The last transmission to Spririt will occur on Wednesday, May 25, 2011. NASA says that it has repeatedly tried to contact the little rover for the past ten months.
However, Spirit has not responded.
NASA stated that it had hoped that '... Spirit might reawaken as the solar energy available to it increased after a stressful Martian winter without much sunlight.'
However, NASA continues that: 'With inadequate energy to run its survival heaters, the rover likely experienced colder internal temperatures last year than in any of its prior six years on Mars. Many critical components and connections would have been susceptible to damage from the cold.'
For more on the story, please read the rest of the before-mentioned NASA article.
The Mars Exploration Rover (MER) website contains further information on the rovers Spirit and Opportunity.
The MER web site states, "Spirit Remains Silent at Troy." So long Spirit!