Opportunity first detected the storm on 1 June and it has now grown to cover an area larger than North America, more than 18 million square kilometres (seven million square miles).
Operators at NASA's Jet Propulsion Laboratory (JPL) attempted to contact the rover on Tuesday but did not receive a signal, which means the craft has likely entered a "low power fault mode" wherein all systems, save a mission clock, are shut down.
The nuclear-powered Curiosity Mars rover, operating in Gale Crater well away from Opportunity, is not expected to be impacted by the current storm. Because of the storm, those panels aren't receiving enough sunlight to fully charge up the rover, meaning that it's now trying to save what little power it does have before getting in touch with Earth again.
Opportunity was in remarkably good health going into the storm, Callas said, with only an arthritic joint in its robotic arm. "We're concerned but we're hopeful that the storm will clear and the rover will begin to communicate to us".
The clock will regularly wake an onboard computer to check power levels.
"Our expectation at this point is that the rover has gone to sleep, it's in this low-power mode and it will remain in that low power mode until there's sufficient energy to charge the batteries back above a certain threshold", Callas said.
It could also pose a threat to the Curiosity rover, which is monitoring the storm from the periphery. There are two related issues here: power and temperature.
Callas said the dust storm had essentially turned day into night for Opportunity as the opacity of the atmosphere, a measure of how effectively the dust is blocking out sunlight, climbed to record levels.More news: Kim takes night tour of Singapore ahead of summit with Trump
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There is a risk that Opportunity will get too cold as it struggles to power its internal heaters, which protect its batteries from Mars' extreme cold. Both were scheduled to complete 90-day geology missions but were able to operate for years beyond that.
Spirit finally went silent on March 22, 2010, stuck in deep sand and unable to favorably orient itself so its solar arrays could face the low-altitude sun during the harsh martian winter. NASA recently launched towards Mars the Insight lander-and two tiny CubeSats, satellites that are 14.4 by 9.5 by 4.6 inches when packed tight. The storm has been growing since the end of May with unprecedented speed. The last such major storm occurred in 2007, which sent NASA's rovers into a "hunker down" mode for several weeks before the skies cleared.
If the solar panels become caked with dust, and thus stop working properly, there won't be enough juice to restart the clock. That leaves the air in the upper atmosphere cooler than below, and an unstable mix emerges; as the warmer air rises, it carries dust into the skies.
The storms' cyclical nature, however, are poorly understood.
The dust storm was first spotted on May 30 by NASA's Mars Reconnaissance Orbiter, and it has been growing ever since.
A massive dust storm raging across Mars has overcome NASA's aging Opportunity rover, putting the unmanned, solar-powered vehicle into sleep mode and raising concerns about its survival, the U.S. space agency said Wednesday.
The first indicators of the storm were reported on May 30 and it is almost circling the planet, NASA scientist Rich Zurek said on Wednesday in a conference call.