The Microsoft co-founder billionaire Paul G Allen's Stratolaunch, a massive aircraft designed to launch rockets into space from high altitude, has been rolled out of its hangar for the first time in preparation for testing. Airborne launches date back decades, most famously to the X-15 program of the 1950s and '60s, when manned rocket planes were carried aloft under the wing of a B-52 bomber and released on hypersonic research flights. In the 1990s, Orbital Sciences Corp. (now Orbital ATK) began releasing rockets from the belly of a converted L-1011 airliner to put small satellites into low Earth orbit. More recently, Paul Allen, the Microsoft co-founder, funded development of the Burt Rutan-designed SpaceShipOne, the first privately developed manned spacecraft. Here are things to know about the program that has been underway since 2011.