The odometer on Spirit has been stuck at 4.8 miles for more than 11/2 years and the Mars rover has been incommunicado since March. This double dose of misfortune hangs over the scrappy spacecraft, which marks its seventh year on Mars today.
NASA doesn't know if Spirit is dead or alive, but it's diligently listening for any peep as the rover remains mired in a sand trap.
"There's a realistic possibility that Spirit may never wake up again," said Dave Lavery, Mars rovers program executive at NASA headquarters.
Two Mars orbiters have been making daily overhead passes listening for a signal from Spirit, which became stuck in April 2009 while driving backward. After several attempts to free it, Spirit got new instructions to conduct science observations while mired in the sand.
It stopped talking with Earth in March and is presumed to be in hibernation to conserve power. During this deep sleep, communications and other activities are suspended so energy can go to heating and battery recharging.
Spirit is designed to try to wake up when its battery gets enough charge. Scientists are disappointed with its silence, but are holding out hope it will spring back to life.
"I'm not ready to say goodbye yet," said mission chief scientist Steve Squyres of Cornell University.
With each passing day on Mars, the sun gets higher in the sky, increasing the amount of sunlight reaching Spirit's solar panels. The sun will be at its highest point in mid March. After that, the chances of hearing from Spirit dwindle. If Spirit doesn't radio back by March, it's "probably not going to," Lavery said.