Comets Comets have a very small nucleus, a coma of gas and dust that is the most visible part and can be very large, a hydrogen envelope, a dust tail, and an ion tail. Comets The comet’s tail always points away from the Sun, driven by the solar wind and radiation pressure.  Comets have very eccentric orbits.  Long-period comets (C/) have periods of hundreds of thousands, or even millions, of years.  Short-period comets (P/) are less common, and have periods of less than 200 years.