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Emily LakdawallaApril 24, 2008

Opportunity's arthritic shoulder is flaring up again

Late yesterday evening a message landed in my inbox with the dreaded headline: "Mars Exploration Rover Status Report." This headline almost never yields good news. But anyone who is in the habit of checking the Spirit and Opportunity images on a regular basis probably noticed the issue last week: it looks like the shoulder joint on Opportunity's robotic arm may have failed.

Beginning in late 2005, the shoulder joint on Opportunity's robotic arm (which, like a human arm, has a shoulder joint, an elbow joint, and a wrist joint) has been prone to stalling. It wasn't broken -- they could still make the motor work by supplying it with increased voltage -- but clearly there was some problem, and the possibility of a more serious failure in the future. Because of the possibility of failure, the team changed their daily housekeeping procedures. Instead of always keeping the arm in its stowed position (in which its weight rests on a hook under the deck of the rover), the rover drivers programmed Opportunity to unstow and deploy the arm at the end of each driving day. The arm was still being stowed for driving.

As a part of this change in procedure, Opportunity was also programmed to take a photo documenting the position of the arm after unstowing the arm in the afternoon. So those of us who follow the rovers through their images have grown accustomed to seeing these daily sets of forward hazcam images with the arm deployed, usually alternating with images taken as a normal part of Opportunity's driving efforts. Here are some sets of these images taken over the last several sols: post-drive images on the left, "idd_unstow" images on the right. ("IDD" stands for Instrument Deployment Device, that is, the arm.) The first three sets are normal. The last is an "uh-oh." It looks like the arm barely moved.
Three unstows and an 'uh-oh' for Opportunity

NASA / JPL / montage by Emily Lakdawalla

Three unstows and an 'uh-oh' for Opportunity
At the end of each day of driving, the Opportunity rover unstows its robotic arm in case a problem develops with the shoulder joint over the cold Martian night. On Sol 1502 (April 15, 2008), the arm barely budged.
Mars Exploration Rover

NASA / JPL / Dan Maas

Mars Exploration Rover
The problem joint is the one located immediately below the rover deck, on the left side of the rover, which rotates the arm out from its stowed position and allows the arm to move horizontally through the space between the wheels. If this motor is frozen, the rover will be able to place the arm in only a limited part of this space.
According to the status report, the rover engineers haven't yet fully diagnosed the problem. Here's what they have to say about it so far:
A small motor in the robotic arm of NASA's Mars Exploration Rover Opportunity that began stalling occasionally more than two years ago has become more troublesome recently.

Rover engineers at NASA's Jet Propulsion Laboratory, Pasadena, Calif., are diagnosing why the motor, one of five in the robotic arm, stalled on April 14 after much less motion that day than in the case of several earlier stalls. They are also examining whether the motor can be used and assessing the impact on Opportunity's work if the motor were no longer usable....

The performance of the motor in the past week is consistent with increased resistance in the electrical circuit, such as from degrading of wire in the winding, rather than a mechanical jam. Additional tests are planned for checking whether the apparent resistance is localized or intermittent.

The fact that the stall is unusual is bad news. However, what's good news is that the arm was at least unhooked from its stowed position before the motor stalled. So even if the motor is totally dead, the rest of the joints in the arm can still be used to position the science instruments. Without the shoulder motor, they'll have a far more limited space in which they can operate the tools, but any ability to use those tools is better than having them hooked permanently on the underside of the rover.

And we don't even know yet if the shoulder motor is actually dead. It will probably take the rover team some time, I expect weeks, to perform a full set of tests and diagnoses and move attempts and so forth in order to figure out exactly what is going on. So Opportunity's drive to Cape Verde is probably on hold for a while. That's too bad, but there's no reason for hurry; we all need to be patient and let the engineers do their careful work, and Opportunity will be off and driving again before we know it.

As a reminder, if you'd like to follow along with the rovers' missions through your images, the photos are easy to access via JPL's Mars Exploration Rover website. But if you have a spare 20 GB of space on your hard drive and a high-speed Internet connection, I highly recommend Mike Howard's Midnight Mars Browser, which downloads all the images and can automatically assemble them into color images, 3D anaglyphs, and panoramas. And I'll remind you also that the next rover update should be posted on this site by A. J. S. Rayl a week from today; I'm sure she'll have much more information on what is happening to troubleshoot the arm problem.

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Emily Lakdawalla

Solar System Specialist for The Planetary Society
Read more articles by Emily Lakdawalla

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