By Irene Klotz

PASADENA, Calif. May 25 (Reuters) - A robotic probe careening toward Mars was perfectly positioned for a landing at the planet's frozen north pole on Sunday, where it will search for water and assess conditions for sustaining life, U.S. space scientists said.

Flight controllers at NASA's Jet Propulsion Laboratory in Pasadena passed up a final opportunity to tweak the spacecraft's path earlier on Sunday, convinced the 1,400-pound (635 kg) probe, named Phoenix, was on target for touch down.

After a do-or-die plunge through Mars' thin atmosphere, Phoenix was expected to reach the surface at 7:53 p.m. EDT (2353 GMT).

"We have nothing left to do but land," mission manager Barry Goldstein said. "We're just watching and waiting."

Pulled by Mars' gravity, Phoenix was traveling at 85,000 miles (136,800 km) per hour. By the time it reaches the red planet's atmosphere at 7:46 p.m. EDT (2346 GMT), the probe was expected to be tearing along at 12,700 mph (20,400 kph).

Within a period of seven minutes, a heat shield, parachute and thruster rockets must work perfectly for Phoenix to land gently in Mars' arctic circle.

Scientists found in 2002 that Mars' polar regions have vast reservoirs of water frozen beneath a shallow layer of soil. Phoenix was launched Aug. 4, 2007, to sample the water and determine if the right ingredients for life are present.

NASA attempted a landing on Mars' south pole in 1999, but a problem during the final minutes of descent ended the mission.

The U.S. space agency canceled its next Mars lander but successfully dispatched two rovers, Spirit and Opportunity, to the planet's equatorial region to search for signs of past surface water.

Phoenix was created out of spare parts from the failed Polar Lander mission and the mothballed probe. Unlike the rovers, Phoenix will not be bouncing to the planet's surface in airbags, which are not suitable for larger spacecraft.

Instead, like the 1970s-era Viking probes and the failed Polar Lander mission, it uses a jet pack to lower itself to the ground and fold-out legs to land on.

"We haven't landed successfully on legs and propulsive rockets in 32 years," NASA's space sciences chief Ed Weiler told Reuters. "When we send humans there, women and men, they're going to be landing on rockets and legs, so it's important to show that we still know how to do this." (Editing by Dan Whitcomb)

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