What can North Korean missiles hit? Can we stop them?

As Pyongyang's warlike rhetoric continues, South Korean government sources have suggested that North Korea's next moves may include a ballistic missile test, which could take place as soon as 10 April. New Scientist assesses the threat from North Korea's arsenal of rockets.

If North Korea does conduct a missile test this week, what is likely to be involved?
Most speculation surrounds a missile known as the Musudan. This is based on the Soviet R-27, designed in the 1960s to be launched from submarines. The North Korean variant, displayed at a military parade in Pyongyang in October 2010, would be deployed from a large vehicle.

The Musudan would be an important advance over North Korea's workhorse ballistic missile, the No-dong, which has a range of up to 1300 kilometres. No-dongs are fuelled by kerosene, but the Soviet missile on which the Musudan is based used a more potent fuel: unsymmetrical dimethylhydrazine.

If the North Koreans have perfected the technology to use this fuel, the Musudan could have a range of 2500 to 4000 kilometres, which might just put the US Pacific territory of Guam in its sights. Test-fired from North Korea's east coast, a missile with this range would fly high over Japan and out into the Pacific Ocean.

Has the Musudan flown before?
Some reports have suggested that the Musudan, also known as the No-dong B, was tested from Iran in 2006, but most security experts regard this as speculation. It seems unlikely that Pyongyang would use an untested missile for a pre-emptive military strike. "It could blow up on the launch pad," says David Wright of the Union of Concerned Scientists in Cambridge, Massachusetts. So a test flight seems like the obvious next step.

Does North Korea have any other missiles up its sleeve?
In April 2012, six new missiles, dubbed the KN-08, were displayed at a parade in Pyongyang. At first glance, these looked like multi-stage intercontinental ballistic missiles, with a range of up to 10,000 km ? which would put many US cities in range.

However, two rocket engineers, Markus Schiller and Robert Schmucker of the consulting firm Schmucker Technologie in Munich, Germany, scrutinised images of the parade and concluded from subtle variations between the missiles that they were probably mock-ups. The KN-08, the consultants said, was a "dog and pony show" intended to confuse foreign analysts.

North Korea recently put a satellite into orbit. Doesn't that suggest it can build an intercontinental ballisitic missile?
If anything, the successful launch of a satellite into low-Earth orbit in December 2012 on its Unha-3 rocket indicates that Pyongyang may be further from building an ICBM than was previously thought.

South Korea recovered pieces from the rocket's first stage, which showed it to be powered by a cluster of four No-dong engines. The Unha-3 might still be an ICBM in civilian space clothing, if its second stage was based on the Musudan, as had been thought. But its performance suggests that it relied on a Scud-class engine, smaller than those used on the No-dong.

That's fine for lofting a small satellite, but of no use for sending a warhead across the Pacific. "The configuration they flew was not what you'd expect to see for a ballistic missile," says Wright.

But even No-dongs could reach cities in South Korea and Japan. Could one of those be fitted with a nuclear warhead?
Some security experts believe that North Korea probably could put a nuclear weapon on a No-dong.That would help explain the low explosive yields of its 2006 and 2009 nuclear tests: rather than indicating faltering progress in building a large Manhattan Project-style bomb, Pyongyang may have been trying from the start to build a device small enough to put on a missile.

So could the US and its allies shoot down a No-dong?
US officials seem confident that they can. The main line of defence would the Aegis antimissile system, deployed on both US and Japanese ships, which sends an interceptor to destroy an incoming missile while it is still above the atmosphere.

According to the US Missile Defense Agency, in tests Aegis has eliminated a target missile in 24 of 30 attempts. It was also used in February 2008 to destroy a failed US spy satellite that was slowly falling back to Earth and so posed a small threat to Pacific islanders.

That sounds reassuring. But do the tests prove that the system would work in the event of a real attack?
We don't know for sure. "These tests are designed to succeed," says George Lewis of the Judith Reppy Institute for Peace and Conflict Studies at Cornell University in Ithaca, New York. Shooting down a target you've launched yourself under controlled conditions is quite different from responding to a surprise attack, he points out.

According to Ted Postol of the Massachusetts Institute of Technology, North Korea probably won't deploy sophisticated antimissile countermeasures, such as decoy warheads. But he says that it wouldn't be hard for North Korean engineers to cause the spent missile body to break into a series of pieces. It remains unclear from the tests conducted so far whether the Aegis system could then identify and target a warhead from the multiple objects falling back to Earth. "There are so many unknowns," Postol says.

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