What is a comet?

A comet is one of the many kinds of bodies that make up our solar system. Comets are different from planets, asteroids, and meteors because they are largely made up of ice, with some rocks and dust mixed in. A comet is a lot like a dirty snowball — except that it's as big as a mountain! While most comets can be a mile or two across, Comet Hale-Bopp seems especially huge. Scientists think it may be five miles or more across.

Comets spend most of their time out in the farthest parts of the solar system, where it's cold and dark. When a comet is that far from the Sun, we can't see it. A comet's orbit is usually a very long oval, known as an "ellipse," looping around the Sun. It takes the comet thousands of years to travel along this path. But when it comes into our part of the solar system, closer to the Sun, the sunlight heats the ice and causes it to "sublimate" (go directly from ice to gas). This makes a huge cloud of gas, called the "coma," which gives the comet its fuzzy appearance. The coma may be 50,000 miles across! Some of the gas is blown away by the solar wind, causing it to stream outward and form the comet's tail. The dust gets pushed away by radiation pressure but it follows a somewhat different path, making a second tail! The dusty tail shines by reflecting sunlight, so it looks a little yellowish. The gas tail shines by glowing fluorescent light (similar to fluorescent lamps!) and looks a little blue. The tails can be millions of miles long! Because the tails are caused by the Sun, they always point away from the Sun. So if the comet is moving away from the Sun, the tail is pointing out in front — not trailing behind!

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Mercury
Why does Mercury have such extreme temperature differences and Earth does not? Astronomer Cathy Imhoff answers students' questions about the planet Mercury

The Moon
How far is the moon from the Earth? Why does the moon change its shape (as in full, half, and quarter moon)? Astronomer Cathy Imhoff answers students' questions about the moon.

Moon Shadow
Learn what happens when the moon passes directly between the Earth and the Sun: a total solar eclipse.