THE TRIDENT NUCLEAR MISSILE SYSTEM
The Trident missile, named after the trident, is an intercontinental ballistic missile (ICBM) which is armed with nuclear warheads and is launched from submarines (SSBNs), making it a SLBM.

The Trident was built in two variants: the I (C4) UGM-96A and II (D5) UGM-133A. The C4 and D5 designations put the missiles within the "family" that started in 1960 with Polaris (A1, A2 and A3) and continued with the 1971 Poseidon (C3).

Both Trident versions are three-stage, solid-propellant, inertially guided missiles whose range is increased by an aerospike, a telescoping outward extension that halves frontal drag.

The Trident is carried by fourteen active US Ohio class submarines and (with British warheads) four UK Vanguard class submarines.

The launch from the submarine occurs below the ocean surface. The missiles are ejected from their tubes by gas pressure created by a "gas generator", a solid-fuel rocket motor attached to the bottom of the missile tube which heats a pool of water creating steam. After the missile leaves the tube and rises through the water over the submarine, the first stage motor ignites, the aerospike extends, and the boost stage begins. Ideally, the missile is "sheathed" in gas bubbles for its entire time in the water, so liquid never touches its fuselage. Within about two minutes, after the third stage motor fires, the missile is traveling faster than 20,000 ft/s (6,000 m/s).

Trident I (C4) was deployed in 1979 and phased out in the 1990s and early 2000s. Trident II (D5) was deployed in 1990; it is planned to be in service past 2020. As of 2005, a decision is expected soon about whether or not to replace the UK's missiles and submarine fleet. Critics from the peace movement and within the military establishment have questioned the usefulness of such a weapon in the current military climate. The use of (or threat of use of) such weapons would generally be contrary to the rules of international law applicable in armed conflict, according to an Advisory Opinion issued by the International Court of Justice in 1996.