An intense and long-running summertime dust storm has imperiled the Mars Rover Opportunity and, to a lesser degree, its sister rover Spirit, NASA reported.

The craft already have lasted more than three years, and they only were supposed to endure for three months on the Red Planet.

Dust blown about by the storm has blocked 99 percent of the direct sunlight that otherwise would reach Opportunity, light that the rover needs to generate electricity for survival. And the forecast is for the dust storm to last far longer: days, if not weeks.

The rovers “were never designed for conditions this intense,” said Alan Stern, associate administrator of the Science Mission Directorate.

If the sunlight is further cut back for an extended period, the rovers will not be able to generate enough power to keep themselves warm and operate at all, even in a near- dormant state, according to NASA. Rovers use electric heaters to keep some of their vital core electronics from becoming too cold.

When dust in the air reduced the panels’ daily output to less than 400 watt hours, the rover team suspended driving and most observations, including use of the robotic arm, cameras and spectrometers to study the site where Opportunity is located.

On Tuesday, output from Opportunity solar panels dropped to 148 watt hours, the lowest point for either rover. On Wednesday, Opportunity’s solar-panel output dropped even lower, to 128 watt hours.

So NASA has shut down most rover operations, even eliminating communications with Earth on some days, to conserve electricity.

The storm may damage one or both rovers permanently, or even disable them. Engineers will assess the capability of each rover after the storm clears.