but it can still do science.

Spirit, one of the Mars Rovers, is stuck in the sand. Of its six wheels, one has been inoperable for a while; now two wheels are dead.  Thus, getting unstuck is proving to be a vexing problem.  Mission lead scientist Steve Squires has reported that they have been able to move Spirit incrementally (on the order of a few centimeters) occasionally, but there has been no breakthrough.

But that doesn’t mean that its mission is over.  The clever folks at NASA are excited about research they can do with Spirit as a stationary outpost.  Squires is most intrigued by trying to determine if Mars has a liquid or solid core.  Okay, how will they do that?

Mars, like lots of planets, wobbles a little as it spins on its axis.   If it has a liquid core it will wobble in one fashion; if it has a solid core it will wobble a little differently.  Scientists know the orbit of Mars with great precision, so they can track the spacecraft very accurately.  If it is still, they will be able to track the axis spin and determine the wobble, thus discovering the likelihood of the nature of the core, liquid or solid.