Flight has been divided into four main speed groups, subsonic, transonic, supersonic, and hypersonic. All of the speeds of flight are in reference to the speed of sound which is 340 meters per second or about 761 miles per hour, this speed is also referred to as mach 1. Subsonic flight has to do with flying at any speed below the speed of sound. Transonic flight is speeds between mach 0.8 and mach 1. Supersonic flight is defined as flying at speeds above mach 1. Hypersonic flight is any speed above mach 5.5 or about 4200 miles per hour. The majority of passenger and commercial aircraft such as the Boeing 737-900 (pictured above center) fly at transonic speeds while military jets  like the F-35 Lightning (pictured above right) fly at supersonic speeds. The number of hypersonic aircraft is limited to a very small handful which includes such planes as the North American X-15 (pictured above left) and the space shuttle on reentry. As the speed of an aircraft increase beyond mach 1, complications begin to arise with factors such as drag, lift, heat expansion, and thrust.