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.
This object is on display in Rockets & Missiles at the Steven F. Udvar-Hazy Center in Chantilly, VA.