Mars researchers are keeping a watchful eye on a large dust storm that could – if it grows into a raging global event that shrouds the whole planet – complicate efforts to land the first of two NASA rovers less than three weeks from now. “Usually they appear and die out in a few days to a week, or they expand into global storms. We don’t know what’s going to happen with this one,” said Bruce Jakosky, a University of Colorado geologist who served on the site selection committee for NASA’s $820 million Mars Exploration Rover mission. The dust storm was spotted late last week and now covers a large region of the planet’s northern hemisphere, Jakosky said Sunday during a Mars exploration workshop for reporters.