CAPE CANAVERAL, Fla. -- A NASA rover on Mars has fallen silent as a gigantic dust storm envelops the planet and blots out the sun.

Flight controllers tried late Tuesday night to contact Opportunity, but the rover did not respond. The storm has been growing since the end of May and now covers one-quarter of the planet.

Controllers expect it will be several more days before there's enough sunlight to recharge Opportunity's battery through its solar panels. NASA says the battery is likely so low that only a clock is still working, to wake the spacecraft for periodic power-level checks.

NASA launched the twin rovers Opportunity and Spirit in 2003 to study Martian rocks and soil. Spirit hasn't worked for several years. Opportunity, however, has kept exploring well past its expected mission lifetime.