Fred Whipple made the first modern description of a comet nucleus in the 1950s. In his theory, a comet was a roughly homogeneous entity he referred to as an “icy conglomerate,” made up of different ices combined with dust. It was known as the “dirty snowball” theory in the media. Even though this would be able to explain the cometary activity, it did not fit with our developing knowledge of comets as bodies that occasionally disintegrate on their own when they are simply warmed by the Sun.
Paul Weissman suggested in 1986 that comets were actually “rubble piles” of variously sized, loosely linked smaller particles. For these pieces of debris to avoid harming one another, they would have needed to encounter at extremely low speeds and be in orbits that were substantially similar.
One of two theories forms the basis of contemporary models for comet production. The first is that instabilities formed in the material-filled disk around the Sun as it developed. Numerous processes may have led to these instabilities, but the end consequence is high-density regions where clusters of smaller objects are gravitationally drawn to one another and combined.
Radial drift, the second phenomenon, appears as forming particles enlarge to meters in diameter. They are now slowly drifting toward the Sun due to drag, picking up more tiny bodies along the way.
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