CAMBRIDGE, Mass. - 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...