Charon (moon)
Charon (/ˈkɛərɒn, -ən/ KAIR-on, -ən or /ˈʃærən/ SHAIR-ən), known as (134340) Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 km (377 mi). Charon is the sixth-largest known trans-Neptunian object after Pluto, Eris, Haumea, Makemake, and Gonggong. It was discovered in 1978 at the United States Naval Observatory in Washington, D.C., using photographic plates taken at the United States Naval Observatory Flagstaff Station (NOFS).
Charon as imaged by the New Horizons spacecraft, July 2015. A massive fault system, Serenity Chasma, crosses Charon's face, while Charon's north pole is covered by Mordor Macula, a region covered in dark organic material | |
| Discovery | |
|---|---|
| Discovered by | James W. Christy |
| Discovery date | June 22, 1978 |
| Designations | |
Designation | Pluto I |
| Pronunciation | /ˈkɛərɒn, -ən/ KAIR-on, -ən or /ˈʃærən/ SHAIR-ən |
Named after | Discoverer's wife, Charlene, and Χάρων Kharōn |
Alternative names | S/1978 P 1 |
| Adjectives | Charonian Charontian, -ean Charonean |
| Orbital characteristics | |
| Epoch 2452600.5 (2002 Nov 22) | |
| Periapsis | 19,587 km |
| Apoapsis | 19,595 km |
Semi-major axis | 19591.4 km (planetocentric) 17181.0 km (barycentric) |
| Eccentricity | 0.0002 |
Orbital period (sidereal) | 6.3872304±0.0000011 d (6 d, 9 h, 17 m, 36.7 ± 0.1 s) |
Average orbital speed | 0.21 km/s |
| Inclination | 0.080° (to Pluto's equator) 119.591°±0.014° (to Pluto's orbit) 112.783°±0.014° (to the ecliptic) |
Longitude of ascending node | 223.046°±0.014° (to vernal equinox) |
| Satellite of | Pluto |
| Physical characteristics | |
Mean radius | 606.0±0.5 km (0.095 Earths, 0.51 Plutos) |
| Flattening | <0.5% |
Surface area | 4.6×106 km2 (0.0090 Earths) |
| Volume | (9.32±0.14)×108 km3 (0.00086 Earths) |
| Mass | (1.586±0.015)×1021 kg (2.66×10−4 Earths) (12.2% of Pluto) |
Mean density | 1.702±0.017 g/cm3 |
Surface gravity | 0.288 m/s2 |
| 0.59 km/s 0.37 mi/s | |
Synodic rotation period | synchronous |
| Albedo | 0.2 to 0.5 at a solar phase angle of 15° |
| Temperature | −220 °C (53 K) |
| 16.8 | |
| 1 | |
| 55 milli-arcsec | |
With half the diameter and one-eighth the mass of Pluto, Charon is a very large moon in comparison to its parent body. Its gravitational influence is such that the barycenter of the Plutonian system lies outside Pluto, and the two bodies are tidally locked to each other. The dwarf planet systems Pluto–Charon and Eris–Dysnomia are the only known examples of mutual tidal locking in the Solar System, though it is likely that Orcus–Vanth is another.
The reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be essential ingredients of life. These tholins were produced from methane, nitrogen, and related gases which may have been released by cryovolcanic eruptions on the moon, or may have been transferred over 19,000 km (12,000 mi) from the atmosphere of Pluto to the orbiting moon.
The New Horizons spacecraft is the only probe that has visited the Pluto system. It approached Charon to within 27,000 km (17,000 mi) in 2015.