Acomet tail is a projection of material from acomet that often becomes visible when illuminated by theSun, while the comet passes through the innerSolar System. As a comet approaches the Sun,solar radiation causes thevolatile materials within the comet tovaporize and stream out of thecomet nucleus, carrying dust away with them.
Blown by thesolar wind, these materials typically form two separate tails that extend outwards from the comet's orbit: the dust tail, composed ofcomet dust, and the gas or ion tail, composed ofionized 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 ananti-tail (or antitail) extending in the opposite directions to the main tail.
In the outerSolar 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 theKuiper belt have been reported from theHubble 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 ofdust and gas thus released form a huge, extremely tenuous atmosphere around the comet called thecoma, and the force exerted on the coma by the Sun'sradiation pressure andsolar wind cause an enormoustail 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]
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 largerdust particles left behind by the comet. These dust particles are less affected by the Sun'sradiation pressure and tend to remain roughly in the comet'sorbital 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.[6]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.[7][8]
Most comets do not develop sufficiently for an anti-tail to become visible, but notable comets that did display anti-tails includeArend–Roland in 1957,[9]Kohoutek in 1973,[10]Hale–Bopp in 1997, C/1999 H1 (Lee) in 1999,[11]Lulin in 2009,PANSTARRS andC/2022 E3 (ZTF) in 2023,[12]12P/Pons–Brooks andC/2023 A3 Tsuchinshan–ATLAS in 2024,[13][14] and3I/ATLAS in 2025.[15][16]
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 3.8astronomical units (570 Gm; 350×10^6 mi).[17]
TheUlysses spacecraft made an unexpected pass through the tail of the cometC/2006 P1 (Comet McNaught), on February 3, 2007.[18] Evidence of the encounter was published in the October 1, 2007, issue ofThe Astrophysical Journal.[19][relevant?]
The observation of antitails contributed significantly to the discovery ofsolar wind.[20] The ion tail is the result ofultraviolet radiation ejecting electrons off particles in the coma. Once the particles have been ionised, they form a plasma which in turn induces amagnetosphere 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 abow 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 withplasma. The field lines "drape" around the comet forming the ion tail.[21] (This is similar to the formation of planetary magnetospheres.)
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".[21] This has been observed on a number of occasions, notable among which was on April 20, 2007, when the ion tail ofcomet Encke was completely severed as the comet passed through acoronal mass ejection.[22] This event was observed by theSTEREO spacecraft.[23] A disconnection event was also seen withC/2009 R1 (McNaught) on May 26, 2010.[24]
Venus possesses a similar tail due to theinduced magnetosphere formed by interaction of the solar wind with the venusian atmosphere. On January 29, 2013,ESA scientists reported that theionosphere of the planetVenus streams outwards in a manner similar to "the ion tail seen streaming from acomet under similar conditions."[25][26] While Mercury lacks an atmosphere, theMESSENGER 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'smagnetotail.[27][citation needed]
[drawing on page 403] the Skylab crewmen observed a brilliant spike or anti tail projecting toward the Sun from the head of Kohoutek [...] Dr Zdenek Sekanina concluded that the spike was not wholly due to a perspective or geometrical effect as has been assumed in previous comet studies.
The comet displayed a strikingly narrow antitail the night of October 14th
this approximately sunward feature is reminiscent of the distant activity of other comets
This type of anti-tail, not a result of perspective, may not have been previously observed. We explain the anti-tail as an anisotropic extension of the snow line, or survival radius of a sublimating ice grain, in the direction of the Sun.
.png)
