Fred Whipple - the astronomer who first correctly described comets as "dirty snowballs" - has died aged 97.
By Dr David Whitehouse
BBC News Online science editor
He revolutionised the study of comets when in 1950-51 he proposed that they were not "sandbags" but small bodies made of rock, dust and ice.
He also predicted the coming of artificial satellites and was prepared with a satellite tracking network when Sputnik was launched in 1957.
He discovered six comets, all of which were named after him.
Chief of Chaff
Fred Whipple began work at the Harvard College Observatory in 1931 and from 1955 to 1973 directed the Smithsonian Astrophysical Observatory, helping it to become the renowned Harvard Smithsonian Center for Astrophysics.
Working for the US Air Force during World War II he perfected the idea for chaff - little bundles of shredded aluminium foil that could be dropped from US aircraft to confuse German radar. Air Force wits dubbed him the "Chief of Chaff" as a result.
But it was his work on the icy conglomerate model for comets that is regarded as the highlight of his distinguished career.
"This was a complete paradigm switch since back then the consensus model for a comet was a flying cloud of particles," Don Yeomans, of the Jet Propulsion Laboratory in California, told BBC News Online.
"It had been so since the late 19th Century when comets were identified with meteor showers."
But Whipple knew that some comets have been orbiting the Sun a thousand times and more. If they were nothing but sand, they would have broken up.
In 1950, he published a paper suggesting they were "icy conglomerates"; what the media later called "dirty snowballs."
As the comet gets closer to the Sun, the ice vaporises and forms a spectacular coma, or tail. When in 1986, the Giotto Mission came close enough to Halley's Comet to photograph its nucleus, it looked just the way Whipple described it 36 years before.
Whipple's model allowed the so-called nongravitational forces acting on comets to be understood. The rocket-like thrusting when the ices began to vaporise produced a small but noticeable thrust that needed to be accounted for so that the motions of active comets could be modelled far more accurately.
Flow of knowledge
Whipple also made contributions in fields as diverse as variable stars, galaxies, stellar evolution, supernovas, the interstellar medium, radio astronomy and astronomical instrumentation.
He was also the first to compute an accurate orbit for the then recently discovered Pluto.
While his collected works were published in two large volumes in 1972, he continued to publish research studies for at least another quarter century.
"Fred was one of the truly great forces in astronomical research, and our field has gained immeasurably as a result of his insights," says Don Yeomans.
"While a bit in awe of him, I quickly realised he was completely unaffected by his fame.
"It really didn't matter whether one was a very young and inexperienced scientist or a well established, distinguished scientist.
"He treated everyone with the same kindness and respect."