On June 10, with a dense, planet-wide dust storm approaching its position in Mars' Perseverance Valley, NASA's Opportunity rover shut down all systems and entered hibernation mode. More than 90 days later, as the dust storm begins to abate, NASA engineers have yet to hear back from Opportunity, and concern is growing.

"[We're] starting to approach a range where there should be energy being generated by the solar arrays," Project Manager John Callas at NASA's Jet Propulsion Laboratory in Pasadena, California, told CBS News on Aug. 21. "So we're starting to get close to the time frame where the vehicle should start to charge. We're looking every day. People are making bets as to when they think we'll hear from it."

satellite image of Mars NASA's Opportunity rover appears as a blip in the center of this square. (Photo: NASA/JPL-Caltech/Univ. of Arizona)

There was a glimmer of hope though on Sept. 20 when HiRISE, a high-resolution camera onboard NASA's Mars Reconnaissance Orbiter, captured a satellite image showing Opportunity. Look closely at the image above and you can see a tiny white dot in the center of the square. NASA officials said the fact that the rover appears in the image means the dust storm has "substantially cleared." Now, the waiting game continues to see if Opportunity will come back online.

Unlike the Curiosity rover, which runs off a nuclear-powered battery, Opportunity relies purely on solar cells to charge its lithium batteries. While the rover has endured massive dust storms before, the intensity of this one — described by NASA officials as a "dark, perpetual night" –– coupled with its unprecedented length, could prove to be too much for the plucky little robot.

The little rover that could

Opportunity in Endurance crater (simulated view based on actual imagery). Opportunity in Endurance crater (simulated view based on actual imagery). (Photo: NASA)

Engineered for a mission that was expected to last only 90 days, Opportunity has defied all odds by surviving and conducting explorations on the surface of Mars for nearly 15 years. Even its twin, Spirit, which landed three weeks before Opportunity in January 2004, managed to function until 2010. As it currently stands, Opportunity holds the off-Earth roving record with a distance of more than 28 miles.

The biggest concern facing engineers is whether or not Opportunity's batteries had enough reserves to help protect its internal components from Mars' freezing surface temperatures. One of the last signals sent NASA on June 10 showed the rover's temperature to be about minus 20 degrees Fahrenheit (minus 28 Celsius).

"The rover needs to balance low levels of charge in its battery with sub-freezing temperatures," the team explained. "Its heaters are vitally important to keeping it alive, but also draw more power from the battery. Likewise, performing certain actions draws on battery power, but can actually expel energy and raise the rover's temperature."

While an analysis by engineers of the rover's systems relative to the freezing cold showed that it had the potential to keep warm for the duration of the storm, the rover's age makes any positive outcome less than certain.

"It's like you have a loved one in a coma in the hospital," Callas told The Washington Post. "The doctors are telling you, 'You just gotta give it time, and she'll wake up.' But if it's your 97-year-old grandmother, you're going to be very concerned, and by all means we are."

All ears on Mars

An image of the dust storm sweeping across the surface of Mars as captured by the ESA's Mars Express in April 2018. An image of the dust storm sweeping across the surface of Mars as captured by the ESA's Mars Express in April 2018. (Photo: ESA/DLR/FU Berlin)

To listen for a possible sign of life from Opportunity, engineers use NASA's Deep Space Network to ping the rover during scheduled "wake up" windows. They also sift through radio signals emanating from Mars to see if one might happen to be Opportunity's "voice." The team warns, however, that should Opportunity regain contact, there's no guarantees it will be the same robot it was before.

"The rover's batteries could have discharged so much power — and stayed inactive so long — that their capacity is reduced," they explain. "If those batteries can't hold as much charge, it could affect the rover's continued operations. It could also mean that energy-draining behavior, like running its heaters during winter, could cause the batteries to brown out."

For now, all the team can do is listen and hope that Opportunity has once more defied the odds.

Beginning on Sept. 11, NASA will ping Opportunity three times a day for 45 days instead of the usual three times a week in hopes it will wake up. The pings will instruct the rover to create a signal at a specific frequency. If it doesn't respond within this time period, NASA will re-evaluate what to do next. Though, the agency did say it would passively listen until January 2019.

"It remains to be seen," Deputy Principal Investigator Ray Arvidson told the Planetary Society. "But I wouldn't bet against this rover."

Editor's note: This article has been updated since it was originally published in August 2018.

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.

Mars Opportunity rover: Please phone home!
With a planet-wide dust storm finally abating, NASA is anxiously listening for a signal from Opportunity, its longest-serving Mars rover.