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xvi PREFACE s­o-­called greenhouse effect on Venus leading to its extraordinarily hot surface temperature, precise tests of general relativity, the rotation of Mercury, the first exoplanets, radio galaxies, quasars, pulsars, cosmic masers, and cosmic evolution. The discovery of the microwave background by Arno Penzias and Bob Wilson in 1965 revolutionized cosmology and, interestingly, was made at the same AT&T Bell Laboratories as the initial discovery by Jansky of cosmic radio emission some three decades earlier.

The early pioneering radio astronomy observations were largely carried out by a single individual or small group who conceived of an experiment, designed and built the equipment, carried out the observations, and analyzed the resulting data. The 1956 establishment in the USA by the National Science Foundation (NSF) of the National Radio Astronomy Observatory (NRAO), operated by Associated Universities, Inc. (AUI), changed the culture and ultimately impacted all astronomy, not just radio astronomy. Before NRAO, US astronomical observatories were primarily privately funded through generous gifts from wealthy individuals like Charles Yerkes, James Lick, Percival Lowell, Andrew Carnegie, and John D. Rockefeller, and the philanthropic foundations which they established. The European-based observatories were often state supported, but they, like their private American counterparts, were used nearly exclusively by their own staff members.

Following the lead of the physicists who had created the Brookhaven National Laboratory (BNL), the NRAO was established to provide scientific instruments that were too costly for individual universities to build and operate. Although originally intended to provide opportunities for American radio astronomers to compete in the rapidly developing new field, with the availability of its first instrument, the 85 Foot Howard E. Tatel Telescope, NRAO developed a more broadly welcoming policy and announced its visiting scientist program on page 1179 of the October 30, 1959, issue of Science:

"The National Radio Astronomy Observatory was established by the National Science Foundation to make available to scientists from any institution facilities for research in radio astronomy…. The facilities of the observatory are open to any competent scientist with a program of work in radio astronomy, regardless of institutional affiliation."

Under the wise leadership of its young Director, Dave Heeschen, as additional telescopes were completed—the 300 Foot, the 140 Foot, the Green Bank Interferometer, the 36 Foot millimeter telescope, and eventually the Very Large Array (VLA)—NRAO facilities continued to be available to any scientists with a good program, independent of their institutional or national affiliation. This concept, which has become known as “Open Skies,” after the Open Sky agreements which regulate international airline traffic, ultimately was adopted by nearly all major ground- and space-based American as well as international astronomical facilities. When two of us (Kellermann and Bouton) interviewed Dave Heeschen on July 13, 2011 (see https://science.nrao.edu/about/pub-