The US has the technology to shoot down a North Korean missile but the success of its defence systems can never be guaranteed, a leading expert claims.

Global security expert Joe Cirincione said it was time to have an honest conversation about the reliability of the world's missile defence systems.

Writing in Defense One, the president of public grantmaking foundation Ploughshares Fund explained some of the flaws in missile defence technology and argued the main reason the US and its allies do not shoot down North Korea's weapons is simply because they can't, reports News.com.au.

The author of Nuclear Nightmares: Securing the World Before It Is Too Late, wrote there was no chance Japan or the US could have shot down the North's most recent missile.

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That missile flew over the island of Hokkaido, landing roughly 3700km from its launch site.

South Korea's military reported the weapon reached an altitude of about 770km and was in the air about 19 minutes.

Mr Cirincione argued none of the theatre ballistic missile defence weapons in existence are capable of reaching a missile that high.

"It is hundreds of kilometres too high for the Aegis interceptors deployed on Navy ships off Japan," he wrote.

"Even higher for the THAAD systems in South Korea and Guam. Way too high for the Patriot systems in Japan, which engage largely within the atmosphere."

In a video posted to Twitter, he goes on to explain we could shoot one down if the missile wasn't flying as high, or if the North Koreans aimed it directly at the missile defence system.

He also explained things like rain, lack of light and decoys could interfere with the defence system's success.

'Take a shot'

Research Associate at California's James Martin Centre for Nonproliferation Studies Shea Cotton said the points Mr Cirincione raised were fairly accurate.

Mr Cotton said the systems in place across Japan, South Korea and Guam are designed to protect small areas against direct attack rather than large swathes of the ocean.

"If we had a system in the ocean near where the missile came down, that might be able to make an attempt but otherwise no," he said.

Mr Cotton said it would be both interesting and scary to see what would happen if North Korea ever followed through on its threat to bracket Guam with missiles.

"Theoretically, the THAAD systems there should be able to take a shot at it," he said.
"I don't think they will, but they might."

Mr Cotton pointed out that out of all our systems, the Terminal High Altitude Area Defense (THAAD) has the most impressive success rate.

"But it has never been tested (to my knowledge) under actual combat conditions like the sort it would be expected to face in the event of a DPRK launch," he said.

The Terminal High-Altitude Area Defense system, or THAAD, as pictured in Seongju, South Korea earlier this month. Photo / AP
The Terminal High-Altitude Area Defense system, or THAAD, as pictured in Seongju, South Korea earlier this month. Photo / AP

Mr Cotton also highlighted how when it comes to shooting down a missile there remained a lot of coulds and shoulds.

"Shooting down a DPRK missile might prompt North Korea to build even more missiles or deploy countermeasures designed to thwart missile defence systems," he said.

"If it doesn't work it'll be really embarrassing to the US.

"Also I'm worried if it works that it would give US policy makers way too much confidence in a system that is only partially tested and is only capable against very specific threats."

'Not a write off'

Dr Euan Graham, International Security Program director at the Lowy Institute, said while missile defence wasn't an exact science we shouldn't dismiss it as a write off.

Dr Graham said each system had its strengths and overlapping capabilities and one system could potentially take down a missile when another system couldn't.

"The Aegis system has been designed to intercept a missile up in space so can take a shot at this but it's technically difficult and also has a reasonable risk of failure," he said.

However, Dr Graham also said if Japan or the US tried to shoot down a missile and it failed there would be serious questions raised about the billions invested in missile defence technology.

"Missile defence systems do have a useful role," he said.

"It's often a competition between offence and defence."