Because X-rays are rapidly absorbed by even very small thicknesses of matter, observations from near the top or above the atmosphere are essential. So Giacconi persisted and sent his exploratory proposal to the Air Force Cambridge Research Laboratory, a sponsor of defense-related research at AS&E. The proposal was accepted and funded for a series of rocket experiments. The first two suffered rocket failures. The third, on 12 June 1962, cast an exploratory net with a large-area detector and wide field of view. It caught an extrasolar source of X-rays of astounding and totally unexpected brilliance in the southern sky. Within a year its position was located by Friedman's group in the constellation of Scorpius, and came to be called Sco X-1. That was the beginning of extrasolar X-ray astronomy.

I observed Rossi huddled with Giacconi, Herbert Gursky, and Frank Paolini as they meticulously considered every other possibility they could imagine as an explanation for the surge in count rate that had occurred when the detector scanned across the southern sky. They concluded it was an extrasolar source, and sent their letter of discovery to Physical Review Letters. When the editor, Samuel Goudsmit, a distinguished physicist who knew Rossi well, phoned Rossi to ask whether he was sure of the results, Rossi gave his assurance and the letter was immediately published: Evidence for Sources of X-rays from Outside the Solar System, by Giacconi, Gursky, Paolini, and Rossi.