Incredibly, in 1997 NASA began a remarkable reversal of exploration fortunes when the agency safely landed the first probe on Mars in 22 years. Not only did the touchdown snap the exploration losing streak, but it also signaled the start of a steady march of NASA spacecraft that would prove Mars to be much more than just the "red planet."

Golombek's Gamble
1994, when geologist Matt Golombek accepted the challenge of sending NASA's Pathfinder robotic explorer to Mars, he knew the trail to the planet had long grown cold. "No one had been back to Mars since the '70s," said Golombek, the project's lead scientist. The last time was 1975, when Viking 1 and 2 visited the planet. Golombek could never have guessed the trail of triumph his spacecraft would blaze.

In 1994, when geologist Matt Golombek accepted the challenge of sending NASA's Pathfinder robotic explorer to Mars, he knew the trail to the planet had long grown cold. "No one had been back to Mars since the '70s," said Golombek, the project's lead scientist. The last time was 1975, when Viking 1 and 2 visited the planet. Golombek could never have guessed the trail of triumph his spacecraft would blaze.

Pathfinder was a mission to test a cost-conscious and compact lander with an ingenious system for cushioning its touchdown. "The lander was clad in an airbag, which was the first time this had been attempted by Americans," said Golombek. Prior to Pathfinder, Mars probes used complex and temperamental rocket thrusters to precisely control their landing. The new airbag system was far simpler and more forgiving, because it allowed a probe to gently bounce and roll like a beach ball until it came to a stop.

Tucked inside the airbag was a pyramid-shaped lander with hinged petals that opened and flattened out to reveal the enclosed Pathfinder science platform. Pathfinder was designed as an electronic base station equipped with cameras, scanners and communications equipment to relay its findings back to Earth.

Alongside Pathfinder was the intrepid Sojourner "microrover." Weighing a scant 23 pounds, Sojourner arrived on Mars with a very special assignment. "Sojourner's job was to go out and learn stuff about the rocks," said Golombek. "The question was, where could you learn the most from this little 1-foot geologist?"

To select a landing site, Golombek could only rely on blurry, 20-year-old scouting images left over from the Viking era. Taking his best guess, he chose to land the spacecraft in a washed-out basin he believed would have interesting rocks and provide a wide area for the risky landing attempt. "We needed a big runway," said Golombek.

Golombek's gamble paid off with a safe landing and incredible attention back home on Earth. "In a sense, it was the first surface landing for a whole generation; Pathfinder had the front page of the New York and L.A. Times for a week," said Golombek.

The rocks Sojourner found offered plenty to be thrilled about, too. For one, the shape and types of rocks examined by the rover resembled those millions of miles away on Earth. But then came a discovery of something even more significant.

"There were hints of a watery past," said Golombek. The tiny rover found little, round pebbles resembling a type of rock that forms in water called "conglomerate." Golombek and his colleagues began to wonder if the dry and lifeless Mars had once been more like our moist and fertile Earth. "This was the opening era of field geologists on Mars," concluded Golombek.

Demystifying Mars Part II: Surveying the Future
While Pathfinder and Sojourner were digging in the martian dirt, a new satellite -- Mars Global Surveyor -- was on its way to study Mars from the sky above. "The idea for Surveyor was to put a spacecraft in polar orbit and systematically study the weather and surface of the planet," said Surveyor project scientist Arden Albee.

The satellite left its Florida launch pad on Nov. 7, 1996. Zipping through space, the craft needed to shed speed in order to be gripped by the orbit-inducing gravity of Mars. To do so, Albee and Surveyor's large team of scientists and engineers used a radical and experimental technique to slow the racing satellite. "We pioneered aerobraking with Surveyor," said Albee.

During aerobraking, a spacecraft lightly skims a planet's atmosphere with each oversized orbit to create drag that gradually scrubs off speed. The entire process of aerobraking takes place over hundreds of passes and gradually reduces the size of the orbit. Aerobraking eliminates the need for extra fuel that would be used to power engines during a braking maneuver.

While ultimately successful, the early phases of Surveyor's aerobraking process proved to be a bit tricky. "When our solar panels came up against the pressure of the atmosphere, we ended up with what I'll call a sprained wing," said Albee. In response, engineers helped to lessen pressure on the ailing panel by easing the satellite into orbit at a slower pace.

Once Surveyor was circling Mars in its targeted orbit, the spacecraft began using its cameras and sensors to show us the planet's majestic features. From high in space, the craft revealed a planet with dramatic terrain.

Mars was shown to be capped with smooth poles of frozen carbon dioxide, while its mid-latitudes feature dormant volcanoes, wide deserts and rocky plateaus. Perhaps most visually stunning was the discovery of ancient gullies likely carved by running water. Surveyor also detected near the equator a 300-mile-wide patch of ferric oxide, a chemical that forms in standing water.

On a global scale, the satellite determined that Mars has ebbing and flowing magnetic fields too. Early in the planet's history, molten magma appears to have seeped through its floating crust. The rising magma cooled in sporadic pockets, creating fields of erratic magnetic intensity. The shape and arrangement of the fields also indicates the planet's plates of crust shift and crunch together to form riffs and mountains like those on Earth.

In 2001, Surveyor was joined in orbit by the Odyssey satellite. Odyssey was sent to analyze the composition of the planet's surface and radiation levels. Initially, the two satellites had independent missions. But before long, two new spacecraft in need of their cooperation would be on the way: the Mars Exploration Rovers.

Demystifying Mars Part III: Roving Martians
Mars Exploration Rover project scientist Joy Crisp believes her robotic geologists owe much to the work of Mars Global Surveyor and Odyssey. "Mars Global Surveyor and Odyssey were very important in selecting the landing sites. They were key to our success, both scientifically and for landing," said Crisp.

The Mars Exploration Rovers, called Spirit and Opportunity, are direct descendants of the Pathfinder/Sojourner system. But this time, each pillowy lander carried a larger all-terrain rover. The golf cart-sized rovers were self-sufficient, robotic dune buggies that carried their own science, power and communications equipment.

Spirit and Opportunity landed on opposite sides of the martian globe in January 2004. Spirit set down in a large meteor impact crater called Gusev Crater. Half a world away, Opportunity targeted the wide, open desert plain of an area called Meridiani Planum. Both rovers arrived to continue looking for geological evidence that Mars had once been wet with standing or flowing water.

Opportunity divined the first signs of water on a thin outcrop of rocks. "Water was around for a while because it deposited the rocks and later soaked them," said Crisp. It turns out the chunk of bedrock had once been swimming in water.

What tipped off Crisp and her team were small, spherical bits dotting the bedrock. They suspected these bits are most likely another form of concretions that form when iron precipitates out of water that soaked the rocks. Found alongside the concretions were little nooks and crevices identified as "lenticular voids."

The pockmarks are often left behind after water-borne crystals break down from moving water or chemical changes. Perhaps the strongest evidence for water came when chemical analysis showed the rock was rich with a type of salt called jarosite. On Earth, rocks containing that much jarosite either form in water or are highly altered by long exposures to water. The presence of the salt suggested the rock may once have rested in an acidic lake.

A short time later, Spirit also found evidence of past water on the other side of Mars. While exploring Gusev Crater, the rover studied a rock named Mazatzal that was made up of layers and stripes of different minerals. Geologists suspected the rock had once been wetted by groundwater and the stripes were cracks filled by minerals carried in by a thin trickle of water that squeezed through.

Along with discovering evidence of water, the two rovers have amazed scientists and Mars watchers by managing to explore the planet far longer than expected. "The plan was for 90 days, but we've been pleasantly surprised that they've kept going and going," said Crisp. Both Spirit and Opportunity have each survived more than 600 days. The truth is, though, some days the right to keep going have been hard-won.

After its first 17 days on Mars, Spirit abruptly and mysteriously stopped communicating with Earth. And NASA was doubly worried knowing that Opportunity was landing only five days later and could fall prey to the same failure as its twin. Initial indications were that Spirit's flight software began to malfunction. The glitch caused the rover to cease communications and reboot its computer 60 times in three days.

NASA initially believed it could take weeks to revive the comatose rover. But just three tireless days later, engineers determined Spirit's problems rested with storing computer files in the rover's "flash memory." Flash memory - like the kind used to store numbers and pictures on cell phones - holds a rover's computer files even when turned off.

Engineers solved the problem by temporarily storing files onboard a different portion of Spirit's memory and reinstalling the flight software into the flash memory. Engineers soon had Spirit back to work and knew how to avoid the same problem with the soon-to-arrive Opportunity.

While Opportunity managed to avoid Spirit's issue, the rover literally ran into trouble of its own. On its 446th day exploring Mars, Opportunity bogged down in a sand drift. About 8 feet long and 1 foot tall, the drift's 15-degree slope proved a little too steep for the rover to climb.

The dune appeared to be made of fine-grain sand that offered poor traction for Opportunity's wheels to bite into. Not wanting to make matters worse and dig into deeper trouble, NASA engineers stopped the rover's spinning wheels and set to work devising an escape plan.

The engineers constructed a sandbox and experimented by filling it with different mix of materials to simulate the sand trap that ensnared Opportunity. Then they drove their test rover into the sandbox, got it stuck and worked out how to set if free under its own power. Before long, the engineers had a plan in hand and took what they learned to the rover control room to slowly ease Opportunity out of the dune, sending the rover on its way.

Like the Spirit and Opportunity rovers trundling across the martian surface, NASA's march across the sparse and enigmatic planet continues. The Mars Reconnaissance Orbiter -- launched in August 2005 -- is streaking toward the planet to join Mars Global Surveyor and Odyssey in orbit.

In the years to come, more inquisitive spacecraft will follow them to investigate Mars, perhaps solving the mystery of what happened to the planet's water and even preparing for the arrival of the ultimate scientific explorer: humans.

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