How to make your own comet

My homemade 0.00152-km diameter comet threatens to destroy the Earth during an unannounced flyby last night. Photo: Bob King

I teach a community education astronomy class and since last night was the last class, I thought it would be fun to build a homemade comet. What with Comet Elenin in the news, the timing seemed appropriate.

Making a comet is much like making brownies – you just need the right ingredients. It also helps to see it done first by someone else, and what better place than Youtube (see below) for that. There are several variations but they all involve the following ingredients:

* A cup of water. One of the main ingredients of comets is water ice.
* Organics or carbon containing compounds. A cup of dirt will do the job. It also gives your comet a gritty, dirty appearance much like the real thing.
* A cup of sand to represent the silicate or rocky bits found in comets.
* A can of dark-colored soda. I used root beer. This represents organics and it also will darken the color of your comet. Real comets are mixtures of ice, dust and stony grits and as dark as charcoal.
* A few squirts of window cleaner. The ammonia in the cleaner is partly made of nitrogen, compounds of which are also found in comets.
* When you have placed all these in a metal bowl, add the final ingredient – a couple cups of dry ice pellets. Comets contain dry ice or frozen carbon dioxide. Cool, smoky vapor will now bubble up from your mixture.

A real comet! This picture of Comet Hartley 2 was taken a flyby on November 4, 2010 by the EPOXI spacecraft. Vaporizing ice at right sends streamers of dust and water into space. Credit: NASA.

Needless to say, it’s important to wear protective glasses and good gloves when handling dry ice, since it’s 109 degrees below zero. I bought two pounds of pellets for just under $5 from our local industrial gas and welding supply shop (Praxair) here in Duluth, Minn. Kept in a cooler, the ice easily lasted though the afternoon and into the night.

Once all your ingredients are in place, reach into the bowl with your gloved hands and crush the stuff together, holding the mass tightly for a maybe a half minute. When you lift it out for all to see, you’ll be holding an ugly, lumpy, vaporizing, crackling plug of homemade comet. In real comets, sunlight causes ice to vaporize or sublimate, going directly from solid to gas. You’ll see the same in your homemade version. If you blow on it to simulate the sun and solar wind, a misty tail will point away from the ‘sun’ (your mouth).

As a comet swings inward toward the sun, heat, light pressure and the solar wind cause the vaporizing ice and its entrained dust to 'blow' back behind the comet and form two tails, one of dust and the other gas. Credit: NASA

My first attempt with the class was an instructive failure. I lifted the comet out of the bowl and it cracked into a couple pieces. Since real comets are friable objects, they do sometimes fall apart under the heat and gravitational stress caused by the sun.

But serendipity was on my side last night, because after letting the mix sit for 15 minutes without touching it, a very fine comet – with spoon stuck inside – spontaneously formed on its own! I lifted it into view at the end of class, and we were all pleasantly surprised. What a beauty. And just for fun, my buddy Will Wiethoff and I simulated a near collision with the Earth using one of the paintings hanging in the planetarium entryway.

You can simulate the solar wind and light pressure from the sun by blowing over your comet or over a spoonful of dry ice. Photo: Bob King

So if you’d like to make a comet, collect your ingredients, taking special care with dry ice, and have at it. It’s important to use a metal bowl so it won’t crack when you fill it with the bitter cold ice. And if you want to keep your comet for show-and-tell, bag it and store it in a freezer. Creating a comet helps us to better visualize and understand what we’re talking about when we see one through a telescope or in pictures. I’ve included one of the videos below to help you along the way. And here’s a link to another.

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