An asteroid is a minor planet—an object that is neither a true planet nor an identified comet— that orbits within the inner Solar System. They are rocky, metallic, or icy bodies with no atmosphere. The size and shape of asteroids vary significantly, ranging from small rubble piles under a kilometer across and larger than meteoroids, to Ceres, a dwarf planet almost 1000 km in diameter.
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.
2060 Chiron is a ringed small Solar System body in the outer Solar System, orbiting the Sun between Saturn and Uranus. Discovered in 1977 by Charles Kowal, it was the first-identified member of a new class of objects now known as centaurs—bodies orbiting between the asteroid belt and the Kuiper belt. Chiron is named after the centaur Chiron in Greek mythology.
In planetary astronomy, a centaur is a small Solar System body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Centaurs generally have unstable orbits because they cross or have crossed the orbits of the giant planets; almost all their orbits have dynamic lifetimes of only a few million years, but there is one known centaur, 514107 Kaʻepaokaʻawela, which may be in a stable orbit. Centaurs typically exhibit the characteristics of both asteroids and comets. They are named after the mythological centaurs that were a mixture of horse and human. Observational bias toward large objects makes determination of the total centaur population difficult. Estimates for the number of centaurs in the Solar System more than 1 km in diameter range from as low as 44,000 to more than 10,000,000.
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:
Hubble search for transition comets was a study involving amateur astronomers and the use of the Hubble Space Telescope, one of only six studies involving amateur astronomers approved by NASA.
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.
60558 Echeclus is a centaur, approximately 84 kilometers (52 miles) in diameter, located in the outer Solar System. It was discovered by Spacewatch in 2000 and initially classified as a minor planet with provisional designation 2000 EC98 (also written 2000 EC98). Research in 2001 by Rousselot and Petit at the Besançon observatory in France indicated that it was not a comet, but in December 2005 a cometary coma was detected. In early 2006 the Committee on Small Bodies Nomenclature (CSBN) gave it the cometary designation 174P/Echeclus. It last came to perihelion in April 2015, and was expected to reach about apparent magnitude 16.7 near opposition in September 2015.
14827 Hypnos is a highly eccentric, sub-kilometer-sized carbonaceous asteroid that is thought to be an extinct comet. It is classified as near-Earth object and potentially hazardous asteroid of the Apollo group.
7604 Kridsadaporn, provisional designation 1995 QY2, is an unusual, carbonaceous asteroid and Mars-crosser on a highly eccentric orbit from the outer regions of the asteroid belt, approximately 12 kilometers (7.5 miles) in diameter. It was discovered on 31 August 1995, by Australian astronomer Robert McNaught at Siding Spring Observatory near Coonabarabran, Australia. Due to its particular orbit, the C-type asteroid belongs to MPC's list of "other" unusual objects, and has been classified as an "asteroid in cometary orbit", or ACO. The asteroid was named in memory of Thai astronomer Kridsadaporn Ritsmitchai.
A lost comet is one which was not detected during its most recent perihelion passage. This generally happens when data is insufficient to reliably calculate the comet's location or if the solar elongation is unfavorable near perihelion passage. The D/ designation is used for a periodic comet that no longer exists or is deemed to have disappeared.
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.
C/2001 OG108 (LONEOS) is a Halley-type comet with an orbital period of 48.51 years. It was discovered on 28 July 2001 by the LONEOS telescope at Lowell Observatory. Of the short-period comets with known diameters and perihelion inside the orbit of Earth, C/2001 OG108 is the second largest after Comet Swift–Tuttle.
C/2013 UQ4 (Catalina) is a Solar System comet that came close to the Earth on July 10, 2014, at a distance of 0.314 AU (47,000,000 km; 29,200,000 mi).
2000 DG8 is a dark centaur and damocloid on a retrograde and highly eccentric orbit from the outer region of the Solar System. It was first observed on 25 February 2000, by astronomers with the LINEAR program at the Lincoln Lab's ETS near Socorro, New Mexico, United States. It has not been observed since 2001. The unusual object measures approximately 16 kilometers (9.9 miles) in diameter.
2011 MM4, provisional designation: 2011 MM4, is a sizable centaur and retrograde damocloid from the outer Solar System, approximately 64 kilometers (40 miles) in diameter. It was discovered on 24 June 2011, by astronomers with the Pan-STARRS 1 at the Haleakala Obs. in Hawaii.
In planetary science, the term unusual minor planet, or unusual object, is used for a minor planet that possesses an unusual physical or orbital characteristic. For the Minor Planet Center (MPC), which operates under the auspices of the International Astronomical Union, any non-classical main-belt asteroid, which account for the vast majority of all minor planets, is an unusual minor planet. These include the near-Earth objects and Trojans as well as the distant minor planets such as centaurs and trans-Neptunian objects. In a narrower sense, the term is used for a group of bodies – including main-belt asteroids, Mars-crossers, centaurs and otherwise non-classifiable minor planets – that show a high orbital eccentricity, typically above 0.5 and/or a perihelion of less than 6 AU. Similarly, an unusual asteroid (UA) is an inner Solar System object with a high eccentricity and/or inclination but with a perihelion larger than 1.3 AU, which does exclude the near-Earth objects.
Chimera is a NASA mission concept to orbit and explore 29P/Schwassmann-Wachmann 1 (SW1), an active, outbursting small icy body in the outer Solar System. The concept was developed in response to the 2019 NASA call for potential missions in the Discovery-class, and it would have been the first spacecraft encounter with a Centaur and the first orbital exploration of a small body in the outer Solar System. The Chimera proposal was ranked in the first tier of submissions, but was not selected for further development for the programmatic reason of maintaining scientific balance.
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.
(523676) 2013 UL10 (prov. designation:2013 UL10) is a reddish centaur with cometary activity orbiting the Sun between Jupiter and Uranus. It was discovered on 18 August 2010, by a team of astronomers with the Pan-STARRS survey at the Haleakalā Observatory, Hawaii. It is the first centaur known to have both comet-like activity and red surface colors. It is also one of the smallest centaurs, with a nucleus of no more than 10 kilometers (6.2 miles) in diameter. As of 2021, it has not been named.
