PENTAGON -- North Korea claimed its hydrogen bomb was "miniaturized" to fit on a missile -- which would be a huge engineering achievement -- also unlikely.
U.S. aircraft flying out of Japan will collect air samples off the coast of North Korea to determine exactly what kind of nuclear device went off.
But based on analysis of the seismic wave the blast triggered, U.S. intelligence is "highly skeptical" of the North Korean claim it successfully tested a hydrogen bomb. Tuesday night's explosion is estimated to have been less than 10 kilotons -- about half the power of the weapon which destroyed Hiroshima at the end of World War II.
A hydrogen bomb -- which has never been used in war, but was tested in the Pacific during the 1950s -- would be 50 to 100 times more powerful.
Whatever kind of device was detonated, the greatest mystery about the North Korean nuclear weapons program is whether they have the capability to launch one against the United States.
North Korea has a missile capable of reaching that far, but is has never been tested. To be on the safe side, the U.S. keeps its missile defense system, which officials say is capable of shooting down a small number of North Korean missiles, on constant alert.
North Korea seems to specialize in nasty surprises. U.S. intelligence has been expecting another test sooner or later, but had no warning it was going to happen last night.