Trident II D5 is a submarine-launched ballistic missile system that constitutes the UK's nuclear deterrent.
Developed and manufactured by Lockheed Martin in the United States, Trident entered service with the Royal Navy in 1994, 14 years after it was selected as the replacement for the submarine-launched Polaris missile.
Each Trident missile has a range of up to 7,500 miles (12,000km) and is accurate to within a few feet. Their destructive power is estimated as the equivalent of eight Hiroshimas.
The UK deploys 16 Trident missiles on each of its four Vanguard-class submarines, of which one is on patrol at all times. The fleet is based at Faslane in Scotland.
A further 70 missiles can be accessed from a communal pool at the Strategic Weapons facility in Georgia in the United States, where the missiles are also periodically serviced.
Each Trident missile is designed to carry up to 12 nuclear warheads, but the Royal Navy's are armed with three after the 1998 Strategic Defence Review imposed a limit of 48 per submarine.
All the UK's warheads are built at the Atomic Weapons Establishment in Aldermaston, Berkshire.
During a Vanguard patrol, the missile payload is carried upright in launch tubes behind the submarine fin, or conning tower.
At launch, the pressure of expanding gas in the tube forces the missile out and to the ocean surface where, once it is far enough from the submarine, the solid fuel in the first of three stage motors ignites.
At the same time, an aerospike designed to reduce drag by around 50% extends from the tip of the missile.
Once the missile reaches space, its stellar sighting guidance system takes a reading from the stars to work out the missile's position and make any adjustments necessary to the pre-programmed route to its target area.
A second - or boost stage - rocket then fires, followed by the third stage. Within approximately two minutes from launch the missile is travelling at over 20,000ft (6,100 metres) a second.
Once in position over its targets, the missile's third motor separates from the forward section containing the warheads.
The guidance system takes another star reading to confirm its position.
Small thruster rockets then manoeuvre the forward section so each warhead can be individually released in the right place to freefall to its target, where they detonate according to one of a number of pre-set fuse options.
In the UK, the authority for a real (rather than test) Trident launch would have to come from the prime minister via a secure communications network.
Trident has a 30-year lifespan that is due to end in 2024. The UK will need to take a decision soon on whether to extend Trident's lifespan or replace it with an alternative system.