"I'm a little nervous on the inside. ... This is not an easy thing to do," said Phoenix scientist Peter Smith of the landing planned for late Sunday.

"There's a lot of uncertainties left. ... Mars is always there to throw those uncertainties at us," added Doug McCuistion, Mars Exploration Program Director, of what NASA calls "the scariest seven minutes of the mission" -- the period of hyper-deceleration and descent onto the Red Planet.

Mission specialists were reviewing data to decide whether a course-correction maneuver would be needed late Saturday to keep the Phoenix on track for landing in a relatively rock-free, flat region in the Mars arctic after its 679-million-kilometer (422-million-mile) journey from Earth.

Phoenix will enter the Martian atmosphere at around 2331 GMT Sunday at about 21,000 kilometers (13,000 miles) per hour and rely on its thermal shield, then a parachute followed by a bank of pulse thrusters, to slow down to a mere eight kph (five mph) ahead of touchdown on the circumpolar region known as Vastitas Borealis -- akin to northern Canada in Earth's latitude.

Phoenix will become the first spacecraft to land on the Martian arctic surface, digging into the polar ice in a new three-month mission searching for signs of life.

"We are going to a place on the planet that is unexplored and very exciting," Smith, Phoenix principal investigator at the University of Arizona, told reporters Saturday.

"Getting a scoop full of that icy soil is our goal" in searching for a habitable zone, he said.

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 earlier this week.

Given the long distance, the US space agency's Jet Propulsion Laboratory in Pasadena, California, which controls the mission, 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.

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