“The first two sources of X radiation outside our galaxy have been discovered in data obtained by rocket-borne X-ray detectors. The new sources have been identified at the U.S. Naval Research Laboratory as coinciding with two of the most powerful radio-emitting galaxies, Cygnus A and M 87. Because the earth’s atmosphere is essentially opaque to X rays from space, instruments are placed above most of the atmosphere by means of Aerobee rockets fired from the White Sands Proving Ground in New Mexico.”
—Scientific American, April 1966
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