Trying to shoot down an incoming missile or warhead with a physical projectile, by contrast, is much more difficult.
While U.S. officials said there is "nothing to indicate" that North Korea can or will place a nuclear warhead atop a missile right now, most experts say it is very difficult to know the warhead and payload of a missile test in advance.
The latest, smaller version of the missile does have a warhead but is meant to destroy its target primarily by simply hitting it.
Pentagon officials promised to press forward with plans for more tests of the system after an interceptor missile hit a dummy warhead over the Pacific Ocean on Saturday.
In 2007 America concluded, controversially, that Iran had been developing a nuclear warhead for a missile, but had stopped in 2003.
It is not thought to have a missile capable of carrying a nuclear warhead.
Friday's successful advanced missile defense test, which shot down a mock nuclear warhead in space, shows that the US is getting closer to defending the country against ballistic missile attack.
The New Start treaty limits each side to no more than 800 deployed nuclear warhead delivery systems including bombers, missile launchers and nuclear submarines - a cut of about 50%.
The US and other regional allies fear North Korea is working to develop a nuclear warhead small enough to arm a missile, though it is not believed to have achieved this yet.
The US and North Korea's neighbours fear Pyongyang's ultimate goal is to put a nuclear warhead on a long-range missile that could target the west coast of the US, but it is not believed to have mastered the technology yet.
BBC: North Korea in nuclear warning after UN rocket resolution
Regional neighbours and the US fear Pyongyang is working to build a nuclear warhead small enough to put on a missile, but believe it does not yet have the capabilities to do so.
Defense officials now believe that North Korean missiles can reach Hawaii as well as Alaska, but not the continental U.S. At the same time, U.S. officials don't believe North Korea has developed a miniaturized nuclear warhead that could be mounted on any missile.
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An interceptor missile launched over the Pacific Ocean, designed to destroy a dummy warhead, failed to separate from its booster rocket early Saturday.
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America, meanwhile, has again been showing around documents that purport to be Iranian design work on a missile nose-cone of a sort that that could carry a nuclear warhead.
This capacity is also increased because the new, smaller missiles have radars that can distinguish between different parts of a target missile, ensuring that if the target has broken up, the Patriot attacks the warhead rather than the debris.
But a nuclear warhead would have to be small enough to fit onto, say, a Nodong missile - and it would have to be capable of withstanding the buffeting and other forces exerted on it during flight.
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