As winter approaches on Mars, NASA's Opportunity rover continues to
inch deeper into the stadium-sized crater dubbed "Endurance." On the
other side of the planet, the Spirit rover found an intriguing patch
of rock outcrop while preparing to climb up the "Columbia Hills"
backward. This unusual approach to driving is part of a creative plan
to accommodate Spirit's aging front wheel.

Spirit, with an odometer reading of over 3.5 kilometers (2.2 miles),
has already traveled six times its designed capacity. Its right front
wheel has been experiencing increased internal resistance, and recent
efforts to mitigate the problem by redistributing the wheel's
lubricant through rest and heating have been only partially
successful.

To cope with the condition, rover planners have devised a roundabout
strategy. They will drive the rover backward on five wheels, rotating
the sixth wheel only sparingly to ensure its availability for
demanding terrain. "Driving may take us a little bit longer because it
is like dragging an anchor," said Joe Melko, a rover engineer at
NASA's Jet Propulsion Laboratory, Pasadena, Calif. "However, this
approach will allow us to continue doing science much longer than we
ever thought possible."

On Thursday, July 15, Spirit successfully drove 8 meters (26 feet)
north along the base of the Columbia Hills backward, dragging its
faulty wheel. The wheel was activated about 10 percent of the time to
surmount obstacles and to pull the rover out of trenches dug by the
immobile wheel.

Along the way, Spirit drove over what scientists had been hoping to
find in the hills -- a slab of rock outcrop that may represent some of
the oldest rocks observed in the mission so far. Spirit will continue
to drive north, where it likely will encounter more outcrop.
Ultimately, the rover will drive east and hike up the hills backward
using all six wheels.

"A few months ago, we weren't sure if we'd make it to the hills, and
now here we are preparing to drive up into them," said Dr. Matt
Golombek, a rover science-team member from JPL. "It's very exciting."

For the past month, the Spirit rover has been parked near several
hematite-containing rocks, including "Pot of Gold," conducting science
studies and undergoing a long-distance "tuneup" for its right front
wheel.

Driving with the wheel disabled means that corrections might have to
be made to the rover's steering if it veers off its planned path. This
limits Spirit's accuracy, but rover planners working at JPL's rover
test facility have come up with some creative commands that allow the
rover to auto-correct itself to a limited degree.

As Spirit prepares to climb upward, Opportunity is rolling downward.
Probing increasingly deep layers of bedrock lining the walls of
Endurance Crater at Meridiani Planum, the rover has observed a
puzzling increase in the amount of chlorine. Data from Opportunity's
alpha particle X-ray spectrometer show that chlorine is the only
element that dramatically rises with deepening layers, leaving
scientists to wonder how it got there. "We do not know yet which
element is bound to the chlorine," said Dr. Jutta Zipfel, a rover
science-team member from the Max Planck Institute for Chemistry,
Mainz, Germany.

Opportunity will roll down even farther into the crater in the next
few days to see if this trend continues. It also will investigate a
row of sharp, teeth-like features dubbed "Razorback," which may have
formed when fluid flowed through cracks, depositing hard minerals.
Scientists hope the new data will help put together the pieces of
Meridiani's mysterious and watery past. "Razorback may tell us more
about the history of water at Endurance Crater," said Dr. Jack Farmer,
a rover science-team member from Arizona State University, Tempe.

Rover planners are also preparing for the coming Martian winter, which
peaks in mid-September. Dwindling daily sunshine means the rovers
will have less solar power and take longer to recharge. Periods of
rest and "deep sleep" will allow the rovers to keep working through
the winter at lower activity levels. Orienting the rovers' solar
panels toward the north will also elevate power supplies. "The rovers
might work a little bit more every day, or a little bit more every
other day. We will see how things go and remain flexible," said Jim
Erickson, project manager for the Mars Exploration Rover mission at
JPL.

JPL, a division of the California Institute of Technology in Pasadena,
manages the Mars Exploration Rover project for NASA's Office of Space
Science, Washington.

Images and additional information about the project are available on
the Internet at http://marsrovers.jpl.nasa.gov and
http://athena.cornell.edu .
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