Comet
An icy, dusty object from the outer Solar System that grows a glowing head and tail when it comes near the Sun.
A comet is like a dirty snowball, or more often a snowy dirtball: a solid centre called the nucleus, usually a few kilometres wide, made of ice, dust and rock. Far from the Sun it is frozen and dark. As it travels closer, sunlight warms it and the ice turns straight into gas, carrying dust with it. This makes a fuzzy cloud around the nucleus, called the coma, and long tails that always point away from the Sun.
Comets matter because they are some of the least changed leftovers from the birth of the Solar System. They also create meteor showers: the dust they shed spreads along their orbit, and when Earth passes through it, we see shooting stars. Comet Halley, which returns about every 76 years and is next due in 2061, makes the Orionids each October.
Meteorites that people find come from asteroids, not comets, because comet dust is too small and fragile to survive the fall. The European Rosetta spacecraft studied comet 67P up close from 2014 to 2016. A bright comet looks like a fuzzy star with a tail that stays still against the stars over minutes.
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Level: Beginner Ages 7 and up