For the solar-powered robot, the dust is not as dangerous as the darkness. Since the storm began two weeks ago, the amount of light the spacecraft receives has dropped to less than 1 percent of normal levels. Energy production has fallen from hundreds of watt-hours a day to almost nothing. NASA has not heard from the rover since Tuesday.
This is a “spacecraft emergency,” Mars Exploration Rover project manager John Callas said Wednesday. His team is operating under the assumption that the charge in Opportunity's batteries has dipped below 24 volts and that the rover has entered a low-power fault mode, when all subsystems except the mission clock are turned off.
The clock is programmed to rouse the rover at periodic intervals to check whether light levels are sufficient to wake up — a state called “solar groovy.” But the storm is still growing and should encircle the planet in a matter of days. NASA expects that it will be weeks, if not months, before the dust clears enough to allow the spacecraft to turn back on.
Though the rover is expected to survive the storm, “It's like you have a loved one in a coma in the hospital,” Callas said. “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.”
Opportunity, along with Spirit, its twin, landed on the planet in 2004. Though the mission was supposed to last only 90 days, Opportunity is approaching its 15th year there. Before the storm began, it had been rolling down a channel called Perseverance Valley, which scientists think may have been carved by water billions of years ago.
NASA's main worry now is that the rover's batteries may freeze without the heat generated by its normal operations. (Opportunity is designed to withstand temperatures as low as minus 55 degrees Celsius.) Other problems may arise if the rover's energy levels drop even further. That would trigger a “clock fault,” when the rover loses track of time entirely.
Fortunately for NASA, dust storms tend to trap radiation, preventing the kind of wild temperature swings that often occur on Mars. Callas also expressed faith in Opportunity's batteries — even though they are 15 years old, they are still working at 85 percent of their capacity.
“They're the finest batteries in the solar system. I wish my cellphone battery had half of that,” he said.
Local dust storms are a common occurrence on Mars — strong winds lift particles the size of those in talcum powder and carry them into the sky. The thin atmosphere makes these events different from storms on Earth; even the most powerful Martian winds couldn't topple a rover. But that atmosphere also makes it easier for sunlight to heat the dust and loft it higher and higher. It may take weeks for the clouds to clear.
Scientists aren't sure what causes huge, planet-scale dust storms like the one now raging. They seem to occur every three to four Mars years (six to eight Earth years), and they usually happen when the planet is at its closest to the sun.
A similar storm in 2007 severely damaged Spirit, blanketing the little robot's solar panels and cutting its energy production to less than half of normal capacity. Spirit later got stuck in a sand trap, where it could not angle toward the sun in the winter, and has been silent since 2010.
This current storm is “unprecedented in the pace at which it has grown and spread across the globe,” said Rich Zurek, chief scientist for the Mars Program Office at NASA's Jet Propulsion Laboratory. It covers almost a quarter of the surface of Mars, an area the size of North America and Russia combined. Its tau — a measure of how much light is being absorbed — is 10.8, the highest ever recorded.
Even on the opposite side of the planet, where the Curiosity rover is parked at Gale Crater, the thick red haze almost completely obscures the horizon. (Curiosity, which landed in 2012, is nuclear-powered and mostly unaffected by the dust.) The storm is being monitored by a fleet of orbiting spacecraft, particularly the Mars Reconnaissance Orbiter. Its wide-angle camera first detected the event.
NASA is deeply interested in understanding the triggers of these major storms, Zurek said. The agency's next Mars mission, the InSight lander, due to arrive at the Red Planet this fall, depends on solar power. So will an autonomous rotorcraft that is slated fly with the Mars 2020 rover in two years. And if the space agency aims to one day send astronauts to the Martian surface, it would probably like to protect them from dust clouds, too.
Meanwhile, “we're all pulling for Opportunity,” Mars Exploration Program director Jim Watzin said. “It's a remarkably resilient little rover.”
Correction: A previous version of this article incorrectly stated that the Mars 2020 rover was solar powered. The rover will operate on nuclear power, but the mission also includes a solar-powered autonomous rotorcraft, the Mars Helicopter.