SOFIA, the Stratospheric Observatory for Infrared Astronomy, is an airborne infrared astronomy observatory that is an 80/20 joint project of NASA and the German Aerospace Center (DLR). It is based on a Boeing 747SP wide-body aircraft that has been modified to include a large door in the aft fuselage that can be opened in flight to allow a 2.5 m (8.2 ft) diameter reflecting telescope access to the sky.