Cross-posted at School of Doubt.

Comet McNaught over Australia, by Fir0002/Flagstaffotos GFDL v1.2

I love hands-on science demos. One of my favorites is the “Make your own comet” that I often do for public events with kids, adults, anyone! I perfected my comet making skills while I was a part of Dark Skies, Bright Kids in Virginia, and I’ll share the not-so-secret recipe with you here. It’s fairly simple… once you practice it and get the mixtures right. It is also being included in the new CosmoQuest unit called In-VESTA-Gate where we talk about all the “little things” in the solar system. (P.S. We’re looking for teachers to look over and/or beta test the unit, so please check it out!)

Comets are the dirty snowballs of the solar system. You know that lump of YUCK left on the side of the road several days after it snows? Yeah, it’s kind of like that. Though when seen in the night sky, these objects can be quite spectacular, such as when Comet McNaught graced the Southern skies in 2007.

So what is a comet made of? We can look at comets with various telescopes and tell what’s in them by looking at their spectra. They become active as they get close to the sun, sporting tails and jets. These give off emission lines of carbon-containing molecules, or “organics” as well as volatiles (things that turn into a gas at “ordinary” temperatures) such as water, carbon dioxide, and ammonia.

Nuclei of comets Tempel 1 and Hartley 2, by NASA.

We’ve also sent spacecraft to get a close of look at several comet nuclei. And here’s the surprising thing: They are very, very dark. In fact, these generally are the least reflective objects in the solar system. Dark surface full of volatiles that turn to gas as it gets close to the Sun? Sounds like a dirty snowball to me. But it’s not just any ordinary ice that makes a comet, so here is how to do this really cool demo.

First, you need to gather your materials. Comets are made of dirt and rock, so grab some! Powdered charcoal briskets get you an excellent dark color, but I just grad a bag of soil from a home and garden store to use in a pinch. Really desperate? Try not to get caught digging up your neighbor’s yard…

You’ll also want some water, a glass cleaner containing ammonia, some sort of “organic” compound such as soda or corn syrup, and the best ingredient, dry ice. Now, the dry ice needs to be powdered, so whether you get it in pellet or block form, it helps to bang on it for a while with a rubber mallet while it is double, or even triple garbage bagged. You may be able to get dry ice at your local grocery store, a willing university science department, or a local distributer. To finish up materials, you’ll need a large bowl, a cooler in which to store the dry ice, a large serving spoon, gallon sized baggies for every person making a comet, paper towels (for the mess) and, very importantly, protective gloves for anyone handling dry ice. Oven mitts work, despite being unwieldy.

SAFETY NOTE: Dry ice can be dangerous and children should never handle unsupervised. It can and will burn your skin. It can and will explode a sealed container. Seriously, respect the stuff. 

For each comet, show off and describe each ingredient and how it is analogous to what is in a comet: dirt, rocks, water, ammonia, organics. You only need a tiny bit of ammonia and organics for show and add just enough water to make mushy, but not runny, mud. These all go into your gallon-sized plastic baggie. Now get your gloves on and get ready for the fun part. Pour in some of the crushed dry ice, about as much as you have mud mixture, and quickly much the bag around to mix the ingredients. DO NOT SEAL THE BAG. Now crush it into a ball with your gloved hands while still inside the bag and squeeze. At this point, I like to mention that I’m too lazy to wait millions of years for this stuff to accrete on its own, so I put a little pressure on it. Now, still with gloved hands, pull the new comet out of the bag.

Notice that your comet is outgassing a bit, especially where there are still little chunks of dry ice. Go ahead and blow on it to make more! This is a process called sublimation, when a solid turns directly into a gas. This happens to the volatiles in comet when it gets close enough to the sun to be heated up. This is part of what makes the comet tail! Interesting fact: the comet’s tail always points away from the Sun and does not indicate direction of motion.

The most recent, full set of instructions from DSBK can be found here (pdf), while the current draft of that day’s lesson in In-VESTA-Gate can be found here (doc).

And, oh look, my buddy Phil Plait made this comet for his Discovery Channel special. And then he one-ups me by blasting it with a super high-powered laser! Or, trying to, at least.

Be sure to use this demo when talking about Comet ISON, a recently discovered comet that looks like it will put on a lovely show as it gets closer to the Sun through November, and, if it survives, on its way back in December. Check out this viewing guide by David Dickinson of Universe Today. And have fun holding a piece of the cosmos in your hand! Your gloved hand, that is…