By David Hambling, technology expert
An aircraft carrier is a potent tool for force projection, a mobile air base to provide airpower wherever it is needed.
The Royal Navy refers to the new carrier HMS Queen Elizabeth as "four acres of sovereign territory, deployable across the globe to serve the United Kingdom", and other nations measure their strength by the number of carriers they can operate.
However, aircraft carriers are part of a bigger picture and they are far from invulnerable.
Queen Elizabeth has just started 11 weeks of flight trials off the east coast of the US.
This marks the first time that a fast jet has operated from a British warship for eight years; the last time it was a Harrier take-off from HMS Ark Royal, both now retired.
The UK’s new aircraft are F-35B Lightning IIs - stealthy, fifth-generation jets which are extremely difficult to detect on radar.
It can carry out attack missions without being spotted by surface-to-air missiles operators, and in air-to-air combat an F-35 pilot will see the enemy first, and can choose to evade or attack before the opposition is even aware they are there.
Queen Elizabeth will initially host 24 F-35Bs (one squadron) but could eventually have at least twice that number.
An entire squadron will be able to get airborne in just 15 minutes via the ski jump ramp.
HMS Queen Elizabeth will also have a complement of helicopters, the mix depending on the mission.
For blue-water operations, these might include Merlin anti-submarine helicopters as well as Merlin Crowsnest airborne early warning helicopters.
If the requirement is to support land operations, then the helicopter fleet could comprise Chinook heavy transports, Apache gunships and Lynx Wildcat multirole helicopters.
While HMS Queen Elizabeth is in America, the US aircraft carrier USS Harry S Truman is visiting Portsmouth after operations in the Mediterranean and Arabian Gulf.
The contrast between the two ships says something about the approach of the two nations.
And, while there continues to be debate in the UK over whether the £3.1bn cost of Queen Elizabeth is good value, the £10bn price tag for the latest US carrier went practically without comment.
Air power is a key component of modern warfare. In a sense, whoever controls the air controls the land or sea below.
Hence the ability to sail a carrier to any trouble spot makes it a vital tool in projecting power.
But the dominance of aircraft carriers has of course led to weapons to sink them.
These ‘carrier killers’ include submarines like Russia’s Antey-class, cruisers and corvettes packed with guided missiles, and, more recently China’s giant DF-21 and DF-26, land-based ballistic missiles with a range of more than 2,000 miles specifically designed to target ships.
A one-tonne warhead moving at several thousand miles an hour could do huge damage even to something the size of an aircraft carrier.
Queen Elizabeth has limited defensive armament – a trio of ‘Dalek’ computer-controlled guns for shooting down incoming missiles at close range – plus a decoy system which launches countermeasures which can confuse both missiles above the water and torpedoes below it.
However, an aircraft carrier will never sail alone, but will be escorted by other ships as part of a carrier strike group.
HMS Queen Elizabeth and will typically be accompanied by several Type 45 destroyers and Type 23 frigates, as well as one or more Astute-class hunter-killer submarines.
These will deal with any incoming threats, whether they are enemy ships, aircraft, missiles or submarines.
In recent conflicts, aircraft carriers have been able to stay well away from the conflict zone, sending their squadrons to bomb relatively unequal opponents.
Although Royal Navy carriers came under attack off the Falklands in 1982, the last carrier to be sunk was the Japanese Amagi in 1945.
There has not been any test of modern aircraft carriers against modern threats in combat conditions for decades, and the technology on both sides has moved on tremendously.
Some critics claim that HMS Queen Elizabeth is a 'floating target' without adequate protection and that more escorts are needed.
Others maintain that the new carrier is essential to safeguard UK’s interests and will act as a conspicuous presence as well as a deterrent to potential opponents.
The debate will only be resolved when the new carrier is battle-ready and taking part in operations.