NASA may have heard the last from the immobilized Spirit rover for several months as the stranded robot endures the depths of winter on Mars, the space agency announced late Wednesday.

Spirit's rear hazard camera shows the rover's wheels stuck in sand. Credit: NASA/JPL-Caltech

Spirit did not establish a radio link with an orbiting satellite as scheduled Tuesday, missing a once-per-week communications session with engineers at the Jet Propulsion Laboratory in Pasadena, Calif.

NASA's Mars Odyssey spacecraft circling the Red Planet has been used as a communications relay satellite linking Earth with the unmanned Martian outpost.

"We are checking other less-likely possibilities for the missed communication, but this probably means that Spirit tripped a low-power fault sometime between the last downlink on March 22 and yesterday," said John Callas, the project manager for the Spirit and Opportunity rovers.

The golf cart-sized rover has been stuck in a sand pit nicknamed Troy for nearly a year. Controllers at JPL gave up on trying to free Spirit in January and began trying to reposition the vehicle to give the rover a fighting chance to survive winter.

Spirit is tilted 9 degrees toward the south, a poor attitude to capture rays of sunlight from the northern sky. The lack of power was expected to trigger low electricity alarms on the rover that would halt communications with Earth, shut down other systems, and route power to heaters keeping the craft's electronics warm.

"The recent downlinks had indicated that the battery state of charge was decreasing, getting close to the level that would put Spirit into this hibernation," Callas said in a NASA statement.

During the winter, Spirit will be subjected to bone-chilling temperatures colder than the rover has ever experienced on Mars. Earlier this month, instruments aboard Spirit measured a temperature of -42.7 degrees Fahrenheit inside the spacecraft.

NASA says the temperatures are not expected to drop below the rover's design specifications.

Spirit has lived 25 times longer than its original 90-day mission, and the vehicle is currently beginning its 76th month on Mars since parachuting to landing at Gusev crater on Jan. 3, 2004.

The longevity has taken a toll on Spirit, which has lost two of its six wheels after driving 4.8 miles across the crater floor and scaling the Columbia Hills. Spirit's long life could also make the craft more susceptible to the upcoming cold winter.

"The temperature limit was for a new rover. We now have an older rover with thousands of thermal cycles on Mars, so the colder temperatures will be a further stress," Callas said.

The sun will reach the lowest point in the sky at the southern hemisphere's winter solstice in mid-May. Officials say the rover could hibernate for up to six months.

Spirit will remain out of communications until it has enough battery charge to activate its radio transmitter.

"We may not hear from Spirit again for weeks or months, but we will be listening at every opportunity, and our expectation is that Spirit will resume communications when the batteries are sufficiently charged," Callas said.

NASA uplinked a schedule of communications opportunities with Earth and Odyssey, so Spirit will know when it can contact controllers when the rover wakes up.

In this image from March 11, the rim of Bopolu crater appears on the southwest horizon. Opportunity was about 40 miles away from Bopolu crater as the rover captured this image. Credit: NASA/JPL-Caltech/Cornell University

Opportunity continues driving on the other side of the Red Planet. The rover is now trekking across a wind-blasted plain toward Endeavour crater in a region known as Merdiani Planum.

After clicking through 20 kilometers, or 12.43 miles, on its odometer last week, Opportunity is about 7.5 miles from the rim of Endeavour.

Endeavour will be the largest impact site the crater-hopping rover has visited since 2004. The crater stretches about 14 miles across and is about 1,000 feet deep.

Along the way, Opportunity has stopped driving to study features such as boulders, fallen meteorites and smaller craters.

Opportunity has also tested new software permitting the rover to select a target from a wide-angle image and point its panoramic camera at the object for closer observations.