Before Whipple began his research, comets were mysterious objects whose origin and composition were unknown. Most people thought that they were piles of sand and rock held together by gravity. Some thought that they were rocks ejected by volcanoes on Saturn and Jupiter. Nobody was sure how they were able to keep throwing off the material that formed their tails.
Whipple postulated that comets were simply large masses of ice and rock. He called them "icy conglomerates," but the term dirty snowball soon took over the public's imagination. Evaporation of steam produced by the sun's warming could accelerate the comets if the jets were pointed backward or slow it if they were pointed forward.
Whipple was born to a farm family in Red Oak, Iowa, in 1906, but his family moved to Los Angeles when he was 15. He attended Occidental College and UCLA, where he majored in mathematics, then received his doctorate from UC Berkeley. Including the war years, when he did research for the government, he spent all of his career at Harvard, where he was a mentor to Carl Sagan and a close friend of Isaac Asimov.