Opportunity gets out of the sand dune

After nearly a monthof trying, the rover Opportunity
escaped from its sand dune. Engineers were making it
backtrack to escape. They'd command it move ten meters,
but it would move a few inches. After weeks of this
they broke free of the dune.
The soil consistency in the dune was different.
Inpictures it appears like fine, cake-like dust.
While in the dune they couldnt run the spectrometer
to see whether this was a compositional change, e.g.
sulfates, or just a particle size change. Maybe they'll
answer that question soon.
The new operational plan will break formerly long runs of
a hundred meters or greater into shorter ten meter runs.
That way if it gets stuck again, it wont spin the wheels
as long and dig deeply into the sand.
Also they use a "trick" to figure out if they've gone a
full ten menters, or spun out somewhere. The main method
they've had to measure distance was to count wheel rotations,
but that does work when you are slipping. They plan to
make the rover move in arcs, and take compare pictures
at the start and end of runs. If the shadow angle changes
as much as predicted, then they've moved the proper amount.
(I guess they look at rear pics too.)

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