Whipple, Fred Lawrence (1906-2004)

US astronomer whose hypothesis in 1949 that the nucleus of a comet is like a dirty snowball was confirmed in 1986 by space-probe studies of Halley's Comet.

Whipple was born in Red Oak, Iowa, and received his BSc from the University of California at Los Angeles in 1927. In 1931 he was appointed to the staff of the Harvard College Observatory, becoming a professor there 1950-77. He was also director of the Smithsonian Astrophysics Observatory, Cambridge, Massachusetts, 1955-73.

In addition to discovering six new comets, Whipple proposed that the nucleus of a comet consisted of a frozen mass of water, ammonia, methane, and other hydrogen compounds together with silicate minerals, dust, and other materials. As the comet's orbit brought it nearer to the Sun, solar radiation would cause the frozen material to evaporate, producing a large amount of silicate dust that would form the comet's tail.

Whipple, who was sometimes referred to as ‘Dr Comet’, also worked on ascertaining cometary orbits and defining the relationship between comets and meteors. In the 1950s he became active in the programme to devise effective means of tracking artificial satellites. In 2002, at the age of 95, he was the oldest scientist on the Comet Nucleus Tour (Contour) team. He went on to become a professor with the Harvard-Smithsonian Center for Astrophysics.