It seems like this same question has to be answered every time North Korea threatens to fire a long range missile.  According to the article the US military will not shoot down the missile unless it threatens an area protected by US missile defenses.  North Korea has historically fired their long range missiles on test trajectories out into the ocean.  These tests allow intelligence agencies to collect information on the Kim regime’s progress in developing their missiles:

South Korean Defense Ministry retrieved an object believed to be a part of North Korean rocket, which was launched on February 7, 2016.

While the US said this week it would shoot down any North Korean missile that posed a threat, don’t expect to see Pyongyang’s next launch blasted out of the sky.

US Defense Secretary Ash Carter said Tuesday that if launches do not pose a risk to the US or an ally, “it may be more to our advantage… to gather intelligence from the flight.”
The US and other observers “can learn a lot” from any missile test, said Tal Inbar, a North Korea expert at the Fisher Institute for Air and Space Strategic Studies.
“We can analyze the trajectory and conclude some insight about the power of the engines and the amount of fuel, and estimate the potential range of the missile.”
If it is possible to retrieve the missile or rocket from the sea, as South Korea did in February last year, Inbar said there “is a wealth of intelligence in such debris.”
He added that it was generally unwise to shoot down any missile that does not pose a threat as not only would you lose the ability to examine the missile and its flight, “if you try and shoot it down and miss, that’s a huge PR embarrassment.”  [CNN]
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