Spirit Rover is heading to her sixth anniversary on Mars, but it looks like she'll have to sit the dance out. Spirit has been trapped, basically high-centered on a hard spot surrounded with flour-like soil. Some surprising test results show that one of the wheels we thought was jammed has some slight function, but the bad news is one of the other wheels no longer functions at all, the motor is burned-out. Between the high-center, and the dead wheels, it doesn't look like Spirit is going to be able to dislodge herself. On top of this bad news, Winter is coming.
The rovers are solar powered. We've used tricks the past few winters to maximize that incoming power - mostly, parking the rovers so they are angled towards the sun. It's a critical step to making them last so long.
In the warm summer days, temperatures can reach a balmy 70F. The long days generate plenty of power, and we're able to rove about and do science on the surface. The nights get a bit cold, but there are heaters on board which can be run to keep the rovers' brains from freezing. In Summer we have plenty of power to run those heater as needed, except perhaps during the occasional several-month-long dust storm.
In the Winter, temperatures can plummet to below -150F, colder than dry-ice. Temperatures this cold can precipitate mechanical failures - for instance, there are glass lenses in the rovers which could literally break just from getting that cold. Metal fatigue becomes amplified. Though there are heaters, with shorter, colder days, the heaters must run longer to maintain a safe temperature.
Spirit is trapped. She can't reposition herself to angle the panels towards the sun. With the increased needs for power over the Winter, this is not good news.
It's somewhat fitting that in the process of getting trapped, she made more important science discoveries about the surface of Mars. It's sort of a last cry of triumph for the old girl. She's almost six years on-planet, and six years in Rover-time is about 25 lifetimes in people years... :)