NASA's Mars rover Opportunity captured this view of late-afternoon shadows, including one cast by the rover itself, stretching toward the floor of Endeavour Crater. Opportunity's navigation camera recorded the three component images on Nov. 11, 2017, about a week before Opportunity's eighth Martian winter solstice.

The harsh Martian elements still haven't broken NASA's long-lived Opportunity rover.

The solar-powered Opportunity has rolled through the coldest and darkest parts of the Martian winter in good nick, NASA officials said. The lowest-light period of the current Red Planet year, which lasts 687 Earth days, came in October and November, they added.

The rover is therefore well on its way to surviving this winter, the eighth the craft has endured since touching down in January 2004, a few weeks after its twin, Spirit, landed. Both six-wheeled robots were tasked with searching for signs of past water activity on Mars, in missions designed to last just three months. [Latest Mars Rover Photos from Opportunity & Spirit]

"I didn't start working on this project until about Sol 300," Jennifer Herman, power subsystem operations team lead for Opportunity at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, said in a statement. (A sol is a Martian day, which lasts about 24 hours and 40 minutes.) "I was told not to get too settled in, because Spirit and Opportunity probably wouldn't make it through that first Martian winter. Now, Opportunity has made it through the worst part of its eighth Martian winter."

The rover's solar panels are quite free of dust at the moment, she added.

This view from within Perseverance Valley, on the rim of Endeavour Crater, includes wheel tracks from the Opportunity Mars rover's descent of the valley. The rover's panoramic camera took component images in September and October of 2017 for this approximately true-color scene. (Image credit: NASA/JPL-Caltech/Cornell University/Arizona State University)

"If Opportunity's solar arrays keep getting cleaned [by wind] as they have recently, she'll be in a good position to survive a major dust storm," Herman said. "It's been more than 10 Earth years since the last one, and we need to be vigilant."

Spirit also far outlasted its warranty, but that rover is no longer with us. After losing the use of two wheels, Spirit got bogged down in soft Martian dirt in 2009. The rover's handlers couldn't reorient it to catch enough sun during the ensuing winter, and the rover essentially froze to death in 2010.

The crucial winter orientation, which Spirit's handlers were striving for, is to face the rover northward. Both Spirit and Opportunity landed (at separate sites) in Mars' southern hemisphere, so they've needed to find north-facing slopes to stay warm and powered up during the coldest months.

The impact of the season on the rovers' science work has varied widely, depending on the nature of the terrain they were exploring when winter hit. For example, Opportunity spent its first winter on the north-facing slopes of a crater, so it was able to stay quite active. But the rover had to camp out for 19 weeks in a single spot during its fourth winter, in 2009 and 2010, NASA officials said.

Opportunity has managed to work through much of this year's late fall and early winter. It's been exploring the water-carved Perseverance Valley, which runs down the inner slope of the western rim of the 14-mile-wide (22 kilometers) Endeavour Crater.

This enhanced-color image, captured by Opportunity in October 2017, of ground sloping downward to the right in Perseverance Valley shows textures that may be due to abrasion by wind-driven sand. (Image credit: NASA/JPL-Caltech/Cornell University/Arizona State University)

As Opportunity drives through the valley, the rover has been stopping at north-facing sites to catch some rays, a strategy the team likens to a frog hopping from lily pad to lily pad. The team plans to continue its exploration of Perseverance over the next few months, searching for clues about how the valley was carved into the crater rim.

"We have not been seeing anything screamingly diagnostic in the valley itself about how much water was involved in the flow," Opportunity project scientist Matt Golombek, also of JPL, said in the same statement. "We may get good diagnostic clues from the deposits at the bottom of the valley, but we don't want to be there yet, because that's level ground with no more lily pads."

The other active rover on the Martian surface, NASA's car-size Curiosity, doesn't have to worry about angling itself toward the sun to make it through the winter. That rover is powered by a radioisotope thermoelectric generator, which converts the heat generated by the radioactive decay of plutonium-238 into electricity.

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