It seems nothing can hold NASA's Mars Rovers back.

Opportunity, one of NASA's two twin rovers, has lost the two encoders that operate motors on its rock abrasion tool, but team members have worked together in test beds and computer sequencing rooms to create a new way for the robot to grind and brush surfaces without its encoders.

And it works. On October 7, the rover successfully completed a new seek-scan procedure. Using this technique, the rover locates a rock surface by simultaneously spinning its grind teeth and wire brush while also extending toward the rock surface. Normally, the rock abrasion tool software monitors the safe operation of the grind or brush using the two encoders, which detect stalls that can occur during grinding and encoding. In the event of a stall, the encoders measure the z-axis position (the point where the rock abrasion tool contacts the rock surface). But now, without the encoders, engineers must rely on current limits and contact switches to know when grind teeth come into contact with a rock surface.

Opportunity followed a command to run both the grind and revolve motors along with a parallel command to move in toward the rock surface. When the rock abrasion tool made contact with the surface, contact switches disengaged, ending the activity. The following day, the team successfully directed the rover to retract the rock abrasion tool 1 millimetre and brush the surface.

Opportunity's most recent project on November 8 called for it to complete an encoder-less brush of the surface of "Smith" (inside Victoria Crater), to acquire microscopic images, place the alpha-particle X-ray spectrometer, and study composition of the area with the spectrometer. The rover was also instructed to take panoramic camera images of "Cabo Frio," a promontory at the rim of Victoria Crater. It was to acquire full-colour images, using all 13 filters of the panoramic camera, of a layered rock target known as "Brongniart."

On the other side of the planet Opportunity's twin, "Spirit", is also a wounded but carrying on. It has been gradually losing power, with energy levels dropping to 320 watt-hours per Martian day (100 watt-hours is the amount of energy needed to light a 100-watt bulb for one hour).

Measurements of atmospheric dust, known as Tau, have also been averaging 0.65. Even though this is typical of levels measured throughout most of the mission, the rover's power levels are lower than in previous years because of higher dust accumulation on the solar panels.

Regardless of these hindrances, the rover's latest announced project on November 9 was to run diagnostic tests of the rock abrasion tool and acquire a mosaic of images of West Valley View with its panoramic camera. The rover was to spend 22 hours acquiring data with its Mossbauer spectrometer, be on the lookout for morning dust devils, and acquire movie frames at 8-minute intervals to record the progress of dust devils if they occurred.

See an interview with one of the Mars rover drivers.