The American space agency says it is concerned but hopeful that its veteran robot rover on Mars can survive a big dust storm on the planet.
The 15-year-old vehicle, known as Opportunity, has gone into an emergency mode because there is not enough sunlight reaching its solar panels to power onboard systems.
All contact was lost at the weekend.
The only function likely running off the batteries currently is a clock.
The expectation is that the robot is using this timepiece to wake itself periodically to check whether skies have cleared.