Comet tail

A comet tail and coma are visible features of a comet when they are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System. 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.

Diagram of a comet showing the dust tail, gas tail, and the dust trail (or anti-tail)

The Great Comet of 2024 with tail (left) and anti-tail visible

Blown solar downwind, two separate tails are formed: one composed of dust and the other of gases. They become visible through different phenomena: the dust reflects sunlight directly, and the gases glow from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.

Tail formation

Diagram of a comet's orbit showing how the gas and dust tails develop as the comet passes the Sun

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,^[1][2] but these detections have been questioned,^[3][4] 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.^[5]

Size

Animation of a comet's tail

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 3.8 astronomical units (570 Gm; 350×10^⁶ mi).^[6]

The Ulysses spacecraft made an unexpected pass through the tail of the comet C/2006 P1 (Comet McNaught), on February 3, 2007.^[7] Evidence of the encounter was published in the October 1, 2007, issue of The Astrophysical Journal.^[8]

Magnetosphere

The observation of antitails contributed significantly to the discovery of solar wind.^[9] The ion tail is the result of ultraviolet radiation ejecting electrons off particles in the coma. Once the particles have been ionised, they form a plasma which in turn induces a magnetosphere around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. The comet is supersonic relative to the solar wind, so a bow shock is formed upstream of the comet (i.e. facing the Sun), in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma. The field lines "drape" around the comet forming the ion tail.^[10] (This is similar to the formation of planetary magnetospheres.)

Tail loss

Comet Encke loses its tail

If the ion tail loading is sufficient, then the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event".^[10] This has been observed on a number of occasions, notable among which was on the 20th of April 2007 when the ion tail of comet Encke was completely severed as the comet passed through a coronal mass ejection.^[11] This event was observed by the STEREO spacecraft.^[12] A disconnection event was also seen with C/2009 R1 (McNaught) on May 26, 2010.^[13]

Anti-tail

Diagram of a comet's tails and anti-tail

The anti-tail (or antitail), 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.^[14] 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.^[15][16]

Most comets do not develop sufficiently for an anti-tail to become visible, but notable comets that did display anti-tails include Comet Arend–Roland in 1957,^[17] Comet Kohoutek in 1973,^[18] Comet Hale–Bopp in 1997, C/1999 H1 (Lee)^[19] in 1999, Comet Lulin in 2009, Comet PANSTARRS in 2013, C/2022 E3 (ZTF) in 2023,^[20] 12P/Pons–Brooks^[21] in 2024 and C/2023 A3 Tsuchinshan–ATLAS^[22] in 2024.

Analogues

Venus possesses a similar tail due to the induced magnetosphere formed by interaction of the solar wind with the venusian atmosphere. On January 29, 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to "the ion tail seen streaming from a comet under similar conditions."^[23][24] While Mercury lacks an atmosphere, the MESSENGER mission observed magnesium and sodium flowing off the planet, along the magnetic field lines trailing behind the planet, making them the primary components of Mercury's magnetotail.^{[25][citation needed]}