The Ananke group is a group of retrograde

irregular satellite In astronomy, an irregular moon, irregular satellite or irregular natural satellite is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They have been captured by their parent planet, unlike regular s ...

s of

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

that follow similar

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

s to

Ananke In ancient Greek religion, Ananke (; grc, Ἀνάγκη), from the common noun , "force, constraint, necessity") is the personification of inevitability, compulsion and necessity. She is customarily depicted as holding a spindle. One of the ...

and are thought to have a common origin. Their semi-major axes (distances from Jupiter) range between 19.3 and 22.7 Gm, their

orbital inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth ...

s between 145.7° and 154.8°, and their orbital eccentricities between 0.02 and 0.28. The core members include (negative period indicates retrograde orbit):

Scott S. Sheppard Scott Sander Sheppard (born 1977) is an American astronomer and a discoverer of numerous moons, comets and minor planets in the outer Solar System. He is an astronomer in the Department of Terrestrial Magnetism at the Carnegie Institution for Scie ...

David C. Jewitt David Clifford Jewitt (born 1958) is a British-American astronomer who studies the Solar System, especially its minor bodies. He is based at the University of California, Los Angeles, where he is a Member of the Institute for Geophysics and Pl ...

Carolyn Porco Carolyn C. Porco (born March 6, 1953) is an American planetary scientist who explores the outer Solar System, beginning with her imaging work on the Voyager missions to Jupiter, Saturn, Uranus and Neptune in the 1980s. She led the imaging scienc ...

''Jupiter's outer satellites and Trojans'', In: ''Jupiter. The planet, satellites and magnetosphere.'' Edited by Fran Bagenal, Timothy E. Dowling, William B. McKinnon. Cambridge planetary science, Vol. 1, Cambridge, UK: Cambridge University Press, , 2004, p. 263 - 28
Full text(pdf).
David Nesvorný, Cristian Beaugé, and Luke Dones ''Collisional Origin of Families of Irregular Satellites'', The Astronomical Journal, 127 (2004), pp. 1768–178
Full text.
/ref> The

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

(IAU) reserves names ending in -e for all retrograde moons, including this group's members.

Origin

The Ananke group is believed to have been formed when an

asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

was captured by Jupiter and subsequently fragmented by a collision. This belief is founded on the fact that the

dispersion Dispersion may refer to: Economics and finance *Dispersion (finance), a measure for the statistical distribution of portfolio returns *Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variatio ...

of the

mean There are several kinds of mean in mathematics, especially in statistics. Each mean serves to summarize a given group of data, often to better understand the overall value (magnitude and sign) of a given data set. For a data set, the ''arithme ...

¹ orbital parameters of the core members is very small and can be accounted for by a small velocity impulse (15 < δV < 80 m/s), compatible with a single collision and breakup.David Nesvorný, Jose L. A. Alvarellos, Luke Dones, and Harold F. Levison ''Orbital and Collisional Evolution of the Irregular Satellites'', The Astronomical Journal,126 (2003), pages 398–429
(pdf)
/ref> Based on the sizes of the satellites, the original asteroid may have been about 28 km in diameter. Since this value is near the approximate diameter of Ananke itself, it is likely the parent body was not heavily disrupted.(pdf).
Available photometric studies put this in doubt, however, and suggest that

secular resonance A secular resonance is a type of orbital resonance between two bodies with synchronized precessional frequencies. In celestial mechanics, secular refers to the long-term motion of a system, and resonance is periods or frequencies being a simple nu ...

has mixed the Ananke and Pasiphae groups: three of the moons of the former family ( Harpalyke,

Praxidike In Greek mythology, Praxidice (Ancient Greek: Πραξιδίκη, ) may refer to the following characters: * Praxidice, goddess of judicial punishment and the exactor of vengeance, which were two closely allied concepts in the classical Greek ...

and

Iocaste In Greek mythology, Jocasta (), also rendered Iocaste ( grc, Ἰοκάστη ) and also known as Epicaste (; ), was a daughter of Menoeceus, a descendant of the Spartoi Echion, and queen consort of Thebes, Greece, Thebes. She was the wife of ...

) display similar grey colours (average colour indices: B−V = 0.77 and V−R = 0.42) while Ananke itself is on the boundary between grey and light red. Grav, Tommy; Holman, Matthew J.; Gladman, Brett J.; Aksnes, Kaare ''Photometric survey of the irregular satellites'',Icarus, 166,(2003), pp. 33-45
Preprint
/ref> ¹ Osculating orbital parameters of irregular satellites of Jupiter change widely in short intervals due to heavy perturbation by the Sun. For example, changes of as much as 1 Gm in semi-major axis in 2 years, 0.5 in eccentricity in 12 years, and as much as 5° in 24 years have been reported. Mean orbital elements are the averages calculated by the numerical integration of current elements over a long period of time, used to determine the dynamical families.

References

External links

{{Moons of Jupiter Moons of Jupiter Irregular satellites Moons with a retrograde orbit