On Monday (December 16, 2009), NASA will begin transmitting commands to the Mars rover Spirit in an attempt to free it. Spirit has been stuck in soft soil since May 1, 2009 – there’s a great picture (created from combined images) of its track through the Martian soil here. NASA has been monitoring it and testing extraction scenarios with a duplicate rover on Earth, but there are three major difficulties.
Spirit's view of its own situation. Note the circled wheel partially buried in loose "talcum-powder-like" sand.
The first is the different conditions here – gravity is lower and there is much less of an atmosphere on Mars. The second is the time necessary for commands transmitted from Earth to reach the rover – there is about a three to five minute delay, depending on the respective position of the planets in their orbits.

The third is that Spirit is dragging its right front wheel. The rover was designed with one motor powering each of its six wheels, but one motor stopped working in March, 2006. Spirit now has to drag that wheel against the resistance of a non-functioning motor.

The Mars rover Spirit landed January 3, 2004 PST with a planned mission length of 90 days. 2142 days later – over 23 times longer than the planned task – Spirit is still transmitting data and responding to commands. I’m cheering for the little robotic miracle of science to free itself!