Comet tail
A comet tail is a projection of material from a comet that often becomes visible when illuminated by the Sun, while the comet passes through the inner Solar System. As a comet approaches the Sun, solar radiation causes the volatile materials within the comet to vaporize and stream out of the comet nucleus, carrying dust away with them.
Blown by the solar wind, these materials typically form two separate tails that extend outwards from the comet's orbit: the dust tail, composed of comet dust, and the gas or ion tail, composed of ionized gases. They become visible through different mechanisms: the dust tail reflects sunlight directly, while the gas tail glows because of the ionization.
Larger dust particles are less affected by solar wind and tend to persist along the comet's trajectory, forming a dust trail which, when seen from Earth in certain conditions, appears as an anti-tail extending in the opposite directions to the main tail.
Tail formation
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.
Anti-tail
The anti-tail is an apparent spike extending from the coma towards the Sun, and therefore in the opposite direction to the gas and dust tails. The anti-tail consists of larger dust particles left behind by the comet. These dust particles are less affected by the Sun's radiation pressure and tend to remain roughly in the comet's orbital plane and eventually form a disc along the comet's orbit due to the ejection speed of the particles from the comet's surface. As Earth passes through the comet's orbital plane, this disc is seen side on, and appears as the characteristic spike.The other side of the disc can sometimes be seen, though it tends to be lost in the dust tail. The anti-tail is therefore normally visible for a brief interval only when Earth passes through the comet's orbital plane.
Most comets do not develop sufficiently for an anti-tail to become visible, but notable comets that did display anti-tails include Arend–Roland in 1957, Kohoutek in 1973, Hale–Bopp in 1997, C/1999 H1 in 1999, Lulin in 2009, PANSTARRS and C/2022 E3 in 2023, 12P/Pons–Brooks and C/2023 A3 Tsuchinshan–ATLAS in 2024, and 3I/ATLAS in 2025.
Size
While the solid nucleus of comets is generally less than 30 km across, the coma may be larger than the Sun, and ion tails have been observed to extend.The Ulysses spacecraft made an unexpected pass through the tail of the comet C/2006 P1, on February 3, 2007. Evidence of the encounter was published in the October 1, 2007, issue of The Astrophysical Journal.