An Atlas 5 rocket blasted off with the unmanned space plane Saturday afternoon from Cape Canaveral.
The Air Force has launched a second experimental space plane that resembles a small shuttle.
Air Force officials aren't saying much about the X-37B orbital test vehicle. It's the second of its type to be launched. The first rocketed into orbit last spring. It landed in California in December following a 270-day mission.
The X-37B is 29 feet long with a wing span of 15 feet.
The Air Force says the newest craft will serve as a test platform for satellite sensors and systems. Officials say the voyage will build upon what was learned during the first mission, though they won't say what that was.
The ultimate purpose of the X-37B and details about the craft largely remain a mystery.