This is the Pegasus XL, a version of the U.S.'s first air-launched orbital launch vehicle. The Pegasus is carried by a modified Lockheed L-1011 or other plane up to its launch altitude of 12,000 m. (39,000 ft.). The three-stage, solid-fuel Pegasus is then released, and its rocket engine ignites and accelerates it to orbital velocity. It can place a small 450 kg (1,000 lb)-class satellite into Earth orbit.
Developed by Orbital Sciences Corporation, Pegasus was first used in 1990 and has since launched dozens of satellites. This vehicle includes the wing of a Pegasus flown into space and recovered in 2000, as well as the first-stage rocket motor used in ground testing the XL version in 1994. This object was donated to the Smithsonian in 2004 by Orbital Sciences.