Making a Comet

This past week I took part in Journey through the Universe for the second time. Journey through the Universe is an annual event, running for the past eleven years, where astronomers and engineers from the various observatories on Mauna Kea go into classrooms and give talks to the students there. Most of the time these talks include some sort of physical demonstration or hands-on activity. This year I talked about comets, and as part of my talk I created a miniature comet.

Comets are basically, in the immortal words of Fred Whipple, “dirty snowballs.” They are composed of varying amounts of rock, dust, and various ices such as water, carbon dioxide, and ammonia. In preparation for my presentation I did a few test runs of the comet-making recipe I found, including two at work which proved quite popular with my coworkers. I took some pictures of the process (though in poor light, unfortunately) and thought I'd share them here.

We start with a plastic garbage bag inside a large plastic mixing bowl. (When dealing with dry ice, you really want to keep it away from metal as the extremely fast cooling of the metal causes it to emit nails-on-a-chalkboard-level screeches.)

Next, we add some dirt and stir it in.

Then, we add a dash of ammonia and some corn syrup, to represent the assorted organic molecules detected in comets. Organic, in this context, is used in the chemistry sense, meaning “a molecule containing carbon.”

Now, we crush up two cups of dry ice by pounding it with a meat tenderizer, add it to the dirty water, and stir vigorously. This produces an impressive amount of fog, making it hard to see what's happening, but you can feel the ice start to freeze up pretty quickly.

When it's mostly frozen, you can pick up the bag and form the comet like a snowball (something I have basically zero experience doing, I should note). When that's done, we've got a comet!

(Man, that light is poor.)

I don't usually take pictures of myself, so this actually took quite a bit of practice, and I got a bunch of melting dirty comet-water on my hand and a cramp in my leg in the process. Here's a close-up look at the comet:

This comet may actually be the best one I've made so far. I initially thought some of the chunks of dry ice were too big, but thinking about it now I think that may actually be a good representation of real comets. After making the comet I stuck it in a pan in front of me on my computer desk and watched it as it melted over the course of a few hours. This last picture was taken about half an hour after making the comet, and show pits where the water ice froze around some dry ice which has now melted, leaving a pitted surface.

This is quite a fun project to do. The part where you mix the dry ice and water is especially visually impressive, and quite popular with bystanders. You'll most likely have dry ice leftover, too, so you can have more fun with it after the fascination of watching a comet melt has worn off. A hui hou!