Lander Zeroes in On Martian North Pole
An artist's rendition of the Mars lander Phoenix traveling through space, just before unfurling its solar cells.
Credit: JPL

NASA?s next spacecraft to visit Mars has changed course to zero in on its red planet landing site.

The Phoenix Mars Lander fired its thrusters for 35 seconds Thursday to fine-tune its heading for a planned May 25 landing near the Martian north pole.

?This is our first trajectory maneuver targeting a specific location in the northern polar region of Mars,? said Brian Portock, chief of NASA?s Phoenix navigation team at the Jet Propulsion Laboratory in Pasadena, Calif., in a statement.

Phoenix?s targeted drop zone is an area that mission scientists have dubbed ?Green Valley.? The region is a broad, flat valley where mission planners plan to land Phoenix somewhere within a 62-mile by 12-mile (100-km by 20-km) ellipse.

"Our landing area has the largest concentration of ice on Mars outside of the polar caps,? said Phoenix principal investigator Peter Smith of the University of Arizona, Tucson. ?If you want to search for a habitable zone in the arctic permafrost, then this is the place to go.?

Ray Arvidson, chair of the Phoenix landing site working group and veteran Mars scientist, told that the lander?s target zone offers smooth terrain with a few scattered rocks. It is also home to so-called ?polygonal? plains that are expected to harbor dirty water ice beneath their surface, added Arvidson, who is a co-investigator for Phoenix?s robotic arm at Washington University in St. Louis, Mo.

Some five million rocks have been mapped in the region by spacecraft orbiting Mars, mission managers said.

"We have never before had so much information about a Mars site prior to landing," Arvidson said in a statement.

NASA launched the $420-million Phoenix last August on a mission to the martian arctic, where it is expected to use a robotic arm-mounted scoop to dig into the red planet?s surface to study Mars water ice and soil.

Researchers hope the probe?s onboard ovens, wet chemistry lab and other instruments will determine if its landing site may have once been habitable for microbial life. Phoenix is also designed to double as a Mars arctic weather and atmosphere-monitoring station.

But first, the probe has to reach Mars.

Thursday?s thruster firing was the second of five trajectory tweaks planned during Phoenix?s 422 million-mile (679 million-km) trek to Mars. The spacecraft first changed course just after its August launch, with three more maneuvers planned between now and landing day.

Unlike NASA?s Mars rovers Spirit and Opportunity, which used airbags to make a bouncy landing on the red planet in 2004, Phoenix?s touchdown will rely on a set of rocket thrusters that will fire in pulses to slow the craft. They rockets are designed to begin firing just 3,000 feet (914 meters) above the Martian surface and slow Phoenix to about 5 mph (8 kph) before its three metal legs touch down.

Similar powered landing approaches were used for NASA?s successful Viking landers in the 1970s, as well as the ill-fated Mars Polar Lander, which was lost just before landing near the martian south pole in 1999.

"Landing on Mars is extremely challenging. In fact, not since the 1970s have we had a successful powered landing on this unforgiving planet,? said Doug McCuistion, director of NASA?s Mars exploration program. ?There's no guarantee of success, but we are doing everything we can to mitigate the risks." special correspondent Leonard David contributed to this report from Boulder, Colo. Editor?s note: This story has been corrected to reflect the proper intended landing site of the lost Mars Polar Lander.

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