comet tailsTails of gas and dust that point in the antisolar direction away from the cometary nucleus and appear only when a comet is near the Sun, i.e. closer than about 2 AU (see illustration). Not all comets have tails. The luminosity is due to both molecular and atomic emission and to reflection of sunlight. The gas and dust is forced away by radiation pressure and by solar-wind interactions. There are two markedly different types of comet tails. The ion tail (or plasma tail) is usually straight and bluish, showing many diverse visible and rapidly changing structures. It consists exclusively of ionized molecules, moving at velocities between 10 and 100 km s–1, the repulsive force being 20 to 100 times greater than gravity. The tail can be a million km wide and lengths of 10 million km are not uncommon. Sometimes large sections of the ion tail can break away (see disconnection event). Dust tails are more strongly curved than ion tails. They contain solid particles, which simply reflect sunlight. Particles are ejected from the nucleus along curved paths known as syndynames. The tail is an envelope over the reflection regions of particles of similar mass. The closer the dust tail is to the ion tail in curvature the smaller are the dust particles responsible for reflecting sunlight; they are usually 10–100 micrometers in diameter.