Science: Bomb in Space

Long after midnight, bathers on Honolulu's Waikiki Beach were startled by a brilliant, soundless flash above the horizon to the southwest. The flash faded to reddish clouds that hung luminously in the night sky. Thousands of other Hawaiian residents saw it, and telephone switchboards were swamped with excited calls.

The flash was the explosion of a nuclear weapon over Johnston Island, 700 miles from Honolulu. Unquestionably, it was the highest ever exploded by the U.S. To be seen direct in Honolulu, it must have occurred many miles above the earth, and estimates put it as high as 100 miles. The AEC announced only: "the test detonation of a nuclear warhead missile." Speculation was that the warhead had been hurled aloft by the Army's Redstone missile, providing Hawaii with a preview of what the explosion will look like when an anti-missile attacks an invading missile.

The test's purpose was to find what happens when a nuclear warhead explodes in a virtual vacuum above the bulk of the atmosphere. The behavior of a nuclear explosion near sea level is known precisely. The nuclear fireball expands very fast at first, but both its temperature and pressure fall as it gets bigger. When its pressure equals that of the air, the ball stops expanding (for a megaton explosion, at a diameter of about one mile). The air also absorbs gamma and ultraviolet rays, confines radioactive particles to a comparatively small cloud.

In space, a nuclear explosion will behave very differently. Its gamma rays will not be absorbed; traveling at the speed of light, they may do damage to humans and to delicate electrical apparatus—including missiles—miles away. Just behind them will come fast-expanding concentric shells of radioactive beta particles (electrons), alpha particles (charged helium nuclei) and neutrons. Bringing up the rear will be the hot gases of the ball of fire, which will expand indefinitely. Some of the residue of an explosion above the atmosphere will presumably shoot out of the solar system. But the amount of lethal fallout on the earth's surface will probably be negligible, since by the time the radioactive particles descend to earth, they will be widely dispersed in both time and geography.