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What is an ICBM and how would the US defend against it?

North Korea has finally announced it has developed a missile capable of hitting the United States. Pyongyang has reported that their latest Intercontinental Missile (ICBM), which was launched in the early hours of Wednesday, 29th November South Korean time, was its most powerful yet. The US Government has confirmed that the trajectory of the new missile launch was 8,100 miles, just enough to hit the United States.

This comes after a summer of heightened tension, with North Korea announcing it would strike American territory Guam in August, although this came to nothing. In Hollywood style, president Donald Trump retaliated with aggressive and chilling rhetoric.

Here, WIRED takes a closer look at ICBMs and what nuclear missile strike would actually look like.

What is an Intercontinental Ballistic Missile?

As opposed to bombs, torpedoes and rockets, missiles are guided. If unguided, it would technically be a rocket. A bomb is neither powered nor guided whilst a torpedo is powered from underwater.

Unlike a regular missile, Ballistic Missiles can travel along a suborbital trajectory just above the Earth's atmosphere, allowing it to travel much further than any other missile.

Intercontinental Ballistic Missiles (ICBMs) have a range of 6,000-plus miles (that's London to New York) at the speed of approximately 15,000 mph. The ICBM can consequently travel much further around the Earth than any other missile.

Three phases of deployment:

Phase 1: Ignition

First, the missile is aimed towards the target. Once the missile is ignited it cannot be extinguished. If unprotected personnel are nearby at the point of detonation, small levels of hydrochloric acid could be inhaled by them, but it is unlikely that much more than eye and upper airway irritation will be experienced, according to John E. Pike, who now directs GlobalSecurity.org.

Once in the air, the missile is aimed towards the target. Ten seconds later, the first roll manoeuvre occurs, meaning the missile turns 45 degrees on its axis.

After 19 seconds, the missile is at an altitude of 8,300ft.

By 39 seconds, the missile is at 38,000ft. At 45 seconds, the second roll manoeuvre occurs, turning the missile 45 degrees again.

Phase 2: Separation

By 62 seconds, the second phase begins. The missile begins to separate and the skirt of the missile starts to detach from the middle body and top shroud.

By 74 seconds the missile is now at 100,000ft.

After 121 seconds, the shroud is jettisoned from the main body of the vehicle, leaving just the nuclear warhead attached to it. The shroud and the warhead now travel together as the separate reentry vehicle at 315,000ft, which will now return to the atmosphere. The post boost vehicle now acts as the engine that steers the reentry vehicle and puts it on a more accurate ballistic path. It can also be used just before the reentry vehicle comes back into the atmosphere to correct any errors in the flight path that have occurred.

The reentry vehicle now begins to drop to 240,000ft.

Phase 3: Termination

The reentry vehicle (the shroud and the warhead) manoeuvres again in space to focus on its target as it re-enters the atmosphere.

That is when the spin-gas generator, located inside the reentry vehicle, is fired up. This acts to stabilise the the warhead as it plummets back down to Earth.

The final stage is detonation on impact.

How would the United States respond to an ICBM deployment?

If North Korea has the capability to use ICBMs and it did, in fact, aim them at Guam or any other base, the US would first attempt to intercept the incoming ballistic missile(s) with missile defences. However, Patricia Lewis, the research director of International Security at Chatham House explains, "this type of interception is notoriously difficult and cannot be relied on".

Bruce Blair, nuclear security expert and a research scholar at the Program on Science and Global Security at Princeton University, also warns that it is unlikely the US would respond with its own ICBM. "ICBMs would have to fly over Russia and China to reach North Korea. There is no way we would do that lest we trigger a mistaken retaliation by one or both of these countries," Blair explains.

Instead, US Trident D-5 missiles on submarines would be the most effective retaliation to a missile strike by North Korea. Blair advises that these strategic bombers would execute any war plan against North Korea.