Comets are simply loose dirty snowballs made mostly of volatile ice and dust.

Fred Whipple’s “dirty snowball” model was successful because it explained the defining behavior of comets. A nucleus rich in volatile ice warms near the Sun, releasing gas and entrained dust that form the coma and tails. For decades, that was a useful physical picture of an object no spacecraft had yet seen up close.

The first encounters complicated it. Images of Halley’s nucleus in 1986 showed an extraordinarily dark object rather than a bright exposed snowball, with activity concentrated in jets. Later missions found porous surfaces, pits, cliffs, layering, dust-rich material, minerals, and complex organic chemistry. Deep Impact excavated material from comet Tempel 1, while Rosetta followed comet 67P for years and revealed a geologically varied, structurally complex nucleus.

The original model survives at its core: sublimating ice really does power cometary activity. What changed is the idea that “dirty snowball” is a complete description of what a comet nucleus is made of and how it is built.