Ensign John Gay could see the fighter plane drop from the sky heading toward the port side of the aircraft carrier Constellation. At 1,000 feet, the pilot drops the F/A-18C Hornet to increase his speed to 750 mph, vapor flickering off the curved surfaces of the plane. Sonic Boom! We usually hear sounds that move through air, but sound can travel through any material. When sound travels through materials (like air), the molecules move back and forth. When an object vibrates, the air molecules around the object travel out in waves in all directions from the object. The speed at which sound moves depends on the material through which sound waves travel. Sound travels faster through liquids and solids than through gases because the molecules in liquids and solids are closer together. In air, the speed of sound varies depending on such things as altitude, density, and temperature. In normal air at room temperature, the speed of sound is 758 miles per hour (1220 kilometers per hour). When an object travels at the speed of sound, this speed is known as Mach 1. While it's true that the Space Shuttle travels many times faster than the speed of sound, the shuttle is traveling in a near vacuum (where there is an absence of matter). For sound to travel, there must be something to carry the vibrations--such as air. Because there's virtually nothing in space across which sound can be conducted, no shock waves are made and no sonic booms are created. In fact, it's completely silent out there! Visit pbs.org for more information. F/A-18 Hornet breaking the sound barrier