Editing Adrastea (moon)

{{Short description|Moon of Jupiter}}
{{For|the moon of Jupiter known by this name between 1955 and 1975|Ananke (moon)}}
{{For|the main-belt asteroid|239 Adrastea}}
{{Good article}}
{{Infobox planet
| name         = Adrastea
| pronounced   = {{IPAc-en|æ|d|r|ə|ˈ|s|t|iː|ə}}<ref>as "Adrastia" in Noah Webster (1884) ''A Practical Dictionary of the English Language''</ref>
| adjectives   = Adrastean {{IPAc-en|æ|d|r|ə|ˈ|s|t|iː|ə|n}}<ref>A.H. Clough (1905) ''Plutarch's lives: the translation called Dryden's'', vol. 3, p. 238.</ref><!--also 'Adrastæan Nemesis' from 1809 is the same name-->
| named_after  = [[Adrasteia|Ἀδράστεια]] ''Adrasteia''
| image        = File:adrastea.jpg
| image_scale   =
| caption      = Image of Adrastea taken by [[Galileo (spacecraft)|''Galileo'' spacecraft]] between November 1996 and June 1997
| discoverer =
  {{plainlist |
* [[David Jewitt|David C. Jewitt]]
* G. Edward Danielson
  }}
| discovered   = 8 July 1979
| mean_orbit_radius = {{val|129000|u=km}}{{sfn|Evans Porco et al.|2002}}{{sfn|Burns Simonelli et al.|2004}}
| eccentricity = {{val|0.0015}}{{sfn|Evans Porco et al.|2002}}{{sfn|Burns Simonelli et al.|2004}}
| period       = {{val|0.29826|u=d}} <br> (7 h, 9.5 min){{sfn|Evans Porco et al.|2002}}{{sfn|Burns Simonelli et al.|2004}}
| avg_speed    = 31.378 km/s{{efn|name=calculated}}
| inclination  = 0.03° <br> (to Jupiter's equator){{sfn|Evans Porco et al.|2002}}{{sfn|Burns Simonelli et al.|2004}}
| satellite_of = [[Jupiter]]
| mean_radius  = {{val|8.2|2.0|u=km}}{{sfn|Thomas Burns et al.|1998}}
| dimensions   = 20&thinsp;×&thinsp;16&thinsp;×&thinsp;14 km{{sfn|Thomas Burns et al.|1998}}
| volume       = {{val|p=≈&thinsp;|2345|u=km<sup>3</sup>}}{{efn|name=calculated}}
| mass         = 
| density      = 
| surface_grav = 
| escape_velocity = 
| rotation     = [[synchronous rotation|synchronous]]
| axial_tilt   = zero{{sfn|Thomas Burns et al.|1998}}
| albedo       = {{val|0.10|0.045}}{{sfn|Thomas Burns et al.|1998}}
| single_temperature = ≈&thinsp;122 K
}}

'''Adrastea''' ({{IPAc-en|æ|d|r|ə|ˈ|s|t|iː|ə}}), also known as '''{{nowrap|Jupiter XV}}''', is the second by distance, and the smallest of the four [[Inner satellites of Jupiter|inner moons of Jupiter]]. It was discovered in photographs taken by ''[[Voyager&nbsp;2]]'' in 1979, making it the first [[natural satellite]] to be discovered from images taken by an interplanetary [[spacecraft]], rather than through a telescope.{{sfn|IAUC 3454}} It was officially named after the mythological [[Adrasteia]], foster mother of the [[Greek god]] [[Zeus]]—the equivalent of the [[Roman god]] [[Jupiter (mythology)|Jupiter]].{{sfn|IAUC 3872}}

Adrastea is one of the few moons in the [[Solar System]] known to orbit its planet in less than the length of that planet's day. It orbits at the edge of [[Rings of Jupiter#Main ring|Jupiter's main ring]] and is thought to be the main contributor of material to the [[rings of Jupiter]]. Despite observations made in the 1990s by the [[Galileo (spacecraft)|''Galileo'' spacecraft]], very little is known about the moon's physical characteristics other than its size and the fact that it is [[tidally locked]] to [[Jupiter]].

== Discovery and observations ==
[[File:Adrastée FDS 20630.png|thumb|280px|left|Discovery image of Adrastea, taken on July 8, 1979, by ''Voyager&nbsp;2''. Adrastea is the fainter dot, in the very middle, straddling the line of the Jovian rings.]]

Adrastea was discovered by [[David Jewitt|David C. Jewitt]] and [[G. Edward Danielson]] in ''[[Voyager 2|Voyager&nbsp;2]]'' probe photographs taken on July 8, 1979, and received the designation '''{{nowrap|S/1979&nbsp;J&nbsp;1}}'''.{{sfn|IAUC 3454}}{{sfn|Jewitt Danielson et al.|1979}} Although it appeared only as a dot,{{sfn|Jewitt Danielson et al.|1979}} it was the first moon to be discovered by an interplanetary spacecraft. Soon after its discovery, two other of the inner [[moons of Jupiter]] ([[Thebe (moon)|Thebe]] and [[Metis (moon)|Metis]]) were observed in the images taken a few months earlier by ''[[Voyager 1|Voyager&nbsp;1]]''. The [[Galileo spacecraft|''Galileo'' spacecraft]] was able to determine the moon's shape in 1998, but the images remain poor.{{sfn|Thomas Burns et al.|1998}} In 1983, Adrastea was officially named after the Greek nymph [[Adrastea (mythology)|Adrastea]], the daughter of [[Zeus]] and his lover [[Ananke (mythology)|Ananke]].{{sfn|IAUC 3872}}

Although the ''[[Juno (spacecraft)|Juno]]'' orbiter, which arrived at Jupiter in 2016, has a camera called [[JunoCam]], it is almost entirely focused on observations of Jupiter itself. However, if all goes well, it should be able to capture some limited images of the moons [[Metis (moon)|Metis]] and Adrastea.<ref>[http://adsabs.harvard.edu/abs/2015AGUFM.P41B2066H JunoCam: Science and Outreach Opportunities with Juno<!-- Bot generated title -->] Hansen, C. J.; Orton, G. S. American Geophysical Union 12/2015</ref>
{{clear|left}}

== Physical characteristics ==
Adrastea has an irregular shape and measures 20×16×14&nbsp;km across.{{sfn|Thomas Burns et al.|1998}} A surface area estimate would be between 840 and 1,600 (~1,200) km<sup>2</sup>. This makes it the smallest of the four inner moons. The bulk, composition, and mass of Adrastea are not known, but assuming that its mean density is like that of [[Amalthea (moon)|Amalthea]],{{sfn|Burns Simonelli et al.|2004}} around 0.86&nbsp;g/cm<sup>3</sup>,{{sfn|Anderson Johnson et al.|2005}} its mass can be estimated at 2{{E-sp|15}}&nbsp;kg. Amalthea's density implies that the moon is composed of water [[ice]] with a [[porosity]] of 10–15%, and Adrastea may be similar.{{sfn|Anderson Johnson et al.|2005}}

No surface details of Adrastea are known, due to the low resolution of available images.{{sfn|Thomas Burns et al.|1998}}

== Orbit ==
Adrastea is the smallest and second-closest member of the [[inner satellites of Jupiter|inner Jovian satellite family]]. It orbits [[Jupiter]] at 70,200&nbsp;mph at a radius of about {{Convert|129,000|km|mi|abbr=on|sp=us|sigfig=1}} (1.806&nbsp;Jupiter radii) at the exterior edge of the planet's [[Rings of Jupiter#Main ring|main ring]].{{sfn|Burns Simonelli et al.|2004}} Its orbit has a very small [[orbital eccentricity|eccentricity]] of around 0.0015 and an [[inclination]] relative to Jupiter's equator of 0.03°, respectively.{{sfn|Burns Simonelli et al.|2004}}

Due to [[tidal locking]], Adrastea rotates synchronously with its orbital period, keeping one face always looking toward the planet. Its long axis is aligned towards Jupiter, this being the lowest energy configuration.{{sfn|Thomas Burns et al.|1998}}

The orbit of Adrastea lies inside [[Jupiter]]'s [[synchronous orbit]] radius (as does [[Metis (moon)|Metis]]'s), and as a result, [[tidal deceleration|tidal force]]s are slowly causing its orbit to decay so that it will one day impact Jupiter. If its density is similar to Amalthea's then its orbit would actually lie within the fluid [[Roche limit]]. However, since it is not breaking up, it must still lie outside its rigid Roche limit.{{sfn|Burns Simonelli et al.|2004}}

== Relationship with Jupiter's rings ==
Adrastea is the largest contributor to material in [[rings of Jupiter|Jupiter's rings]]. This appears to consist primarily of material that is ejected from the surfaces of Jupiter's four small inner satellites by meteorite impacts. It is easy for the impact ejecta to be lost from these satellites into space. This is due to the satellites' low density and their surfaces lying close to the edge of their [[Hill sphere]]s.{{sfn|Burns Simonelli et al.|2004}}

It seems that Adrastea is the most copious source of this ring material, as evidenced by the densest ring (the main ring) being located at and within Adrastea's orbit.{{sfn|Burns Showalter et al.|1999}} More precisely, the orbit of Adrastea lies near the outer edge of Jupiter's main ring.{{sfn|Ockert-Bell Burns et al.|1999}}  The exact extent of visible ring material depends on the phase angle of the images: in [[forward scattering|forward-scattered light]] Adrastea is firmly outside the main ring,{{sfn|Ockert-Bell Burns et al.|1999}} but in [[Backscatter|back-scattered light]] (which reveals much bigger particles) there appears to also be a narrow ringlet outside Adrastea's orbit.{{sfn|Burns Simonelli et al.|2004}}

== Notes==
{{notes
| notes =
{{efn
| name = calculated
| Calculated on the basis of other parameters.
}}
}}
==References==
{{reflist|20em}}

'''Cited sources'''
* {{cite journal| doi = 10.1126/science.1110422| last1 = Anderson | first1 = J. D.| last2 = Johnson | first2 = T. V.| last3 = Schubert | first3 = G.| last4 = Asmar | first4 = S.| last5 = Jacobson | first5 = R. A.| last6 = Johnston | first6 = D.| last7 = Lau | first7 = E. L.| last8 = Lewis | first8 = G.| last9 = Moore | first9 = W. B.| date = 27 May 2005| last10 = Taylor | first10 = A.| last11 = Thomas | first11 = P. C.| last12 = Weinwurm | first12 = G.| title = Amalthea's Density is Less Than That of Water| journal = Science| volume = 308| issue = 5726| pages = 1291–1293| pmid = 15919987| bibcode = 2005Sci...308.1291A| s2cid = 924257 | ref = {{sfnRef|Anderson Johnson et al.|2005}}}}
* {{cite journal| doi = 10.1126/science.284.5417.1146| last1 = Burns| first1 = Joseph A.| last2 = Showalter| first2 = Mark R.| last3 = Hamilton| first3 = Douglas P.| last4 = Nicholson| first4 = Philip D.| last5 = de Pater| first5 = Imke| last6 = Ockert-Bell| first6 = Maureen E.| last7 = Thomas| first7 = Peter C.| date = 14 May 1999| title = The Formation of Jupiter's Faint Rings| journal = Science| volume = 284| issue = 5417| pages = 1146–1150| pmid = 10325220| bibcode = 1999Sci...284.1146B| ref = {{sfnRef|Burns Showalter et al.|1999}}}}
* {{cite encyclopedia
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  }}
* {{cite journal
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  | date=September 2002
  | title = The Orbits of Metis and Adrastea: The Origin and Significance of their Inclinations
  | journal = Bulletin of the American Astronomical Society
  | volume = 34
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  | ref = {{sfnRef|Evans Porco et al.|2002}}
  }}
* {{cite journal| doi = 10.1126/science.206.4421.951| last1 = Jewitt| first1 = David C.| last2 = Danielson| first2 = G. Edward| last3 = Synnott| first3 = Stephen P.| date = November 23, 1979| title = Discovery of a New Jupiter Satellite| journal = Science| volume = 206| issue = 4421| pages = 951| pmid = 17733911| bibcode = 1979Sci...206..951J| s2cid = 6391249| ref = {{sfnRef|Jewitt Danielson et al.|1979}}}}
* {{cite journal
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 |title=Editorial Notice 
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 |volume=3454 
 |url=http://www.cbat.eps.harvard.edu/iauc/03400/03454.html 
 |access-date=2012-03-28 
 |ref={{sfnRef|IAUC 3454}} 
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 |archive-url=https://web.archive.org/web/20110725035158/http://www.cbat.eps.harvard.edu/iauc/03400/03454.html 
 |archive-date=2011-07-25 
}} (discovery)
* {{cite journal
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* {{cite journal| doi = 10.1006/icar.1998.6072| last1 = Ockert-Bell | first1 = M. E.| last2 = Burns | first2 = J. A.| last3 = Daubar | first3 = I. J.| last4 = Thomas | first4 = P. C.| last5 = Veverka | first5 = J.| last6 = Belton | first6 = M. J. S.| last7 = Klaasen | first7 = K. P.| date = 1 April 1999| title = The Structure of Jupiter's Ring System as Revealed by the Galileo Imaging Experiment| journal = Icarus| volume = 138| issue = 2| pages = 188–213| bibcode = 1999Icar..138..188O| ref = {{sfnRef|Ockert-Bell Burns et al.|1999}}| doi-access = free}}
* {{cite journal| doi = 10.1006/icar.1998.5976| last1 = Thomas | first1 = P. C.| last2 = Burns | first2 = J. A.| last3 = Rossier | first3 = L.| last4 = Simonelli | first4 = D.| last5 = Veverka | first5 = J.| last6 = Chapman | first6 = C. R.| last7 = Klaasen | first7 = K.| last8 = Johnson | first8 = T. V.| last9 = Belton | first9 = M. J. S.| author10 = Galileo Solid State Imaging Team| date =September 1998| title = The Small Inner Satellites of Jupiter| journal = Icarus| volume = 135| issue = 1| pages = 360–371| bibcode = 1998Icar..135..360T| ref = {{sfnRef|Thomas Burns et al.|1998}}| doi-access = free}}

==External links==
* [https://web.archive.org/web/20130317224600/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Adrastea Adrastea Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
{{Spoken Wikipedia|date=2021-03-01|En-Adrastea (moon)-article.ogg}}{{Moons of Jupiter}}

{{DEFAULTSORT:Adrastea (Moon)}}
[[Category:Moons of Jupiter]]
[[Category:Astronomical objects discovered in 1979|19790708]]
[[Category:Discoveries by David C. Jewitt]]
[[Category:Discoveries by G. Edward Danielson]]
[[Category:Moons with a prograde orbit]]