NASA'S PHOENIX Mars Lander is depicted moments before its scheduled touchdown on May 25 in this artist's representation.
NASA

"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.