The links below will get you started.
Some people think when they see a comet it is bad luck.
This site tells us about what early man thought about comets. Assume a Spherical Comet - This site is about how a farmer couldn't stop his chickens from getting in the road. If you build a fence to block off the road the chickens won't cross. Mankind has the same problem as the farmer. Man has to decide how to block comets from entering Earth Authmosphere.