View source for Binary asteroid

{{short description|System of two asteroids orbiting their common center of mass}}
{{See also|Minor-planet moon}}
[[File:243 ida crop.jpg|thumb|280px|Binary asteroid [[243 Ida]] with its small [[minor-planet moon]], [[Dactyl (moon)|Dactyl]], as seen by ''[[Galileo (spacecraft)|Galileo]]'']]
[[File:Didymos-Dimorphos true orientation.png|thumb|Binary near-Earth asteroid [[65803 Didymos]] and its moon [[Dimorphos]] imaged by the [[Double Asteroid Redirection Test]] spacecraft]]

A '''binary asteroid''' is a system of two [[asteroid]]s orbiting their common [[barycenter]]. The binary nature of [[243 Ida]] was discovered when the [[Galileo (spacecraft)|Galileo spacecraft]] flew by the asteroid in 1993. Since then [[List of minor-planet moons|numerous binary asteroids]] and [[Minor-planet moon#Triple systems|several triple asteroids]] have been detected.<ref>{{cite book |last1=Margot|first1=Jean-Luc|title=Asteroids IV|last2=Pravec|first2=Petr|last3=Taylor|first3=Patrick|last4=Carry|first4=Benoît|last5=Jacobson|first5=Seth |pages=355|arxiv=1504.00034 |date=2015|doi=10.2458/azu_uapress_9780816532131-ch019 |chapter=Asteroid Systems: Binaries, Triples, and Pairs|isbn=9780816532131|bibcode=2015aste.book..355M |s2cid=56278100 |editor1-last=Michel |editor1-first=Patrick |editor2-last=DeMeo |editor2-first=Francesca E. |editor-last3=Bottke |editor3-first=William F.}}</ref>

The mass ratio of the two components – called the "primary" and "secondary" of a binary system – is an important characteristic. Most binary asteroids have a large mass ratio, i.e. a relatively small satellite in orbit around the main component. Systems with one or more small [[minor-planet moon|moons]] – also called "companions" or simply "satellites" – include [[87&nbsp;Sylvia]], [[107&nbsp;Camilla]] and [[45&nbsp;Eugenia]] (all triples), [[121&nbsp;Hermione]], [[130&nbsp;Elektra]] (a quadruple), [[22&nbsp;Kalliope]], [[283&nbsp;Emma]], [[379&nbsp;Huenna]], [[243 Ida]] and [[4337 Arecibo]] (in order of decreasing primary size). Some binary systems have a mass ratio near unity, i.e., two components of similar mass. They include [[90&nbsp;Antiope]], {{mpl-|300163|2006 VW|139}}, {{mpl|2017 YE|5}} and [[69230&nbsp;Hermes]], with average component diameters of 86, 1.8, 0.9 and 0.8&nbsp;km, respectively.

In August 2024 [[Gaia (spacecraft)|Gaia]] reported 352 new binary asteroid candidates.<ref>{{Cite journal |last=Liberato |first=L. |last2=Tanga |first2=P. |last3=Mary |first3=D. |last4=Minker |first4=K. |last5=Carry |first5=B. |last6=Spoto |first6=F. |last7=Bartczak |first7=P. |last8=Sicardy |first8=B. |last9=Oszkiewicz |first9=D. |last10=Desmars |first10=J. |date=August 2024 |title=Binary asteroid candidates in Gaia DR3 astrometry |url=https://www.aanda.org/10.1051/0004-6361/202349122 |journal=Astronomy & Astrophysics |volume=688 |pages=A50 |doi=10.1051/0004-6361/202349122 |issn=0004-6361|arxiv=2406.07195 }}</ref><ref>{{Cite web |last=Graham-Shaw |first=Kate |title=These 352 Asteroids Likely Have Cute Little Moonlets |url=https://www.scientificamerican.com/article/these-352-asteroids-likely-have-cute-little-moonlets/ |access-date=2024-08-14 |website=Scientific American |language=en}}</ref>

== Description ==

Several theories have been posited to explain the formation of binary-asteroid systems. Many systems have significant macro-porosity (a "[[Rubble pile|rubble-pile]]" interior). The satellites orbiting large [[Asteroid belt|main-belt asteroids]] such as 22&nbsp;Kalliope, 45&nbsp;Eugenia or 87&nbsp;Sylvia may have formed by disruption of a parent body after impact or fission after an oblique impact. [[Trans-Neptunian object|Trans-Neptunian]] binaries may have formed during the formation of the Solar System by mutual capture or three-body interaction. [[Near-Earth object|Near-Earth asteroids]], which orbit in the inner part of the Solar System, most likely form by spin-up and mass shedding,<ref name="binary NEAs">{{cite journal | doi=10.1126/science.1072094 | author=Margot, Jean-Luc | s2cid=8768432 | date=2002 | title= Binary Asteroids in the Near-Earth Object Population. | journal=Science | volume=296 | issue=5572 | pages=1445–1448 | pmid=11951001 |bibcode = 2002Sci...296.1445M |display-authors=etal}}</ref> likely as a result of the [[YORP effect]]. Numerical simulations suggest that when solar energy spins a “rubble pile” asteroid to a sufficiently fast rate by the YORP effect, material is thrown from the asteroid's equator.<ref name="Walsh08">{{cite journal|date=June 2008|title=Rotational breakup as the origin of small binary asteroids. |journal=Nature |volume=454|issue=7201 |pages=188–191|doi=10.1038/nature07078|author=Walsh, Kevin J.|pmid=18615078|last2=Richardson|first2=DC|last3=Michel|first3=P|bibcode = 2008Natur.454..188W |s2cid=4418744 }}</ref> This process also exposes fresh material at the poles of the asteroid.<ref name="Walsh08"/><ref>[http://newswise.com/articles/view/542454/ Study Puts Solar Spin on Asteroids, their Moons & Earth Impacts] Newswise, Retrieved 14 July 2008.</ref>

== Gallery ==
<gallery>
2006VW139.gif|Time-lapse video of binary main-belt comet {{mpl-|300163|2006 VW|139}} (288P)<ref>{{cite web|title=Hubble discovers a unique type of object in the Solar System|url=https://www.spacetelescope.org/news/heic1715/|website=www.spacetelescope.org|access-date=21 September 2017}}</ref>
The double asteroid 90 Antiope - Eso0718a (no tagline).jpg|Artist's impression of the double asteroid [[90&nbsp;Antiope]]
</gallery>

== See also ==
* {{section link|Minor-planet moon|List of minor planets with moons}}
* {{annotated link|Subsatellite|aka=Moonmoon}}
* [[Contact binary (small Solar System body)]]
* Triple asteroids {{mpl|(153591) 2001 SN|263}} and {{mpl|(136617) 1994 CC}}

== References ==
{{Commons category|Binary asteroids|Binary&nbsp;asteroids}}
{{reflist}}

{{Asteroids}}
{{Small Solar System bodies}}

[[Category:Binary asteroids| ]]