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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.
The Oort cloud, sometimes called the Öpik–Oort cloud, is theorized to be a vast cloud of icy planetesimals surrounding the Sun at distances ranging from 2,000 to 200,000 AU. The concept of such a cloud was proposed in 1950 by the Dutch astronomer Jan Oort, in whose honor the idea was named. Oort proposed that the bodies in this cloud replenish and keep constant the number of long-period comets entering the inner Solar System—where they are eventually consumed and destroyed during close approaches to the Sun.
Near-Earth Asteroid Tracking (NEAT) was a program run by NASA and the Jet Propulsion Laboratory, surveying the sky for near-Earth objects. NEAT was conducted from December 1995 until April 2007, at GEODSS on Hawaii, as well as at Palomar Observatory in California. With the discovery of more than 40 thousand minor planets, NEAT has been one of the most successful programs in this field, comparable to the Catalina Sky Survey, LONEOS and Mount Lemmon Survey.
Damocloids are a class of minor planets such as 5335 Damocles and 1996 PW that have Halley-type or long-period highly eccentric orbits typical of periodic comets such as Halley's Comet, but without showing a cometary coma or tail. David Jewitt defines a damocloid as an object with a Jupiter Tisserand invariant (TJ) of 2 or less, while Akimasa Nakamura defines this group with the following orbital elements:
166P/NEAT is a periodic comet and centaur in the outer Solar System. It was discovered by the Near Earth Asteroid Tracking (NEAT) project in 2001 and initially classified a comet with provisional designation P/2001 T4 (NEAT), as it was apparent from the discovery observations that the body exhibited a cometary coma. It is one of few known bodies with centaur-like orbits that display a coma, along with 60558 Echeclus, 2060 Chiron, 165P/LINEAR and 167P/CINEOS. It is also one of the reddest centaurs.
In astronomy, the Hills cloud is a vast theoretical circumstellar disc, interior to the Oort cloud, whose outer border would be located at around 20,000 to 30,000 astronomical units (AU) from the Sun, and whose inner border, less well defined, is hypothetically located at 250–1500 AU, well beyond planetary and Kuiper Belt object orbits—but distances might be much greater. If it exists, the Hills cloud contains roughly 5 times as many comets as the Oort cloud.
2012 DR30 is a trans-Neptunian object and centaur from the scattered disk and/or inner Oort cloud, located in the outermost region of the Solar System. The object with a highly eccentric orbit of 0.99 was first observed by astronomers with the Spacewatch program at Steward Observatory on 31 March 2009. It measures approximately 188 kilometers (120 miles) in diameter.
2013 BL76 is a trans-Neptunian object and centaur from the scattered disk and Inner Oort cloud approximately 30 kilometers in diameter.
2013 AZ60 is a small Solar System body (extended centaur) from the scattered disk or inner Oort cloud. 2013 AZ60 has the 8th-largest semi-major axis of a minor planet not detected outgassing like a comet (2013 BL76, 2005 VX3 and 2012 DR30 have a larger semi-major axis).
In observational astronomy, the observation arc of a Solar System body is the time period between its earliest and latest observations, used for tracing the body's path. It is usually given in days or years. The term is mostly used in the discovery and tracking of asteroids and comets. Arc length has the greatest influence on the accuracy of an orbit. The number, spacing of intermediate observations, and timestamps have a lesser effect.
1996 PW is an exceptionally eccentric trans-Neptunian object and damocloid on an orbit typical of long-period comets but one that showed no sign of cometary activity around the time it was discovered. The unusual object measures approximately 10 kilometers in diameter and has a rotation period of 35.4 hours and likely an elongated shape.
C/2015 ER61 (PanSTARRS) is a comet and inner Oort cloud object. When classified as a minor planet, it had the fourth-largest aphelion of any known minor planet in the Solar System, after 2005 VX3, 2012 DR30, and 2013 BL76. It additionally had the most eccentric orbit of any known minor planet, with its distance from the Sun varying by about 99.9% during the course of its orbit, followed by 2005 VX3 with an eccentricity of 0.9973. On 30 January 2016, it was classified as a comet when it was 5.7 AU from the Sun. It comes close to Jupiter, and a close approach in the past threw it on the distant orbit it is on now.
C/2016 U1 (NEOWISE) is a hyperbolic comet discovered 21 October 2016 by NEOWISE, the asteroid-and-comet-hunting portion of the Wide-Field Infrared Survey Explorer (WISE) mission. The comet brightened to magnitude +6.8 and could be observed with binoculars, during the first week of 2017 and it was closest to the Sun on 14 January 2017.
C/2017 K2 (PanSTARRS) is an Oort cloud comet with an inbound hyperbolic orbit, discovered in May 2017 at a distance beyond the orbit of Saturn when it was 16 AU (2.4 billion km) from the Sun. Precovery images from 2013 were located by July. It had been in the constellation of Draco from July 2007 until August 2020. As of June 2022, the 3-sigma uncertainty in the current distance of the comet from the Sun is ±6000 km.
C/2017 U7 (PanSTARRS) is a hyperbolic comet, first observed on 29 October 2017 by astronomers of the Pan-STARRS facility at Haleakala Observatory, Hawaii, United States when the object was 7.8 AU (1.2 billion km) from the Sun. Despite being discovered only 10 days after interstellar asteroid 1I/'Oumuamua, it was not announced until March 2018 as its orbit is not strongly hyperbolic beyond most Oort Cloud comets. Based on the absolute magnitude of 10.6, it may measure tens of kilometers in diameter. As of August 2018, there is only 1 hyperbolic asteroid known, ʻOumuamua, but hundreds of hyperbolic comets are known.
C/2018 C2 (Lemmon) is a hyperbolic comet. It was first observed on 5 February 2018 by the Mount Lemmon Survey conducted at the Mount Lemmon Observatory near Tucson, Arizona, in the United States. The discovery was announced on 4 March 2018 along with another hyperbolic object, A/2017 U7. Based on the absolute magnitude of 15.1, it may measure several kilometers in diameter. On 22 March 2018 it was determined to be a hyperbolic comet.
C/2018 F4 (PanSTARRS) is a hyperbolic comet. It was discovered on 17 March 2018 when it was beyond the orbit of Jupiter, 6.4 AU (960 million km) from the Sun. It was quite far from the Sun and turned out to simply be an asteroidal object that was discovered before cometary activity was noticeable. As perihelion is inside the orbit of Jupiter, this object should become more active. In April 2018 it was determined to be a hyperbolic comet. Given that the incoming velocity was similar to that of an Oort cloud object, we can very confidently say that it is not of interstellar origin. C/2018 F4 fragmented around August 2020.
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.
C/2021 O3 (PanSTARRS) is perhaps an Oort cloud comet, discovered on 26 July 2021 by the Pan-STARRS sky survey. It came to perihelion on 21 April 2022 at 0.287 AU (42.9 million km). from the Sun.
References
- ↑ "Orbit like a comet, rocky like an asteroid". earthsky.org. 3 May 2016.
- ↑ Meech, Karen (November 2014). "C/2013 P2 Pan STARRS - The Manx Comet". AAS/Division for Planetary Sciences Meeting Abstracts #46. 46: 200.02. Bibcode:2014DPS....4620002M.
- ↑ "First Observations of the Surfaces of Objects from the Oort Cloud". Institute for Astronomy, University of Hawaii at Manoa. 10 November 2014.
- ↑ Meech, Karen (29 April 2016). "Inner solar system material discovered in the Oort cloud". Science Advances. 2 (4): e1600038. Bibcode:2016SciA....2E0038M. doi:10.1126/sciadv.1600038. PMC 4928888 . PMID 27386512.
- ↑ "Comet with Stunted Tail Hints at How Solar System Formed". Astrobiology at NASA. 23 May 2016. Archived from the original on April 27, 2017. Retrieved 23 January 2019.
