It's my first sol driving the rovers since I came back from jury duty, and I've switched to Opportunity -- my first sol on this rover since, well, since before they encountered Purgatory Ripple.
Yestersol's drive was a 20-degree turn in place followed by a whopping 50cm drive forward, in two 25cm arcs. But only one of the two arcs completed. Opportunity sensed that she'd slipped 32% on that arc, and we'd set a limit of 30%, so she stopped. Opportunity is scared of bogging down again.
But Khaled and Paolo analyze the drive and conclude that it's safe for us to proceed. What happened was that as we did the turn in place, the right-front wheel -- whose steering actuator is broken -- was mostly just dragged along, digging itself a small trench in the process. Then when we tried to step forward, we slipped a touch more than expected as we climbed out of that trench. But the rover's safe, on top of the soil rather then underneath it, and the way ahead is clear.
Our goal here is simply to drive back to a part of the tracks we left as we exited Purgatory. Yestersol's drive was supposed to leave us about 50cm from them, but, well ... so today we'll drive a whopping 75cm; if it goes well, we'll be right where we need to be to IDD the tracks over the weekend. Then next week we'll pack up and leave.
Khaled and I do spend a fair amount of time working on the drive -- everyone's paranoid about re-approaching this ripple -- but we're ready in plenty of time. Jeff Favretto notices the level of effort we're putting in, and during one of the meetings, he just grins and holds his hands a little more than shoulder-width apart. "Seventy-five centimeters," he grins, shaking his head. "This is how far we're going. It takes all that ...."
Meanwhile, Spirit's exhibiting a little bit of unusual behavior. During her last drive, the left bogie angles changed in unusual ways, indicating that the middle wheel kept lifting well into the air and then dropping back down. One hypothesis is that something's stuck on the outside of the wheel. If this is true, we'll never be able to see it: they're not visible from any of the cameras. Jeff thinks he knows what's stuck in the wheel: "It's that dinosaur bone we've been looking for all this time!"
Courtesy NASA/JPL-Caltech. Post-drive front HAZCAM image showing the track created by the right front wheel when we turned in place on the previous drive.
 As you can see, we hadn't then thought of using the MI to view the middle wheels. We didn't think of that trick until much later, when Spirit was embedded in Troy.