Opportunity, also known as MER-B (Mars Exploration Rover – B) or MER-1, and nicknamed "Oppy", is a robotic rover that was active on Mars from 2004 until the middle of 2018. Launched on July 7, 2003, as part of NASA's Mars Exploration Rover program, it landed in Meridiani Planum on January 25, 2004, three weeks after its twin Spirit (MER-A) touched down on the other side of the planet.