A comet is a small-sized (10 kms or less in diameter) celestial body composed of rock, ice, dust and organic compounds. This composition makes comets very dark, and hard to be seen. Sometimes comets become visible while passing near a sun: the heat is absorbed by the dark colour and the superficial ice layer sublimates, creating a flow of gas and dust that shines by reflicting light. This phenomenon has caused sentient beings to say that comets have a tail. Comet tails can be very long, up to 150 million kilometres.
Comets often travel along an elliptical and very long orbit in a star system. They can also travel along straight paths in deep space, but if they go near a gravity field of another medium- to large-sized body they can be attracted and impact on their surface or their path can bend and become elliptical.
There is not consesus on a sharp distinction between comets and asteroids. Some scientists define comets as having an extended, gravitationally unbound atmosphere surrounding their central nucleus. However, extinct comets that passed close to a sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.
Sentient beings have managed to colonise comets as well. It is not rare to see comets auctioned or raffled.
Comets as life bringers
Many scientists speculate that comets bombarding planets and moons brought quantities of water and organic molecules in significant quantities. It was also suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis. These theories lead to the possibility that the formation of life on those celestial bodies was caused by comets.