How A Satellite Is Launched

Satellites are launched into space by various types of rockets, called launch vehicles. A single launch vehicle can be used to place two or more satellites into orbit at one time. Some launch vehicles are expendablethey are designed to be used only once. The space shuttle, a type of manned spacecraft, serves as a reusable launch vehicle. A few satellites have been launched from high-flying airplanes.

A major consideration in the choice of a launch vehicle for a particular flight, or mission, is the amount of thrust generated by the vehicle. The greater the payload to be lifted and the higher the orbit, the greater the thrust must be. (The payload consists of the satellite, its protective coverings, and any other hardware, such as separation devices, and sometimes a rocket engine that boosts the satellite into its proper orbit.)

The launch vehicle may be a single-stage rocket (having one main set of rocket engines) with or without auxiliary boosters, or it may have two or three stages stacked one on top of another.

Preparation for a mission begins months in advance of the anticipated launch date. The launch vehicle must be assembled and its various partsincluding pumps, fuel tanks, engines, and steering mechanismstested. The satellite is checked to determine, as completely as possible, if it will function as planned in space. In this check the satellite is subjected to, among other things, intense vibration (simulating rocket flight), near vacuum, and extremes of heat and cold.

In the hours prior to a launch, many preparations and checks must be made. Launch vehicles that use liquid propellants must be fueled during this time. The sequence of events leading up to the launch is referred to as the countdown.

For a few seconds after the rocket engines of the launch vehicle are ignited, the vehicle is held down by restraints to permit the engines' thrust to build. The restraints are then released and lift-off occurs. The launch vehicle rises slowly at first, but rapidly gains speed as it gains altitude.

With a multistage launch vehicle, only the engines of the first stage are ignited for lift-off. After its fuel is exhausted, the first stage is jettisoned and the second stage is fired. Each stage is fired and jettisoned in turn. The space shuttle is a single-stage launch vehicle with three liquid-fuel rocket engines and two solid-fuel rocket boosters. The liquid-fuel engines are supplied with fuel from a large external tank. The boosters and the liquid-fuel tank are jettisoned before the shuttle reaches orbit.

A launch vehicle travels straight up only for the first few seconds of its flight. To enter earth orbit, the satellite must be traveling parallel or nearly parallel to the earth's surface at the end of powered flight. Shortly after lift-off, therefore, the launch vehicle begins to tilt over, entering a long curved path that will bring it to orbital altitude in a nearly horizontal position.

In addition to carrying a satellite to a given altitude, a launch vehicle must impart to the satellite enough horizontal speed to keep it in orbit. The speed required depends on the satellite's altitude; the lower the satellite, the faster it must travel to stay in orbit. For example, a satellite must travel at 17,253 miles per hour (27,766 km/h) to stay in a circular orbit at an altitude of 200 miles (322 km) and it must travel 15,802 miles per hour (25,431 km/h) to stay in a circular orbit at an altitude of 1,000 miles (1,609 km). In the lower orbit the satellite will circle the earth in 90.96 minutes; in the higher orbit, in 118.41 minutes.

The space shuttle is designed to orbit the earth at a relatively low altitudetypically about 200 miles above the earth. Satellites carried into orbit by the space shuttle can simply be released into space from the shuttle's cargo bay. These satellites continue in the same general orbit and can be recovered by later shuttle missions to service them or return them to earth. A satellite intended for an orbit higher than that of the shuttle is equipped with a small single-stage or multistage rocket of its own. The satellite is ejected from the cargo bay by a spring mechanism. The satellite's rocket is then fired to propel the satellite into its proper orbit.

Virtually all United States satellites are launched from either Cape Canaveral on the eastern coast of Florida or from Vandenberg Air Force Base on the southern coast of California. The Cape Canaveral launch site is used for launching satellites into orbits that circle the earth above or nearly above the Equator. They are launched eastward, across the Atlantic Ocean. Launching a satellite eastward takes advantage of the speed imparted to it by the earth's rotation. At the latitude of the cape, this speed is 900 miles per hour (1,448 km/h). The Vandenberg launch site is used for launching satellites southward into polar or near-polar orbits. Most United States military satellites are launched from Vandenberg.

The Commonwealth of Independent States also has two major launch sites. Most of the commonwealth's satellites with general west-to-east orbits are launched from a site in Kazakhstan east of the Aral Sea. Most satellites placed in polar or near-polar orbits are launched from a site in Russia south of Archangel, a city near the White Sea.

Other launch sites include one built by France near Kourou, French Guiana, and a Japanese launch site on Tanega Island in southern Japan.