Rover Team Tests Mars Moves on Earth
Mars rover engineers are using a testing laboratory to simulate
specific Mars surface conditions where NASA's rover Opportunity
has spun its wheels in a small dune. Careful testing is
preceding any commands for Opportunity to resume moving to get
out of the dune and continue exploring.
Image right: Rover engineers in JPL's In-Situ Instrument Lab check how a test rover moves in material chosen to simulate some difficult Mars driving conditions. These tests in early May 2005 were designed to help plan the best way for the rover Opportunity to drive off of a soft-sand dune that the rover dug itself into the previous week. The mixture of sandy and powdery material brought in for these specific tests matched the way the soil underneath Opportunity caked onto wheels, filling the spaces between the cleats on the wheels. Image credit: NASA/JPL.
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The rover team at NASA's Jet Propulsion Laboratory, Pasadena,
Calif., has cooked up recipes combining various sandy and
powdery materials for the best simulation on Earth of the dune
where Opportunity dug itself in to wheel-hub depth last week.
The team has not asked Opportunity to turn its wheels at all
since the rover bogged down during a drive on April 26.
"We choose to proceed cautiously, so we don't expect to begin
actually driving out of the dune before next week, possibly
later," said Jim Erickson, rover project manager at JPL. "Both
Opportunity and Spirit have already provided many more months
of scientific exploration than anyone expected. By taking good
care of them, we hope to keep them exploring for more months to
come. Tests so far have sustained our optimism about
Opportunity's ability to drive out of this dune, but we have
more testing ahead to understand how robust that capability
Opportunity had driven about 40 meters (131 feet) of a planned
90-meter (295-foot) drive on the rover's 446th martian day when
its wheels began slipping. The rover was driving backwards at
the time. The team frequently alternates between backwards and
forwards driving to keep wheel lubrication well distributed.
The wheels kept rotating enough times to have covered the rest
of the distance if they hadn't been slipping, but the rover
eventually barely inched forward. After a turn at the end of
the planned drive, Opportunity sensed that it had not turned
properly and stopped moving.
Opportunity is positioned across the ridge of an elongated dune
or ripple of soft sand that is about one-third meter (one foot)
tall and 2.5 meters (8 feet) wide. "We've climbed over dozens
of ripples, but this one is different in that it seems to be a
little taller and to have a steeper slope, about 15 degrees on
part of its face," said Mark Maimone, a JPL rover mobility
Last week, engineers arranged a simulated dune using sand that
was already at JPL's rover testing facility and put a test
rover into a comparably dug-in position. The test rover had no
difficulty driving away, even when sunk in belly-deep. However,
that sand offered better traction than the finer, looser
material that appears to make up the surface at Opportunity's
current position. "We needed to do tests using material more
like what Opportunity is in, something that has a fluffier
texture and cakes onto the wheels," said JPL rover engineer
Rick Welch, who is leading the tests.
Experimenting with different mixtures, engineers and scientists
came up with a recipe that includes play sand for children's
sandboxes, diatomaceous earth for swimming pool filters and
mortar clay powder. Then they went to several home supply and
hardware stores to find enough bags and boxes of the
ingredients to make more than 2 tons of the simulated Mars sand
for more realistic mobility tests, said JPL rover mobility
engineer Jeff Biesiadecki.
Dr. Robert Sullivan of Cornell University, Ithaca, N.Y., a
rover science team member, worked with engineers in the JPL
testbed to match the properties of the test sand as closely as
possible with those of the sand beneath Opportunity, based on
images of wheels and wheel tracks on Mars. "We found that when
the wheels dig in, the material we're using does stick to the
wheels and fills the gaps between the cleats, but it doesn't
stick when you're just driving over it. That's good because
it's the same as what we see in the images from Opportunity,"
Experiments indicate that in this more powdery material, the
test rover positioned comparably to Opportunity can drive out
after some initial wheel-spinning. More testing, analysis,
planning and review will precede any actual commands for
Opportunity to begin driving away from the dune.
Meanwhile, Opportunity has been using its cameras to study its
surroundings at the edge of a region called "Etched Terrain."
Since landing more than 15 months ago, it has driven 5.35
kilometers (3.32 miles). Spirit, halfway around Mars, has
recently been using all of its research tools to examine an
outcrop called "Methuselah," the first outcrop of layered rock
that Spirit has found. The rover has also been taking short
movies of dust-carrying whirlwinds called "dust devils." On
some afternoons, the rover sees several at once moving across
the plain. Spirit has driven a total of 4.31 kilometers (2.68
JPL, a division of the California Institute of Technology in
Pasadena, has managed NASA's Mars Exploration Rover project
since it began in 2000. Images and additional information about
the rovers and their discoveries are available on the Internet
Guy Webster (818) 354-6278
Jet Propulsion Laboratory, Pasadena, Calif.