The Mars rover Opportunity resumed rolling freely across the Martian surface Saturday after scientists freed it from a sand dune where it had been mired for nearly five weeks, NASA officials said. Engineers at the Jet Propulsion Laboratory, which manages the mission, cheered when images beamed back to Earth showed the rover's wheels were free. "We've got a working rover on Mars that cost $400 million to build and ... keep working," project manager Jim Erickson said. "I'd like t...