(CNN) A massive dust storm has hit Mars, and NASA's Opportunity rover is right in the middle of it.

Opportunity relies on energy from sunlight to operate, and with the light blocked, day has turned to night. The rover has put itself to sleep -- and remained out of contact -- in order to cope with historic energy lows.

"We're concerned but hopeful that the storm will clear and the rover will begin to communicate with us," John Callas, Opportunity project manager at NASA's Jet Propulsion Laboratory, said Wednesday.

Engineers last received a transmission from the 15-year-old Opportunity on Sunday morning.

It weathered another storm in 2007, but that one wasn't this intense. During that storm, the rover was limited to minimal operations for two weeks and went for several days without contact.

"The team has a strong bond and tight emotional connection with the rover," Callas said. "It's like you have a loved one in a coma in the hospital. The doctors are telling you to give it time, and the vital signs are good. You just have to wait it out. If it's your 97-year-old grandmother, you're going to be concerned, and we are. We are by no means out of the woods. It's a threatening storm, and we don't know how long it will last or what the environment will be like when it clears."

This is one of the most intense storms ever observed on Mars, NASA said. It was first detected May 30 and now covers a quarter of the planet, or more than 15.8 million square miles. That's about the size of North America and Russia combined.

Opportunity is stranded in the dark, rather poetically, in Perseverance Valley.

These two views from NASA's Curiosity rover -- from June 7, left, and June 10 -- show how dust has increased over three days from a major Martian dust storm.

This is the area Opportunity has been observing in order to learn what created the valley, which is a channel carved in the rim by Endeavor Crater. It's been testing to determine whether the valley was sculpted by flowing water, wind erosion or combination of factors. Learning how it was formed could provide insight into the history of the Red Planet.

The rover has not responded to attempts at contact from NASA engineers, which means its batteries have probably dipped to the point at which it is in low power fault mode. That means everything effectively shuts off, except the mission clock, which can wake the computer and check the rover's power levels each day to see whether it can communicate.

If the dust storm continues at this rate, the rover won't be able to charge for the next few days and will put itself back to sleep.

Simulated views of a darkening Martian sky blotting out the sun from the Opportunity rover's point of view, with the right side simulating Opportunity's current view.

However, Opportunity was designed for only a 90-day mission and has lasted 15 years, so it's pretty hardy.

The scientists also said that the Curiosity rover, which is in Gale Crater, should be fine.

One positive is that the dust swirl is helping raise the rover's temperature; while the rover can't receive heat from the sun, the dust itself can. Right now, the rover is at about negative 20 degrees Fahrenheit. The rover needs help from heaters to protect its batteries, much like running a car in the winter to protect battery life.

Luckily, it's approaching summer on Mars now, so it shouldn't hit the lowest temperatures Opportunity was designed to handle, which is minus 131 degrees Fahrenheit.

"We think we can ride this out for a while," Callas said.

Although the rover needs solar energy, it has about 8 watts of thermal energy available in its insulated box.

And there isn't any danger of the rover being buried by dust, although clearing it off once the storm subsides may be another challenge.

It's not like a desert storm shifting large sand dunes, more like talcum powder lofted into atmosphere and distributed around the globe, said Jim Watzin, director of the Mars Exploration Program at NASA Headquarters.

The storm is not expected to last or worsen. At most, it could endure for a month or two. As long as the rover stays warm enough, it can endure. But the pace at which the storm grew is unprecedented. Mission scientists also wonder whether another storm could happen later.

Another positive is the rare chance to study this storm. Normally, these storms, which can grow to become planet-encircling, start out locally and pass the valley before growing. But this one stalled out over the site.

The mission scientists are intrigued: Why do these massive widespread events happen in some years but not all?

"The current dust storm is providing an unprecedented chance to study Mars," Watzin said. "Knowing and understanding how these storms behave ahead of more ambitious missions, it is essential that we learn to monitor and predict storms.

"We're all pulling for Opportunity. It's been a remarkably resilient rover. Its longevity has taught us much about operating on Mars. Regardless of what happens, this little rover has been a great investment to help us explore the Red Planet."