Adrastea_(moon)

:''For the [[asteroid]], see [[239 Adrastea]]. For the moon of Jupiter known by this name 1955-1975, see [[Ananke (moon)]]''.

{{Infobox Planet
| name = Adrastea
| image = [[Image:adrastea.jpg]]
| bgcolour = #a0ffa0
| caption = Image of Adrastea taken by [[Galileo probe|Galileo's]] solid state imaging system between November 1996 and June 1997.
| discovery = yes
| discoverer = [[David Jewitt|David C. Jewitt]]<br/> [[G. Edward Danielson]]
| discovered = [[July 8]], [[1979]]
| mean_orbit_radius = 129,000 [[kilometre|km]]<ref name=Evans2002>{{cite journal|last=Evans|first=M.W.|coauthors=Porco, C.C.; Hamilton, D.P.|title=The Orbits of Metis and Adrastea: The Origin and Significance of their Inclinations|journal=Bulletin of the American Astronomical Society |year=2002|volume=34|pages=883|url=http://adsabs.harvard.edu/abs/2002DPS....34.2403E}}</ref><ref name=Burns2004>{{Harvard reference|Surname1=Burns|Given1=J.A.|Surname2=Simonelli|Given2=D.P.|Surname3=Showalter|Given3=M.R.|Surname4=Hamilton|Given4=D.P.|Surname5=Porco|Given5=C.C.|Surname6=Esposito|Given6=L.W.|Surname7=Throop|Given7=H.|Chapter=Jupiter’s Ring-Moon System| Title=Jupiter: The planet, Satellites and Magnetosphere|Year=2004|Publisher=Cambridge University Press|Editor= Bagenal, F.; Dowling, T. E.; McKinnon, W. B.|url=http://www.astro.umd.edu/~hamilton/research/preprints/BurSimSho03.pdf }}</ref>
| eccentricity = 0.0015<ref name=Evans2002/><ref name=Burns2004/>
| period = 0.29826 d (7 h 9.5 min)<ref name=Evans2002/><ref name=Burns2004/>
| avg_speed = 31.378 km/s<ref name=stub>Calculated on the basis of other parameters</ref>
| inclination = 0.03° (to Jupiter's equator)<ref name=Evans2002/><ref name=Burns2004/>
| satellite_of = [[Jupiter]]
| physical_characteristics = yes
| mean_radius = 8.2 ± 2.0 km<ref name=Thomas1998>{{cite journal|last=Thomas|first=P.C.|coauthors=Burns, J.A.; Rossier, L.; et.al.|title=The Small Inner Satellites of Jupiter|journal=ICARUS|year=1998|volume=135|pages=360–371|doi=10.1006/icar.1998.5976 |url=http://adsabs.harvard.edu/abs/2000Icar..147..353S}}</ref>
| dimensions = 20×16×14 km<ref name=Thomas1998/>
| volume = ~2,345 [[square kilometre|km³]]<ref name=stub/>
| mass = ~2{{e|15}} [[kilogram|kg]]<ref name=stub/>
| density = 0.86 g/cm³ (assumed)
| surface_grav = ~0.002 [[Acceleration|m/s²]] (0.0004 g)<ref name=stub/>
| escape_velocity = ~0.008 km/s<ref name=stub/>
| rotation = [[synchronous rotation|synchronous]]
| axial_tilt = zero<ref name=Thomas1998/>
| albedo = ~0.1 ± 0.045<ref name=Thomas1998/>
| single_temperature = ~122 K
}}
'''Adrastea''' ({{pronEng|ˌædrəˈstiːə}} ''ad'-rə-stee'-ə,'' Greek ''Αδράστεια)'' or '''{{nowrap|Jupiter XV}}''', is the second of [[Jupiter (planet)|Jupiter's]] known [[natural satellite|moons]] (counting outward from the planet). It was discovered by [[David Jewitt|David C. Jewitt]] and [[G. Edward Danielson]] in ''[[Voyager 2]]'' probe photographs taken in 1979 and received the designation '''{{nowrap|S/1979 J 1}}'''.<ref>[http://cfa-www.harvard.edu/iauc/03400/03454.html IAUC 3454: ''Editorial Notice''] [[1980]] [[February 25]] (discovery)</ref><ref name=Jewitt1979>{{cite journal|last=Jewitt|first=D.C.|coauthors=Danielson, G.E.; Synnott, S.P. |title=Discovery of a New Jupiter Satellite|journal=Science|year=1979|volume=206|pages=951|url=http://links.jstor.org/sici?sici=0036-8075%2819791123%293206:4421%3c951:DOANJS%3e2.0.CO%3b2-V&origin=ads }}</ref> In 1983, it was officially named after the mythological [[Adrastea (mythology)|Adrastea]],<ref>[http://cfa-www.harvard.edu/iauc/03800/03872.html IAUC 3872: ''Satellites of Jupiter and Saturn''] [[1983]] [[September 30]] (naming the moon)</ref> who was a daughter of [[Jupiter (god)|Jupiter]] and [[Ananke (mythology)|Ananke]].

Adrastea was the first natural satellite to be discovered from images taken by an interplanetary spacecraft, rather than through telescopic photography.

==Physical characteristics==
Adrastea has an irregular shape and measures 20x16x14 km³ across.<ref name=Thomas1998/> The bulk composition and mass of Adrastea are not known, but assuming that its mean density is like that of Amalthea (~0.86 g/cm³)<ref name=Anderson2005>{{cite journal|last=Anderson|first=J.D.|coauthors=Johnson, T.V.; Shubert, G.; et.al.|title=Amalthea’s Density Is Less Than That of Water |journal=Science |year=2005|volume=308|pages=1291–1293|doi=10.1126/science.1110422| url=http://adsabs.harvard.edu/abs/2005Sci...308.1291A}}</ref> its mass can be estimated at ~2×10<sup>15</sup> kg. Amalthea's density implies that that moon is composed of water [[ice]] with a [[porosity]] of 10-15%, and Adrastea may be similar.<ref name=Anderson2005/>

No surface details of Adrastea are known, due to the low resolution of available images.<ref name=Thomas1998/>
[[Image:Adrastée FDS 20630.png|thumb|300px|left|Discovery image of Adrastea, taken on [[July 8]], [[1979]] by Voyager 2. Adrastea is the dot in the very middle, straddling the line of the Jovian rings.]]

==Orbit==
Adrastea is the smallest and second closest member of the [[inner satellites of Jupiter|inner Jovian satellite family]]. It orbits [[Jupiter (planet)|Jupiter]] at a radius of ~129,000 km (1.806 Jupiter radii) within the planet's [[Rings_of_Jupiter#Main_Ring|Main Ring]]. The orbit has very small [[orbital eccentricity|eccentricity]] ~0.0015 and [[inclination]] ~ 0.03° relative to the equator of [[Jupiter (planet)|Jupiter]].<ref name=Burns2004/>

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 (planet)|Jupiter]], this being the lowest energy configuration.<ref name=Thomas1998/>

The orbit of Adrastea lies inside [[Jupiter (planet)|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]]. In any case, however, since it is not breaking up, it must still lie outside its rigid [[Roche limit]].<ref name=Burns2004/>

==Relationship with Jupiter's rings==
Adrastea is the largest contributor to material in [[rings of Jupiter|Jupiter's rings]]. This material appears to consist primarily of material that is ejected from the surfaces of [[Inner satellites of Jupiter|Jupiter's four small inner satellites]] by meteorite impacts. It is easy for the impact ejecta to be lost from the satellites into space because the satellites' surfaces lie fairly close to the edge of their [[Roche sphere]]s due to their low density.<ref name=Burns2004/>

It appears that Adrastea is the most copious source of this ring material, as evidenced by the most dense ring (the Main Ring) being located at and within Adrastea's orbit.<ref name=Burns1999>{{cite journal|last=Burns|first=J.A.|coauthors=Showalter, M.R.; Hamilton, D.P.; et.al.|title=The Formation of Jupiter's Faint Rings|journal=Science|year=1999|volume=284|pages=1146-1150|doi=10.1126/science.284.5417.1146| url=http://adsabs.harvard.edu/abs/1999Sci...284.1146B}}</ref> Precisely, the orbit of Adrastea lies near the outer edge of Jupiter's Main Ring. The exact location of the visible ring material depends on the phase angle of the images: in [[light scattering|forward-scattered light]] Adrastea is firmly outside the Main Ring,<ref name=Ockert-Bel1999>{{cite journal|last=Ockert-Bel|first=M.E.|coauthors=Burns, J.A.; Daubar, I.J.; et.al.|title=The Structure of Jupiter’s Ring System as Revealed by the Galileo Imaging Experiment| journal=ICARUS|year=1999|volume=138|pages=188–213|doi=10.1006/icar.1998.6072| url=http://adsabs.harvard.edu/abs/1999Icar..138..188O}}</ref> but in back-scattered light (which reveals much bigger particles) there appears to also be a narrow ringlet outside Adrastea's orbit.<ref name=Burns2004/>

==Exploration==
Adrastea was discovered in ''[[Voyager program|Voyager 1 and 2]]'' images, but appeared only as a dot.<ref name=Jewitt1979/> The [[Galileo spacecraft|''Galileo'' spacecraft]] was able to determine its shape, but the images remain poor.<ref name=Thomas1998/>

==See also==
* [[Inner satellite]]
* [[Rings of Jupiter]]

==References==
{{reflist|2}}

==External links==
[http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Adrastea Adrastea Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]

{{Moons of Jupiter}}

[[Category:Jupiter's moons]]

[[als:Adrastea (Mond)]]
[[bs:Adrasteja (mjesec)]]
[[br:Adrastea (loarenn)]]
[[bg:Адрастея (спътник)]]
[[ca:Adrastea (satèl·lit)]]
[[cs:Adrastea (měsíc)]]
[[co:Adrastea]]
[[da:Adrastea (måne)]]
[[de:Adrastea (Mond)]]
[[es:Adrastea (luna)]]
[[eo:Adrasteo (luno)]]
[[fi:Adrastea (kuu)]]
[[fr:Adrastée (lune)]]
[[hr:Adrasteja (mjesec)]]
[[it:Adrastea (astronomia)]]
[[he:אדרסטאה]]
[[lv:Adrasteja (pavadonis)]]
[[lt:Adrastėja (palydovas)]]
[[hu:Adraszteia (hold)]]
[[nl:Adrastea (maan)]]
[[ja:アドラステア (衛星)]]
[[nn:Jupitermånen Adrastea]]
[[nds:Adrastea (Maand)]]
[[pl:Adrastea (księżyc)]]
[[pt:Adrasteia]]
[[ru:Адрастея (спутник)]]
[[sk:Adrastea (mesiac)]]
[[sl:Adrasteja (luna)]]
[[sr:Адрастеа]]
[[sv:Adrastea]]
[[tl:Adrastea (buwan)]]
[[uk:Адрастея (супутник)]]
[[zh:木卫十五]]