Huge dust storms raging on Mars pose the worst threat yet to Nasa's robot rovers, the US space agency has said.
Martian dusts storms are making it difficult for the rovers to operate
Dust is starving the rovers of power by blocking out the sunlight needed to charge their batteries.
The six-wheeled, solar-powered rovers - Opportunity and Spirit - are operating at two distant sites just south of the Martian equator.
A series of dust storms have dogged the rovers for a month, and could continue for several more days, if not weeks.
If the sunlight is further reduced over an extended period, the rovers will not be able to generate enough power to operate or keep themselves warm.
In an effort to protect the rovers from power loss that has the potential to leave one or both permanently disabled, the US space agency has been scaling back their functions to the bare minimum, leaving them in near-dormant states.
"We're rooting for our rovers to survive these storms, but they were never designed for conditions this intense," said Nasa's Alan Stern.
Low on charge
The dust storms have been worst at Opportunity's locale on Mars' Meridiani plains.
Dust in the atmosphere over Meridiani has blocked 99% of direct sunlight to Opportunity, leaving only diffuse light to power it.
Before the dust storms began last month, Opportunity's solar panels had been producing about 700 watt hours of electricity per day, enough to light a 100-watt bulb for seven hours.
On 17 July, the output from Opportunity's solar panels dropped to 148 watt hours, the lowest point ever for either rover.
Rover operations have been scaled back to conserve power
Then, on Wednesday, Opportunity's solar-panel output dropped even lower, to 128 watt hours.
"The threat to the rover is that it doesn't have the energy to stay warm and that its sensitive electronics would become too cold. Things would get so cold that something would break inside the electronics," said rover project manager John Callas.
Electric heaters prevent the rovers' vital electronics from getting too cold.
One concern is that absence of sunlight could make the rovers drain their batteries. That worst-case scenario is still weeks off at a minimum.
Mr Callas said that because it was now Martian summer for the rovers, there was a chance temperatures would not fall low enough to ruin the electronics even if the rovers were starved of power.
Opportunity had been poised to start a dramatic and dangerous descent into Victoria Crater. The rover was to explore the 60m-deep depression to glean further insights into the geological history of Mars.
Since landing on the Red Planet in early 2004, the rovers have gathered important data about Martian geology, including evidence parts of it were once soaked by water.