On August 30, planetary science lost one of its guiding lights when Fred Whipple died at age 97 in a Cambridge, Massachusetts hospital. The most notable of his many accomplishments in a career that spanned most of the 20th Century was his 1950 “dirty snowball” theory that revolutionized the study of comets.
Born in 1906 in Red Oak, Iowa, Whipple studied at Occidental College and received an undergraduate mathematics degree from UCLA. A talented tennis player, his dreams of becoming a champion were cut short by a bout with polio; it was only after this that he turned his attention to astronomy. As a graduate student at UC Berkeley, he helped calculate the orbit of newly discovered Pluto. Whipple worked at Harvard from 1931-1977, and directed the Smithsonian Astrophysical Observatory (SAO) from 1955 to 1973, helping in the 1973 merger of the two observatories that formed the Harvard-Smithsonian Center for Astrophysics. Between 1932 and 1942, he discovered six comets that now bear his name.
In the 1930s, Whipple determined that meteors were made up of fragile material from comets. During World War II, he co-invented chaff, aluminum fragments used to foil radar and protect planes; for this, the U.S. Air Force dubbed him “Chief of Chaff.” In 1946, he invented the Meteor Bumper or Whipple Shield, a thin, protective metal shield that pro-tects spacecraft from meteor impacts, which is still in use today. Whipple was the author of several books on astronomy, and in the early 1950s he co-wrote a series of popular articles for Collier's magazine with Wernher von Braun that helped spark the U.S. involve-ment in space exploration.
When the Russians launched Sputnik 1 in October 1957, Whipple already had a network of amateur astronomers in place to observe and track satellites. In 1963, President Kennedy gave Whipple the President’s Award for Distinguished Public Service, the highest U.S. civilian honor for government service, for his leadership in creating this worldwide satellite observation network. Later, Whipple helped the SAO to develop a camera-based satellite tracking system; it was so accurate that astronomers were able to determine the exact shape of Earth through examining its effects on satellite orbits.
In 1950, Whipple proposed the “icy conglomerate” (a.k.a. “dirty snowball”) cometary model, in which he suggested comets have icy cores surrounded by thin, insulating cores of dust and rocky particles, and that jets of material ejected as the result of solar heating can change a comet’s orbit. (Prior to that, comets had been thought of as “sandbags” or gravel banks held together by gravity.) This was based largely on studies of Comet Encke. Whipple’s model was confirmed by the observations of spacecraft that flew by Halley’s comet in 1986. Whipple’s 1950 and 1951 papers describing his theory were the most-cited astronomy articles in the past 50 years. Whipple’s comet work continued nearly until his death; from 1999-2002, Whipple was involved in NASA’s ill-fated Contour mission; designed to visit two comets, it instead broke apart after launch.