In a stroke of luck, the NASA rover Opportunity has discovered a basketball-size metal meteorite sitting on the surface of Mars, the mission’s main scientist said Tuesday. Scientists believe the meteorite might lead to clues about how martian winds are reshaping the planet’s surface. Opportunity came upon the meteorite last week while performing other tasks. Tests confirmed it was a nickel-iron meteorite, said Steve Squyres, a Cornell University scientist who is the principal investigator for NASA’s Mars Exploration Rovers mission.