The view of the Martian skyline from the Opportunity rover, as the storm growsNASA / JPL-Caltech / TAMU

Millions of miles from Earth, tucked inside a rust-colored, rocky valley, a space robot is sleeping.

NASA’s Opportunity rover is currently hunkered down on Mars as a dust storm of unprecedented size swirls around the planet. As of this week, the tempest spans 14 million square miles, about one quarter of the entire planet, according to NASA’s Jet Propulsion Laboratory. The storm blocked sunlight from reaching the surface of continent-sized regions, including the valley where Opportunity resides. And it could last for weeks.

Warning: the rest of this story features detailed descriptions of a small, hard-working space robot, which some people call “Oppy” for short, that is trying to survive a storm, all alone, on another planet. This may prompt feelings of affection and attachment that don’t make sense because it’s a robot. But this is what we humans do.

That includes the humans who operate Opportunity.

“We are all concerned,” John Callas, the Opportunity project manager, told reporters on a call Wednesday. “This team has a very strong bond with the rover. We have a very tight emotional connection with it. It’s like you have a loved one in a coma in the hospital. The doctors are telling you that you just got to give it time and she’ll wake up, all the vital signs are good, it’s just waiting it out. But if it’s your 97-year-old grandmother, you’re going to be very concerned.”

The Opportunity rover resides in Mars’s southern hemisphere, which is entering the planet’s summer season, a time when dust storms are common. During summer, sunlight warms the air closest to the Martian surface and leaves the upper layers of the atmosphere cooler. This mismatch causes warm air to rise, taking surface dust with it, and winds scatter the tiny particles around the planet. Scientists and engineers weren’t surprised when they spotted one such storm brewing on May 30. But within days, the storm had grown to record-breaking proportions.

With the atmosphere padded with dust, Opportunity could no longer use its solar panels to charge its batteries. It’s just too dark, according to Mark Lemmon, an associate professor at Texas A&M University who works on daily operations of the Opportunity rover. Lemmon is an atmospheric-opacity expert, which means he has spent nearly 15 years estimating how bright or dark the sky looks to rovers on Mars. The scientific term for this is “tau,” a measure of the opaqueness of an atmosphere.

Usually around this time of the Martian year, tau oscillates between 1 and 2. During an intense dust storm in 2007, the tau value rose to 5.5. On Sunday, as this storm swirled, Lemmon recorded a tau of 10.8 on Mars. The greater that value, the less sunlight permeates the atmosphere to reach the surface. Under these conditions, “the brightness of the sun was 99.9995 percent fainter than it normally would be,” Lemmon said. It was so dark on the surface that engineers could command the rover to take pictures of the sun without the use of its solar filters, which act as eclipse glasses.

“During the middle of the day like this, just like on Earth, it’s not advisable to look directly at the sun, but there’s so much dust in the atmosphere that you wouldn’t even have to squint,” Lemmon said. “That’s the scary thing: Even looking straight at the sun, it’s not bright.”

If humans were to stand next to Opportunity right now—clad in the appropriate protective spacesuit, of course—the sky would look overcast rather than pitch-black. But for a rover that depends on sunlight to work, it may as well be nighttime.

Here’s how the Martian sky looks to another rover on Mars, Curiosity, which runs on nuclear power. (NASA officials said Wednesday that the Curiosity team isn’t worried about that rover, which is located on the opposite side of Mars.)

NASA engineers haven’t heard from Opportunity since Sunday, which suggests that the rover, in an attempt to preserve its dwindling power supply, has slipped automatically into a mode that shuts off nearly all of its functions. This mode prevents Opportunity from taking more pictures for Lemmon to analyze, which means scientists don’t know how dark it is for the rover right now. “I doubt that it’s gotten much better than that 10.8,” Lemmon said. “And it might have gotten worse.”

In this low-power mode, the only working system is the mission clock, which is programmed to wake up the rover’s computer, check whether the spacecraft has enough battery power to operate, and then go back to sleep if it doesn’t. NASA officials say that the mission clock could die eventually, too. “These are historic, low-energy levels for the vehicle,” Callas said.

It’s possible that Opportunity will reach such power levels that it won’t be able to recover even when the storm passes and sunlight returns. It’s happened before. In 2010, extreme winter conditions pushed the power supply on the Spirit rover, already stuck in a sand pit, to critical levels. When spring returned, the rover didn’t wake up.

Scientists hope that the conditions in Perseverance Valley, where spring is well on its way, will protect Opportunity from this fate. Callas said Wednesday that engineers believe the rover’s hardware and systems will be able to maintain the minimum temperature necessary to stay live. “We should be able to ride out this storm,” he said. Opportunity is a hardy rover, they point out. Both Opportunity and Spirit were designed for 90-day missions on Mars, and yet they lasted years, uncovering clues of the water that once flowed on the surface.

NASA’s next Mars space robot should be unharmed, too. The InSight lander, launched last month, is scheduled to touch down on the surface in November. Callas said the spacecraft’s landing procedure will be unaffected by the dust storm.

Like Opportunity, its stewards at NASA are stuck. As the storm drags on, all they can do is point their antennas into space and wait for them to hear the faint ping of a dusty rover calling home.

“When you have a robot on another planet and it stops talking to you because it can’t, you just have to hope that everything works out right,” Lemmon said.

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