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 (JPL), Pasadena, Calif. “However, this
approach will allow us to continue doing science much longer
than we ever thought possible,” he said.

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 have 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,” he added.

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 bed 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, Phoenix.

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 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.