NASA's Opportunity rover successfully landed on Mars late Saturday, arriving at the Red Planet exactly three weeks after its identical twin set down. "We're on Mars everybody," mission scientist Wayne Lee declared as fellow members monitoring the landing at NASA's Jet Propulsion Laboratory burst into wild applause. The unmanned, six-wheeled rover landed at 9:05 p.m. PST in Meridiani Planum, NASA said. The smooth, flat plain lies 6,600 miles and halfway around the planet from where its twin, Spirit, set down on Jan. 3. Together, the twin rovers make up a single $820 million mission to determine if Mars ever was...