In May 2008, NASA's Phoenix lander will slam into the atmosphere of Mars at a blistering 12,750 mph and descend to the frozen surface in a nail-biting seven minutes. Unlike NASA's hugely successful Mars Exploration Rovers, Phoenix will not bounce to a landing cocooned in protective airbags. Instead, it will use a dozen pulsing, computer-controlled rocket motors to make its final descent to a soft landing near the martian polar cap. The last time NASA tried that, the ill-fated Mars Polar Lander simply disappeared, the presumed victim of a premature engine shutdown.

This time around, using hardware left over from the polar lander project and a subsequent mission that was canceled in the wake of the 1999 failure, NASA managers believe they have done all that can reasonably be expected to ensure success and carry the agency's on-going "follow the water" strategy of Mars exploration to a new level.

Along with fixing the sensor/software glitch believed to be responsible for the Mars Polar Lander's demise, engineers with spacecraft builder Lockheed Martin and NASA's Jet Propulsion Laboratory found and fixed a variety of other shortcomings in an exhaustive effort to ensure success.

"This team has put an enormous amount of energy into retiring and finding all sorts of problems," said Barry Goldstein, Phoenix project manager at JPL in Pasadena, Calif. "We are very confident that we basically have retired everything that we can think of. But the simple fact of the matter is, landing on Mars is difficult. Probably the most difficult thing about it is not the things we can think of, it's things we can't think of.

"I'm confident we have worked as hard as any group of people can. Am I confident in the landing? I'll be nervous. If my fingernails survive that day, it will be a miracle. It's not the things that we know that will hurt us. It's the things we don't know."

But if the appropriately named Phoenix mission works, NASA will open a new chapter in its on-going exploration of Mars.

"The Mars Exploration Rovers ... went to study the past history of water and the discoveries they have made have been very successful in finding out where the water was," Goldstein said. "What Phoenix is doing is using a stationary lander and going to where the water is."

Data collected by NASA's orbiting Mars Odyssey indicates vast amounts of ice within a few feet of the surface of Mars at extreme northern latitudes, possibly the remnant of an ancient, now vanished ocean. Phoenix is equipped with an eight-foot robot arm that can dig nearly two feet into the soil next to the lander to tap into that ice and deliver samples to compact instruments and furnaces that will look for signs of organic compounds.

While the relatively low-cost Phoenix is not equipped with instruments to search for signs of biological activity, mission scientists are hopeful the spacecraft will be able to determine whether the environment at the ice-soil boundary represents a modern habitable zone.

"I see this mission as a stepping stone towards the search for life on other planets," said Peter Smith, principal investigator at the University of Arizona. "We're hoping to find a place that we consider really a habitable zone on Mars. To me, if we can find that out, that would be a tremendous success. We're also really interested in following up on the discovery of water ice by the Odyssey spacecraft in 2002 and trying to understand how the ice got there and what it's source was and what its history has been."

NASA's Mars Pathfinder lander and rover and the two Mars Exploration Rovers currently weathering a global dust storm on opposite sides of the red planet were built to study the geology of Mars and to confirm the presence of surface water in the distant past. They have done that with stunning success. Phoenix will concentrate on water known to exist today and help scientists determine whether the frozen reservoir is the remnant of a once-vast sea.

"We want to understand the ice properties," Smith said. "This is a big part of Mars. It's part of the NASA theme of follow the water. Well, we're for the first time getting kind of a fist-full, so to speak, of water and soil and we're going to analyze it.

"We intend to go where we know there's ice near the surface. Our entire mission is trying to understand the history of this ice, in particular does it melt over time and provide a habitat for some sort of Martian biology?"

The Phoenix mission is scheduled to get underway Saturday with launch atop a Delta 2 rocket. Liftoff from complex 17 is targeted for 5:26:34 a.m. with a second opportunity available a half-hour later at 6:02:59 a.m. Forecasters are predicting an 80 percent chance of good weather.

Opportunities to launch spacecraft to Mars occur every 26 months as Earth catches up with and passes the red planet in their orbits around the sun. Close approach this time around occurs in December and by launching now, Phoenix can achieve the most energy efficient trajectory. This year's launch period ends Aug. 24.

The Mars Pathfinder lander and Mars Global Surveyor, both launched in 1996, were the first two missions in a long-range scientific assault on Mars and the mystery of its missing water. NASA's original Mars program called for launching two missions every two years, 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 Climate Orbiter crashed into the planet due to an embarrassing English-to-metric conversion oversight while the Polar Lander simply disappeared during powered descent to the surface. Engineers believe a sensor-software problem led to the premature shutdown of its engines.

Wide-ranging NASA and independent investigations identified major management and technical shortcomings in the implementation of former NASA Administrator Daniel Goldin's "faster, better, cheaper" approach to planetary exploration.

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 Reconnaissance Orbiter, but a lander virtually identical to the lost Polar Lander was mothballed and its mission canceled.

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.

The Spirit and Opportunity rovers successfully bounced down on opposite sides of Mars in January 2004. With a design life of just 90 days, both rovers are still in operation, genuine rock stars in the history of Mars exploration.

In the meantime, the Phoenix project was born in 2003, when Smith's team submitted a proposal to incorporate the mothballed 2001 lander hardware in a new mission. And now, it's finally ready to go.

The Delta 2 and a solid-fuel upper stage will boost the 1,477-pound spacecraft and its solar-powered interplanetary cruise stage to a departure velocity of around 24,657 mph about 84 minutes after launch. A half-dozen trajectory correction maneuvers are planned and if all goes well, the spacecraft will reach Mars on May 25, 2008.

Based on high-resolution images from the Mars Reconnaissance Orbiter, mission planners ruled out one landing site because of boulder fields that could cause major problems for a legged lander. The current landing zone is located at 67 degrees north latitude, roughly equivalent to Iceland or northern Siberia on Earth. MRO pictures indicate a very smooth terrain. The target landing ellipse measures 12 miles wide by 93 miles long.

The entry, descent and landing profile is complex, fast-paced and computer controlled. At the time of landing, Earth and Mars will be 171 million miles apart, so far it will take radio signals 15.3 minutes to make a one-way trip. Data from Phoenix will be relayed back to Earth throughout the descent by NASA's Mars Odyssey and Mars Reconnaissance Orbiter, but realtime commanding will not be possible. Phoenix's survival will depend on its flight software, the martian weather, the terrain at the landing site and a certain amount of luck.

"Mars has been known as a spacecraft eater," Goldstein said. "We have been fairly successful recently with the MER (landers). But that doesn't make it any easier the next time around."

Seven minutes before it hits the discernible atmosphere, Phoenix will separate from its interplanetary cruise stage and reorient itself to put its heat shield forward. Atmospheric entry will occur five minutes later at an altitude of 77.7 miles and a velocity of 3.5 miles per second. Over the next three minutes, the spacecraft will experience peak heating and a deceleration of 9.3 times the force of Earth's gravity at sea level.

Now at an altitude of 7.8 miles, a large braking parachute will deploy and 15 seconds later, the no-longer-needed heat shield will fall away. The spacecraft's three landing legs will snap open and a radar altimeter will fire up to compute the spacecraft's altitude and sink rate.

Then, in the most dramatic phase of the descent, the lander will separate from its parachute about 3,000 feet above the martian surface. Three seconds later, 12 small rocket motors will begin firing to slow the descent and bleed off horizontal velocity. Unlike the throttled engines on the Viking landers three decades ago, Phoenix relies on pulsing on-off cycles to control the craft's descent rate and orientation. The engines will shut down when sensors on the landing legs contact the surface.

For 15 minutes, Phoenix will wait for any dust kicked up by landing to settle out before deploying its two fan-like circular solar arrays. A meteorology mast and the lander's main stereo camera will deploy and engineers will begin a complex sequence of operations to thoroughly check out the robot's systems.

A successful soft landing "is a very difficult thing to do," Goldstein said. "We have gone away from the Pathfinder and MER (airbag) landing system for a reason. We have to have an ability to land a larger payload on the surface as we try to expand the Mars program. Eventually, there will have to be a landing system for people. They are not going to want to bounce around in airbags."

Phoenix is designed to operate for at least three months and possibly longer. But engineers say the sort of extended life enjoyed by the Mars Exploration Rovers is not an option for Phoenix.

"The real killer is the ice caps coming down," Goldstein said. "We expect to be encased in solid CO2 at some point, according to what the scientists are telling us. Will we check to see if it thaws out and comes back to life (the following summer)? Sure, but we didn't design this vehicle to survive cryogenic freezing."