The two Mars rovers that have been carefully conserving critical power supplies since June, when the summer dust-storm season began on the red planet, are now springing back to work as the storms subside.

Jake Matijevic, engineering team chief at NASA's Jet Propulsion Laboratory at the California Institute of Technology in Pasadena, California, said the dust storms have been easing over the past two weeks, allowing the two rovers to collect more sunlight with their solar panels to recharge their batteries. When the dust storms were at their strongest in June and July, as much as 99 percent of the sunlight in the planet's southern hemisphere was blocked -- drastically affecting the Opportunity rover. The storms meant cutbacks in operations as both Opportunity and its sibling, Spirit, on the other side of the planet, drastically conserved power so their systems would survive and continue to operate.

"Everything survived just fine," Matijevic said. "We came out of the storms without a problem."

So far, power levels on both rovers remain below prestorm levels, he said, but they are enough for the rovers to again move about and continue to explore the planet's surface.

Both rovers are resuming work after a series of certification tests to be sure that all systems are still operating properly. Mission engineers are again able to send commands to the rovers and receive data back on a daily basis, which wasn't possible for the past two months. "We were spreading those out over a couple of days during the dust storms just to try to conserve energy," Matijevic said.

Past storms on Mars affected the rovers, but those were only short-term events, Matijevic said. This year's storms were especially strong.

Typically, the solar panels on each rover produce about 700 watt-hours of electricity per day -- enough to light a 100-watt bulb for seven hours, according to NASA. But this year's dust storms reduced that to as little as 128 watt hours per day. When daily power generation is down to less than 400 watt-hours, the rovers suspend their driving on the planet and stop using their robotic arms, cameras and other instruments.

Electrical power is critical on the rovers in the deep cold of space, where subfreezing temperatures can destroy sensitive electronics. If the batteries completely die, heating systems on the rovers that keep those sensitive electronic parts within a safe temperature range would fail, leaving the rovers so cold that their onboard electronics would likely fail. The rovers are certified for operating temperatures as low as -40 degrees Fahrenheit, while the low temperatures on Mars can plummet to as much as -103 degrees Fahrenheit.

The two rovers, which landed on Mars three weeks apart in January 2004, were designed to collect information, perform experiments and travel the planet for 90 days. Instead, they have collected data on Mars for more than three years, giving scientists and researchers a wealth of information.

A few weeks after landing, the Spirit rover suffered computer memory problems that were later resolved. Two months later, the rovers needed operating system updates to improve their performance. Last year, Spirit had a problem with one of its six wheels, but that also was corrected, allowing the mission to continue.

"We've long since recognized that these vehicles are very capable and resilient after three-and-a-half years on Mars," Matijevic said. "I think we probably can go on for another calendar year at least and maybe longer, too."

According to Matijevic, the rover Opportunity will continue its exploration of the Victoria Crater on Mars to learn more about the terrain, while Spirit will soon explore Gusev Crater near where it landed.

The dust-storm season, while easing, will continue through mid-November, so the rovers won't return to full power right away, Matijevic said.