CAPE CANAVERAL, Fla. - NASA's seemingly unstoppable Mars rover Opportunity has been knocked out by a gigantic dust storm that is enveloping the red planet and blotting out the sun. Officials said Wednesday they're hopeful the rover will survive the storm, which already covers one-quarter of Mars and is expected to encircle the planet in another few days. It could be weeks or even months, though, until the sky clears enough for sunlight to reach the Martian surface and recharge Opportunity's batteries through its solar panels. For now, Mars' oldest working rover is stuck in the middle of the raging...