Fred Lawrence Whipple

In 1950, Whipple proposed that a comet's nucleus is a solid body composed of a mixture of volatile water ice, frozen gases, and rocky dust — hence the nickname 'dirty snowball.' This replaced the earlier theory that comets were loose swarms of particles, and it explained how solar heating produces a comet's glowing coma and tail.

The model was strongly supported by space mission observations, including the European Space Agency's Giotto probe flyby of Halley's Comet in 1986 and NASA's Deep Impact mission to Comet Tempel 1 in 2005. These missions provided close-up evidence of solid, icy nuclei consistent with Whipple's prediction.

Beyond the dirty snowball model, Whipple discovered six comets and made major contributions to the study of meteors, proposing that meteors are largely debris shed by comets. He later directed the Smithsonian Astrophysical Observatory, where he helped establish programs for tracking artificial satellites after the launch of Sputnik in 1957.

Whipple spent much of his career at Harvard University and the Smithsonian Astrophysical Observatory, serving as director of the observatory from 1955 to 1973. He remained associated with Harvard as a professor of astronomy and continued to be active in research long into his later years.