Ten Things You Don't Know About Comets

By Phil Plait | April 20, 2010 6:00 am

 

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Comets are dirty snowballs.

gail_comet

So what makes up the nucleus of a comet? That’s not simple to answer, because every comet is different, and there are things that straddle the line between comet and asteroid.

In general, comet nuclei are mountain-size dirty snowballs: rock, dust, gravel and other bits of stuff all mixed up with what I like to call "frozen gases" — I know, gas isn’t solid, but we’re talking ammonia, carbon dioxide, methane… stuff we usually think of as gases. Astronomers call them "ice", but I want to distinguish them from water ice.

And there’s water. Lots and lots of water. It’s frozen solid, of course, until the comet gets near the Sun. Then these ices sublimate (turn from a solid directly into a gas), forming the giant head of the comet and also stream away to form the tail. When these gases are released we can study them with telescopes and determine what’s in them. In the 1980s, the Giotto spacecraft found that 80% of the material blown out from Comet Halley was water! That’s true of most comets studied, too.

The picture above shows my friend Gail Zasowski, an astronomy grad student at my alma mater, the University of Virginia. She and fellow grad student Nicole Gugliucci made comets for kids as part of the Dark Skies, Bright Kids program. They used dry ice, water, window cleaner, dirt, and other products to make model comets. In the picture, Gail is blowing on the "comet" to make it outgas. I’ve done this as well, and it’s a fun activity (though a wee bit dangerous since dry ice is so cold). You can find more pictures on Nicole’s Flickr page.

But that means…

 

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