So what happens when a comet’s elliptical orbit brings it by our neck of the woods? That’s when the action can really kick in. Some experts refer to a comet as a "dirty snowball." Its core is made up of rock and ice made from water, ammonia, methane and more. Surrounding the core is a coma made up of heated gases and dust. When the comet approaches the sun, heat and wind push the gases of the coma away from the comet, and this dust forms the tail.
Pictured here is the Comet Siding Spring, also known as C/2007 Q3, which was first discovered in 2007. On Oct. 7, 2009, it passed by the Earth and sun before heading back to colder parts of outer space. Its tail to thought to be 10 million miles long.