Polaris and the history of Britain's nuclear weapons

In December 1962, President John F Kennedy told the British Prime-Minister Harold Macmillan that the US would sell its most advanced nuclear missile technology to the UK - so long as it remained nominally a NATO deterrent capability.

Britain was given the then-revolutionary UGM-27 Polaris submarine-launched ballistic missile (SLBM) under the 1962 Nassau Agreement. The US agreed to sell the missile, launch compartment and guidance technology to Britain, which in turn would manufacture her own nuclear warheads and the submarines to carry the system.

This arrangement, formalised under the Polaris Sales Agreement on 6 April 1963, still provides the basis for Britain's Trident nuclear deterrent in the 21st century. However, the door to advanced American nuclear technology was not always open to Britain in this way.

In spite of an early British lead in the theoretical development of nuclear weapons during the Second World War, after American entry into the conflict and the subsequent birth of the Manhattan Project, Britain agreed to play a small but significant supporting role in the massive US effort which produced the world's first nuclear weapons.

After the atomic bombing of Hiroshima and Nagasaki, it was clear to the world's major powers that a nation without this technology would be all but helpless against a nuclear-armed foe.

Helped by spies in the Manhattan Project, the Soviet Union was not far behind the United States and, by 1949, had its own nuclear weapons.

	On December 21, 1962, Macmillan (l) and Kennedy announced Britain would join the Polaris programme [Getty]

However, much to British dismay, the United States passed the McMahon Act in 1946 which banned the transfer of Atomic materials and expertise to foreign nations, including Britain.

As a result, the Attlee government decided Britain had no choice but to develop its own independent nuclear weapons programme.

This produced the first British nuclear fission (atomic) bomb in 1952 - two and a half years behind the Soviet Union - and a true thermonuclear (hydrogen) bomb in the 'Grapple' trials in 1957-8. Early atomic weapons used fission - the splitting of heavy uranium or plutonium atoms - to produce explosions in the kiloton-range. A kiloton is the equivalent of the explosive force of a thousand tons of conventional TNT.

However, thermonuclear weapons use fission to trigger nuclear fusion - where light hydrogen atoms are fused together under intense temperatures and pressures. While much more complex to develop than atomic bombs, thermonuclear hydrogen bombs have no theoretical upper limit in terms of power and are often megaton-range; with the power of millions of tons of TNT.

After Britain showed it had mastered megaton-class thermonuclear weapons development in 1957, the Eisenhower administration passed the 1958 US–UK Mutual Defence Agreement which re-established transatlantic nuclear-weapons cooperation.

While it developed nuclear weapons during the late 1940s and 1950s, Britain also produced three different advanced jet-powered heavy bombers to carry them. These were the Vickers Valiant, Handley-Page Victor and Avro Vulcan; collectively known as the V-bombers. These bombers could fly at close to supersonic speeds and at high altitudes to targets in the Soviet Union.

They were the main British deterrent force during the late 1950s and early 1960s. Unfortunately, the high-flying bombers were increasingly vulnerable to Soviet developments in surface-to-air missile defences and supersonic jet fighters. They could also be destroyed on the ground at their bases with less than four minutes warning by Soviet intermediate range nuclear missiles in occupied East Germany.

Britain developed a land-based intercontinental ballistic missile (ICBM) called Blue Streak to compensate for the vulnerability of the V-bombers, but it was cancelled due to its high cost and vulnerability to destruction on the ground while being readied for launch in a crisis.

Britain instead hoped to procure an advanced air-launched nuclear missile from the United States for the V-bombers to carry, called Skybolt. When Skybolt was cancelled by the US after repeated test failures, the British government was left with few options and a huge gap in capability. Polaris filled that gap.

Unlike land-based bombers or ICBMs, submarine-launched ballistic missiles, such as Polaris and its successor Trident, are almost impossible for a potential enemy to destroy in a first strike. The nuclear submarines which carry and, if necessary, launch the missiles are quiet, fast and can stay underwater for months at a time.

	The US military upgraded from the Polaris missile (l) to the Poseidon, while Britain upgraded directly to Trident [Getty]

Polaris and Trident are powered by solid fuel and so can be stored ready-to-fire for years, unlike liquid-fuelled missiles which have to be stored empty and then fuelled up prior to launch - hardly ideal during a nuclear crisis or standoff.

ICBM and SLBM warheads are also extremely hard to intercept in flight due to the hypersonic speeds at which their warheads re-enter the atmosphere onto a target, and the fact that each missile carries multiple nuclear warheads.

Polaris was accurate to around 2-3km at its maximum range of 4,600km. This meant that the British and American submarines could patrol large potential launch zones in the arctic or open ocean while keeping many Soviet cities under threat to maintain deterrence. However, they were not accurate enough to credibly target specific military facilities as a first-strike capability, being designed instead to be able to threaten a level of devastation upon cities, even if a Soviet surprise attack destroyed all British and/or American land-based capability.

The UK, like the US, Soviet Union and France also developed and deployed smaller tactical nuclear weapons such as the WE.177 for use against battlefield targets, enemy submarines, and others for use by aircraft and even artillery units.

These were small bombs and depth charges using thermonuclear warheads with "moderate" kiloton-range yields. After the end of the Cold War, Britain retired the last of its tactical nuclear weapons in 1998.

In 1982, the existing Polaris Sales Agreement was used as the basis for the sale of the new American UGM-133A Trident II (D5) missile for Britain's next generation of SLBM-carrying submarines.

In contrast to Polaris, Trident D5 has over twice the range, is much faster in re-entry, carries up to twelve independently targetable warheads per missile, and is much more accurate. However, the principles remain the same; missiles with nuclear warheads on a nuclear-powered submarine that is very hard to detect and is constantly at sea ready to retaliate against a nuclear aggressor to the UK and/or NATO.

The safety of the system and its ability to guarantee the destruction of almost any target on earth regardless of where the submarine happens to be at the moment of launch, has enabled the UK to dispose of all its other nuclear weapons and reduce the number of warheads for Trident itself to between 140 and 160 operational weapons - a substantially lower number than any other nuclear nation.

Justin Bronk is a Research Fellow in Military Sciences at the Royal United Services Institute in London. Follow him on Twitter: @Justin_Br0nk