But with the nearly five decades of Mars exploration fraught with failures -- about half of the three dozen tries have crashed, disappeared or missed the planet altogether -- there is little room for error.

"This is not a trip to grandma's house. Putting a spacecraft safely on Mars is hard and risky," Ed Weiler of NASA's Science Mission Directorate said this past week.

Given the long distance, the JPL will have to wait an agonizing 15 minutes for the radio signal confirming the safe landing to reach Earth.

One minute after Phoenix confirms arrival, its radio will go silent for 20 minutes to save its batteries before deploying its two solar antennas. Its first images will reach Earth only after two hours.

The probe will work under harsh conditions with temperatures ranging between minus 73 degrees and minus 33 degrees Celsius (minus 99 to minus 27 degrees Fahrenheit).

NASA wants to assess whether the Martian arctic has ever had conditions favorable to microbial life, Smith said.

Given that Mars' polar region is subject to Earth-like seasonal changes, Smith said, the scientists are looking to see whether there is a point where the region warms and changes into a water-rich soil with life-supporting minerals.

Phoenix is equipped with a camera and a 2.35-meter (7.7-foot) robotic arm that can dig as deep as one meter to find ice and heat up samples to detect carbon and hydrogen molecules, essential elements of life.

With its two solar panels unfurled, Phoenix is five meters wide and 1.52 meters long (16 feet wide and five feet long). It weighs 350 kilograms (772 pounds), including 25 kilograms (55 pounds) of scientific instruments.

A NASA orbiter tracked a Martian dust cloud moving across the landing zone Saturday, but the JPL said it was not expected to pose a hazard to the landing.

(Xinhua News Agency via agencies May 26, 2008)