The Phoenix
Mars Lander is set to scope out Mars' frozen pole for life-supporting
conditions next May, but researchers are predicting a tricky soil-sampling
mission.
If the
spacecraft's rockets don't blast the topsoil away, the exhaust fumes could
contaminate it with fuel. Or, Martian surface winds might simply blow away the
samples before they can be dumped into the lander.
To combat
the looming technical troubles, a team led by engineer and atmospheric
scientist Nilton Renno at the University of Michigan has developed several
unique tests to prepare the three-legged spacecraft's operators for success.
Nutty
soil conditions
The lander's first hurdle
will be to avoid blasting the icy soil away when it touches down, Renno said.
"Phoenix
will be traveling around 10 meters per second when it lands, so it's going to
use thrusters to soften the landing," Renno said. The price for this relatively
soft descent is that the thrusters will be ejecting exhaust at more than twice
the speed of sound, he explained.
To measure the effects of
the 12 thrusters on Mars' surface, where gravity is just 38 percent as strong
as on Earth, Renno and his colleagues created a glorified shower stall armed
with a downward-pointing air cannon. At the stall's base, they placed sawdust
and crushed walnut shells to mimic the Martian soil
and blasted it with air for several seconds to simulate the thrusters' blasts.
"The
walnuts help us simulate not only the weight, but also the density of the
soil," Renno said. "It's going to be pretty darn close," he said, when the
laboratory performs the experiment again under vacuum conditions at NASA's Ames Research Center in California.
So far,
Renno explained, the results show that if the lander travels horizontal to the
ground while
landing, it should leave an intact "wake" of soil for chemical,
conductivity and microscope analyses. Another benefit is that thruster exhaust would
spread over a larger area and cut down onany soil contamination that would be more concentrated
in a vertical landing.
So long, sample
Assuming a top-notch
landing, Renno said Phoenix's next challenge will be for its 8-foot robotic arm
to collect and fully deliver the samples of icy soil.
Mission engineers had planned to dump the scooped-up samples into the lander's
mini-laboratory from a height of about 4 inches, but Martian winds
of up to 11 mph are expected at the landing site.
"The finer material will
blow away at that height, leaving only the heavier, denser soil to sample,"
Renno said. "To make the trip worthwhile, we need the whole sample." The more
clearance the arm has from the craft, however, the less likely the arm is to
snag on or damage the spacecraft
Renno thinks shrinking that
distance to about 1.2 inches from the drop-zone should address the competing concerns,
but the team is planning tests in a wind tunnel to be certain. In the upcoming
experiment, they'll drop bits of ebony, bamboo and balsa to represent Martian
ice, soil and dust
in the wind and determine the optimum drop distance using high-speed camera
images.
Renno said addressing such
problems may seem simple, but the number of issues to fix add up quickly. "With
missions like these, there are so many things to worry about," Renno said. "But
you solve them one by one and get it right, until it's perfect."
The team's
experiments were funded by NASA as well as Lockheed Martin, the spacecraft's
manufacturer.
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