It is boosted into space by a launch vehicle, then re-enters Earth's atmosphere and lands as a spaceplane. The X-37 is operated by the United States Air Force for orbital spaceflight missions intended to demonstrate reusable space technologies. It is a 120%-scaled derivative of the earlier Boeing X-40. The X-37 began as a NASA project in 1999, before being transferred to the U.S. Department of Defense in 2004. It is capable of long duration flights for classified purposes, likely including materials testing. The X-37 launches atop an Atlas V 501. Dream Chaser is under development for NASA's Commercial Resupply Services program. In December 2010, Sierra Nevada Corporation made a commercial proposal to NASA to develop the lifting-body Dream Chaser in response to NASA's Commercial Crew Development (CCDev) solicitation. The vehicle would be launched on a human-rated Atlas V rocket but would land on a runway. Three piloted suborbital rocket powered aircraft have reached space: the North American X-15, SpaceShipOne, and SpaceShipTwo. All these crafts were not capable of entering orbit and were first lifted to high altitude by a carrier aircraft. On 7 December 2009, Scaled Composites and Virgin Galactic unveiled SpaceShipTwo, along with its atmospheric mothership "Eve". On 13 December 2018 SpaceShipTwo VSS Unity successfully crossed the boundary of space. The Mikoyan-Gurevich MiG-105 was an atmospheric prototype of an intended orbital spaceplane, with the suborbital BOR-4 subscale heat shield test vehicle successfully reentering the atmosphere before program cancellation. HYFLEX was a miniaturized suborbital demonstrator launched in 1996, flying to 110 km altitude, achieving hypersonic flight, and successfully reentering the atmosphere. Various types of spaceplanes have been suggested since the early twentieth century. Notable early designs include a spaceplane equipped with wings made of combustible alloys that it would burn during its ascent, and the Silbervogel bomber concept. World War II Germany and the postwar US considered winged versions of the V-2 rocket, and in the 1950s and '60s winged rocket designs inspired science fiction artists, filmmakers, and the general public. The U.S. Air Force invested some effort in a paper study of a variety of spaceplane projects under their Aerospaceplane efforts of the late 1950s, but later reduced the scope of the project. The result, the Boeing X-20 Dyna-Soar, was to have been the first orbital spaceplane, but was canceled in the early 1960s in lieu of NASA's Project Gemini and the U.S. Air Force's crewed spaceflight program.