Aurora (also credited as the SR-91 Aurora) is the popular name for a hypothesised United States reconnaissance aircraft, believed by conspiracy theorists to be capable of hypersonic flight (speeds of over Mach 5). According to the hypothesis, the Aurora was developed in the 1980s or 1990s as a replacement for the aging and expensive SR-71 Blackbird. A British Ministry of Defence report, released under the Freedom of Information Act, from May 2006 refers to USAF priority plans to produce a Mach 4-6 highly supersonic vehicle. In September 2007, DARPA and the USAF signed a memo of understanding to build a Mach-6 unmanned aircraft called "Blackswift" under the Force Application and Launch from Continental United States (FALCON) program, but that does not explain the earlier reports. It is believed by some[who?] that the Aurora project was canceled due to a shift from spyplanes to high-tech unmanned aerial vehicles and reconnaissance satellites which can do a similar job as a spyplane, but with less risk of casualties or loss of highly expensive, sensitive equipment.