Gordon Pettengill

1997 Charles A. Whitten Medal Winner

Massachusetts Institute of Technology, Cambridge, Mass.

Gordon H. Pettengill was honored as the recipient of the Charles A. Whitten Medal at AGU’s Spring Meeting in Baltimore on May 28, 1997. The award is given for outstanding achievements in research on the form and dynamics of the Earth and the planets. The citation and Pettengill’s response are given here.


“‘Rock solid’ is the best description of Gordon Pettengill’s research. Always in the forefront, never trivial, Gordon’s publications set the highest standards for scientific content and clarity of presentation. His preparation was equally solid, from an undergraduate physics education at the Massachusetts Institute of Technology to graduate school in physics at the University of California, Berkeley, with only a brief interlude partly spent in Europe in 1944-1945 settling a certain conflict there.

“The common thread through almost all of Gordon’s research is radio or, more specifically, radar. His experiences as a radio ham during his youth helped give Gordon a firm grasp of the nuts and bolts; theoretical underpinning followed in his formal education, with sophisticated applications being the hallmark of his subsequent research. Throughout, Gordon has been the antithesis of a `fuzzy thinker’; when he speaks or writes, his statements can be relied upon to be backed by careful, rigorous thinking.

“Let me note just a few highlights of his contributions. Gordon’s career blossomed in the space age. 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, Gordon used this radar to `skin track’ Sputnik I, the first such observation of a satellite. His earliest research extending well beyond the Earth’s environment 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 even 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.

“Gordon first became famous for observations closer to home: 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.

“Gordon went to the Arecibo Observatory in the early 1960s and was largely responsible for its use as a radar astronomical tool. Most notable from this phase of Gordon’s work was the spectacular discovery that Mercury’s spin period was about 59 Earth days, not the 88 days that had been widely believed for nearly a century. In typical Gordon 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, Gordon 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). Gordon 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, Gordon made use of radar systems at MIT’s Haystack Observatory and Cornell’s Arecibo Observatory, systems whose development he had guided for astronomical applications.

“Over the last two decades, Gordon returned to an early love, concentrating most heavily on Venus, this time utilizing radars aboard spacecraft, first the Pioneer Venus Orbiter and most recently, Magellan. Because of Gordon’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 with vigor, intelligence, and perseverance the idea for using a radar altimeter to map Venus. He also contributed key technical ideas and corrected many errors of contractors along the way. It is hard to overestimate the importance of the incredible attention Gordon gave to every detail that resulted in these superbly functioning radar instruments. 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. The one individual that many planetary scientists feel is most responsible for our present knowledge of Venus (aside from its atmosphere) and for the puzzles it presents is Gordon.

“As he goes through middle age, Gordon has been turning some of his talents toward theory. In a remarkable piece of analytical detective work, he developed a model to explain the anomalous monostatic and bistatic radar scattering properties of Venus’ highest land, Maxwell Montes – a thin layer of the rare Earth metal tellurium.

“I am also happy to report that Gordon has an excellent heredity, and with his razor-sharp mind, I expect that he will be making unexpected fundamental contributions to planetary science for at least another two decades (he is currently involved in altimeter experiments aboard the Mars Global Surveyor mission, now underway).

“It has been my pleasure to have known Gordon for over 40 years as a dear friend as well as a close colleague; there is no one I admire more. It thus gives me great pleasure to be able to describe some of the accomplishments for which Gordon is receiving the Whitten Medal of the American Geophysical Union.”

—IRWIN SHAPIRO, Harvard Smithsonian, Center for Astrophysics, Cambridge, Mass.


“Thank you, Irwin; I feel very honored and flattered to have been awarded the Charles A. Whitten Medal! It is a particular pleasure to have you as citationist, since our professional and personal paths have crossed and paralleled each other so often over the last four decades. Even 43 years ago, while I was observing the scattering of high-energy protons as a graduate student at Berkeley, you were working out theoretical aspects of the same problem (unbeknownst to each other) at Harvard. Since then, our work has taken both of us far afield, from the nuclear force to radio frequencies at the opposite reach of the energy spectrum.

“In his response to receiving the 1996 Bowie Medal at last fall’s AGU, Gene Shoemaker remarked on the lucky accident of being in the right field and the right place at the right time. I owe a similar debt to the gods of chance! At Berkeley (and earlier at Los Alamos), I felt like a very small fish in a very large pond: in those days just after World War II, nuclear physics was thought to be at the cutting edge of scientific research, and all the brightest people seemed to be elbowing their way into it. Part of my motivation in moving back east in 1954 and joining the Massachusetts Institute of Technology’s Lincoln Laboratory as a newly fledged Ph.D. was to get into a younger and less mature field where there might be less competition. I remember my employment interview with the then director of Lincoln, where I voiced my hopes of becoming involved with spacecraft exploring the solar system only to be rebuffed with: “`you don’t believe in that Buck Rogers stuff, do you?’” Fortunately, my remarks were not held against me, and they hired me anyway! Moreover, Buck Rogers indeed soon became one of the icons of space exploration, following the launch of Sputnik a few years later.

“Those were the glory years, when the military seemed willing to spend endless sums of money to explore new technical avenues for defense, and if useful science could be done along the way that was just great! The thinking was that exercising the new toys this way might help in their teething process! So exercise we did! Even the 1000-ft. Arecibo telescope that came along 10 years later in 1964 was initially built with military (ARPA) money in order to study the properties of the upper levels of the ionosphere, which could not be observed by ordinary short-wave-radio ionosounding. Boy, did we find an application for it that went far beyond the Earth!

“In the 1970’s, NASA missions began to replace defense projects as the way to study the planets, and I became interested in putting a small radar in orbit around Venus, rather than standing back on Earth and blasting away with a giant one! This era of my life brought me into contact with Peter Ford, whose contributions to the radar mapping of Venus were enormous; without his help the results we achieved would not have been possible. Finally, I must acknowledge the sacrifice that chasing a space mission inevitably requires from one’s family, particularly when the project is located on the other side of the country! Without the support of my wife, Pam, I would never have been able to concentrate so single-mindedly on seeing the experiments through their inevitable challenges and difficulties. Again, let me express my appreciation for being selected as the Whitten Medalist!”

—GORDON H. PETTENGILL, Massachusetts Institute of Technology, Cambridge, Mass.