"Follow the water" has been NASA's mantra as it has explored Mars for signs of present or past life. It will be no different later this month when the Phoenix Mars Lander touches down on the Red Planet for what researchers hope will be their closest encounter yet with extraterrestrial water.

Powered by solar panels, Phoenix is set to take a three-month tour of the plains near the north pole of Mars, enduring surface temperatures from –100 to –28 degrees Fahrenheit (–73 to –33 degrees Celsius). The craft is designed to dig into the cementlike layer of ice that researchers believe lies buried a few inches below the surface in the planet's polar regions, scanning for signs of past liquid water and organic compounds, the carbon-rich molecules that make life on Earth possible.

If all goes according to plan, "Phoenix will touch water for the first time" on Mars, Doug McCuistion, director of NASA's Mars Exploration Program, said at a press conference this week at NASA Headquarters in Washington, D.C.

Assuming, that is, it survives entry. Only five of 13 attempts to land on Mars have succeeded. (Although five of six U.S. probes have made it.) The $420-million Phoenix is the sibling mission of the doomed Mars Polar Lander (MPL), which crashed during landing in 1999.

Since then, however, NASA has successfully carried out four missions to Mars, including landing the twin rovers Spirit and Opportunity. And NASA says that it has taken steps to prevent Phoenix from suffering its predecessor's fate.

If so, Phoenix will be the first craft to touch down using rocket thrusters since the Viking 2 lander, some 32 years ago. MPL, launched in 1998, was to have landed less than 600 miles (1,000 kilometers) from the planet's south pole to collect soil and ice samples for analysis and to gauge meteorological conditions.

The agency lost contact with the craft just prior to its entry into the Martian atmosphere on December 3, 1999. A NASA review board found that the craft most likely crashed when it misinterpreted vibrations (caused by lowering its landing gear) as a signal that it had reached the ground, triggering its thrusters to shut down about 130 feet (40 meters) above the surface.

Phoenix was to be the follow-up mission in the Mars Surveyor Program, but it was scuttled in 2001 following the accident review. "We weren't certain that we were quite ready to do this again," McCuistion said.

Mars exploration got its mojo back with the spectacularly successful rovers, which bounced to a landing on Mars in January, 2004, cushioned by airbags. Together they have explored Mars's equatorial region for evidence of water long since dried up.

To encounter water directly, researchers would have to look toward the poles again. Mars Odyssey, an orbiting craft launched in 2001, and Mars Reconnaissance Orbiter (MRO), sent into space four years later in 2005, paved the way for Phoenix by identifying a safe landing spot from orbit. Odyssey's Gamma-Ray Spectrometer mapped out the spread of water ice under the Martian surface.

For Phoenix's landing site, mission planners settled on an extremely flat elliptical swath roughly 60 miles by 10 miles (100 kilometers by 20 kilometers) in area. MRO spent four years mapping the region for rocks as small as 1.5 yards across that might damage the craft, Ray Arvidson, chair of the Phoenix landing site working group at Washington University in St. Louis's Earth and Planetary Remote Sensing Laboratory, said at the news conference.

Phoenix launched on August 4, 2007, when Mars was 121 million miles (195 million kilometers) from Earth. It still has 12 million miles (19 million kilometers) to go to reach Mars, said Barry Goldstein, Phoenix project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The journey has been so remarkably uneventful, it's scary how clean it's been," he said.

The craft shut itself down once during its journey when a cosmic ray struck its computer memory, which was the way it was designed to behave in such an event, Goldstein said. He added that a scheduled course correction was canceled last week because of smooth sailing but that NASA plans to fine-tune Phoenix's aim this weekend.

The real test will come during landing. Just before 5 P.M. Pacific time on May 25, the Phoenix lander is scheduled to separate from its rocket-powered "cruise stage" and dive into Mars's atmosphere at a speed of 12,600 miles (20,300 kilometers) per hour relative to the Martian surface.

What comes next is referred to as "seven minutes of hell." After four minutes, the craft will have slowed to 1,100 miles (1,170 kilometers) per hour under the protection of a heat shield. The lander will deploy a parachute at a distance of 7.8 miles (12.6 kilometers) from the surface, then jettison its heat shield, flip over to face its thrusters toward the planet and finally fire them in short, coordinated bursts, touching down at 4:53 P.M. Pacific time (taking into account the 15-minute communication lag between Mars and Earth).

Goldstein said his biggest worry is a three-second communications delay following the separation of the craft, after which Phoenix will begin transmitting to Earth via the orbiting Odyssey and MRO craft. He said that in an emergency, NASA has arranged with the European Space Agency to use its Mars Express orbiter to relay signals. Once Phoenix has landed, it will go silent for an hour and a half as it unfolds its solar panels and powers up, at which point Goldstein said it could send back its first images of the landing site.

Sample collection is set to begin during touchdown, even before the solar panels unfold. The vehicle's sampling tool will stay open during landing to catch the dust that Phoenix will surely kick up, said Peter Smith, Phoenix principal investigator at the University of Arizona in Tucson. (That same dust might interfere with the unfolding process, which therefore is slated to begin 15 minutes after landing.)

As soon as two days later, Phoenix could deploy its robotic arm to begin scraping away the five- or six-inch (13- to 15-centimeter) layer of loose topsoil using a backhoe-like motion. If a blade on the edge of the scraper proves too weak to chip away at the uncovered ice, a drill the size of a pinky finger will flick down to slice and slip a few shreds of ice into the sampler.

Phoenix has eight single-use ovens that will slowly bake soil and ice samples separately to 1,800 degrees F (1,000 degrees C) and sniff the resulting vapors for evidence of water, carbon-rich compounds and sulfates. The lander will also mix a soil sample with water carried from Earth to test the soil's chemistry in the presence of liquid water.

To assess whether liquid water flowed over the region during the heat spells Mars experiences every 50,000 years, an onboard microscope will examine cracks in the subsurface ice that should form during cold times. If liquid water flowed across the slits during the summer, it would pool in them; otherwise they would be filled with dust and sand, Smith said.

Phoenix's primary mission will last three months. Unlike the rovers, which have had their missions extended repeatedly, Phoenix will have at most until early 2009 before the sun sets on Mars's north pole for the winter. During that time, Phoenix might succumb to the harsh Martian winter temperatures.

Goldstein noted, however, that Phoenix would still have a chance. If it powered up again in the spring, its computers would resume communication with Earth—and Phoenix would become more like Lazarus.