A self-portrait of Spirit shows the solar panels still gleaming in the Martian sunlight and carrying only a thin veneer of dust in 2005.

Credit: NASA

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WASHINGTON: After three years on Mars, NASA's rovers are facing their toughest foe: Martian dust storms blocking the sunlight that charges their batteries.

As a result, the U.S. space agency has ordered rovers Spirit and Opportunity – which were originally designed for just a three-month mission – nearly into hibernation. Communication with Earth has been cut to save vital energy for heaters that keep their delicate innards from freezing.

Intense conditions

"We're rooting for our rovers to survive these storms, but they were never designed for conditions this intense," said NASA associate administrator Alan Stern.

By Tuesday last week, NASA stopped any driving and all Opportunity's science observations, but the rover was still using more energy than its solar panels could generate, and drew down its battery.

"The only thing left to cut were some of the communication sessions," said John Callas, project manager for the twin rovers at the Jet Propulsion Laboratory, Pasadena, California.

That was the first time either rover had been told to skip communications for a day or more to save energy. "We are taking more aggressive action with both rovers than we needed before," Callas said.

Engineers calculate that communications blackouts should lower daily energy use to less than 130 watt hours. That leaves little juice for the rovers' heaters, which prevent damage to their vital core electronics.

Communications blackout

Spirit, Opportunity's antipode, is suffering less from the storms, but produced below the rovers' average 700 watt hours prior to the storms. That is enough electricity to burn a 100-watt bulb for seven hours.

When dust storms cut daily output to 400 watt hours, NASA cut driving and observations, which make use of the robotic arm, cameras and spectrometers.

The unmanned, remote-controlled rovers landed on Mars to scan its surface in search of signs of water and other components, making use of their instruments to take close-up looks at rocks along the ridges and craters of the planet most like Earth.

A possible outcome of this storm is that one or both rovers could be damaged permanently or even disabled. Engineers will assess the capability of each rover after the storm clears.