This Saturday, at our annual Carroll College Astronomy Day event, well have observatory tours, well look for sunspots with our solar telescope and, best of all, youll get a chance to watch us build a model comet, right in front of your eyes. I love watching the kids laugh with delight as we shape the steaming, stinking, ice-cold mass and hold it up for all to see. So, what is a comet? And how can we build one?

For thousands of years, most people around the world saw comets as frightening mysteries. The sun, the moon and the stars go through their cycles regularly and predictably. But then suddenly a comet would appear in the sky for a few weeks or months. People thought they were bad omens, warnings of war, plague and the death of kings. And then science was born.

Today, we know that comets are something like dirty snowballs, typically a few miles across, roughly the size of a mountain. They spend most of their time far from the sun in the outer reaches of our solar system, where its cold and dark, living in Plutos neighborhood. However, a comet will occasionally fall in close to the sun, and then things get interesting. The suns heat melts the comets outer layers, causing them to spew out an enormous cloud of gas and dust, thousands or millions of miles across. This gas ball, called the coma, is usually the first thing that we can see from here on Earth. Then, the suns wind and radiation begin pushing the comets gas away, giving the comet a tail which can stretch for a hundred million miles across space.

The comet that we build on Saturday will be a lot smaller, but the ingredients will be scientifically correct. Well begin with a big chunk of dry ice, frozen carbon dioxide. Normally, carbon dioxide is a gas, part of the air around us. However, when cooled to a temperature of minus 109 degrees, carbon dioxide becomes solid. Well smash up our dry ice with a hammer then mix in a bunch of water and ammonia, just like a real comet. The dry ice will freeze them together into a big steaming icy ball, with a horrible smell from the ammonia. As we combine them, well mix in some sand and dirt: Real comets leave a trail of dust and gravel behind them as they move through space. Comets also contain a mix of complex organic molecules, so well stir in a dash of dark corn syrup for good measure. Our final result will be spewing out a stinking dry ice fog as it drips and melts its way to oblivion.

Our annual astronomy day event will run from 10 a.m. until 3 p.m. on Saturday, April 5, in Simperman Hall Room 114 on the Carroll College campus. Comet building will begin at 1 p.m., so please stop by and see for yourself what a comet looks like up close!

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See a comet up close on Astronomy Day