Here's the latest update at the website for Spirit (aka MER-A, or Mars Exploration Rover-A), one of two exploration vehicles poking around on Mars (along with its companion vehicle Opportunity, MER-B) since January 2004. If you work through the dry mission language, it's somewhat ominous:
SPIRIT UPDATE: Spirit Remains Silent at Troy - sols 2321-2329, July 14-22, 2010:
Spirit remains silent at her location called "Troy" on the west side of Home Plate. No communication has been received from the rover since Sol 2210 (March 22, 2010).
It is likely that Spirit has experienced a low-power fault and has turned off all sub-systems, including communication and gone into a deep sleep. While sleeping, the rover will use the available solar array energy to recharge her batteries. When the batteries recover to a sufficient state of charge, Spirit will wake-up and begin to communicate.
There is the additional risk that the rover may trip a mission clock fault. If that happens, the rover would remain asleep until the batteries have recharged sufficiently, and there is enough sunlight on the solar arrays to wake the rover. With the southern winter solstice back on May 13, 2010, solar energy levels and temperatures are expected to be improving.
Mixed feelings here: Spirit was designed to complete its mission in three months. By means of its rechargeable solar batteries, and with careful shepherding from the mission team who are in constant touch with it, Spirit is now in the sixth year of that three-month mission. It usually powers down to a sleep mode during the Martian winter, but it looks like there's some chance that this time it may not wake up.
(Hat-tip to Doctor Beyond.)