A comet is basically a dirty iceball composed of rock and ice.
Asteroids, on the other hand, are primarily just rocks.
Yet these two space objects are not always as mutually exclusive as this would suggest. Let me introduce Phaethon, a “rock comet” that blurs the definitions between asteroid and comet, and let me tell you why it will be worth paying attention to this fascinating object in the coming years.
Unlike typical comets that shed substantial amounts of dust when they heat up near the Sun, Phaethon doesn’t seem to be releasing enough dust today to account for the Geminids. This absence of significant dust emissions generates an interesting problem.
Phaethon’s orbit brings it extremely close to the Sun, much closer than Mercury, our innermost planet. At its closest approach (termed perihelion), its surface temperature reaches extremes of around 730°C.
You would expect such intense heat to strip away any volatile materials that exist on Phaethon’s surface. This should either expose fresh, unheated layers and shed huge volumes of dust and gas each time it passes close to the Sun, or form a barren crust that protects the volatile-rich interior from further heating, leading to an absence of gas or dust release.
Neither of these processes seem to be occurring, however. Instead, Phaethon continues to exhibit comet-like activity, emitting gas but not an accompanying dust cloud.