A comet is a chunk of ice, rock, and dust that orbits the sun, often traveling from the far edges of our solar system into the inner regions where Earth is. When a comet gets close enough to the sun, the sun's heat causes the ice to vaporize, creating a glowing cloud around the comet's center and often a spectacular tail that can stretch millions of miles across space.

Ancient people noticed comets appearing in the night sky and often saw them as omens or messages from the gods, since they looked so different from ordinary stars. We now know that comets are leftover material from when our solar system formed over four billion years ago, making them like frozen time capsules from the early solar system.

The most famous comet is probably Halley's Comet, which swings past Earth every 75 years or so. The last time it visited was 1986, and it won't return until 2061. When people say something appeared “like a comet,” they mean it showed up suddenly and spectacularly, then disappeared just as quickly.

Comets follow long, stretched-out orbits called ellipses. Some take just a few years to complete their journey around the sun, while others take thousands of years, spending most of their time in the cold, dark outer reaches of the solar system before making a brief, brilliant appearance near Earth.