With the darkest days of the Martian winter now over, NASA took its Opportunity Mars Rover for a drive this week. The rover had been stationary while its solar panels lacked enough sunlight to power its batteries.

The rover's drive Tuesday was a short one: "about 12 feet northwest and downhill," according to NASA. The agency says Opportunity has driven 21.4 miles since it landed on Mars in January of 2004.

Opportunity's current area of southern Mars reached its winter solstice at the end of March. And that means the rover's "break time" is over.

For NPR's Newscast, Joe Palca filed this report:

"Opportunity has been sitting still since last December. It uses panels to charge its batteries, and they just can't generate enough electricity during the winter. It takes a lot of energy to drive across the Martian sand.""The rover has been exploring the Meridiani region of Mars for more than 8 years — far longer than the three months it was nominally supposed to operate. It's now perched at the top of a large crater known as Endeavor.""Opportunity's twin rover hasn't been heard from in two years... but if all goes according to plan, a new, much larger rover called the Mars Science Laboratory will land on Mars this summer."

The new rover is called Curiosity; it was launched last November. Curiosity is expected to arrive at the Gale Crater on Mars in August.

In the meantime, NASA expects Opportunity to have plenty of material to study in the Endeavour Crater, which is "more than 20 times wider than Victoria Crater, which Opportunity examined for two years," according to the agency.

A NASA release explains, "One type of deposit detected from orbit at some locations on Endeavour's rim contains ancient clay minerals, interpreted as evidence of ancient, wet conditions with less acidity than the ancient, wet environments recorded at sites Opportunity visited during its first seven years on Mars."

Copyright 2016 NPR. To see more, visit http://www.npr.org/.