Fred Whipple Dies.

Fred L. Whipple, a pioneer in astronomy who proposed the “dirty snowball” theory for the substance of comets, has died. He was 97.

Whipple died Monday at a Cambridge hospital, the Harvard-Smithsonian Center for Astrophysics said Tuesday.

Whipple proposed the theory in 1950, saying that comets consisted of ice with some rock mixed in, rather than sand held together by gravity, as was widely believed. Whipple’s theory was an attempt to explain why some comets seemed to arrive at destinations earlier or later than predicted.

Whipple believed that as a comet approached the sun, its light vaporized ice in the comet’s nucleus. The jets of particles that resulted acted like a rocket engine that either slowed or accelerated the comet.

He also theorized that the glowing comet tails contained particles that originated from frozen reservoirs in comet nuclei.

Whipple’s theories were proven correct in 1986 by close-up photographs of Haley’s comet by the European Space Agency’s Giotto spacecraft.

Rest In Peace, Fred.