The United States placed its interceptor missiles in Alaska and California on alert for the long-range North Korean missile that failed less than a minute after being launched, U.S. officials said on Wednesday.
The launch of Pyongyang's intercontinental Taepodong 2 missile, and at least five shorter-range missiles, marked the first time the fledgling U.S. anti-missile shield has been officially reported to have been primed in response to a specific event.
The U.S. Northern Command, which operates the interceptors in Alaska and California as part of its homeland defense mission, said it was "able to determine quickly the missiles posed no threat to (the) United States or its territories."
The Northern Command said its personnel detected the launches immediately after North Korea's first series of test-firings.
Bryan Whitman, a Pentagon spokesman, told reporters, "What I will tell you is that each and every launch was detected and monitored and that the interceptors were operational during the missile launches that took place."
The anti-missile shield includes early-warning satellites for launch detection, ground-based and sea-based radar stations for surveillance and tracking, the interceptors in California and Alaska silos to destroy incoming warheads plus command nodes in Alaska and Colorado.
The shield, while still being tested, has been made "operational" many times since the end of 2004 but is never known to have been activated in response to a perceived threat.
In a typical engagement scenario, infrared sensors aboard satellites detect the hot plume of a missile launch and alert operations centers of a possible attack.
Land- and sea-based sensors are then directed to search for the incoming missile and pick out the warhead from among decoys and space debris, according to the U.S. Government Accountability Office, the investigative arm of Congress.
Based on this tracking data, an interceptor, consisting of a barrel-sized "kill vehicle" mounted atop a booster rocket, may be launched to engage the threat -- a decision that must be made within minutes.
In the case of the Taepodong 2, which fell harmlessly into the Sea of Japan about 40 seconds after launch, there was "far too little time to make a decision on whether to shoot it down," said Victoria Samson, an expert on missile defense at the private Center for Defense Information.