“Without any doubt, the regularity which astronomy shows us in the movements of the comets takes place in all phenomena. The trajectory of a simple molecule of air or vapour is regulated in a manner as certain as that of the planetary orbits; the only difference between them is that which is contributed by our ignorance. Probability is relative in part to this ignorance, and in part to our knowledge.” -Pierre-Simon Laplace
Originating from well out beyond the planets we're accustomed to, the cold, icy worlds of the outer Solar System normally roam in isolation, hardly noticed by anything at all in the Universe. But once in a while, a gravitational interaction will cause one of those bodies to fly towards the Sun.
And when it does, all sorts of interesting phenomena will develop, including tails, a coma, and a variety of spectacular colors, all taking place at breakneck speeds. What causes all of this?