Comets are essentially big dirty snowballs, the kind you throw at someone you really don't like. They are mostly made of muddy stinky ice, and bits of dust and little rocks.
Comets are about the size of cities and towns, sort of like frozen lakes in space. We've had a few that were the size of big cities like L.A. and one that was even bigger. But the majority are Smallvilles. Some are mere villages, neighborhoods.
I'm guessing there are at least a billion comets way out there in the far reaches of the solar system, maybe a couple billion, in a region known as the Oort Cloud. The cloud is basically a swarm of the dirty little buggers surrounding all the planets and they are just orbiting around the Sun way out there where it's dark and cold. Some people say the cloud is a light year across. Some say more. Some say less. Whatever. People say a lot of things.
Sometimes the orbit of a comet gets perturbed. That's astronomy talk for when gravity does something annoying. It could be something like when several of the planets are lining up which might also explain that occasional hip pain you've been having. Or maybe comets in a bad neighborhood are having some sort of comet gang war. Or maybe our Oort cloud interacts with the Oort cloud of another star system. It's conceivable that as we bob and weave our way around the galaxy we can trade comets with another system, give them some of ours, take some of theirs, a sort of comet foreign exchange program.
Anyway, a comet's orbit can be altered in a way that brings it into the inner solar system. When comets get close enough to the Sun, the heat makes them melt and even start to evaporate. Jets of gas and dust and other crap form long tails that we can see from Earth. Usually there are separate tails for the dust and the gas, since dust and gas are of such differing densities. These tails can sometimes be millions of miles long. This is a comet's chance at long last to finally get noticed, and the same goes for astronomers. Suddenly buying a telescope wasn't so dumb after all.
When the water and gas in a comet evaporate they release and leave behind a trail of gravel in the wake. Most of the chunks are the size of your fingernails, but even dirtier. If the Earth crosses that trail of debris, we have a meteor shower as some of the bits of rock enter and burn up in our atmosphere.
Once a comet starts heading in towards the Sun it's kind of hard to stop. It either plows straight into the Sun in a screaming blaze of glory, or it winds up circling back around as a periodic event, setting up return engagements for as long as it can keep coming up with new material. It's like a blog.
A FEW FAMOUS COMETS
- Halley - regular period of 76.1 years. Last pass was in 1986. Next pass is in 2061 which I hear won't be that great, but the one in 2137 is supposed to be killer.
- Hale-Bopp - discovered in 1995 (seen before that in 2213 BC). Next pass in 4380. Hale-Bopp is freakin huge, 37 miles in diameter.
- Hyakutake - bright and fairly close in the spring 1996. This one is a slow returner. Next pass predicted to be somewhere between the years of 72,000 and 116,000.
- Encke - orbit of only 3.3 years, Recent passes: December 2003, April 2007...
- West - a very bright comet. Seen in 1975-76. Broke into pieces on last close approach of the Sun.
Here's something fun to do on a rainy day and a guarantee to make you geek of the week. It can be kind of messy, maybe really messy, so be sure to do it at your mom's house and use her best china.
Water, 2 cups
Dirt, 2 heaping tablespoons
Alcohol, 1 teaspoon
Amino acids (sugar), 1 teaspoon
Ammonia, 1 teaspoon
CO2 - frozen, crushed, 1 pound
6 quart mixing bowl
2 plastic shopping bags
heavy pair of gloves
measuring cups and spoons
Line the mixing bowl with one of the plastic bags.
Pour the water in the bowl.
Add the dirt and mix with the spoon until uniform consistency is achieved.
Add the three A’s - alcohol, ammonia, amino acids. Use non-detergent ammonia or there will be bubbles surging out everywhere.
Put about a pound of dry ice in the other plastic bag (wear the gloves). Pound the holy crap out it with the hammer until thoroughly powdered. It might help if you shout out "I am Thor and I have judged thee unworthy!"
Add the ice to the mixture and mix it in until the whole mess hardens. Pick it up and compress it firmly (again use the gloves) into a solid shape somewhere between a sphere and a potato. If you have done it right it should look like a big smokey turd. Serve on a platter (the one mom uses for turkey), observe and discuss. Since you are the first to see it, you get to name it. I named mine Big Smokey Turd.
Like any comet only 93 million miles from the Sun, it will start to melt. It will likely emit puffs of carbon dioxide gas while it sizzles, and small pockets will pop out ejecta material. Eventually it will melt completely leaving a lovely plate full of brown goo.