Oort cloud

An artist's rendering of the Oort cloud and the Kuiper belt (inset). Sizes of individual objects have been exaggerated for visibility.

The Oort cloud /ˈɔrt/^[1] (named after the Dutch astronomer Jan Oort), or Öpik–Oort cloud,^[2] is a hypothesized spherical cloud of predominantly icy planetesimals that may lie roughly 50,000 AU, or nearly a light-year, from the Sun.^[3] This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth of the Oort cloud's distance. The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the region of the Sun's gravitational dominance.^[4]

The Oort cloud is thought to comprise two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud, or Hills cloud. Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane.

Astronomers argue that the matter composing the Oort cloud formed closer to the Sun and was scattered far out into space by the gravitational effects of the giant planets early in the Solar System's evolution.^[3] Although no confirmed direct observations of the Oort cloud have been made, astronomers argue that it is the source of all long-period and Halley-type comets entering the inner Solar System and many of the centaurs and Jupiter-family comets as well.^[5] The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way itself. These forces occasionally dislodge comets from their orbits within the cloud and send them towards the inner Solar System.^[3] Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud.^[3][5] Although the Kuiper belt and the scattered disc have been observed and mapped, only four currently known trans-Neptunian objects—90377 Sedna, 2000 CR₁₀₅, 2006 SQ₃₇₂, and 2008 KV₄₂—are considered possible members of the inner Oort cloud.^[6][7]

Hypothesis

In 1932, the Estonian astronomer Ernst Öpik postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System.^[8] In 1950, the idea was independently revived by the Dutch astronomer Jan Hendrik Oort as a means to resolve a paradox:^[9] over the course of the Solar System's existence, the orbits of comets are unstable; eventually, dynamics dictate that a comet must either collide with the Sun or a planet, or else be ejected from the Solar System by planetary perturbations. Moreover, their volatile composition means that as they repeatedly approach the Sun, radiation gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents further outgassing. Thus, Oort reasoned, a comet could not have formed while in its current orbit, and must have been held in an outer reservoir for almost all of its existence.^[9][10][11]

There are two main classes of comet, short-period comets (also called ecliptic comets) and long-period comets (also called nearly isotropic comets). Ecliptic comets have relatively small orbits, below 10 AU, and follow the ecliptic plane, the same plane in which the planets lie. Nearly all isotropic comets have very large orbits, on the order of thousands of AU, and appear from every direction in the sky.^[11] Oort noted that there was a peak in numbers of nearly isotropic comets with aphelia (their farthest distance from the Sun) of roughly 20,000 AU, which suggested a reservoir at that distance with a spherical, isotropic distribution.^[11] Those relatively rare comets with orbits of about 10,000 AU have probably gone through one or more orbits through the Solar System and have had their orbits drawn inward by the gravity of the planets.^[11]

Structure and composition

The presumed distance of the Oort cloud compared to the rest of the Solar System

The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly)^[11] to as far as 50,000 AU (0.79 ly)^[3] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly).^[11] The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0.32–0.79 ly), and a doughnut-shaped inner Oort cloud of 2,000–20,000 AU (0.03–0.32 ly). The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune.^[3] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.^[12] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;^[12][13][14] it is seen as a possible source of new comets to resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.^[15]

The outer Oort cloud is thought to contain several trillion individual objects larger than approximately 1 km (0.62 mi),^[3] and many billions with absolute magnitudes^[16] brighter than 11 (corresponding to approximately 20-kilometre (12 mi) diameter), with neighboring objects typically tens of millions of kilometres apart.^[5][17] Its total mass is not known with certainty, but, assuming that Halley's Comet is a suitable prototype for all comets within the outer Oort cloud, the estimated combined mass is 3×10²⁵ kilograms (6.6×10²⁵ pounds), or roughly five times the mass of the Earth.^[3][18] Earlier it was thought to be more massive (up to 380 Earth masses),^[19] but improved knowledge of the size distribution of long-period comets has led to much lower estimates. The mass of the inner Oort Cloud is not currently known.

If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of various ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide.^[20] However, the discovery of the object 1996 PW, an asteroid in an orbit more typical of a long-period comet, suggests that the cloud may also contain rocky objects.^[21] Analysis of the carbon and nitrogen isotope ratios in both the long-period and Jupiter-family comets shows little difference between the two, despite their presumably vastly separate regions of origin. This suggests that both originated from the original protosolar cloud,^[22] a conclusion also supported by studies of granular size in Oort-cloud comets^[23] and by the recent impact study of Jupiter-family comet Tempel 1.^[24]

Origin

The Oort cloud is thought to be a remnant of the original protoplanetary disc that formed around the Sun approximately 4.6 billion years ago.^[3] The most widely accepted hypothesis is that the Oort cloud's objects initially coalesced much closer to the Sun as part of the same process that formed the planets and asteroids, but that gravitational interaction with young gas giant planets such as Jupiter ejected the objects into extremely long elliptic or parabolic orbits.^[3][25] Recent research has been cited by NASA hypothesizing that a large number of Oort cloud objects are the product of an exchange of materials between the Sun and its sibling stars as they formed and drifted apart, and it is suggested that many—possibly the majority—of Oort cloud objects were not formed in close proximity to the Sun.^[26] Simulations of the evolution of the Oort cloud from the beginnings of the Solar System to the present suggest that the cloud's mass peaked around 800 million years after formation, as the pace of accretion and collision slowed and depletion began to overtake supply.^[3]

Models by Julio Ángel Fernández suggest that the scattered disc, which is the main source for periodic comets in the Solar System, might also be the primary source for Oort cloud objects. According to the models, about half of the objects scattered travel outward towards the Oort cloud, while a quarter are shifted inward to Jupiter's orbit, and a quarter are ejected on hyperbolic orbits. The scattered disc might still be supplying the Oort cloud with material.^[27] A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.^[28]

Computer models suggest that collisions of cometary debris during the formation period play a far greater role than was previously thought. According to these models, the number of collisions early in the Solar System's history was so great that most comets were destroyed before they reached the Oort cloud. Therefore, the current cumulative mass of the Oort cloud is far less than was once suspected.^[29] The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material.^[3]

Gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This explains the nearly spherical shape of the outer Oort cloud.^[3] On the other hand, the Hills cloud, which is bound more strongly to the Sun, has yet to acquire a spherical shape. Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster of 200–400 stars. These early stars likely played a role in the cloud's formation, since the number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations.^[30]

In June 2010 Harold F. Levison and others suggested on the basis of enhanced computer simulations that the Sun "captured comets from other stars while it was in its birth cluster". Their results imply that "a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from the protoplanetary discs of other stars".^[31]

Comets

Comet Hale–Bopp, an archetypal Oort-cloud comet

Comets are thought to have two separate points of origin in the Solar System. Short-period comets (those with orbits of up to 200 years) are generally accepted to have emerged from either the Kuiper belt or the scattered disc, which are two linked flat discs of icy debris beyond Neptune's orbit at 30 AU and jointly extending out beyond 100 AU from the Sun. Long-period comets, such as comet Hale–Bopp, whose orbits last for thousands of years, are thought to originate in the Oort cloud. The orbits within the Kuiper belt are relatively stable, and so very few comets are thought to originate there. The scattered disc, however, is dynamically active, and is far more likely to be the place of origin for comets.^[11] Comets pass from the scattered disc into the realm of the outer planets, becoming what are known as centaurs.^[32] These centaurs are then sent farther inward to become the short-period comets.^[33]

There are two main varieties of short-period comet: Jupiter-family comets (those with semi-major axes of less than 5 AU) and Halley-family comets. Halley-family comets, named for their prototype, Halley's Comet, are unusual in that while they are short-period comets, it is hypothesized that their ultimate origin lies in the Oort cloud, not in the scattered disc. Based on their orbits, it is suggested they were long-period comets that were captured by the gravity of the giant planets and sent into the inner Solar System.^[10] This process may have also created the present orbits of a significant fraction of the Jupiter-family comets, although the majority of such comets are thought to have originated in the scattered disc.^[5]

Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as "cometary fading", has yet to be resolved. No known dynamical process can explain this undercount of observed comets. Hypotheses for this discrepancy include the destruction of comets due to tidal stresses, impact or heating; the loss of all volatiles, rendering some comets invisible, or the formation of a non-volatile crust on the surface.^[34] Dynamical studies of Oort Cloud comets have shown that their occurrence in the outer-planet region is several times higher than in the inner-planet region. This discrepancy may be due to the gravitational attraction of Jupiter, which acts as a kind of barrier, trapping incoming comets and causing them to collide with it, just as it did with Comet Shoemaker–Levy 9 in 1994.^[35]

Tidal effects

Most of the comets seen close to the Sun seem to have reached their current positions through gravitational distortion of the Oort cloud by the tidal force exerted by the Milky Way. Just as the Moon's tidal force bends and deforms the Earth's oceans, causing the tides to rise and fall, the galactic tide also bends and distorts the orbits of bodies in the outer Solar System, pulling them towards the galactic centre. In the charted regions of the Solar System, these effects are negligible compared to the gravity of the Sun. At the outer reaches of the system, however, the Sun's gravity is weaker and the gradient of the Milky Way's gravitational field plays a far more noticeable role. Because of this gradient, galactic tides can deform an otherwise spherical Oort cloud, stretching the cloud in the direction of the galactic centre and compressing it along the other two axes. These small galactic perturbations may be enough to dislodge members of the Oort cloud from their orbits, sending them towards the Sun.^[36] The point at which the Sun's gravity concedes its influence to the galactic tide is called the tidal truncation radius. It lies at a radius of 100,000 to 200,000 AU, and marks the outer boundary of the Oort cloud.^[11]

Some scholars theorise that the galactic tide may have contributed to the formation of the Oort cloud by increasing the perihelia—closest distances to the Sun—of planetesimals with large aphelia.^[37] The effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system. Cumulatively, however, the effect can be quite significant: up to 90% of all comets originating from the Oort cloud may be the result of the galactic tide.^[38] Statistical models of the observed orbits of long-period comets argue that the galactic tide is the principal means by which their orbits are perturbed toward the inner Solar System.^[39]

Star perturbations and stellar companion hypotheses

Besides the galactic tide, the main trigger for sending comets into the inner Solar System is thought to be interaction between the Sun's Oort cloud and the gravitational fields of nearby stars^[3] or giant molecular clouds.^[35] The orbit of the Sun through the plane of the Milky Way sometimes brings it in relatively close proximity to other stellar systems. For example, during the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud is Gliese 710.^[40] This process also serves to scatter the objects out of the ecliptic plane, potentially also explaining the cloud's spherical distribution.^[40][41]

In 1984, Physicist Richard A. Muller postulated that the Sun has a heretofore undetected companion, either a brown dwarf or a red dwarf, in an elliptical orbit within the Oort cloud. This object, known as Nemesis, was hypothesized to pass through a portion of the Oort cloud approximately every 26 million years, bombarding the inner Solar System with comets. However, to date no evidence of Nemesis has been found, and many lines of evidence (such as crater counts), have thrown its existence into doubt.^[42][43] Recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals.^[44] Thus, the Nemesis hypothesis is no longer needed.^[44]

A somewhat similar hypothesis was advanced by astronomer John J. Matese of the University of Louisiana at Lafayette in 2002. He contends that more comets are arriving in the inner Solar System from a particular region of the Oort Cloud than can be explained by the galactic tide or stellar perturbations alone, and that the most likely cause is a Jupiter-mass object in a distant orbit.^[45] This hypothetical gas giant planet has been nicknamed Tyche. One all-sky survey using parallax measurements in order to clarify local star distances, the WISE mission, is currently underway with part of its mission being to help with either proving or disproving the Tyche hypothesis.^[44]

Oort cloud objects (OCOs)

Sedna, a possible inner Oort cloud object discovered in 2003

Apart from long-period comets, only five known objects have orbits which suggest that they may belong to the Oort cloud: 90377 Sedna, 2000 CR₁₀₅, 2006 SQ₃₇₂, 2008 KV₄₂, and 2010 GB₁₇₄. The first two, and 5th, unlike scattered disc objects, have perihelia outside the gravitational reach of Neptune, and thus their orbits cannot be explained by perturbations from the gas giant planets.^[46] If they formed in their current locations, their orbits must originally have been circular; otherwise accretion (the coalescence of smaller bodies into larger ones) would not have been possible because the large relative velocities between planetesimals would have been too disruptive.^[47] Their present-day elliptical orbits can be explained by a number of hypotheses:

1. These objects could have had their orbits and perihelion distances "lifted" by the passage of a nearby star when the Sun was still embedded in its birth star cluster.^[6]
2. Their orbits could have been disrupted by an as-yet-unknown planet-sized body within the Oort cloud.^[48]
3. They could have been scattered by Neptune during a period of particularly high eccentricity or by the gravity of a far larger primordial trans-Neptunian disc.
4. They could have been captured from around smaller passing stars.

Of these, the stellar disruption and "lift" hypothesis appears to agree most closely with observations.^[6] Some astronomers prefer to refer to Sedna and 2000 CR₁₀₅ as belonging to the "extended scattered disc" rather than to the inner Oort cloud.^[47]

Modified Newtonian dynamics within the Oort cloud

Modified Newtonian dynamics (MOND)^[54][55] suggests that at their distances from the Sun, the objects comprising the Oort cloud should experience accelerations of the order of 10⁻¹⁰ m/s², and thus should be within the realms at which deviations from Newtonian predictions come into effect. According to this hypothesis, which was proposed to account for the discrepancies in the galaxy rotation curve, which are more commonly attributed to dark matter, acceleration ceases to be linearly proportional to force at very low accelerations.^[54] If correct, this would have significant implications regarding the formation and structure of the Oort cloud. However, the majority of cosmologists do not consider MOND a valid hypothesis.^[56]

Future exploration

Space probes have yet to reach the area of the Oort cloud. One proposal is to use a craft powered by a solar sail that would take around 30 years to reach its destination.^[57]

See also

Book: Solar System

• Heliosphere
• Interstellar comet
• Kuiper belt
• List of possible dwarf planets
• List of trans-Neptunian objects
• Scattered disc
• Tyche (hypothetical planet)

References

External links

Listen to this article (info/dl)

This audio file was created from a revision of the "Oort cloud" article dated 2012-04-02, and does not reflect subsequent edits to the article. (Audio help)

More spoken articles

Wikimedia Commons has media related to Oort cloud.

• Oort Cloud Profile by NASA's Solar System Exploration
• The Kuiper Belt and The Oort Cloud
• The effect of perturbations by the Alpha Cen A/B system on the Oort Cloud

v t e The Solar System The Sun Mercury Venus Earth Mars Ceres Jupiter Saturn Uranus Neptune Pluto Haumea Makemake Eris Moons Terrestrial Martian Jovian Saturnian Uranian Neptunian Plutonian Haumean Eridian Rings Jovian Saturnian Uranian Neptunian Rhean Meteoroids Minor planets Asteroids Asteroid belt Centaurs TNOs Kuiper belt Scattered disc Comets Oort cloud Astronomical object Star Planet Double planet Dwarf planet Small Solar System body Gas giant Ice giant Planetary system List of Solar System objects List of Solar System objects by size List of minor planets Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Virgo Supercluster → Pisces–Cetus Supercluster Complex → Observable universe → Universe Each arrow (→) may be read as "within" or "part of". PORTALS: Astronomy Earth sciences Solar System

v t e Comets Features Nucleus Coma Tails Antitail Dust trail Dust Types Names Antimatter Encke-types Exocomets Extinct Great Halley-types Interstellar Lost Main-belt Sungrazing (Kreutz Sungrazers) Related Asteroid Centaur Comet discoverers LINEAR Extraterrestrial atmospheres Oort cloud Small Solar System body See also Comet vintages Comets in fiction Fictional comets Space missions Comet Hopper CONTOUR CRAF Deep Impact/EPOXI Deep Space 1 Giotto Hayabusa Mk2 ICE Marco Polo MarcoPolo-R Rosetta Philae Sakigake Stardust/NeXT Suisei Ulysses Vega program Vega 1 Vega 2 Vesta Latest C/2013 R1 (Lovejoy) C/2013 A1 (Siding Spring) P/2013 P5 (PANSTARRS) C/2012 S1 (ISON) C/2012 K1 (PANSTARRS) C/2012 F6 (Lemmon) C/2012 E2 (SWAN) 276P/Vorobjov 2P/Encke   Lists (more) Periodic comets Number 1P/Halley 2P/Encke 3D/Biela 4P/Faye 5D/Brorsen 6P/d'Arrest 7P/Pons–Winnecke 8P/Tuttle 9P/Tempel 10P/Tempel 11P/Tempel–Swift–LINEAR 12P/Pons–Brooks 13P/Olbers 14P/Wolf 15P/Finlay 16P/Brooks 17P/Holmes 18D/Perrine–Mrkos 19P/Borrelly 20D/Westphal 21P/Giacobini–Zinner 22P/Kopff 23P/Brorsen–Metcalf 24P/Schaumasse 25D/Neujmin 26P/Grigg–Skjellerup 27P/Crommelin 28P/Neujmin 29P/Schwassmann–Wachmann 30P/Reinmuth 31P/Schwassmann–Wachmann 32P/Comas Solá 33P/Daniel 34D/Gale 35P/Herschel–Rigollet 36P/Whipple 37P/Forbes 38P/Stephan–Oterma 39P/Oterma 40P/Väisälä 41P/Tuttle–Giacobini–Kresák 42P/Neujmin 43P/Wolf–Harrington 44P/Reinmuth 45P/Honda–Mrkos–Pajdušáková 46P/Wirtanen 47P/Ashbrook–Jackson 48P/Johnson 49P/Arend–Rigaux 50P/Arend 51P/Harrington 52P/Harrington–Abell 53P/Van Biesbroeck 54P/de Vico–Swift–NEAT 55P/Tempel–Tuttle 56P/Slaughter–Burnham 57P/du Toit–Neujmin–Delporte 60P/Tsuchinshan 62P/Tsuchinshan 65P/Gunn 67P/Churyumov–Gerasimenko 68P/Klemola 69P/Taylor 71P/Clark 73P/Schwassmann–Wachmann 74P/Smirnova–Chernykh 75D/Kohoutek 77P/Longmore 78P/Gehrels 81P/Wild 82P/Gehrels 84P/Giclas 85P/Boethin 87P/Bus 88P/Howell 91P/Russell 94P/Russell 95P/Chiron 96P/Machholz 98P/Takamizawa 99P/Kowal 100P/Hartley 101P/Chernykh 102P/Shoemaker 103P/Hartley 105P/Singer Brewster 107P/Wilson–Harrington 109P/Swift–Tuttle 111P/Helin–Roman–Crockett 112P/Urata–Niijima 113P/Spitaler 114P/Wiseman–Skiff 115P/Maury 116P/Wild 117P/Helin–Roman–Alu 118P/Shoemaker–Levy 119P/Parker–Hartley 120P/Mueller 121P/Shoemaker–Holt 122P/de Vico 125P/Spacewatch 128P/Shoemaker–Holt 129P/Shoemaker–Levy 130P/McNaught–Hughes 132P/Helin–Roman–Alu 133P/Elst–Pizarro 136P/Mueller 138P/Shoemaker–Levy 139P/Väisälä–Oterma 143P/Kowal–Mrkos 144P/Kushida 147P/Kushida–Muramatsu 152P/Helin–Lawrence 153P/Ikeya–Zhang 157P/Tritton 158P/Kowal–LINEAR 159P/LONEOS 160P/LINEAR 161P/Hartley–IRAS 163P/NEAT 164P/Christensen 165P/LINEAR 166P/NEAT 167P/CINEOS 168P/Hergenrother 169P/NEAT 170P/Christensen 171P/Spahr 172P/Yeung 173P/Mueller 174P/Echeclus 176P/LINEAR 177P/Barnard 178P/Hug–Bell 206P/Barnard–Boattini 208P/McMillan 209P/LINEAR 238P/Read 246P/NEAT 255P/Levy 271P/van Houten–Lemmon 276P/Vorobjov 289P/Blanpain None D/1770 L1 D/1993 F2 P/1997 B1 P/2007 R5 P/2010 A2 P/2010 B2 P/2010 V1 P/2011 NO1 P/2013 P5 Non periodic comets Before 1910 C/-43 K1 (Comet Caesar) X/1106 C1 C/1577 V1 (Great Comet of 1577) C/1652 Y1 C/1680 V1 (Kirsch's Comet) C/1702 H1 C/1729 P1 (Comet of 1729) C/1743 X1 C/1760 A1 C/1769 P1 C/1811 F1 (Great Comet of 1811) C/1823 Y1 (Great Comet of 1823) C/1843 D1 (Great March Comet of 1843) C/1847 T1 (Miss Mitchell's Comet) C/1858 L1 (Comet Donati) C/1861 G1 (Comet Thatcher) C/1861 J1 (Great Comet of 1861) X/1872 X1 C/1882 R1 (Great September Comet of 1882) C/1887 B1 (Great Southern Comet of 1887) C/1910 A1 (Great January Comet of 1910) After 1910 C/1911 O1 (Brooks) C/1911 S3 (Beljawsky) C/1927 X1 (Skjellerup–Maristany) C/1948 V1 (Eclipse) C/1956 R1 (Arend–Roland) C/1961 R1 (Humason) C/1965 S1 (Ikeya-Seki) C/1969 Y1 (Bennett) C/1970 K1 (White–Ortiz–Bolelli) C/1973 E1 (Kohoutek) C/1975 V1 (West) C/1980 E1 (Bowell) C/1989 X1 (Austin) C/1989 Y1 (Skorichenko–George) C/1993 Y1 (McNaught–Russell) C/1995 O1 (Hale–Bopp) C/1996 B2 (Hyakutake) C/1997 L1 (Zhu–Balam) C/1998 H1 (Stonehouse) C/1999 F1 (Catalina) C/1999 S4 (LINEAR) C/2000 U5 (LINEAR) C/2000 W1 (Utsunomiya-Jones) C/2001 Q4 (NEAT) C/2002 T7 (LINEAR) C/2004 Q2 (Machholz) C/2006 A1 (Pojmański) C/2006 M4 (SWAN) C/2006 P1 (McNaught) C/2007 F1 (LONEOS) C/2007 N3 (Lulin) C/2007 Q3 (Siding Spring) C/2007 W1 (Boattini) C/2009 F6 (Yi–SWAN) C/2009 R1 (McNaught) C/2010 X1 (Elenin) C/2011 L4 (PANSTARRS) C/2011 W3 (Lovejoy) C/2012 E2 (SWAN) C/2012 F6 (Lemmon) C/2012 K1 (PANSTARRS) C/2012 S1 (ISON) C/2013 A1 (Siding Spring) P/2013 P5 (PANSTARRS) C/2013 R1 (Lovejoy) After 1910 (abc's) Arend–Roland Austin Beljawsky Bennett Boattini Bowell Brooks Catalina Eclipse Elenin Hale-Bopp Humason Hyakutake Ikeya-Seki ISON Kohoutek Lemmon LINEAR C/1999 S4 C/2000 U5 C/2002 T7 LONEOS Lovejoy C/2011 W3 C/2013 R1 Lulin Machholz McNaught C/2006 P1 C/2009 R1 McNaught–Russell NEAT Pan-STARRS C/2011 L4 C/2012 K1 P/2013 P5 Pojmański Siding Spring C/2007 Q3 C/2013 A1 Skjellerup–Maristany Skorichenko–George Stonehouse SWAN C/2006 M4 C/2012 E2 Utsunomiya–Jones West White–Ortiz–Bolelli Yi–SWAN Zhu–Balam Misc Asteroid comets 2060 Chiron (95P/Chiron) 7968 Elst–Pizarro (133P/Elst–Pizarro) 4015 Wilson–Harrington (107P/Wilson-Harrington) 60558 Echeclus (174P/Echeclus) 118401 LINEAR (176P/LINEAR, prev 1999 RE70) Lost comets 11P/Tempel–Swift–LINEAR 15P/Finlay 17P/Holmes 27P/Crommelin 54P/de Vico–Swift–NEAT 69P/Taylor 73P/Schwassmann–Wachmann 113P/Spitaler 107P/Wilson–Harrington 177P/Barnard 206P/Barnard–Boattini D/1770 L1 (Lexell) 3D/Biela 5D/Brorsen 18D/Perrine–Mrkos 20D/Westphal 25D/Neujmin 34D/Gale Unknown orbits X/1106 C1 X/1872 X1