For the solar-powered robot, the dust is not as risky as the darkness.
The rover's batteries are likely to be so low that only a clock is still working, to wake the spacecraft for periodic power-level checks, according to officials. Opportunity's power levels had dropped significantly by June 6, requiring the rover to shift to minimal operations and later to temporarily suspend science operations. Normally, these storms, which can grow to become planet-encircling, start out locally and pass the valley before growing. As Calla said, "It's like having a loved one in a coma in the hospital".
While the rover has proved more resilient than expected by lasting almost 15 years despite being only designed for a 90-day mission, the end could now be nigh for the durable device.
It was trying to establish whether the valley had been sculpted by water or wind erosion, or both.
At last word, the robot's temperature was down to minus-20 degrees Fahrenheit.
Opportunity would normally be able to generate over 600 watt-hours of energy per day with its panels at this time of the Martian year - which at its station is entering the northern hemisphere summer.
This set of images from NASA's Mars Reconnaissance Orbiter shows a fierce dust storm is kicking up on Mars, with rovers on the surface indicated as icons. Unfurled, they contain full propulsion systems. "I wish my cellphone battery had half of that", he said.
Astronauts living on Mars would not want to get caught outside in a fierce dust storm, where winds can reach 70mph - nearly hurricane force.
Mars features a very thin atmosphere that is conducive to dust storms. It may take weeks for the clouds to clear.
Dust storms are a well known phenomenon on Mars, but they are infrequent. These massive, planet-scaled storms are estimated to happen about once every three to four Mars years (six to eight Earth years); the last one was in 2007. Spirit ceased communications in 2010 after becoming stuck in sand. It now covers 15.8 million square miles, which is about the size of north America and Russian Federation combined.
Scientists gauge the strength of a dust storm with the unit tau, a measurement of the atmosphere's opacity.
NASA's Curiosity rover is elsewhere on the planet, on the edge of the current dust storm, but it also doesn't need sunlight.
NASA is deeply interested in understanding the triggers of these major storms, Zurek said. Solar power is essential for the rover to operate. 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.
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