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In modern language, a missile, also known as a guided missile, is a guided airborne ranged weapon capable of self-propelled flight usually by a jet engine or rocket motor. Missiles have four system components: targeting/guidance system, flight system, engine and warhead. Missiles come in types adapted for different purposes: surface-to-surface and air-to-surface missiles (ballistic, cruise, anti-ship, anti-tank, etc.), surface-to-air missiles (and anti-ballistic), air-to-air missiles, and anti-satellite weapons.
Airborne explosive devices without propulsion are referred to as shells if fired by an artillery piece and bombs if dropped by an aircraft. Unguided jet-propelled missiles are usually described as rocket artillery.
In ordinary language, the word missile can refer to any projectile that is thrown, shot or propelled toward a target.
The first missiles to be used operationally were a series of missiles developed by Nazi Germany in World War II. Most famous of these are the V-1 flying bomb and V-2 rocket, both of which used a mechanical autopilot to keep the missile flying along a pre-chosen route. Less well known were a series of Anti-Ship and Anti-aircraft missiles, typically based on a simple radio control (command guidance) system directed by the operator. However, these early systems in World War II were only built in small numbers.
The most common method of guidance is to use some form of radiation, such as infrared, lasers or radio waves, to guide the missile onto its target. This radiation may emanate from the target (such as the heat of an engine or the radio waves from an enemy radar), it may be provided by the missile itself (such as a radar), or it may be provided by a friendly third party (such as the radar of the launch vehicle/platform, or a laser designator operated by friendly infantry). The first two are often known as fire-and-forget as they need no further support or control from the launch vehicle/platform in order to function. Another method is to use a TV guidance, with a visible light or infrared picture produced in order to see the target. The picture may be used either by a human operator who steering the missile onto its target or by a computer doing much the same job. One of the more bizarre guidance methods instead used a pigeon to steer a missile to its target. Some missiles also have a home-on-jam capability to guide itself to a radar-emitting source. Many missiles use a combination of two or more of the methods to improve accuracy and the chances of a successful engagement.
Another method is to target the missile by knowing the location of the target and using a guidance system such as INS, TERCOM or satellite guidance. This guidance system guides the missile by knowing the missile's current position and the position of the target, and then calculating a course between them. This job can also be performed somewhat crudely by a human operator who can see the target and the missile and guide it using either cable- or radio-based remote control, or by an automatic system that can simultaneously track the target and the missile. Furthermore, some missiles use initial targeting, sending them to a target area, where they will switch to primary targeting, using either radar or IR targeting to acquire the target.
Whether a guided missile uses a targeting system, a guidance system or both, it needs a flight system. The flight system uses the data from the targeting or guidance system to maneuver the missile in flight, allowing it to counter inaccuracies in the missile or to follow a moving target. There are two main systems: vectored thrust (for missiles that are powered throughout the guidance phase of their flight) and aerodynamic maneuvering (wings, fins, canard (aeronautics), etc.).