Dysnomia (moon)
Dysnomia (formally (136199) Eris I Dysnomia) is the only known moon of the dwarf planet Eris and is the second-largest known moon of a dwarf planet, after Pluto I Charon. It was discovered in September 2005 by Mike Brown and the Laser Guide Star Adaptive Optics (LGSAO) team at the W. M. Keck Observatory. It carried the provisional designation of S/2005 (2003 UB313) 1 until it was officially named Dysnomia (from the Ancient Greek word Δυσνομία meaning anarchy/lawlessness) in September 2006, after the daughter of the Greek goddess Eris.
Low-resolution image of Eris and Dysnomia as imaged by the Hubble Space Telescope, August 2006 | |
| Discovery | |
|---|---|
| Discovered by | Brown et al. |
| Discovery date | 10 September 2005 |
| Designations | |
Designation | Eris I |
| Pronunciation | /dɪsˈnoʊmiə/, /daɪˈsnoʊmiə/ |
Named after | Δυσνομία Dysnomia |
Alternative names | S/2005 (2003 UB313) 1 Dy /ˈdaɪ/ (nickname) Gabrielle (nickname) |
| Adjectives | Dysnomian |
| Orbital characteristics: 5 | |
| Epoch 31 August 2006 (JD 2453979.0) | |
Semi-major axis | 37273±64 km |
| Eccentricity | 0.0062±0.0010 |
Orbital period (sidereal) | 15.785899±0.000050 d |
Average orbital speed | 0.172 km/s |
| Inclination | ≈ 0° (to Eris's equator; assumed) 78.29°±0.65° (to Eris's orbit) 45.49°±0.15° (to celestial equator) 61.59°±0.16° (to ecliptic) |
Longitude of ascending node | 126.17°±0.26° |
| 180.83° | |
| Satellite of | Eris |
| Physical characteristics | |
Mean diameter | 615+60 −50 km: 7 |
| Mass | (8.2±5.7)×1019 kg: 6 |
Mean density | 0.7±0.5 g/cm3: 7 |
Synodic rotation period | synchronous |
| ≈ 0° to orbit (assumed) | |
| Albedo | 0.05±0.01: 7 |
| 25.4 | |
| 5.6 | |
With an estimated diameter of 615+60
−50 km, Dysnomia spans 24% to 29% of Eris's diameter. It is significantly less massive than Eris, with a density consistent with it being mainly composed of ice.: 8 In stark contrast to Eris's highly-reflective icy surface, Dysnomia has a very dark surface that reflects 5% of incoming visible light, resembling typical trans-Neptunian objects around Dysnomia's size. These physical properties indicate Dysnomia likely formed from a large impact on Eris, in a similar manner to other binary dwarf planet systems like Pluto and Orcus, and as well as the Earth–Moon system.