Praxidike (moon)
Second-largest member of Jupiter's retrograde Ananke group.
Praxidike (Jupiter XXVII, formerly S/2000 J 7) is a small moon of Jupiter. It is an irregular satellite, meaning it travels far from the planet on a path that is steeply tilted and stretched out, moving in the retrograde direction—opposite to Jupiter's spin. The moon belongs to the Ananke group, a set of irregular moons thought to be pieces of an asteroid or trans-Neptunian object that Jupiter captured and that later broke apart in a collision billions of years ago. Praxidike is the second-largest member of this group.
Discovered on 23 November 2000 by Scott S. Sheppard and a team from the University of Hawaiʻi, it was initially labeled S/2000 J 7. In October 2002, it was named after Praxidike, the Greek goddess of punishment.
Praxidike orbits Jupiter at an average distance of about 21 million kilometers, completing one lap in roughly 609 days. Its orbit has an inclination of 149° relative to the ecliptic and an eccentricity of 0.230. This path is constantly altered by gravitational pulls from the Sun and other planets.
Because it is small—roughly 7 kilometers in diameter—Praxidike is very faint, with an apparent magnitude around 21.2, visible only through large, sensitive telescopes. Its surface is dark and gray, with an albedo of just 2.9% as measured by the WISE space telescope, making it one of the darkest objects in the solar system. Its shape is unknown, but given its size, it is likely irregular. Color indices (B−V=0.77, R–V=0.34) suggest it resembles a C-type asteroid.
No space probe has imaged Praxidike up close. The European Space Agency's Juice mission plans to observe all of Jupiter's irregular moons, including Praxidike, from a distance, measuring their rotation periods and shapes by tracking changes in brightness over time.
Quick Facts
- Pronounced
- p · r · æ · k · ˈ · s · ɪ · d · ə · k · iː
- Adjective
- Praxidikean ˌ · p · r · æ · k · s · ə · d · ə · ˈ · k · iː · ə · n
- Named After
- Πραξιδίκη Praxidikē
- Mpc Name
- Jupiter XXVII
- Discoverer
- Scott S. Sheppard et al.
- Discovered
- 23 November 2000
- Discovery Site
- Mauna Kea Observatory
- Observation Arc
- 2000 · 2025 / 2025-12-21 (last obs)
- Inclination
- 149.0°
- Eccentricity
- 0.230
- Arg Peri
- 209.7°
- Asc Node
- 285.2°
Facts from the source article.
Lore & Background
Praxidike was discovered on 23 November 2000 by a team of astronomers from the University of Hawaiʻi led by Scott S. Sheppard, along with ten other Jovian moons, and given the temporary designation S/2000 J 7. In October 2002, it was named after Praxidike, the Greek goddess of punishment. The moon orbits Jupiter in the retrograde direction at a mean distance of 21,147,000 km, completing one orbit in 609.25 days. Its orbit has an inclination of 149° relative to the ecliptic and an eccentricity of 0.230, and is subject to continuous changes due to gravitational perturbation from the Sun and other planets. Praxidike belongs to the Ananke group, whose members orbit Jupiter at distances of approximately 19 to 22 million km with inclinations from 144° to 156° and eccentricities between 0.10 and 0.30. The group is believed to be fragments of an asteroid or trans-Neptunian object that was gravitationally captured by Jupiter and destroyed by a collision several billion years ago. Most of Praxidike's physical properties are unknown; it is estimated to be about 7 km in diameter, with a dark and gray surface. Measurements by the WISE space telescope determined an albedo of only 2.9%, making it one of the darkest known bodies in the Solar System. Its shape is unknown but likely irregular, and its grey color (B−V=0.77, R–V=0.34) is typical of C-type asteroids.
Reader's Guide
Praxidike's significance lies in its membership in the Ananke group, which provides evidence for a major collision event in Jupiter's past. As a fragment of a larger captured body, it helps scientists understand the history of irregular moon formation and the dynamics of the Jovian system. Its extremely dark surface (albedo 2.9%) makes it one of the darkest objects in the Solar System, offering clues about the composition of primitive bodies. Praxidike has not been imaged up close by any space probe, but it is planned as a distant observation target for the European Space Agency's Jupiter Icy Moons Explorer (Juice) mission, which will measure its rotation period and shape by watching brightness changes over time. Its faint apparent magnitude of about 21.2 means it can only be observed by very large, sensitive telescopes, limiting direct study. As the second-largest member of the Ananke group after Ananke itself, Praxidike serves as a key reference for understanding the group's physical properties and origin.
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