Comet Hale-Bopp.Fred Espenak, NASA GSFC
The tails of Comet Hale-Bopp.
Fred Espenak / NASA GSFC

Why do comets have tails?

Comets develop tails as they approach perihelion—the place in their orbits when they are closest to the Sun. The Sun’s heat vaporizes some of the comet's material, releasing dust particles that were trapped in the ice.

A combination of solar radiation pressure and solar wind blow away gas and dust from the comet’s nucleus, forming two separate tails: the ion tail and the dust tail.

Two Types of Tails

Ultraviolet light ionizes the neutral gas blown off the comet, and the solar wind carries these ions straight out from the Sun to form the ion tail, which typically glows blue. The dust tail on the other hand is neutral, composed of small dust particles (similar in size to those found in cigarette smoke). Pressure from the Sun's radiation pushes these particles away from the comet’s nucleus. These particles continue to follow the comet’s orbit around the Sun, and form a diffuse, curved tail that typically appears white or pink from Earth.

Comets aren't the only objects in the solar system with tails — recent observations show that even asteroids can sprout dust tails on occasion.


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July 27, 2017 at 12:43 am

I've been watching the Electric Universe on youtube, the claim the comets have tails because of electrical differences as it comes from way out there to the inner solar system. To be honest, it sounds just a little more believeable than a dirty snowballs melting away as it gets closer to our sun. Can you comment on this? Again to be honest, after billions of years, the dirty snowball theory shouldn't hold up, even 1 trip around the sun.

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Frank C

November 29, 2019 at 4:51 am

Why would there be a tail of a comet if there is nothing forcing the gases or debris to slow down more than the comet? Why would not the debris continue at the same speed, with the comet?

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March 18, 2023 at 5:23 pm

More interesting moment is that the comet's tail changes its length while comet gets closer to the Sun. I do explain it as the comet's own gravity decreases while it gets closer to the Sun, and, vice versa, increases while comet goes away from the Sun.

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