If Spirit landed safely, the airbags would deflate, the landing shell would right itself, and the lander's petals would open like a flower, revealing the 384-pound (175-kilogram) rover within. The rover would raise its camera-equipped observation mast, and Spirit's 90-day mission to study Gusev Crater would begin.
Looking for water and lifeToday, Mars is a cold and dry place, inhospitable to life as we know it — at least on the surface. But based on the evidence collected by past Mars missions, scientists believe that the Red Planet was once warmer and wetter, with liquid water flowing through channels and pooling in lakes or even oceans. Gusev Crater is thought to have been an ancient lakebed, and Spirit would analyze the rocks and soil to determine whether liquid water persisted there long enough to support the development of life.
"We don't know that there's life there, or that there ever was," Cornell University astronomer Steve Squyres, principal investigator for the rovers' scientific payloads, told MSNBC.com in advance of the landing. "I would be willing to bet a year's pay that water flowed there on the surface sometime in the past. But that's as far as I'd be willing to go."
The rover would start by collecting images and other scientific data from where it sits. Even if the spacecraft lands in perfect condition, it will be at least nine days before the rover rolls off its landing platform to begin its travels.
Spirit's twin, the Opportunity rover, is due to touch down on Mars just after 9 p.m. PT (midnight ET) Jan 24, rounding out the $820 million mission. Opportunity is targeting a geologically complex region called Meridiani Planum, which contains significant deposits of hematite, a mineral that could have been formed through the action of ancient hot springs. On Earth, such springs provide a home for microbial life.
Previously, two in every three attempts to land spacecraft on Mars has failed. The latest apparent failure was the British Beagle 2 lander, which has not been heard from since its Christmas descent to the Martian surface. Japan's Nozomi orbiter also missed going into Martian orbit last month.
NASA’s last attempt at landing on Mars, in 1999, failed when a software glitch sent the Polar Lander crashing to the ground. Since then, the space agency has increased oversight of its missions.