Rock comet

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A rock comet is a rare type of small Solar System body that exhibits features of both a comet and an asteroid, mainly in that it outgasses material primarily made up of grains of rock.

Small Solar System body object in the Solar System that is neither a planet, nor a dwarf planet, nor a satellite

A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first defined in 2006 by the International Astronomical Union (IAU) as follows: "All other objects, except satellites, orbiting the Sun shall be referred to collectively as 'Small Solar System Bodies' ".

Comet Icy small Solar System body

A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° across the sky. Comets have been observed and recorded since ancient times by many cultures.

Asteroid minor planet that is not a comet

Asteroids are minor planets, especially of the inner Solar System. Larger asteroids have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not resemble a planet-like disc and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered they were typically found to have volatile-rich surfaces similar to comets. As a result, they were often distinguished from objects found in the main asteroid belt. In this article, the term "asteroid" refers to the minor planets of the inner Solar System including those co-orbital with Jupiter.

The cause of rock comets' outgassing is similar to how mud in a dry lake bottom cracks. Cracks in the mud - geograph.org.uk - 1271501.jpg
The cause of rock comets' outgassing is similar to how mud in a dry lake bottom cracks.

Rock comets, unlike other comets, which outgas primarily ice, have a nucleus made of rock. As a result, their outgassing can be unpredictable.

Ice water frozen into the solid state

Ice is water frozen into a solid state. Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaque bluish-white color.

Comet nucleus the solid, central part of a comet

The nucleus is the solid, central part of a comet, popularly 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 sublimate 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.

Outgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. Outgassing can include sublimation and evaporation, as well as desorption, seepage from cracks or internal volumes, and gaseous products of slow chemical reactions. Boiling is generally thought of as a separate phenomenon from outgassing because it consists of a phase transition of a liquid into a vapor of the same substance.

The reason this happens is thought to be similar to how mud cracks at the bottom of a dry lake bed, leaving small chunks loose on the surface of the body, which later get pushed off it due to radiation pressure sweeping. [1]

Radiation pressure pressure exerted upon any surface exposed to electromagnetic radiation

Radiation pressure is the pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength which is absorbed, reflected, or otherwise emitted by matter on any scale.

Sometimes, an object can be mistakenly identified as a rock comet, such as P/2010 A2, an asteroid in the asteroid belt that collided with another object, causing it to have a field of debris trailing it for a short time. [2]

Asteroid belt the circumstellar disk (accumulation of matter) in an orbit between those of Mars and Jupiter

The asteroid belt is the circumstellar disc in the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The asteroid belt is also termed the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such as near-Earth asteroids and trojan asteroids. About half the mass of the belt is contained in the four largest asteroids: Ceres, Vesta, Pallas, and Hygiea. The total mass of the asteroid belt is approximately 4% that of the Moon, or 22% that of Pluto, and roughly twice that of Pluto's moon Charon.

Rock comets are rare because a rocky small Solar System body must come very close to the Sun, close enough for both the solid ground to crack, and for radiation pressure sufficient to push the separate pieces off the asteroid. However, if the asteroid is close enough to the Sun for the former, the latter will also happen.

Examples

Until 2016, the only known example of a rock comet was 3200 Phaethon, which has a perihelion (closest point to the Sun) closer than any other named asteroid (0.14 AU). It has been known to brighten on occasions, implying outgassing, and has been observed ejecting dust by the STEREO spacecraft. [3] [4] In 2016, observations of 322P/SOHO by the Spitzer Space Telescope showed it to most likely be a rock comet as well, which would make it the lowest-perihelion "asteroid", with a perihelion of 0.054 AU, even though it was initially identified as a comet. [5]

3200 Phaethon Apollo asteroid notable for its very small perihelion and for being the progenitor of the Geminid meteor shower

3200 Phaethon, provisional designation 1983 TB, is an Apollo asteroid with an orbit that brings it closer to the Sun than any other named asteroid. For this reason, it was named after the Greek myth of Phaëthon, son of the sun god Helios. It is 5.8 km (3.6 mi) in diameter and is the parent body of the Geminids meteor shower of mid-December. With an observation arc of 30+ years, it has a very well determined orbit. The 2017 Earth approach distance of about 10 million km was known with an accuracy of ±40 km.

Astronomical unit mean distance between Earth and the Sun, common length reference in astronomy

The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year. Originally conceived as the average of Earth's aphelion and perihelion, since 2012 it has been defined as exactly 149597870700 metres or about 150 million kilometres. The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. It is also a fundamental component in the definition of another unit of astronomical length, the parsec.

STEREO Pair of NASA spacecraft tasked with observing the Sun

STEREO is a solar observation mission. Two nearly identical spacecraft were launched in 2006 into orbits around the Sun that cause them to respectively pull farther ahead of and fall gradually behind the Earth. This enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections.

Several unnamed asteroids travel closer to the Sun than 3200 Phaethon, but none have been observed to outgas, which does not imply they do not, because none of these have been observed in detail, and it is possible that detailed observations could show some outgassing.

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Comet Hale–Bopp comet

Comet Hale–Bopp is a comet that was perhaps the most widely observed of the 20th century, and one of the brightest seen for many decades.

2060 Chiron minor planet in the outer Solar System

2060 Chiron, provisional designation 1977 UB, and also known as 95P/Chiron, is a 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.

7968 Elst–Pizarro asteroid

Comet Elst–Pizarro is a body that displays characteristics of both asteroids and comets, and is the prototype of main-belt comets. Its orbit keeps it within the asteroid belt, yet it displayed a dust tail like a comet while near perihelion in 1996, 2001, and 2007.

1566 Icarus, provisional designation 1949 MA, is an extremely eccentric asteroid, approximately 1.4 km (0.87 mi) in diameter. It is a near-Earth object of the Apollo group and the lowest numbered potentially hazardous asteroid. In 1968, it became the first asteroid ever observed by radar. Its orbit brings it closer to the Sun than Mercury and further out than the orbit of Mars, which also makes it a Mercury-, Venus-, and Mars-crosser. This stony asteroid and relatively fast rotator was discovered by German astronomer Walter Baade at the Palomar Observatory, California, on 27 June 1949. It was named after the mythological Icarus.

Sungrazing comet comet that is extremely close to the sun during part of its orbit

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.

596 Scheila main-belt asteroid

Scheila is a main-belt asteroid and main-belt comet orbiting the Sun. It was discovered on 21 February 1906 by August Kopff from Heidelberg. Kopff named the asteroid after a female English student with whom he was acquainted.

Main-belt comet bodies orbiting within the main asteroid belt which have shown cometary activity during part of their orbit

Main-belt comets (MBCs) are bodies orbiting within the asteroid belt that have shown comet-like activity during part of their orbit. The Jet Propulsion Laboratory defines a main-belt asteroid as an asteroid with a semi-major axis of more than 2 AU but less than 3.2 AU, and a perihelion of no less than 1.6 AU. David Jewitt from UCLA points out that these objects are most likely not comets with sublimating ice, but asteroids that exhibit dust activity, and hence he and others started calling these class of objects active asteroids.

Extinct comet Comet that lacks typical activity

An extinct comet is a comet that has expelled most of its volatile ice and has little left to form a tail and coma. In a dormant comet, rather than being depleted, any remaining volatile components have been sealed beneath an inactive surface layer.

354P/LINEAR small solar system body

354P/LINEAR is a small Solar System body that displayed characteristics of both an asteroid and a comet, and thus, was initially given a cometary designation. Because it has the orbit of a main-belt asteroid and showed the tail of a comet, it was listed as a main-belt comet. But within a month of discovery, an analysis of images by the Hubble telescope suggested that its tail was generated by dust and gravel resulting from a recent head-on collision between asteroids rather than from sublimation of cometary ice. This was the first time a small-body collision had been observed; since then, minor planet 596 Scheila has also been seen to undergo a collision, in late 2010. The position of the nucleus was remarkable for being offset from the axis of the tail and outside the dust halo, a situation never before seen in a comet. The tail is created by millimeter-sized particles being pushed back by solar radiation pressure.

238P/Read is a main-belt comet discovered on 24 October 2005 by astronomer Michael T. Read using the Spacewatch 36-inch telescope on Kitt Peak National Observatory. It has an orbit within the asteroid belt and has displayed the coma of a traditional comet. It fits the definition of an Encke-type comet with.

C/2011 W3 (Lovejoy) Kreutz Sungrazer comet discovered in November 2011 by Terry Lovejoy

Comet Lovejoy, formally designated C/2011 W3 (Lovejoy), is a long-period comet and Kreutz sungrazer. It was discovered in November 2011 by Australian amateur astronomer Terry Lovejoy. The comet's perihelion took it through the Sun's corona on 16 December 2011, after which it emerged intact, though greatly impacted by the event.

311P/PANSTARRS comet

311P/PANSTARRS also known as P/2013 P5 (PANSTARRS) is an asteroid discovered by the Pan-STARRS telescope on 27 August 2013. Observations made by the Hubble Space Telescope revealed that it had six comet-like tails. The tails are suspected to be streams of material ejected by the asteroid as a result of a rubble pile asteroid spinning fast enough to remove material from it. This is similar to 331P/Gibbs, which was found to be a quickly-spinning rubble pile as well.

C/2017 K2 comet discovered in 2017

C/2017 K2 (PANSTARRS) is a non-periodic comet with a hyperbolic orbit, discovered in May 2017 at a distance beyond the orbit of Saturn when it was 16.09 AU from the Sun. Precovery images from 2013 were located by July. It has been in the constellation of Draco since July 2007. As of April 2018, the 3-sigma uncertainty in the current distance of the comet from the Sun is ±30000 km.

ʻOumuamua interstellar object passing through the Solar System, discovered in October 2017

ʻOumuamua is the first and currently only interstellar object detected passing through the Solar System. Formally designated 1I/2017 U1, it was discovered by Robert Weryk using the Pan-STARRS telescope at Haleakala Observatory, Hawaii, on 19 October 2017, 40 days after it passed its closest point to the Sun. When first seen, it was about 33,000,000 km from Earth, and already heading away from the Sun.

2014 FZ71 is a trans-Neptunian object, scattered disc, classified as scattered and detached object, located in the outermost region of the Solar System. It was first observed on 24 March 2014, by a team led by American astronomer Scott Sheppard at the Cerro Tololo Inter-American Observatory in Chile. With its perihelion of almost 56 AU, it belongs to a small and poorly understood group of very distant objects with moderate eccentricities. The object is not a dwarf planet candidate as it only measures approximately 150 kilometers (93 miles) in diameter.

2014 FC72 is a trans-Neptunian object, classified as a scattered and detached object, located in the outermost region of the Solar System. It was first observed on 24 March 2014, by astronomers with the Pan-STARRS survey at Haleakala Observatory, Hawaii, United States. With its perihelion distant from Neptune, it belongs to a small and poorly understood group of objects with moderate eccentricities. The possible dwarf planet measures approximately 500 kilometers (310 miles) in diameter.

2015 KQ174 is a trans-Neptunian object, both considered a scattered and detached object, located in the outermost region of the Solar System. The object with a moderately inclined and eccentric orbit measures approximately 154 kilometers (96 miles) in diameter. It was first observed on 24 May 2015, by astronomers at the Mauna Kea Observatories in Hawaii, United States.

2013 FQ28 is a trans-Neptunian object, both considered a scattered and detached object, located in the outermost region of the Solar System. It was first observed on 17 March 2013, by a team of astronomers at the Cerro Tololo Inter-American Observatory in Chile. It orbits the Sun in a moderate inclined, moderate-eccentricity orbit. The weak dwarf planet candidate measures approximately 260 kilometers (160 miles) in diameter.

References

  1. ↑ Sutherland, Paul (10 September 2013). "Why an asteroid is crumbling into meteor dust". Skymania. Retrieved 7 February 2014.
  2. ↑ Harrington, J.D.; Ray Villard (2 February 2010). "Suspected Asteroid Collision Leaves Trailing Debris". www.nasa.gov. NASA . Retrieved 8 February 2014.
  3. ↑ Jewitt, David; Li, Jing; Agarwal, Jessica (17 June 2013). "The Dust Tail of Asteroid (3200) Phaethon". The Astrophysical Journal. 771 (2): L36. arXiv: 1306.3741 . Bibcode:2013ApJ...771L..36J. doi:10.1088/2041-8205/771/2/L36.
  4. ↑ Jewitt, David; Jing Li (14 September 2010). "Activity in Geminid Parent (3200) Phaethon". The Astronomical Journal. 140 (5): 1519. arXiv: 1009.2710 . Bibcode:2010AJ....140.1519J. doi:10.1088/0004-6256/140/5/1519.
  5. ↑ Knight, Matthew M.; Fitzsimmons, Alan; Kelley, Michael S. P.; Snodgrass, Colin (23 April 2016). "COMET 322P/SOHO 1: AN ASTEROID WITH THE SMALLEST PERIHELION DISTANCE?". The Astrophysical Journal Letters. arXiv: 1604.07790 . Bibcode:2016ApJ...823L...6K. doi:10.3847/2041-8205/823/1/l6.

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