NASA has decided to cease communications with the rover Spirit today, ending a seven-year mission that was supposed to last 90 days.

Spirit is one of a pair of vehicles called Mars Exploration Rovers, originally launched in 2003. The other, Opportunity, is still functioning. Both rovers have covered several miles of the Martian surface, and offered a look at the planet that is much more comprehensive than previous missions because they weren't limited to one place.

Of the two rovers, Spirit seemed to run into more problems. At one point one two of its six wheels were damaged, and it had to limp along for the rest of the mission. On May 1, 2009 - five years, three months and 27 Earth days into the mission -- the rover was trapped in soft sand. For six months mission control sent commands to the rover, telling it to try and pull itself free. When that failed,  

On March 22, 2010, communication was lost. At that time, Mars was entering its winter season. The rover had survived previous winters by parking on a sunny slope and keeping its temperature above the -40 Celsius mark. But that couldn't be done this time, as the rover was stuck where it was. Temperatures hit -55, which is cold enough that some components would have trouble working.

Scientists on the ground had speculated that the combination of a lack of sunlight and extreme cold -- that might have made the rover go into a powered-down mode. They decided to wait until the Martian spring. At that point the solar panels might be able to generate enough power to reestablish communications.

But such was not to be. Today NASA sent its last message to Spirit, and absent an answer, the ground teams decided to stop waiting.

The rover accomplished a number of firsts, such as grinding a rock on Mars to investigate its composition. One of the rear wheels scraped off a layer of soil, and revealed a patch of ground that looked like a good environment for microbial life.

While Spirit is no longer functioning, its twin, the rover Opportunity is still going strong, wending its way to a crater called Endeavour. Both have already set records, operating longer than the Viking lander did (Viking lasted six years).

NASA is also planning another mission to the Red Planet, to be launched in November. Called Curiousity, the new rover will be larger and be able to cover much more ground than Spirit or Opportunity did.