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Probe closing in on Mars
14/05/2008 10:43 - (SA)
Cape Canaveral - 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% 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's north pole at 23:36 GMT, but
no one will know whether it succeeded until about 15 minutes
later. That's how long it will take radio signals, travelling at
light speed, to reach Earth, 276m kilometres 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's atmosphere
zooming along at 20 160km/h relative to the
planet, then dissipate most of its speed and come to a 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 2.3m 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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