Hypersonic speed commonly refers to travel at five times the speed of sound or faster, although there is no sharp boundary between supersonic flight—defined precisely as traveling faster than the speed of sound (Mach 1)—and hypersonic flight. Instead, hypersonic speed is more nebulously defined as traveling fast enough so that strong shock waves and friction in the gas through which a body travels raise the gas temperature high enough that it begins to chemically react (for example, oxygen and nitrogen dissociate from naturally occurring diatomic molecules into atomic species), and at higher speeds even to ionize (when electrons are stripped from gas molecules) and emit significant radiation. At hypersonic speed the heating of vehicle body surfaces due to air friction also becomes significant and can heat those surfaces to extreme temperatures.