NASA kicked off an $800 million mission to Mars today, launching the first of two golf cart-sized robots that will creep across the red planet's surface looking for the geological traces of a warmer, wetter past and the environmental conditions necessary for the evolution of life.

The Mars Exploration Rover Spirit lifts off aboard a Boeing Delta 2 rocket from pad 17A. Credit: NASA

After back-to-back weather delays Sunday and Monday, the "Spririt" rover thundered away from the Cape Canaveral Air Force Station atop a Boeing Delta 2 rocket at 1:58:47 p.m. EDT (1758:47 GMT).

A camera mounted on the side of the rocket's first stage provided spectacular views of the Florida peninsula dropping away below as it thundered toward a preliminary parking orbit. Then, a half-hour after liftoff, a Thiokol Star 48B solid-fuel motor ignited, boosting Spirit out of Earth orbit and onto a 311-million-mile trajectory to Mars.

If all goes well, Spirit, named by the third grade winner of a "name the rovers" contest, will slam into the martian atmosphere Jan. 4 after a seven-month voyage. A second rover, named Opportunity, is scheduled for launch June 25. It will arrive at Mars on Jan. 25.

Scientists can't wait.

Based on data streaming down from two spacecraft currently orbiting Mars, scientists know water once flowed on the martian surface and they know vast amounts of water remain today, locked up in the planet's frigid soil. The question is how long did liquid water exist on the surface and did life have time to evolve? If so, do any pockets of primitive life exist today?

"This is going to be humanity's first great voyage of exploration of this century," said Steve Squyres, principal investigator for the Mars Exploration Rovers project. "It is a powerful mission of scientific exploration.

"Looking at Mars today, it's cold, it's dry, it's barren, it is not the kind of place you'd think would be suitable for life. And yet when looked from above, you see this compelling evidence that once upon a time conditions were different, it was warmer, it was wetter, we see these dried up river beds and lake beds. What we're trying to do with this mission is determine whether or not the conditions at these places would have been suitable for life."

Said NASA science chief Edward Weiler: "If you're really looking for life, it's not just having water, but it's the perseverance of water that counts. And that's what these missions do uniquely, they're really going to tell us about the perseverance of water on Mars, whether it was there for a few minutes, a few hours, millions of years.

"If we have any hope of finding an answer to the question is there life on Mars, was there life on Mars, we have to show that water not only exists on Mars but persevered for long, long periods of time."

And if it did, and if evidence for past or present life on Mars ultimately is found, humanity would finally have solid evidence Earth is not unique in a vast and forbidding universe.

Such a discovery would be "hugely important," Squyres said. "Right now, we only have one demonstrated instance of life in the universe, and that's us. And we don't know how alone we are. To me, that's a fairly profound question. Say we find one more instance in this solar system. Then it requires no great leap of imagination to believe it's present in other solar systems as well. There's a huge difference between one example and two examples."

Spirit and Opportunity will crawl up to 330 feet per day across two carefully selected landing sites, poring over rocks and soil like robot field geologists and, like any tourist, snapping panoramic pictures as they creep along.

Both solar-powered rovers are equipped with an articulated robot arm carrying the equivalent of a rock hammer and a geologist's lens in the form of high-resolution cameras and instruments to measure the chemical composition of selected samples. Using powerful computer software, stereoscopic vision and six independently driven and independently steerable wheels, the rovers will autonomously navigate around obstacles while out of contact with ground controllers at the Jet Propulsion Laboratory in Pasadena, Calif.

Commands and engineering data will be beamed directly to and from the rovers during relatively short communications sessions during the start of each martian day, or "sol." Scientific data will be relayed back to Earth twice each afternoon via UHF radio links with the Mars Global Surveyor and Mars Odyssey spacecraft currently in orbit around Mars.

"Essentially, we'll be using MER as a time machine to take us back into the past and to tell us was water there long enough and perhaps warm enough to have supported life," said Cathy Weitz, MER program scientist at NASA headquarters.

Compared to the small Sojourner rover that caught the world's fancy during the 1997 Mars Pathfinder mission, the Mars Exploration Rovers are full-fledged sport utility vehicles, weighing in at nearly 400 pounds each.

"This is the monster truck of Mars rovers," joked Squyres, a geologist at Cornell University.

The Spirit and Opportunity rovers are the result of an extensive overhaul of NASA's Mars exploration program in the wake of two devastating failures in 1999. One spacecraft, the Mars Polar Lander, crashed because of a software oversight that resulted in a premature engine shutdown. The other, the Mars Climate Orbiter, burned up in the martian atmosphere because of an embarrassing metric-to-English conversion snafu.

An independent investigation concluded both failures primarily were the result of poor communications, overly aggressive development schedules and tight budgets, hallmarks of the agency's "faster, better, cheaper" approach to planetary exploration.

This time around, NASA left no stone unturned to ensure success, spending nearly a billion dollars on the two rovers, their Delta launch vehicles, software development and mission support. Even so, Weiler said he doesn't plan to celebrate until both rovers are safety on the martian surface and collecting scientific data.

"I won't say money was no object, but this time we won't be accused of scraping and scrounging on this program," he said. "We spent $800 million on these two rovers to get them right. We have done everything humanly possible to eliminate as many of the risks as possible in this mission.

"But still, Mars is the death planet. It's a graveyard of many, many spacecraft. Despite all these efforts, the rovers remain high risk missions. We've done everything we can to reduce the risks, but there is still risk. ... We could have a bad day on Mars. A lot of people have had bad days on Mars. We don't call it the death planet for nothing."

Only 12 of the 30 U.S. and Russian Mars missions attempted over the past four decades succeeded. Of eight landing attempts, only three spacecraft made it to the surface intact: NASA's twin Viking landers and the Mars Pathfinder.

But NASA is more optimistic about the latest mission, primarily because the landers will not rely on sophisticated descent rockets and control systems that have little margin for error. Instead, both will drop to the surface encased in giant airbags like those successfully tested in the Pathfinder mission.

If all goes well, Spirit will slam into the atmosphere of Mars on Jan. 4, 2004, at an angle of 11.5 degrees, an altitude of about 80 miles and a velocity of 12,000 mph. Over the next four minutes, the spacecraft will slow to a velocity of about 960 mph and descend to an altitude of just 5.3 miles above Gusev Crater. Scientists believe the crater harbored a vast lake in the distant past.

At that point in the descent, a mortar will fire and a stabilizing parachute will deploy, reducing the craft's downrange velocity and slowing its descent rate. Twenty seconds later, the lander's heat shield will be jettisoned and 10 seconds after that, the rover will be quickly lowered on a 66-feet-long tether, or bridle, below the upper aeroshell where the parachute is anchored.

Eight seconds before touchdown, based on data from a radar altimeter, giant airbags will suddenly inflate, encapsulating the spacecraft in a protective cocoon. Two seconds after that, braking rockets will fire to briefly kill the craft's downward velocity and correct for crosswinds. The support bridle then will be cut and the lander will free fall the final 50 feet to the surface.

Rebounding like some alien beach ball, the spacecraft will bounce as high as 66 feet on its first bounce, taking several minutes to come to rest somewhere in a landing "footprint" measuring 68 miles by nine miles. The airbags then will deflate, the lander will right itself and protective panels will unfold like the petals of a flower.

Unlike the descents of Pathfinder and the Mars Polar Lander, flight controllers expect to receive telemetry up until the point the bridle is cut and the airbag-encased Spirit drops to the surface.

"The good news is we're going to see this thing all the way to 10 meters above the ground," Weiler said. "The bad news is we're going to see it all the way to 10 meters above the ground. Hopefully, we won't see it die, we'll see it living."

Depending on the orientation of the lander immediately after touchdown, and whether or not one of two antennas can "see" Earth, controllers may or may not receive a signal until the following day. That's because Earth will set on the martian horizon within 15 minutes or so of touchdown.

"There is a chance that if we land in just the right orientation (we'll get a signal)," Weiler said. "I'll let you do the experiment. Take a piece of bread, put some jelly on it and randomly flip it and see which side it falls on. That's the way the antenna will be pointed."

The main threat to the airbags during landing is the martian wind velocity and the transverse speed of the descending spacecraft. If the wind speed is too high, the bags can rupture on impact.

"Clearly the one thing you worry about is the actual terrain you hit when we land. Because that's the one thing we can't control," Theisinger said. "If you look at the big Pathfinder panorama, there are places in that photograph where if Pathfinder had landed there, they die. So there are places where we're going where if we land on that rock or that hill or whatever, we're dead."

The Spirit rover will not roll off its support panels right away. It first will take high-priority panoramas of the landing site. Finally, after four to five days of calibrations and checkout, commands will be sent to unfold the rover's stowed wheels to "stand up" the robot. Only then will the rover be ready to roll off onto the martial soil.

"The science mission really can't start until that event happens," Theisinger said. "And that's the one thing we haven't done before. We have practiced it. ... But that's what you worry about."

The Opportunity rover will reach Mars on Jan. 25 and follow a virtually identical descent profile. Both rovers are expected to remain operational for at least 90 days and to crawl up to a six tenths of a mile or so when all is said and done.

A high-resolution panoramic camera mounted atop each rover will be capable of taking pictures three times sharper than those from the Mars Pathfinder, providing Earth-bound viewers with the equivalent of 20-20 vision on the surface of another planet. An associated infrared camera will be used to identify minerals near the rover that may have been formed by interactions with water.

Pictures from both cameras will be used to identify targets worthy of up-close investigation.

Each rover features a robot arm that will be equipped with three instruments for such close-up observations: Two spectrometers and a microscopic camera. In addition, the arms will be equipped with a rock abrasion tool, or RAT, that will grind into selected rocks to reveal their interior structure and composition.

Sophisticated computer equipment will enable the rovers to autonomously move from point A to point B without intervention from ground controllers. They will steer around rocks or other obstacles standing more than 10 inches high and simply drive over smaller obstructions.

"Mars can always surprise you," said Scott Hubbard, currently director of NASA's Ames Research Center and a member of the board investigating the Columbia disaster. "The landing system has been designed to be tolerant of a whole lot of problems, so I think they've done everything they can to get to the surface of Mars safely. After that, it will depend on whether Mars has some surprises no one has accounted for."

Spirit's target, Gusev Crater, is 15 degrees south of the martian equator. Opportunity is heading for Meridiani Planum, two degrees below the equator on the other side of Mars. The landing sites were culled from a list of 155 potential targets.

"Gusev is a big hole in the ground, a crater, and it's got this huge dried up river bed flowing into it," Squyres said. "Now if you can come up with an explanation that says there wasn't a lake there, I'd like to hear it. I mean it was water and it flowed into a hole in the ground.

"This is a big crater, it may have been there for a considerable period of time and you're going to have liquid water standing there for long periods of time, potentially, and then you've got sediments, stuff got eroded out of that valley, it had to go someplace. Sedimentary rocks can do a wonderful job of preserving a record of environmental conditions to a geologist who's properly equipped to read that record."

Opportunity is bound for a different sort of target, one known to have a high concentration of iron oxide, or gray hematite.

"Hematite forms in a number of different ways on Earth but most of them involve the action of liquid water," Squyres said. "So you can think of the hematite mineral signatures being kind of a beacon that's saying to us, 'hey, water may have been here.' Now we don't know how it formed, it could have been a hydrothermal system, could have been a lake, we're not sure. But it says, mineralogically, water."

Still, there is an element of doubt.

"We could be wrong. We could be dead wrong about either one of these sites," Squyres said. "The beauty of going to two different sites is you basically double your science because Mars is such a diverse place. We can go after two really different sets of scientific questions."

NASA managers are no strangers to risky missions. But in the wake of the Feb. 1 Columbia disaster, the agency is under enormous self-imposed pressure to pull off a successful mission. While the Mars program likely would survive a single failure, many agency managers believe the loss of both rovers, coming on the heels of the 1999 failures, could stop the ambitious exploration program in its tracks.

"It's clear the Mars program would be in deep trouble if not ended (by two failures)," said one manager who asked not to be named.

The Mars Pathfinder and the Mars Global Surveyor orbiter were the first two missions in a long-range scientific assault on Mars and the mystery of its missing water. The original program called for launching two missions every two years, when Mars is favorably positioned, culminating in a robotic sample return mission at the end of this decade.

But the second pair of spacecraft - the Mars Climate Orbiter and the Mars Polar Lander - were lost in 1999. The space agency decided to press ahead with the 2001 launch of the Mars Odyssey, currently mapping the red planet in concert with the Mars Global Surveyor, but a lander virtually identical to the lost Polar Lander was grounded.

After pondering what sort of mission to mount in 2003, NASA managers ultimately settled on a pair of landers equipped with proven, state-of-the-art instruments and a Pathfinder-style parachute and air bag descent system to improve the odds of a safe touchdown.

"We were faced with a very tough time constraint when I took over as Mars program director in March 2000," said Hubbard. "We had to make a decision on something that we thought had a high probability of success. And there was at that time no technology available for final landing, terminal descent, for a legged lander that had obstacle avoidance."

Without a reliable, autonomous way for a powered lander to steer around large boulders or steeply sloped terrain during final approach, the success or failure of a costly mission came down to pure chance. Assuming the spacecraft got that far in the first place.

"So given the demonstration of Pathfinder for the air bag technology ... it seemed like the hazard avoidance technology would probably be for a future mission and this looked like a good marriage between existing instruments and a delivery technology that had been proven by Pathfinder."

Even so, he said in recent interview, "I will be holding my breath with everybody else, first with the launches and particularly when we get to the landing. ... I feel a little bit like an expectant father."

Video coverage for subscribers only:
   VIDEO: MARS ROVER SPIRIT LAUNCHED QT
   VIDEO: SPACEFLIGHT NOW LAUNCH VIEW FROM PRESS AREA QT
   VIDEO: TRACKING CAMERA VIEW FROM PATRICK AFB QT
   VIDEO: POWERFUL TRACKER FOLLOWS THE ROCKET QT
   VIDEO: CAPE CANAVERAL PRESS SITE 1 REPLAY QT
   VIDEO: VIEW FROM THE SKID STRIP RUNWAY QT
   VIDEO: POST-LAUNCH NEWS CONFERENCE QT
   VIDEO: NAMING CEREMONY FOR MARS ROVERS QT
   VIDEO: WEATHER SCRUBS SUNDAY LAUNCH ATTEMPT QT
   VIDEO: PRE-LAUNCH NEWS CONFERENCE QT
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