Adrastea (moon)

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For the moon of Jupiter known by this name between 1955 and 1975, see Ananke (moon).

**Adrastea**
Discovery
Image of Adrastea taken by Galileo's solid-state imaging system between November 1996 and June 1997.
Discovered by	David C. Jewitt G. Edward Danielson
Discovery date	July 8, 1979
Orbital characteristics
Mean orbit radius	129,000 km^[2]^[3]
Eccentricity	0.0015^[2]^[3]
Orbital period	0.29826 d (7 h 9.5 min)^[2]^[3]
Average orbital speed	31.378 km/s^[4]
Inclination	0.03° (to Jupiter's equator)^[2]^[3]
Satellite of	Jupiter
Physical characteristics
Dimensions	20×16×14 km^[5]
Mean radius	8.2 ± 2.0 km^[5]
Volume	~2,345 km³^[4]
Mass	~2×10¹⁵ kg^[4]
Mean density	0.86 g/cm³ (assumed)
Equatorial surface gravity	~0.002 m/s² (0.0004 g)^[4]
Escape velocity	~0.008 km/s^[4]
Rotation period	synchronous
Axial tilt	zero^[5]
Albedo	~0.1 ± 0.045^[5]
Temperature	~122 K

Adrastea (pronounced /ˌædrəˈstiːə/ ad'-rə-stee'-ə, Greek Αδράστεια) or Jupiter XV, is the second of Jupiter's known moons (counting outward from the planet). It was discovered by David C. Jewitt and G. Edward Danielson in Voyager 2 probe photographs taken in 1979 and received the designation S/1979 J 1.^[6]^[7] In 1983, it was officially named after the mythological Adrastea,^[8] who was a daughter of Jupiter and Ananke.

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

1 Physical characteristics
2 Orbit
3 Relationship with Jupiter's rings
4 Exploration
5 See also
6 References
7 External links

Physical characteristics

Adrastea has an irregular shape and measures 20x16x14 km³ across.^[5] 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)^[9] its mass can be estimated at ~2×10¹⁵ kg. Amalthea's density implies that that moon is composed of water ice with a porosity of 10-15%, and Adrastea may be similar.^[9]

No surface details of Adrastea are known, due to the low resolution of available images.^[5]

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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 Jovian satellite family. It orbits Jupiter at a radius of ~129,000 km (1.806 Jupiter radii) within the planet's Main Ring. The orbit has very small eccentricity ~0.0015 and inclination ~ 0.03° relative to the equator of Jupiter.^[3]

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.^[5]

The orbit of Adrastea lies inside Jupiter's synchronous orbit radius (as does Metis’s), and as a result, tidal forces 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.^[3]

Relationship with Jupiter's rings

Adrastea is the largest contributor to material in Jupiter's rings. This material 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 the satellites into space because the satellites' surfaces lie fairly close to the edge of their Roche spheres due to their low density.^[3]

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.^[10] 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 forward-scattered light Adrastea is firmly outside the Main Ring,^[11] but in back-scattered light (which reveals much bigger particles) there appears to also be a narrow ringlet outside Adrastea's orbit.^[3]

Exploration

Adrastea was discovered in Voyager 1 and 2 images, but appeared only as a dot.^[7] The Galileo spacecraft was able to determine its shape, but the images remain poor.^[5]

References

^ [1]
^ ^a ^b ^c ^d Evans, M.W.; Porco, C.C.; Hamilton, D.P. (2002). "The Orbits of Metis and Adrastea: The Origin and Significance of their Inclinations". Bulletin of the American Astronomical Society 34: 883.
^ ^a ^b ^c ^d ^e ^f ^g ^h Burns, J.A.; D.P. Simonelli & M.R. Showalter et al. (2004), "Jupiter’s Ring-Moon System", in Bagenal, F.; Dowling, T. E.; McKinnon, W. B., Jupiter: The planet, Satellites and Magnetosphere, Cambridge University Press, <http://www.astro.umd.edu/~hamilton/research/preprints/BurSimSho03.pdf>
^ ^a ^b ^c ^d ^e Calculated on the basis of other parameters.
^ ^a ^b ^c ^d ^e ^f ^g ^h
^ IAUC 3454: Editorial Notice 1980 February 25 (discovery)
^ ^a ^b Jewitt, D.C.; Danielson, G.E.; Synnott, S.P. (1979). "Discovery of a New Jupiter Satellite". Science 206: 951.
^ IAUC 3872: Satellites of Jupiter and Saturn 1983 September 30 (naming the moon)
^ ^a ^b Anderson, J.D.; Johnson, T.V.; Shubert, G.; et.al. (2005). "Amalthea’s Density Is Less Than That of Water". Science 308: 1291–1293. doi:10.1126/science.1110422.
^ Burns, J.A.; Showalter, M.R.; Hamilton, D.P.; et.al. (1999). "The Formation of Jupiter's Faint Rings". Science 284: 1146-1150. doi:10.1126/science.284.5417.1146.
^ Ockert-Bel, M.E.; Burns, J.A.; Daubar, I.J.; et.al. (1999). "The Structure of Jupiter’s Ring System as Revealed by the Galileo Imaging Experiment". ICARUS 138: 188–213. doi:10.1006/icar.1998.6072.

External links

Adrastea Profile by NASA's Solar System Exploration

v • d • e

Moons of Jupiter

Listed in increasing distance from Jupiter. Temporary names in italics.

Inner moons

Metis · Adrastea · Amalthea · Thebe

Galilean moons

Io · Europa · Ganymede · Callisto

Themisto

Himalia group

Leda · Himalia · Lysithea · Elara · S/2000 J 11

Carpo · S/2003 J 12

Ananke group

core	Ananke · Praxidike · Harpalyke · Iocaste · Euanthe · Thyone
peripheral	Euporie · S/2003 J 3 · S/2003 J 18 · Thelxinoe · Helike · Orthosie · S/2003 J 16 · Hermippe · Mneme · S/2003 J 15

Carme group

S/2003 J 17 · S/2003 J 10 · Pasithee · Chaldene · Arche · Isonoe · Erinome · Kale · Aitne · Taygete · S/2003 J 9 · Carme · S/2003 J 5 · S/2003 J 19 · Kalyke · Eukelade · Kallichore

Pasiphaë group