CAMBRIDGE, Mass. (AP) – 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.

Charles Alcock, director of the Center for Astrophysics, said Whipple had “revolutionized the study of comets.”

“Fred Whipple was a truly extraordinary person among extraordinary people. He was gifted with great scientific imagination, superb analytical skills, and excellent management acumen,” Irwin Shapiro, a former director of the center, said in a statement.

Whipple was born in Red Oak, Iowa, in 1906. He received his bachelor’s degree in mathematics at the University of California, Los Angeles, but didn’t turn to astronomy until a bout with polio ended his dreams of being a tennis champion.

He completed his doctorate in astronomy at the UC-Berkeley, in 1931 and accepted a position at Harvard that year.

During World War II, Whipple invented a device used by Allied planes over Germany to confuse enemy radar. The device cut aluminum foil into thousands of fragments, giving a false impression of a much larger number of planes attacking.

In 1946, in anticipation of the future of space flight, Whipple invented a thin outer skin of metal to protect spacecrafts. Meteors disintegrated when they hit the shield, known as a meteor bumper or Whipple shield, leaving only vapor to hit the spacecraft. The technology is still in use today.

He was also ahead of the curve in 1957, when the Soviet Union launched the Sputnik satellite. At the time, Whipple was setting up a network of cameras to track it and one station was already operational.

President Kennedy honored Whipple with an Award for Distinguished Public Service in 1963 for the project.

Whipple was director of the Smithsonian Astrophysical Observatory in Cambridge from 1955 to 1973, when it merged with the Harvard Observatory and was renamed the Harvard-Smithsonian Center for Astrophysics.

Whipple retired from Harvard in 1977, although he continued to bicycle to the center six days a week until he was 90. The license plate on his car was “COMETS.”