Just as on Earth, the southern hemisphere winter is arriving on Mars; the autumn equinox was on 22 January, and the winter solstice will be on 08 August (there’s a very good table of Martian seasons here). Differently from Earth, though, the Martian winter lasts for six months (in fact, the southern winter is almost one month longer than the southern summer), and this has strong implications for the Mars rovers Spirit and Opportunity.
Both rovers are in the Martian southern hemisphere, but Spirit is much farther south, at a latitude of around 15 degrees (Opportunity is closer to the equator, at less than 2 degrees south). This means that, during the long Martian winter, Spirit will receive much less sun light than it is used to, and this, in turn, will affect the amount of energy it will have available to function. For this reason, NASA spent the last few weeks trying to take Spirit to a safe location where it could park for the winter in a slightly northern inclination.
Last weekend, after spending some time almost stuck in a sandy area, Spirit arrived at such a safe location and is now parked with a Sun-facing tilt of 11.5 degrees; this has already bumped its power reserves up by more than 20%, and it is likely that the rover will remain at this location for the whole winter.
Spirit had such a problem with that sandy area, in part, because one of its six wheels has stopped working weeks ago and is being dragged around without spinning. This increases the rover’s power consumption and affects its ability to negotiate the Martian terrain. Still, both rovers are in excellent shape, and have already exceeded their “warrantly” by over 700 (Earth) days.