Professor Fred Whipple

Astronomer who developed the idea that comets are 'dirty snowballs'

A true giant in the relatively small community of astronomers who study the solar system, Fred Whipple is best known for his development in the early 1950s of the idea that comets are "dirty snowballs", a model that has been dramatically confirmed by the space missions sent to comets during the past two decades, most recently to comet 81P/Wild earlier this year.

Fred Lawrence Whipple, astronomer: born Red Oak, Iowa 5 November 1906; Lick Observatory Fellow 1930-31; staff, Harvard University 1931-2004, Instructor 1932-38, Lecturer 1938-45, Associate Professor 1945-50, Chairman, Department of Astronomy 1949-56, Professor 1950-68, Phillips Professor of Astronomy 1968-77 (Emeritus); Director, Smithsonian Institution Astrophysical Observatory 1955-73, Senior Scientist 1973-2004; married 1928 Dorothy Woods (one son; marriage dissolved 1935), 1946 Babette Samelson (two daughters); died Cambridge, Massachusetts 30 August 2004.

A true giant in the relatively small community of astronomers who study the solar system, Fred Whipple is best known for his development in the early 1950s of the idea that comets are "dirty snowballs", a model that has been dramatically confirmed by the space missions sent to comets during the past two decades, most recently to comet 81P/Wild earlier this year.

Born into a farming family in Red Oak, Iowa, in 1906, Whipple moved to Long Beach, California, as a teenager and obtained his undergraduate degree in mathematics from the University of California at Los Angeles in 1927. Following the advice of his mentor, the astronomer Frederick Leonard, he applied for a fellowship at Berkeley, where he earned his PhD degree in Astronomy in 1931.

Accepting an offer from Harlow Shapley, the director, Whipple then took a position as head of the observing programme at the Harvard College Observatory, which was at the time setting up its new observing station in the town of Harvard, some 40km west of light- polluted Cambridge. This gave him the opportunity routinely to examine the sky-patrol photographs that were taken. During a 10-year period he discovered six comets on these photographs and computed their orbits, a technique he had learned at Berkeley, where he had been particularly involved in deriving the first satisfactory orbit for Pluto.

At this point Whipple also became interested in meteors, tiny dust fragments from comets that enter the Earth's atmosphere as "shooting stars". As early as 1936, he began a programme to record the same meteors from both Harvard and Cambridge. Some researchers, notably Ernst Opik - grandfather of the Liberal Democrat MP Lembit Opik - had suggested that many meteors came from outside the solar system on hyperbolic orbits. By measuring the well-separated photographs, Whipple was able to make it clear that meteors orbited the Sun on elliptical orbits. Although he soon demonstrated unequivocally that Opik was wrong, the reason he continued the "Harvard Meteor Project" for two decades was at least partly the enormous respect he had for Opik's intellect and ideas.

Whipple was less generous to Ray Lyttleton of Cambridge University, who averred that comets were in fact merely conglomerations of meteors, held together but loosely by gravity. To explain cometary spectra Whipple realised that gas was needed too, and how better to accomplish this than to embed the dust in ice to form a dirty snowball a few kilometres wide that would partly vaporise and then refreeze as the comet approached and receded from the Sun?

Furthermore, the expulsion of dust triggered by the vaporisation not only increased the supply of meteors but, as the nucleus rotated, produced a rocket-like force that advanced or retarded the comet's returns over and above the gravitational influence of the planets in just the manner that had been observed for several comets.

Whipple maintained a keen interest in the outer solar system, suggesting that Uranus and Neptune had agglomerated largely from comets and in the 1960s putting together the first coherent picture of a permanent population of cometary bodies orbiting the Sun just beyond Neptune. To his mind, Pluto was the largest member of a "comet ring" whose more mundane companions were just too faint for detection with the technology then available. He computed that the total mass of the transneptunian belt had to be less than that of the Earth. During the past 12 years, modern instrumentation has led to the discovery of several hundred roughly 100km-sized bodies that are indeed remarkably similar to what Whipple envisaged.

While on leave from Harvard for war work during the Second World War, Whipple co-invented a cutting device that converted lumps of aluminum foil into thousands of fragments. The release of these fragments of "chaff" from Allied aircraft flying over Germany confused the radar monitoring by the enemy into thinking they were being attacked by squadrons of aircraft. Whipple was particularly proud of this invention, for which he was awarded a Certificate of Merit in 1948.

Already in 1946, Whipple was taking an interest in the possibility of space flight. Mindful of the damage to spacecraft from meteors, he invented the Meteor Bumper, a thin outer skin of metal. Also known as the Whipple Shield, this mechanism explodes a meteor on contact, and improved versions of it are still in use.

Advancing to the position of an associate professor at Harvard in 1945 and to a full professorship five years later, Whipple became Phillips Professor of Astronomy in 1968, a post he held until taking emeritus status in 1977.

In 1955 a study group was established to examine the feasibility of launching artificial satellites in order to carry out some of the studies planned for the upcoming International Geophysical Year. At around the same time the Smithsonian Institution in Washington was looking for a new director for its Astrophysical Observatory. Faced with the difficulty of enticing to Washington a leading scientist to revive what had become a rather moribund astronomical group, the secretary of the Smithsonian Institution and Donald Menzel, who had been Whipple's thesis adviser at Berkeley but by then was director of the Harvard College Observatory, came up with the idea of moving the Smithsonian Astrophysical Observatory to Cambridge. And Whipple was to become the director of the latter.

Following his conviction that space was the new frontier, Whipple used the Smithsonian opportunity to develop a "Satellite Tracking Program" of a dozen observing stations around the world equipped with the specially designed Baker-Nunn cameras, acting in concert with a host of amateur astronomers who volunteered to participate in the "Moonwatch" programme. This army was ready to track the Soviet Union's "Sputnik" when the launching of that first artificial satellite so startled the western world in October 1957. For this and further ongoing activity President John F. Kennedy awarded Whipple the Distinguished Public Service award in 1963.

Despite this success, Whipple was at the same time aware that the Smithsonian still lacked a "real" observatory that would allow it to make observations of astrophysical objects. In 1966, he took steps to establish such a site on Mount Hopkins, 60km south of Tucson, Arizona. A 1.5-metre telescope was put into operation there in 1970, after which he initiated the design of a much larger "Multi-Mirror Telescope". This was finally dedicated in 1979.

In the mean time, Whipple had retired as director of the Smithsonian Astrophysical Observatory in 1973, at which time it was formally combined with the Harvard College Observatory as the Harvard-Smithsonian Center for Astrophysics under a single director. In accepting a particular position almost half a century ago, Fred Whipple sowed the seed for one of the world's leading astrophysical enterprises today.

After his retirement as director more than three decades ago, Whipple continued to retain his interest in comets, still writing papers and books and participating as a member of the ill-fated Contour (Comet Nucleus Tour) space mission team. In 1982 the Mt Hopkins Observatory was renamed the Fred Lawrence Whipple Observatory.

Whipple maintained an office at Observatory Hill in Cambridge for almost 73 years, cycling between it and his home in Belmont (5km sometimes in under 11 minutes) six days a week until the frailties of his extreme age no longer made this possible.

Brian G. Marsden