Comets look beautiful in the night sky, but for centuries, people were afraid of them. Some of the more well-known comets, like Halley’s Comet, created so much fear that people were afraid to leave their homes when the comet was overhead. Today, we know much more about comets. They’re not signs of doom or destruction, as some ancient cultures believed. Instead, they’re very ordinary chunks of space debris.

Q: What are comets made of?
A: Comets are small chunks of ice and dust, which is why they’re often called “dirty snowballs.” The nucleus, which is the true comet, is made of ice, frozen gases (including methane, carbon monoxide and carbon dioxide), and solid particles (including rock, dust and dirt). The nucleus is surrounded by a thin, dark crust.

Q: Where do comets come from?
A: Most comets come from two places: the Oort Cloud and the Kuiper Belt. These are areas deep within the solar system, far beyond Neptune and Pluto, the farthest planets.

Q: What makes a comet glow?
A: As the comet moves closer to the Sun, heat from the Sun causes the comet’s frozen gases to unfreeze and evaporate. Gas and dust escape through the comet’s crust, forming a cloud around the nucleus. This hazy, glowing cloud, called the coma, can be as large as tens of thousands of miles in diameter. It glows because of two things: dust in the coma reflects sunlight, and gases in the coma give off their own light. Together, the coma and the nucleus are called the comet’s head.

Q: What’s a comet’s tail?
A: A comet’s tail forms when the comet approaches the Sun. The Sun pushes dust and gas away from the comet’s head, forming two tails: the dust tail and the gas tail. The dust tail is curved and pale yellow, while the gas tail is blue, and longer and straighter than the dust tail. The comet’s tails always point away from the Sun. As the comet moves away from the Sun and back towards the deep solar system, the tails disappear.

Q: How often do comets orbit the Sun?
A: It depends. Some comets approach the Sun only once. Others, called periodic comets, make repeated visits to the inner solar system. However, comets lose material each time they pass by the Sun, so even periodic comets don’t last forever.