Pluto's natural satellites

From Wikipedia, the free encyclopedia.

The planet Pluto has three known moons. The largest, Charon is proportionally larger than any other satellite in the solar system compared to its planet. The other two moons, as yet unnamed, are much smaller. Their discovery was announced on 2005 October 31.

1 Pluto's satellite system
2 Table of known moons
3 See also
4 References
5 External links

[edit]

Pluto's satellite system

The orbits of the Plutonian system as seen from Earth, estimated from the discovery images of the outer moons.

Hubble images of the Plutonian system.

The innermost moon, Charon, was discovered by James Christy on 1978 June 22, nearly half a century after Pluto. Two outer moons were imaged by the Hubble Space Telescope Pluto Companion Search Team on 2005 May 15, and precovered from Hubble images taken in June 2002. The team has scheduled further Hubble observations for February 2006. Once the orbits are confirmed, the International Astronomical Union can consider names for the outer moons. Given the sensitivity of the Hubble images, and the fact that the entire region of space dominated by Pluto's gravitational field was imaged, Pluto is not expected to have any other moons larger than approximately 20 km in diameter.

Pluto and Charon have been called a double planet because Charon is larger compared to Pluto (half its diameter and an eighth its mass) than any other moon is to a planet; indeed Charon is massive enough that, despite their proximity, Pluto orbits the system's barycenter at a point outside its surface (see here and barycenter for animations). Charon and Pluto are also tidally locked, so that they always present the same face toward each other.

The Plutonian system has not been visited by spacecraft, but a flyby by the New Horizons mission is planned for 2015.

[edit]

Table of known moons

The Plutonian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted. Pluto has been added for comparison, for it orbits a point outside itself.

**Notes:**
* Awaiting orbital confirmation and naming.
** Astronomers use this variant pronunciation, not the classical *kair'-on*.
Name (spheroidal moons in bold) (Pronunciation key)			Mean diameter (km)	Mass (kg)	Semi-major axis (km)	Orbital period (days)	Discovery date
(Pluto)			(2284)	(13×10²¹)	(2450)	(6.3872)	(1930)
Pluto I	Charon	shar'-on^**	1192	1.6×10²¹	19,410	6.3872	1978
S/2005 P 2^*			32-145 [1] [2]	< 5×10¹⁸	49,400 ± 600	25.5 ± 0.5	2005
S/2005 P 1^*			52-160 [3]	< 5×10¹⁸	64,700 ± 850	38.2 ± 0.8	2005

The diameters of objects can be estimated from their assumed surface reflectivity (albedo). The low estimates correspond to a 35% albedo like Charon, while the larger ones correspond to the 4% albedo of the darkest KBOs.

Further observations will determine if S/2005 P 2 and S/2005 P 1 are in orbital resonance with Charon and Pluto. Their currently estimated periods are four and six times that of the Charon-Pluto orbital period.

[edit]