Wednesday May 14, 2008

NASA probe closing in on Mars, but will it land?

By Irene Klotz

CAPE CANAVERAL, Fla. (Reuters) - Nine months ago, NASA's Phoenix probe blasted off for Mars with an unprecedented mission to sample water on another world.

Before that can happen, however, the space agency faces a formidable challenge: landing.

The odds are not great. Historically, 55 percent of all attempts to land on Mars have failed and the method being used for the touchdown of the Phoenix spacecraft on May 25 hasn't been attempted in 32 years.

"This is no trip to grandma's for the weekend," Ed Weiler, NASA's associate administrator for space science, said during a news conference on Tuesday.

After a nearly glitch-free ride, Phoenix is scheduled to settle near Mars' north pole at 7:36 p.m. EDT (2336 GMT), but no one will know whether it succeeded until about 15 minutes later. That's how long it will take radio signals, traveling at light speed, to reach Earth, 171 million miles (276 million km) away.

Rather than using airbags to cushion and bounce to a stop like the twin Mars exploration rovers Spirit and Opportunity, Phoenix is equipped with steering rockets to descend more precisely on target.

A propulsive landing system also is better suited to the heavier spacecraft that NASA would need to support eventual human expeditions on Mars.

NASA tried a rocket-powered descent on a probe called Mars Polar Lander in 1999. The mission came to an abrupt end during the final approach and landing. What went wrong remains a mystery.

"They've done everything they can do to make this a success, but Mars has been known to cause trouble," Weiler said.

NASA used a similar landing system for its twin Mars Voyager probes in the 1970s.

FIVE OF SIX SUCCESSFUL

Phoenix will be the sixth lander the United States has sent to Mars, five of which touched down successfully. The statistic does little to allay the nervousness of Phoenix flight controllers, who will be stationed at NASA's Jet Propulsion Laboratory in Pasadena, California, to wait out the news from Mars.

The worst part will be what project manager Barry Goldstein calls "the seven minutes of terror" -- the high-speed ride through the planet's thin atmosphere and the landing on the Martian equivalent of northern Alaska.

During those minutes, Phoenix will enter Mars' atmosphere zooming along at 12,600 mph relative to the planet, then dissipate most of its speed and come to screeching halt.

If everything works properly, the spacecraft will come to rest on a relatively rock-free and smooth area directly on top of a ice-rich bed of soil. During the three-month mission, Phoenix is to sample the soil and ice to determine if conditions were suitable for life to take hold.

Phoenix has a 7.7-foot robotic arm to bore down into the ground and retrieve samples for analysis. Its suite of science instruments includes small ovens to melt the ice and spectrometers detect a variety of gases.

While not specifically designed to detect life, Phoenix should be able if Mars has or had the right stuff to support it.

"My greatest hope," said lead scientist Peter Smith, with the University of Arizona in Tucson, "is that we'll change the direction of Mars exploration."

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