A native of Providence, Rhode Island, as a young man, Pettengill was enthralled with radio and electronics, taking apart and building old radios. This was to serve as a beginning to a career using radio and its alter-ego, radar, in many of its most practical and esoteric aspects. Most importantly he had a strong curiosity about the laws governing what he experienced as well as a keenly precise reasoning to answer it.
Pettingill's amateur radio callsign is W1OUN.
His undergraduate study at the Massachusetts Institute of Technology was briefly interrupted by service in Europe at the end of World War II. This was followed by work at Los Alamos and a doctorate from University of California, Berkeley.
Pettengill was one of the first pioneers to take radar from its original military application to its use as a grand tool for astronomy.
He was the driving force behind using the then-new Millstone radar at the MIT Lincoln Laboratory for the earliest work in radar astronomy. When it became operational in late 1957, Pettengill used this radar to "skin track" Sputnik I, the first such observation of a satellite. His earliest research, extending well beyond the Earth's orbit was with this same radar in 1961; he used it to make the first ranging measurements to another planet, Venus, to which he would return with more distinction later in his career. These first observations yielded a value for the astronomical unit in terrestrial units which has stood the test of time and has an accuracy some 3 orders of magnitude greater than had been possible with the armamentarium of classical positional astronomy. Such knowledge was critical for the successful navigation of Mariner 2 to Venus.
Pettengill first became famous for the successful two-dimensional radar mapping of the Moon in 1960, a key step in the U.S. preparations for the Apollo program, insuring, for example, from follow-up data, that the Apollo astronauts would not disappear under a meters-thick layer of dust.
He went to the Arecibo Observatory (in Puerto Rico) in the early 1960s and was largely responsible for its use as a radar astronomical tool. Most notable from this phase of Pettengill's work was the discovery that Mercury's 'day' was about 59 Earth days, not the 88 days that had been widely believed for nearly a century. In his typical "assume nothing" fashion, he had realized that despite the 88-day period's having been "confirmed," the evidence in favor of this period was not all that secure, and he planned to make definitive measurements using the delay-Doppler technique as soon as the Arecibo radar could be instrumented for such observations of Mercury. His "nose for an important problem" was more than amply rewarded. This discovery led to Giuseppe Colombo's realization that Mercury was in an unexpected 3:2 spin-orbit resonance and to a subsequent renaissance in the study of dynamical resonances in the solar system.
Later in the 1960s and early 1970s, Pettengill led ground-based radar studies of the surface properties of all of the inner planets, including the Earth's (via a clever "triple-bounce" experiment: Moon-Earth-Moon). Pettengill also played a leading role in the first radar studies of an asteroid (Icarus, in 1968), a comet (Encke, in 1980), and moons of other planets (the Galilean satellites, starting in 1976). In all of this work, Pettengill made use of radar systems at MIT's Haystack Observatory and Cornell's Arecibo Observatory, systems whose development he had guided for astronomical applications. Also in the 1970s, he was involved in several unmanned missions to Mars (the Viking program, for example).
For over two decades, beginning in 1977, he concentrated most heavily on Venus, this time utilizing radars aboard spacecraft, first the Pioneer Venus orbiter and most recently, Magellan. Because of Pettengill's actions spanning the broadest political fronts to the narrowest technical details, the former was a spectacular success, only to be overshadowed by the latter. For many years, he pursued the idea for using a radar altimeter to map Venus. He also contributed key technical ideas along the way. The results, in part, were detailed reflectivity and topographic maps of virtually the entire planet of Venus, providing geologists and geophysicists, for example, with lifetimes of work to understand the development of Venus' crust and the history of its interior. Many planetary scientists feel he is one of the individuals most responsible for our present knowledge of Venus (aside from its atmosphere).
His observations have embraced Mercury, Venus, Mars, several asteroids and comets, the Galilean satellites of Jupiter and the rings of Saturn.
He is a professor emeritus at MIT and prior to that was Director of MIT's Center for Space Research. Gordon won the Charles A. Whitten Medal from the American Geophysical Union in 1997. The asteroid 3831 Pettengill is named after him.