A great comet is a comet that becomes exceptionally bright. There is no official definition; often the term is attached to comets such as Halley's Comet, which during certain appearances are bright enough to be noticed by casual observers who are not looking for them, and become well known outside the astronomical community. Typically, they are as bright or brighter than a second magnitude star and have tails that are 10 degrees or longer under dark skies.[1] Great comets appear at irregular, unpredictable intervals, on average about once per decade. Although comets are officially named after their discoverers, great comets are sometimes also referred to by the year in which they appeared great, using the formulation "The Great Comet of ...", followed by the year. It can also be used as a generic name when a very bright comet is discovered by many observers simultaneously.[2]
The vast majority of comets are never bright enough to be seen by the naked eye, and generally pass through the inner Solar System unseen by anyone except astronomers. However, occasionally a comet may brighten to naked eye visibility, and even more rarely it may become as bright as or brighter than the brightest stars. The requirements for this to occur are: a large and active nucleus, a close approach to the Sun, and a close approach to the Earth. A comet fulfilling all three of these criteria will certainly be very bright. Sometimes, a comet failing on one criterion will still be bright. For example, Comet Hale–Bopp did not approach the Sun very closely, but had an exceptionally large and active nucleus. It was visible to the naked eye for several months and was very widely observed. Similarly, Comet Hyakutake was a relatively small comet, but appeared bright because it passed very close to the Earth.
Size and activity of the nucleus
Cometary nuclei vary in size from a few hundreds of metres across or less to many kilometres across. When they approach the Sun, large amounts of gas and dust are ejected by cometary nuclei, due to solar heating. A crucial factor in how bright a comet becomes is how large and how active its nucleus is. After many returns to the inner Solar System, cometary nuclei become depleted in volatile materials and thus are much less bright than comets which are making their first passage through the Solar System.
The sudden brightening of Comet Holmes in 2007 showed the importance of the activity of the nucleus in the comet's brightness. On October 23–24, 2007, the comet underwent a sudden outburst which caused it to brighten by factor of about half a million. It unexpectedly brightened from an apparent magnitude of about 17 to about 2.8 in a period of only 42 hours, making it visible to the naked eye. All these temporarily made comet 17P the largest (by radius) object in the Solar System although its nucleus is estimated to be only about 3.4km in diameter.
Close perihelion approach
The brightness of a simple reflective body varies with the inverse square of its distance from the Sun. That is, if an object's distance from the Sun is halved, its brightness is quadrupled. However, comets behave differently, due to their ejection of large amounts of volatile gas which then also reflect sunlight and may also fluoresce. Their brightness varies roughly as the inverse cube of their distance from the Sun, meaning that if a comet's distance from the Sun is halved, it will become eight times as bright.
This means that the peak brightness of a comet depends significantly on its distance from the Sun. For most comets, the perihelion of their orbit lies outside the Earth's orbit. Any comet approaching the Sun to within 0.5AU (75millionkm) or less may have a chance of becoming a great comet.
Close approach to the Earth
For a comet to become very bright, it also needs to pass close to the Earth. Halley's Comet, for example, is usually very bright when it passes through the inner Solar System every seventy-six years, but during its 1986 apparition, its closest approach to Earth was almost the most distant possible. The comet became visible to the naked eye, but was unspectacular. On the other hand, the intrinsically small and faint Comet Hyakutake (C/1996 B2) appeared very bright and spectacular due to its very close approach to Earth at its nearest during March 1996. Its passage near the Earth was one of the closest cometary approaches on record with a distance of 0.1AU (15millionkm; 39LD).
↑ Ramsey, John T. & Licht, A. Lewis (1997), The Comet of 44 B.C. and Caesar's Funeral Games, Atlanta, ISBN0-7885-0273-5{{citation}}: CS1 maint: location missing publisher (link).
↑ The Living Age, Volume 58. Lithotyped by Cowles and Company, 17 Washington St., Boston. Press of Geo. C. Rand & Avery. 1858. p.879.
A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, and sometimes a tail of gas and dust gas blown out from the coma. These phenomena are due to the effects of solar radiation and the outstreaming solar wind plasma acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit. If sufficiently close and bright, a comet may be seen from Earth without the aid of a telescope and can subtend an arc of up to 30° across the sky. Comets have been observed and recorded since ancient times by many cultures and religions.
Comet Hale–Bopp is a long-period comet that was one of the most widely observed of the 20th century and one of the brightest seen for many decades.
Halley's Comet is the only known short-period comet that is consistently visible to the naked eye from Earth, appearing every 72–80 years, though with the majority of recorded apparations occurring after 75–77 years. It last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061. Officially designated 1P/Halley, it is also commonly called Comet Halley, or sometimes simply Halley.
Comet Hyakutake is a comet discovered on 31 January 1996. It was dubbed the Great Comet of 1996; its passage to within 0.1 AU (15 Gm) of the Earth on 25 March was one of the closest cometary approaches of the previous 200 years. Reaching an apparent visual magnitude of zero and spanning nearly 80°, Hyakutake appeared very bright in the night sky and was widely seen around the world. The comet temporarily upstaged the much anticipated Comet Hale–Bopp, which was approaching the inner Solar System at the time.
Comet Ikeya–Seki, formally designated C/1965 S1, 1965 VIII, and 1965f, was a long-period comet discovered independently by Kaoru Ikeya and Tsutomu Seki. First observed as a faint telescopic object on September 18, 1965, the first calculations of its orbit suggested that on October 21, it would pass just 450,000 km (280,000 mi) above the Sun's surface, and would probably become extremely bright.
12P/Pons–Brooks is a periodic comet with an orbital period of 71 years. Comets with an orbital period of 20–200 years are referred to as Halley-type comets. It is one of the brightest known periodic comets, reaching an absolute visual magnitude of about 5 in its approach to perihelion. Comet Pons-Brooks was conclusively discovered at Marseilles Observatory in July 1812 by Jean-Louis Pons, and on its next appearance in 1883 by William Robert Brooks. There are ancient records of comets that are suspected of having been apparitions of 12P/Pons–Brooks.
A sungrazing comet is a comet that passes extremely close to the Sun at perihelion – sometimes within a few thousand kilometres of the Sun's surface. Although small sungrazers can completely evaporate during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong evaporation and tidal forces they experience often lead to their fragmentation.
The Kreutz sungrazers are a family of sungrazing comets, characterized by orbits taking them extremely close to the Sun at perihelion. At the far extreme of their orbits, aphelion, Kreutz sungrazers can be a hundred times farther from the Sun than the Earth is, while their distance of closest approach can be less than twice the Sun's radius. They are believed to be fragments of one large comet that broke up several centuries ago and are named for German astronomer Heinrich Kreutz, who first demonstrated that they were related. These sungrazers make their way from the distant outer Solar System to the inner Solar System, to their perihelion point near the Sun, and then leave the inner Solar System in their return trip to their aphelion.
10P/Tempel, also known as Tempel 2, is a periodic Jupiter-family comet with a 5-year orbital period. It was discovered on July 4, 1873 by Wilhelm Tempel. At the perihelion passage on 2 August 2026 the solar elongation is calculated at 164 degrees, with apparent magnitude approximately 8, with closest approach to Earth on 3 August 2026 at a distance of 0.414 AU (61.9 million km).
The nucleus is the solid, central part of a comet, formerly termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublime and produce an atmosphere surrounding the nucleus known as the coma. 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. A typical comet nucleus has an albedo of 0.04. This is blacker than coal, and may be caused by a covering of dust.
13P/Olbers is a periodic comet with an orbital period of 69 years. It fits the classical definition of a Halley-type comet with a period between 20 and 200 years. The comet last passed perihelion 30 June 2024 and it was previously seen in 1956. The next perihelion is in 2094.
Comet Holmes is a periodic comet in the Solar System, discovered by the British amateur astronomer Edwin Holmes on November 6, 1892. Although normally a very faint object, Holmes became notable during its October 2007 return when it temporarily brightened by a factor of a million, in what was the largest known outburst by a comet, and became visible to the naked eye. It also briefly became the largest object in the Solar System, as its coma expanded to a diameter greater than that of the Sun. Between 1857–2106 perihelion remains between 2.05–2.36 AU.
C/2011 L4 (PanSTARRS), also known as Comet PANSTARRS, is a non-periodic comet discovered in June 2011 that became visible to the naked eye when it was near perihelion in March 2013. It was discovered using the Pan-STARRS telescope located near the summit of Haleakalā, on the island of Maui in Hawaii. Comet C/2011 L4 probably took millions of years to come from the Oort cloud. After leaving the planetary region of the Solar System, the post-perihelion orbital period is estimated to be roughly 107000 years. Dust and gas production suggests the comet nucleus is roughly 1 kilometer (0.62 mi) in diameter, while based on the absolute nuclear magnitude and a geometric albedo of 0.04 the diameter of the nucleus is over 2.4 kilometers (1.5 mi). A method based on coma magnitude decay function estimated the effective radius at 2.317 ± 0.190 km.
Comet ISON, formally known as C/2012 S1, was a sungrazing comet from the Oort cloud which was discovered on 21 September 2012 by Vitaly Nevsky and Artyom Novichonok.
C/2019 Y4 (ATLAS) was a comet with a near-parabolic orbit discovered by the ATLAS survey on December 28, 2019. Early predictions based on the brightening rate suggested that the comet could become as bright as magnitude 0 matching the brightness of Vega. It received widespread media coverage due to its dramatic increase in brightness and orbit similar to the Great Comet of 1844, but on March 22, 2020, the comet started disintegrating. Such fragmentation events are very common for Kreutz Sungrazers. The comet continues to fade and did not reach naked eye visibility. By mid-May, comet ATLAS appeared very diffuse even in a telescope. C/2019 Y4 (ATLAS) has not been seen since May 21, 2020.
C/2020 F3 (NEOWISE) or Comet NEOWISE is a long period comet with a near-parabolic orbit discovered on March 27, 2020, by astronomers during the NEOWISE mission of the Wide-field Infrared Survey Explorer (WISE) space telescope. At that time, it was an 18th-magnitude object, located 2 AU away from the Sun and 1.7 AU away from Earth.
C/2021 A1 (Leonard) was a long period comet that was discovered by G. J. Leonard at the Mount Lemmon Observatory on 3 January 2021 when the comet was 5 AU (750 million km) from the Sun. It had a retrograde orbit. The nucleus was about 1 km across. It came within 4 million km (2.5 million mi) of Venus, the closest-known cometary approach to Venus.
C/2014 UN271 (Bernardinelli–Bernstein), simply known as C/2014 UN271 or Comet Bernardinelli–Bernstein (nicknamed BB), is a large Oort cloud comet discovered by astronomers Pedro Bernardinelli and Gary Bernstein in archival images from the Dark Energy Survey. When first imaged in October 2014, the object was 29 AU (4.3 billion km; 2.7 billion mi) from the Sun, almost as far as Neptune's orbit and the greatest distance at which a comet has been discovered. With a nucleus diameter of at least 120 km (75 mi), it is the largest Oort cloud comet known. It is approaching the Sun and will reach its perihelion of 10.9 AU (just outside of Saturn's orbit) in January 2031. It will not be visible to the naked eye because it will not enter the inner Solar System.
Comet Kohoutek is a comet that passed close to the Sun towards the end of 1973. Early predictions of the comet's peak brightness suggested that it had the potential to become one of the brightest comets of the 20th century, capturing the attention of the wider public and the press and earning the comet the moniker of "Comet of the Century". Although Kohoutek became rather bright, the comet was ultimately far dimmer than the optimistic projections: its apparent magnitude peaked at only –3 and it was visible for only a short period, quickly dimming below naked-eye visibility by the end of January 1974.
C/2023 A3 (Tsuchinshan–ATLAS) also known as the Great Comet of 2024 is a comet from the Oort cloud discovered by the Purple Mountain Observatory in China on 9 January 2023 and independently found by ATLAS South Africa on 22 February 2023. The comet passed perihelion at a distance of 0.39 AU on 27 September 2024, when it became visible to the naked eye. Tsuchinshan-ATLAS peaked its brightest magnitude on 9 October, shortly after passing the Sun, with a magnitude of −4.9 per reported observations at the Comet Observation Database (COBS).
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