SEATTLE — It has been more than a year since NASA’s Mars rover Spirit last contacted Earth, and the odds that the immobilized robot will ever wake up get smaller by the day. Still, NASA engineers have more things to try before they give up all hope.

Spirit’s operators had been hoping that the solar-powered rover would stir from its yearlong hibernation last month, as the martian sunshine strengthened at its locale. But the solar energy available to Spirit peaked around March 10, researchers said, and the rover has shown no sign of life since March 22, 2010.

Rover engineers have been trying to contact Spirit for months, assuming that the increasing solar energy might rouse it from its martian slumber. But that has not worked, and researchers have changed strategies, employing tactics designed to address possible problems with Spirit’s communications systems and internal clock.

“Now we’ve moved into ‘There has to be more than one thing wrong with the rover,’” said John Callas, NASA’s project manager for Spirit and its twin rover, Opportunity, at the Jet Propulsion Laboratory in Pasadena, Calif.

“So that’s the phase we’re in right now — trying to see what combination of things could’ve gone wrong that we can do something about.”

If Spirit remains silent through the end of April, the rover team at JPL will likely give up most remaining hope, Callas said. They will switch to single-rover operations and concentrate on driving Opportunity, which he said is still going strong.

Extended warranty

Spirit and Opportunity landed on the Red Planet in January 2004. The six-wheeled, golf-cart-size rovers were originally tasked with three-month missions to look for signs of past water activity on Mars.

Both rovers delivered in a big way from their separate landing sites, finding lots of evidence that the Red Planet was once a much wetter, warmer place.

In 2007, Spirit even stumbled onto evidence of hydrothermal systems on Mars. The find suggested large amounts of energy had flowed through some wet environments in the ancient past, making the emergence of martian life a likelier possibility.

And the rovers just kept on going, far beyond warranty. Each designed to log just 1 kilometer on the martian surface, they instead traveled 34.4 kilometers between them, researchers said.

Opportunity keeps adding to that total with every passing day. But Spirit’s wheels stopped moving for good last year.

A crippled rover falls asleep

The failure of its right front wheel in 2006 compromised Spirit’s mobility, and it became mired in soft sand in May 2009. In January 2010, NASA declared Spirit’s roving mission over and rededicated it as a stationary science outpost.

The trapped Spirit rover went silent March 22, 2010, after failing to maneuver into a position that would have slanted its solar panels toward the sun during the course of the martian winter.

During that winter, Spirit’s internal electronics probably grew colder than ever before. The frigid temperatures could have damaged any of several components, one possible reason that operators have not been able to re-establish communication with Spirit, researchers said.

Operators had hoped Spirit would wake up last month as it warmed in the increasing periods of sunlight. So NASA listened for signs of life from the rover, and engineers told it to phone home.

After the March 10 solar maximum failed to rouse Spirit, the rover team changed tactics a bit. Now they are sending different commands, hoping to elicit a response from Spirit even if its two radio systems are damaged.

“Maybe the rover’s gone tone-deaf, and it’s not hearing our requests to phone home,” Callas said. “Or maybe it’s mute — it is hearing our requests, but it can’t talk to us because its transmitter is bad.”

So some of the revised commands — which began March 15 — include instructions for Spirit to use a backup transmitter, and to be receptive over UHF relay to the signals from several NASA spacecraft currently orbiting Mars, researchers said.

Another possibility is that Spirit’s internal clock is scrambled.

“It may not know exactly what time to wake up, and it may wake up at unusual times. So we’ve been covering those time periods,” Callas said. “We’ve been going through all these combinations.”

If the new methods do not prod a response from Spirit in the next month or so, the team will try a new set of strategies addressing a new set of possible problems. But hope for Spirit’s recovery will be mostly lost at that point, Callas said.

“Those are things we should try as part of our due diligence,” he said. “But that’s likely to be done at a very low level of activity at that point. We will probably have arrived at a reasonable conclusion that the vehicle is not likely recoverable at that point.”