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Obituary

Fred Lawrence Whipple (1906–2004)

A founding father of modern planetary science

Fred Whipple died on 30 August, two months shy of his ninety-eighth birthday. The breadth of his published research, from 1927 to 2000, is extraordinary. It covers such diverse topics as comets, meteors, satellite tracking, variable stars, supernovae, stellar evolution, radio astronomy and astronomical instrumentation. Whipple's collected works were published in two massive volumes in 1972, shortly before his ‘retirement’. But his research contributions continued for another three decades: an additional volume is planned.

Fred Lawrence Whipple was born on 5 November 1906, on a farm in Red Oak, Iowa. When he was 15, the family moved to California. There, Whipple studied mathematics at Occidental College and at the University of California at Los Angeles. As a graduate student at the University of California at Berkeley in 1930, he was one of the first to compute the orbit of the newly discovered planet Pluto. On receiving his PhD in 1931, he joined the staff of the Harvard College Observatory in Cambridge, Massachusetts. He became chairman of Harvard University's astronomy department in 1949, then director of the Smithsonian Astrophysical Observatory from 1955 until 1973.

At Harvard, Whipple developed a photographic tracking network of professional and amateur observers with the aim of determining the trajectories of meteors from two or more simultaneous observations. To fill the daytime gap when meteors could not be photographed, Whipple organized another programme for the detection of these objects using radio-wavelength observations. Eventually, in collaboration with Richard McCrosky and others, he concluded that most meteors were on comet-like orbits; fewer than 1% of the sporadic meteors visible to the naked eye could be traced to an origin outside the Solar System. During his career, he discovered six comets. He liked to quip that anyone could discover a comet — all it took was time. He also discovered the asteroid 1252 Celestia, which he named after his mother; asteroid 1940 was renamed 1940 Whipple, in his honour.

When the Soviet satellite Sputnik was launched in 1957, Whipple's band of meteor observers was the only network in place that could track its progress visually. Later, he developed a photographic tracking system for meteors and artificial satellites that proved so successful and precise that the satellite-tracking data could be used to model the variation in Earth's shape and density from the observed gravitational effects on the orbits. He once noted that the highlight of his career was having his family and parents present at the White House when in 1963 he received the President's Award for Distinguished Public Service for this work, from John F. Kennedy.

His seminal work in cometary science, starting in 1950, was the ‘dirty snowball’ model for the nuclei of comets. It prompted a paradigm shift. A comet had been thought to be a flying cloud of particles. Whipple instead envisaged the cometary nucleus as a conglomerate of ices (mostly water, ammonia, methane, carbon dioxide and carbon monoxide) embedded in a non-volatile matrix of meteoric material. Part of his rationale was to provide an explanation of the so-called non-gravitational forces acting on comets: the rocket-like thrusting of a comet when the ices vaporize near the Sun introduces a small but noticeable thrust on the comet itself. With this effect properly modelled, the motions of active comets could be predicted far more accurately.

Later spacecraft observations showed that comets are surrounded by enormous atmospheres of hydrogen, confirming that the major cometary ice was likely to be water. In 1986, images from the Giotto spacecraft revealed that the nucleus of comet Halley was indeed a solid body made of dirty ice — a dramatic confirmation of Whipple's model. He was fond of telling the story that, while working on his ‘dirty snowball’ papers, he couldn't remember the value for the tensile strength of water ice; he solved the dilemma in characteristic Whipple fashion, by picturing the longest icicle he had seen while growing up in Iowa.

During the Second World War, Whipple led an effort to develop a means of confusing enemy radar — strips of reflective aluminium (known as ‘chaff’) dropped from Allied aircraft was his solution, for which he received a certificate of merit from US President Harry S. Truman. Eleven years before the launch of Sputnik, he developed the Whipple Shield: a series of thin metallic layers that stands out from a spacecraft and protects it from high-speed interplanetary dust particles. When particles hit this outer layer, they fragment and vaporize, and the debris lacks the energy to penetrate the main spacecraft. This same design successfully protected the Stardust spacecraft during a comet flyby in January of this year.

Late in his career Whipple was responsible for the construction of a major optical observatory on Mount Hopkins in Arizona. In 1981, it was renamed the Fred Lawrence Whipple Observatory. Whipple was successful as both a manager of large science enterprises and as a researcher. One of his secrets, he said, which enabled him to do management and science simultaneously was to spend some mornings in a room adjacent to his office, doing research. His secretary was asked to (correctly) notify callers that “Dr Whipple is not in his office at the moment” and could he or she call again later in the day.

An innovative thinker and a major force in the growth of planetary science in the twentieth century, Whipple was always a gentleman and a friend to all his colleagues. He was just plain ‘Fred’ to all who knew him. He was keenly interested in what the younger generation of scientists was doing, and throughout his life he remained a conscientious mentor to his former students. When asked the secret of his longevity at his ninetieth birthday party, he noted, “you've to start early”. Fortunately for planetary science, Fred Whipple did start early — and he stayed late.

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Yeomans, D., Veverka, J. Fred Lawrence Whipple (1906–2004). Nature 432, 31 (2004). https://doi.org/10.1038/432031a

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