Fred Lawrence Whipple, the oldest living American astronomer and one of the last giants of 20th century astronomy, passed away yesterday at the age of 97 following a prolonged illness. A discoverer of six comets, Whipple may be best known for his comet research. Five decades ago, he first suggested that comets were "icy conglomerates," what the press called "dirty snowballs." The Whipple Shields on the Stardust spacecraft, which protected the spaceraft during its Comet Wild 2 encounter earlier this year, were named in his honor.