Contact binary (small Solar System body)

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Contact binaries of varying probability among the small Solar System bodies:

A contact binary is a small Solar System body, such as a minor planet or comet, that is composed of two bodies that have gravitated toward each other until they touch, resulting in a bilobated, peanut-like overall shape. Contact binaries are distinct from true binary systems such as binary asteroids where both components are separated. The term is also used for stellar contact binaries.

Contents

An example of a contact binary is the Kuiper belt object 486958 Arrokoth, which was imaged by the New Horizons spacecraft during its flyby in January 2019. [1]

History

The existence of contact binary asteroids was first speculated by planetary scientist Allan F. Cook in 1971, who sought for potential explanations for the extremely elongated shape of the Jupiter trojan asteroid 624 Hektor, whose longest axis measures roughly 300 km (190 mi) across and is twice as long as its shorter axes according to light curve measurements. [2] Astronomers William K. Hartmann and Dale P. Cruikshank performed further investigation into Cook's contact binary hypothesis in 1978 and found it to be a plausible explanation for Hektor's elongated shape. [3] [4] : 807  They argued that since Hektor is the largest Jupiter trojan, its elongated shape could not have originated from the fragmentation of a larger asteroid. Rather, Hektor is more likely a "compound asteroid" consisting of two similarly-sized primitive asteroids, or planetesimals, that are in contact with each other as a result of a very low-speed collision. [3] [5] Hartmann theorized in 1979 that Jupiter trojan planetesimals formed close together with similar motions in Jupiter's Lagrange points, which allowed for low-speed collisions between planetesimals to take place and form contact binaries. [6] : 1915  The hypothesis of Hektor's contact binary nature contributed to the growing evidence of the existence of binary asteroids and asteroid satellites, which were not discovered until the Galileo spacecraft's flyby of 243 Ida and Dactyl 1993. [4] : 808 

Until 1989, contact binary asteroids have only been inferred from the high-amplitude U-shape of their light curves. The first visually confirmed contact binary was the near-Earth asteroid 4769 Castalia (formerly 1989 PB), whose double-lobed shape was revealed in high-resolution delay-Doppler radar imaging by the Arecibo Observatory and Goldstone Solar System Radar in August 1989. [7] These radar observations were led by Steven J. Ostro and his team of radar astronomers, who published the results in 1990. [7] In 1994, Ostro and his colleague R. Scott Hudson developed and published a three-dimensional shape model of Castalia reconstructed from the 1989 radar images, providing the first radar shape model of a contact binary asteroid. [8]

In 1992, the Kuiper belt was discovered and astronomers subsequently began observing and measuring light curves of Kuiper belt objects (KBOs) to determine their shapes and rotational properties. In 2002–2003, then-graduate student Scott S. Sheppard and his advisor David C. Jewitt observed the KBO and plutino 2001 QG298 with the University of Hawaiʻi's 2.24-m telescope at Mauna Kea, as part of a survey dedicated to measuring the light curves of KBOs. [9] With their results published in 2004, they discovered that 2001 QG298 exhibits a large, U-shaped light curve amplitude characteristic of contact binaries, providing the first evidence of contact binary KBOs. [9] Sheppard and Jewitt identified additional contact binary candidates from other KBOs known to exhibit large light curve amplitudes, hinting that contact binaries are abundant in the Kuiper belt. [9]

The contact binary nature of comets was first suspected after the Deep Space 1 spacecraft's flyby of 19P/Borrelly in 2001, which revealed a bilobate peanut-shaped nucleus with a thick neck connecting the two lobes. [10] [11] : 2  The nucleus of 1P/Halley has also been described as peanut-shaped by researchers in 2004, based on imagery from the Giotto and Vega probes in 1986. [12] : 501  However, the low bifurcation and thick-necked shapes of both of these comet nuclei made it unclear whether they are truly contact binaries. [12] : 501  In 2008, the Arecibo Observatory imaged the Halley-type comet 8P/Tuttle in radar and revealed a highly bifurcated nucleus consisting of two distinct spheroidal lobes, providing the first unambiguous evidence of a contact binary comet nucleus. [12] : 499  Later radar imaging and spacecraft exploration of the Jupiter family comet 103P/Hartley in 2010 also revealed a thick-necked, peanut-shaped nucleus similar to 19P/Borelly. By that time, half of the comets that have been imaged in detail were known to be bilobate, which implied that contact binaries in the comet population are similarly abundant as contact binaries in other minor planet populations. [11] : 4 

Formation and evolution

Generally, contact binary objects in the Solar System form when two objects collide at speeds slow enough that their shapes do not become disrupted. However, the mechanisms leading to this differ depending on the size and orbital location of the object.

Near-Earth asteroids

Collisional fragments [13] : 218 

Due to their close proximity to the Sun, the evolution of near-Earth asteroid (NEA) shapes and binary systems is dominated by the uneven reflection of sunlight off their surfaces, which causes gradual orbital acceleration by the Yarkovsky effect and gradual rotational acceleration by the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect.

High-mass ratio and doubly-synchronous binary systems such as 69230 Hermes are plausible sources for contact binaries in the NEA population, since they are subject to the binary YORP effect, which acts over timescales of 1,000–10,000 years to either contract the components' orbits until they contact, or expand their orbits until they become gravitationally detached asteroid pairs. [14] : 166–167  [15] : 430  The origin of contact binaries from doubly-synchronous binaries in the NEA population is evident from the fact that very few doubly-synchronous binary NEAs are known, whereas contact binary NEAs are much more common. [14] : 167  For doubly-synchronous binary systems with 1 km (0.62 mi)-diameter components, the tangential and radial impact velocities when they collide are less than 50 mm/s (2.0 in/s), which are low enough to not disrupt the shapes of the two bodies. [14] : 167 

In 2007, Daniel J. Scheeres proposed that contact binary asteroids in the NEA population can undergo rotational fissioning after being rotationally accelerated by the YORP effect. [16] Depending on the relative sizes and shapes of the fissioned components, there are three possible evolutionary pathways for contact binary NEAs. [16] : 384  Firstly, if the primary component is elongated and dominates the mass of the system, the secondary will either escape the system or collide with the primary since the orbits of the fissioned components are unstable. [16] : 384  Secondly, if the primary component is elongated and accounts for roughly half of the system's mass, the secondary can temporarily orbit the primary before it will collide with the primary, reforming the contact binary but with a different distribution of the system mass. [16] : 384  Thirdly, if the primary is spheroidal and dominates the mass of the system, the fissioned components can remain in long-lasting orbits as a stable binary system. [16] : 384  As shown by these cases, it is unlikely that fissioned contact binaries can form stable binaries. [17] : L58 

In 2011, Seth A. Jacobson and Scheeres expanded upon their 2007 theory of binary fission and proposed that NEAs can go through repeated cycles of fissioning and reimpacting through the YORP effect. [14] : 167 

Trans-Neptunian objects

In the trans-Neptunian region and especially the Kuiper belt, binary systems are thought to have formed from the direct collapse of gas and dust from the surrounding protoplanetary nebula due to streaming instability. Through impacts and gravitational perturbations by the outer planets, the mutual orbits of binary trans-Neptunian objects contract and eventually destabilize to form contact binaries. [18] : 59 

Geophysical properties

Impacts on one of the lobes of contact binary rubble pile asteroids do not cause significant disruption to the asteroid as the shockwave produced by the impact is damped by the asteroid's rubble pile structure and then blocked by the discontinuity between the two lobes. [19]

Occurrence

Near-Earth asteroids

In 2022, Anne Virkki and colleagues published an analysis of 191 near-Earth asteroids (NEAs) that were observed by the Arecibo Observatory radar from December 2017–2019. From this sample, they found that 10 out of the 33 (~30%) NEAs larger than 200 m (660 ft) in diameter were contact binaries, which is double the previously estimated percentage of 14% for contact binaries of this diameter in the NEA population. [20] : 24  Although the sample size is small and therefore not statistically significant, it could imply that contact binaries could be more common than previously thought. [20] : 24 

Kuiper belt

In 2015–2019, Audrey Thirouin and Scott Sheppard performed a survey of KBOs from the plutino (2:3 Neptune resonance) and cold classical (low inclination and eccentricity) populations with the Lowell Discovery Telescope and Magellan-Baade Telescope. [21] They found that 40–50% of the population of plutinos smaller than 188–419 km (117–260 mi) in diameter (H  ≥ 6) are contact binaries consisting of nearly equal-mass components, [22] : 12  whereas at least 10–25% of the population of cold classical KBOs of the same size range are contact binaries. [21] : 16  The differing contact binary fractions of these two populations imply they underwent different formation and evolution mechanisms. [21] : 17 

Thirouin and Sheppard continued their survey of KBOs in 2019–2021, focusing on the twotino population in the 1:2 orbital resonance with Neptune. [23] : 2–3  They found that 7–14% of twotinos are contact binaries, which is relatively low albeit similar to the contact binary fraction of the cold classical population. [23] : 9  Thirouin and Sheppard noted that the twotinos' contact binary fraction is consistent with predictions by David Nesvorný and David Vokrouhlický in 2019, who suggested that 10–30% of dynamically excited and resonant Kuiper belt populations are contact binaries. [23] : 9  [18] : 59 

486958 Arrokoth is the first confirmed example of a contact binary KBO, seen through stellar occultations in 2018 and spacecraft imaging in 2019.

A stellar occultation by the KBO 19521 Chaos on 29 March 2023 revealed that it had an apparently bilobate shape 380 km (240 mi) across, which could potentially make it the largest known contact binary object in the Solar System. [24] However, the bilobate shape seen in the occultation could well be two binary components transiting each other during the event; this is supported by the smaller-than-expected size of Chaos measured in the occultation. [25]

Comets

Irregular moons

The Cassini spacecraft observed several irregular moons of Saturn at various phase angles while in it was orbit around Saturn from 2004–2017, which allowed for the determination of rotation periods and shapes of the Saturnian irregular moons. In 2018–2019, researchers Tilmann Denk and Stefan Mottola investigated Cassini's irregular moon observations and found that Kiviuq, Erriapus, Bestla, and Bebhionn exhibited exceptionally large light curve amplitudes that may indicate contact binary shapes, or potentially binary (or subsatellite) systems. [26] : 422  In particular, the light curve amplitude of Kiviuq is the largest of the irregular moons observed by Cassini, which makes it the most likely candidate for a contact binary or binary moon. [26] : 422  [27] : 101  Considering that the irregular moons have most likely undergone or were formed by disruptive collisions in the past, it is possible that the fragments of disrupted irregular moons could remain gravitationally bound in orbit around each other, forming a binary system that would eventually become a contact binary. [26] : 421 

Examples

Comet Churyumov–Gerasimenko and Comet Tuttle are most likely contact binaries, [28] [29] while asteroids suspected of being contact binaries include the unusually elongated 624 Hektor and the bilobated 216 Kleopatra and 4769 Castalia. 25143 Itokawa, which was photographed by the Hayabusa probe, also appears to be a contact binary which has resulted in an elongated, bent body. Asteroid 4179 Toutatis with its elongated shape, as photographed by Chang'e-2 , is a contact binary candidate as well. [30] Among the distant minor planets, the icy Kuiper belt object Arrokoth was confirmed to be a contact binary when the New Horizons spacecraft flew past in 2019. [1] The small main-belt asteroid 152830 Dinkinesh was confirmed to have the first known contact binary satellite after the Lucy probe flew by it on November 1, 2023. [31]

See also

Related Research Articles

<span class="mw-page-title-main">Kuiper belt</span> Area of the Solar System beyond the planets, comprising small bodies

The Kuiper belt is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles, such as methane, ammonia, and water. The Kuiper belt is home to most of the objects that astronomers generally accept as dwarf planets: Orcus, Pluto, Haumea, Quaoar, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region.

<span class="mw-page-title-main">Jupiter trojan</span> Asteroid sharing the orbit of Jupiter

The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each trojan librates around one of Jupiter's stable Lagrange points: either L4, existing 60° ahead of the planet in its orbit, or L5, 60° behind. Jupiter trojans are distributed in two elongated, curved regions around these Lagrangian points with an average semi-major axis of about 5.2 AU.

<span class="mw-page-title-main">624 Hektor</span> Largest Jupiter trojan

624 Hektor is the largest Jupiter trojan and the namesake of the Hektor family, with a highly elongated shape equivalent in volume to a sphere of approximately 225 to 250 kilometers diameter. It was discovered on 10 February 1907, by astronomer August Kopff at Heidelberg Observatory in southwest Germany, and named after the Trojan prince Hector, from Greek mythology. It has one small 12-kilometer sized satellite, Skamandrios, discovered in 2006.

<span class="mw-page-title-main">Steven J. Ostro</span>

Steven Jeffrey Ostro was an American scientist specializing in radar astronomy. He worked at NASA's Jet Propulsion Laboratory. Ostro led radar observations of numerous asteroids, as well as the moons of Jupiter and Saturn, Saturn's rings, and Mars and its satellites. As of May 2008, Ostro and his collaborators had detected 222 near-Earth asteroids and 118 main belt objects with radar.

<span class="mw-page-title-main">216 Kleopatra</span> M-type asteroid

216 Kleopatra is a large M-type asteroid with a mean diameter of 120 kilometers and is noted for its elongate bone or dumbbell shape. It was discovered on 10 April 1880 by Austrian astronomer Johann Palisa at the Austrian Naval Pola Observatory, in what is now Pula, Croatia, and was named after Cleopatra, the famous Egyptian queen. It has two small minor-planet moons which were discovered in 2008 and later named Alexhelios and Cleoselene.

4486 Mithra, is an eccentric asteroid and suspected contact-binary, classified as near-Earth object and potentially hazardous asteroid, approximately 2 kilometers in diameter. It belongs to the Apollo group of asteroids and is a relatively slow rotator.

<span class="mw-page-title-main">44 Nysa</span> Main-belt asteroid

Nysa is a large and very bright main-belt asteroid, and the brightest member of the Nysian asteroid family. It is classified as a rare class E asteroid and is probably the largest of this type.

<span class="nowrap">(55565) 2002 AW<sub>197</sub></span> Classical Kuiper belt object

(55565) 2002 AW197 is a classical, non-resonant trans-Neptunian object from the Kuiper belt in the outermost region of the Solar System, also known as a cubewano. With a likely diameter of at least 700 kilometers (430 miles), it is approximately tied with 2002 MS4 and 2013 FY27 (to within measurement uncertainties) as the largest unnamed object in the Solar System. It was discovered at Palomar Observatory in 2002.

<span class="mw-page-title-main">10199 Chariklo</span> Small body of the outer Solar System

10199 Chariklo is the largest confirmed centaur. It orbits the Sun between Saturn and Uranus, grazing the orbit of Uranus. On 26 March 2014, astronomers announced the discovery of two rings around Chariklo by observing a stellar occultation, making it the first minor planet known to have rings.

<span class="nowrap">(307261) 2002 MS<sub>4</sub></span> Classical Kuiper belt object

(307261) 2002 MS4 (provisional designation 2002 MS4) is a large trans-Neptunian object in the Kuiper belt, a region of icy planetesimals beyond Neptune. It was discovered on 18 June 2002 by Chad Trujillo and Michael Brown during their search for bright, Pluto-sized Kuiper belt objects at Palomar Observatory. 2002 MS4 has a diameter close to 800 km (500 mi), which approximately ties it with 2002 AW197 and 2013 FY27 (to within measurement uncertainties) as the largest unnamed object in the Solar System. 2002 MS4 is large enough that astronomers consider it a possible dwarf planet.

<span class="nowrap">(208996) 2003 AZ<sub>84</sub></span> Plutino

(208996) 2003 AZ84 is a trans-Neptunian object with a possible moon located in the outer regions of the Solar System. It is approximately 940 kilometers across its longest axis, as it has an elongated shape. It belongs to the plutinos – a group of minor planets named after its largest member Pluto – as it orbits in a 2:3 resonance with Neptune in the Kuiper belt. It is the third-largest known plutino, after Pluto and Orcus. It was discovered on 13 January 2003, by American astronomers Chad Trujillo and Michael Brown during the NEAT survey using the Samuel Oschin telescope at Palomar Observatory.

11066 Sigurd, provisional designation 1992 CC1, is a stony, rare-type asteroid and elongated contact binary, classified as near-Earth object of the Apollo group of asteroids, approximately 2.5 kilometers in diameter.

<span class="mw-page-title-main">(35107) 1991 VH</span>

(35107) 1991 VH is a binary near-Earth asteroid and potentially hazardous asteroid of the Apollo group. It was discovered on 9 November 1991, by Australian astronomer Robert McNaught at Siding Spring Observatory. This binary system is composed of a roughly-spheroidal primary body about one kilometre in diameter, and an elongated natural satellite less than half a kilometre in diameter. The 1991 VH system is unusual for its dynamically excited state; the satellite has a tumbling, non-synchronous rotation that chaotically exchanges energy and angular momentum with its precessing, eccentric orbit. This asteroid system was one of the two targets of NASA's upcoming Janus Mayhem mission, until the delay of the rocket launch made both targets inaccessible.

<span class="mw-page-title-main">385446 Manwë</span> Binary Kuiper belt object

385446 Manwë, or (385446) Manwë–Thorondor, is a binary resonant Kuiper belt object in a 4:7 mean-motion resonance with Neptune. It was discovered on 25 August 2003, by American astronomer Marc Buie at Cerro Tololo Observatory in northern Chile. A study of Manwë's light curve in 2019 suggests that it may be a contact binary object.

<span class="mw-page-title-main">486958 Arrokoth</span> Kuiper belt object

486958 Arrokoth (provisional designation 2014 MU69; formerly nicknamed Ultima Thule) is a trans-Neptunian object located in the Kuiper belt. Arrokoth became the farthest and most primitive object in the Solar System visited by a spacecraft when the NASA space probe New Horizons conducted a flyby on 1 January 2019. Arrokoth is a contact binary 36 km (22 mi) long, composed of two planetesimals 21 and 15 km (13 and 9 mi) across, that are joined along their major axes. With an orbital period of about 298 years and a low orbital inclination and eccentricity, Arrokoth is classified as a cold classical Kuiper belt object.

<span class="nowrap">2014 OS<sub>393</sub></span> Classical Kuiper belt asteroid

2014 OS393, unofficially designated e31007AI, e3 and PT2, is a binary trans-Neptunian object in the classical Kuiper belt, the outermost region of the Solar System. It was first observed by the New Horizons KBO Search using the Hubble Space Telescope on 30 July 2014. Until 2015, when the object 486958 Arrokoth was selected, it was a potential flyby target for the New Horizons probe. Estimated to be approximately 42 kilometres (26 mi) in diameter, the object has a poorly determined orbit as it had been observed for only a few months.

<span class="nowrap">341520 Mors–Somnus</span>

341520 Mors–Somnus, provisional designation 2007 TY430, is a binary and plutino. It consists of two components less than 60 kilometers in diameter, orbiting at a distance of 21000 km.

<span class="mw-page-title-main">Dimorphos</span> Moon of asteroid Didymos

Dimorphos is a natural satellite or moon of the near-Earth asteroid 65803 Didymos, with which it forms a binary system. The moon was discovered on 20 November 2003 by Petr Pravec in collaboration with other astronomers worldwide. Dimorphos has a diameter of 177 meters (581 ft) across its longest extent and it was the target of the Double Asteroid Redirection Test (DART), a NASA space mission that deliberately collided a spacecraft with the moon on 26 September 2022 to alter its orbit around Didymos. Before the impact by DART, Dimorphos had a shape of an oblate spheroid with a surface covered in boulders but virtually no craters. The moon is thought to have formed when Didymos shed its mass due to its rapid rotation, which formed an orbiting ring of debris that conglomerated into a low-density rubble pile that became Dimorphos today.

2011 JY31 is a binary trans-Neptunian object from the Kuiper belt, located in the outermost region of the Solar System. It was discovered on 4 May 2011, by a team of astronomers using one of the Magellan Telescopes in Chile during the New Horizons KBO Search for a potential flyby target for the New Horizons spacecraft. Distant observations by New Horizons from September 2018 revealed its binary nature, showing two 50 km (31 mi)-wide components in a tight, mutual orbit 200 km (120 mi) apart. The discovery adds support to streaming instability as the dominant mechanism in the formation of tight and contact binary planetesimals such as 486958 Arrokoth, which appear to be prevalent in the cold classical Kuiper belt population.

References

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  27. ↑ Denk, Tilmann; Mottola, Stefano (April 2019). "Studies of irregular satellites: I. Lightcurves and rotation periods of 25 Saturnian moons from Cassini observations". Icarus. 322: 80–102. Bibcode:2019Icar..322...80D. doi:10.1016/j.icarus.2018.12.040. S2CID   127269198.
  28. ↑ Quick Rosetta update: Churyumov-Gerasimenko is a contact binary!
  29. ↑ Success! A final flawless burn. Rosetta now in tandem with its comet
  30. ↑ The formation mechanism of 4179 Toutatis' elongated bi-lobed structure in a close Earth encounter scenario
  31. ↑ Kretske, Katherine. "NASA's Lucy Surprises Again, Observes 1st-ever Contact Binary Orbiting Asteroid - NASA Science". science.nasa.gov. Retrieved 9 November 2023.
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