A comet is a chunk of ice, dust, and rock that orbits the Sun. Far from the Sun, it is a frozen, inactive body. As it approaches, solar heat vaporizes the ice, releasing gas and dust that form a glowing coma around the nucleus and often a tail that stretches millions of kilometers.

Comets come from two main reservoirs. Short-period comets, with orbits under 200 years, originate in the Kuiper Belt beyond Neptune. Long-period comets come from the Oort Cloud, a distant shell of icy bodies that extends thousands of times farther. Gravitational nudges from passing stars or the galaxy itself can send them inward.

The tail is not trailing behind the comet like smoke from a chimney. It points away from the Sun, pushed by solar wind and radiation pressure. That means a comet's tail can lead its motion as it moves away from the Sun. Some comets have two tails: a dust tail that curves and an ion tail that points straight back.

Comets are important because they preserve primitive material from the solar system's formation. The Rosetta mission orbited comet 67P/Churyumov-Gerasimenko and landed a probe on its surface, revealing organic molecules and water ice that may have seeded Earth's oceans.