They're old and dirty, but NASA's Mars rovers are back in the exploration business after enduring a lengthy Red Planet dust bowl that blocked most of the sunlight they need for power.

With skies gradually brightening, the solar-powered rovers Spirit and Opportunity recently resumed driving and other operations that had been suspended during the dust storm.

"The rovers are in good health and in good shape," said John Callas, the rover project manager at NASA's Jet Propulsion Laboratory in Pasadena. "Things have improved from the more dire conditions that were existing previously due to the dust storm on Mars."

During the storm, each of the rovers spent a couple of weeks sleeping most of the time.

"They were in sort of a hibernation state where we were only communicating with them every few days," Callas said Friday. "The rovers would only be awake a very short amount of time each day to save power."

The major concern was whether the rovers would have enough energy to keep sensitive electronics at proper temperatures on the frigid planet.

"At the darkest part of the storm, Opportunity had only 128 watt-hours of energy. Today, it has about 350 watt-hours of energy, so almost three times as much now," Callas said. "The most energy that the rovers have ever seen in their 3 1/2 years on Mars is about 900 watt-hours of energy."

The biggest problem left by the storm is dust on the instruments at the end of the rovers' robotic arms, he said. Some has fallen off or been blown off, and there are ways to measure how dust contamination is affecting an instrument, he said.

The longer-term concern is how the rovers, particularly Spirit, will deal with the next Martian winter, when the sun is low and less energy reaches their solar panels.

"The solar arrays are dusty on both rovers, but dustier on Spirit, and they are dustier now than they were exactly one Martian year ago. So if they don't get cleaner and they continue to accumulate dust at the same rate they saw last year, it will be a tough Martian winter for Spirit," Callas said.

The six-wheel rovers have been exploring opposite sides of Mars since landing in early 2004, finding geologic evidence that rocks were altered by flowing water in its ancient past. They have long outlasted their planned three-month missions, surpassing or nearing 1,300 "sols," as Martian days are called.

"These are really very old rovers and their mechanisms are well beyond their design life by many, many factors, so we're fortunate that they're still working, but things could break — important components could break at any moment — but absent that, they're in good shape and we're ready to continue exploration of both sites."

Spirit, studying Mars' Gusev Crater region, will soon drive to a spot that has been named "Home Plate."

Opportunity, in the Meridiani Planum region, has been waiting to enter Victoria Crater, a half-mile-wide hole blasted into the plains by a meteor. The rover will roll to an entry point in coming weeks, Callas said.