WASHINGTON — NASA is making a new, and perhaps final, attempt to restore contact with the Mars rover Opportunity, which has not communicated with the Earth for more than six months.

The Jet Propulsion Laboratory announced Jan. 25 that it was transmitting a new set of commands to address what it acknowledged are “low-likelihood” events that could have kept the rover from contacting Earth. These new efforts are in addition to the months-long “sweep and beep” campaign of transmitting other commands and listening for a reply from the rover.

The new commands address scenarios where the rover’s primary X-band radio has failed or both the primary and backup X-band radios have failed, as well as cases where Opportunity’s internal clock has an offset affecting its timing. The commands direct the rover to switch to the backup X-band radio or use its UHF transmitter to contact Earth, as well as resetting its clock.

Those scenarios could explain why the rover has failed to contact Earth, but project officials acknowledge that those scenarios are unlikely. “A series of unlikely events would need to have transpired for any one of these faults to occur,” JPL noted in the statement about the new campaign.

Opportunity last contacted Earth June 10 as a massive dust storm, which eventually spanned the planet, darkened the skies above it and deprived it of solar power. In late August, as skies finally cleared, NASA announced a 45-day campaign to try and restore contact with the rover. That plan was criticized by some, including former engineers involved with the Mars Exploration Rovers program, as being too short.

In late October, as that initial 45-day campaign was drawing to a close, JPL announced it was extending the campaign through at least January. Project officials hoped that increasing winds at the rover’s location could remove any dust that accumulated on the rover’s panels, restoring power.

However, the windy season is now ending in that region, which is now entering winter. That will bring with it less sunlight as well as low temperatures that could cause “irreparable harm” to the rover’s electronics, JPL said.

“While we have not heard back from the rover and the probability that we ever will is decreasing each day, we plan to continue to pursue every logical solution that could put us back in touch,” John Callas, Opportunity project manager at JPL, said in the statement.

The JPL statement said the new campaign to contact the rovers will last “several weeks” but was not more specific. “If Opportunity does not respond, the project team would again consult with the Mars Program Office at JPL and NASA Headquarters to determine the path forward,” the center stated.

Efforts to restore contact continued during the five-week partial government shutdown that ended Jan. 25. With the shutdown now over, some in the planetary science community expect that, if the new effort is not immediately successful, NASA could decide to end efforts to contact the rover by mid-February. NASA is currently funded only through Feb. 15 and runs the risk of another shutdown if a budget deal isn’t reached by then, which would delay a decision by NASA Headquarters on the rover’s future.

JPL announced the new effort to contact Opportunity a day after the 15th anniversary of the rover’s landing on Mars for what was originally intended to be a 90-day mission. The rover operated for more than 5,000 days prior to losing contact last June, traveling more than 45 kilometers across the surface.

Callas, in a separate statement about the anniversary, noted the “bittersweet” nature of the milestone given the lack of communications with the rover. “We are doing everything in our power to communicate with Opportunity,” he said, “but as time goes on, the probability of a successful contact with the rover continues to diminish.”