NEW YORK — The twin rovers on Mars have defied the odds once again, surviving thus far the widespread severe dust storms that threatened to cut off solar power to the robotic explorers.

Steve Squyres, the lead scientist for NASA’s Mars Exploration Rovers project and a professor of astronomy at Cornell  University, Ithaca, N.Y., said both Spirit and Opportunity are in “excellent shape” based on a radio transmission received July 23.

“Both came through the weekend beautifully,” Squyres said in a telephone interview July 23. “They were both power positive over the weekend [July 21-22], meaning they were generating more power than they were consuming.”

The amount of sunlight penetrating the dust-choked martian atmosphere had increased slightly over the July 21 weekend, and the batteries of both rovers are fully charged, said Michael Meyer, lead scientist for the Mars Explorations Program at NASA headquarters in Washington.

Astronomers measure atmospheric opacity in units of “tau.” The lower the tau value, the clearer the sky.

“At its worst, tau was a little over five [for Opportunity],” Meyers told Space News. “It now has dropped down to a little less than four.”

The tau value for Spirit, which was hunkered down half a world away from its twin, has dropped slightly and is currently just less than four, Meyers said July 23.

A tau of five means that less than 1 percent of direct sunlight is reaching the Mars surface, but the rovers have been successful at drawing power from scattered sunlight.
Tau levels on Mars at this time of year when no dust storms are occurring are about one, Squyres said.

Even though the rovers were not designed to weather dust storms of this magnitude, Meyers said mission scientists and engineers are optimistic the rovers will survive the storms, which have been raging for nearly a month. All scientific observations and driving – including a planned descent into Victoria Crater by Opportunity – remain suspended for now to conserve power.

Mission scientists will “wait and make sure that the storm doesn’t kick back up,” Meyers said. “They’re going to wait it out a few more days to make sure.”

“Right now, we’re just keeping both vehicles safe, and we’ve been very successful with that so far,” Squyres said. “We’ll just wait and see what Mars does.”

Scientists predict, however, that it could be weeks before Mars’ dust-clogged air clears.

“In order for us to really resume full scientific operations, even at a reduced rate, we need to have the dust opacity drop by a bit from what we’ve been seeing,” Squyres said. “How long it will take to drop to that level, we cannot predict.”

Spirit and Opportunity have been exploring the Mars’ surface for more than 1,200 martian days, far exceeding the three-month mission for which they originally were designed.