[betterunix2]

NK does not announce where their missiles are going, and NORAD probably wanted to clear some airspace to better track the missile, especially given that NK has missiles that can reach the west coast. The military also probably wanted to avoid dangerously confusing situations, like a civilian aircraft failing to squawk as expected or even a civilian plane crashing (just imagine what might happen -- NK is launching a missile, and suddenly NORAD is also tracking another object that seems to be an airplane without a transponder heading directly toward the Bay Area or receiving a call that some kind of explosion was just reported in LA following the object rapidly losing altitude...). It is not as if confusion during a tense moment leading to needless death is unprecedented in the history of aviation:

https://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007

[dragonwriter]

> NORAD probably wanted to clear some airspace to better track the missile

I can't see any way planes in Western US airspace could possibly interfere with tracking an NK launched ICBM, even by clutter, except maybe in the last minutes of the terminal phase of warhead descent at which point any defense (active or civil or both) would already have to have been committed to do any good.

Grounding planes is to mitigate risk to the people on the planes, and maybe to clear lines of fire for anti-missile systems (which is mostly about protecting people who aren't on the planes, but incidentally also protects those on the planes, too.)

[betterunix2]

Like I said, what if one of those planes had a malfunctioning transponder? It is a somewhat stretched scenario, but the NK military might have gotten bold and decided to shoot a missile all the way to the boundary of US territorial waters or airspace. In the middle of nervously tracking a missile that seems to be headed directly toward the west coast, NORAD suddenly sees another object that looks just like an airplane heading toward LA or SF, but it is not identifying itself. Now NORAD personnel have to take time to figure out if this airplane is just a malfunctioning civilian plane or some kind of NK attack.

Sure, NORAD probably tracked the plane from whatever airport it departed from, but during the missile test they probably want to have their personnel focus every second of their attention on the missile and not on checking some flight path.

[bubbleRefuge]

Maybe it was because we have a secret Anti-Ballistic Missile system that "throws darts" at inbound missiles and we don't want any planes in the cross fire ?

[Johnny555]

NORAD probably wanted to clear some airspace to better track the missile

But NORAD said they didn't ask for it:

The official says it was not a national ground stop and may have been issued by a regional air traffic control facility.

“No warning was issued by NORAD HQ,” regarding a potential threat to the US, according to Captain Pamela Kunze, the chief NORAD spokesperson.

[lobocinza]

The radar signature, altitude and speed would be completely different than any civilian aircraft. That was a mach 10 missile. For reference SR-71, the fastest plane that entered service (at least of those we know) has a speed of 3.2+ mach.

[betterunix2]

...because there is no way that a country would try to use a missile launch as a distraction while conducting an actual attack with an airplane...

[lobocinza]

In war and love everything is possible.