In the outer
Solar System, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the
Kuiper belt have been reported from the
Hubble Space Telescope observations, but these detections have been questioned, and have not yet been independently confirmed. As a comet approaches the inner Solar System,
solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of
dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the
coma, and the force exerted on the coma by the Sun's
radiation pressure and
solar wind cause an enormous
tail to form, which points away from the Sun. The streams of dust and gas each form their own distinct tails, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail, made of gases, always points along the streamlines of the solar wind as it is strongly affected by the magnetic field of the plasma of the solar wind. The ion tail follows the magnetic field lines rather than an orbital trajectory.
Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions. == Size ==