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The dwarf planet Pluto has three known moons.
Pluto has three known moons. The largest of Pluto's three moons is Charon. Pluto's moon Charon is proportionally larger, compared to its primary, than any other satellite of a known planet or dwarf planet in the solar system. Pluto's other two moons, Nix and Hydra, are much smaller.
Pluto's satellite 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. With the orbits confirmed, the moons have been given definitive names: Hydra (Pluto III, formerly S/2005 P 1) and Nix (Pluto II, formerly S/2005 P 2). The names were chosen in part because the initials (NH) allude to the New Horizons mission. Further Hubble observations were made in February and March 2006. Given the sensitivity of the Hubble images, and the fact that the entire region of space dominated by Pluto's gravitational field has been imaged, Pluto is not expected to have any other moons larger than approximately 12 km in diameter with the albedo of Charon, or possibly up to 40 km if they are as dark as the darkest Kuiper Belt objects. The possibility of rings created by impacts on the smaller moons will be investigated by the New Horizons probe.
The Plutonian system is highly compact: The three known satellites orbit within the inner 3% of the region where prograde orbits would be stable.
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. Charon and Pluto are also tidally locked, so that they always present the same face toward each other.
Following Buie and Grundy's recent re-calculations taking into account older images, the orbits of the moons are confirmed to be circular and coplanar, with inclinations differing less than 0.4º and eccentricities less than 0.005. The inclinations are roughly 96º to the ecliptic (so technically the moons' movements are retrograde). The diagram on the right shows the view from the axis of the moons' orbits (Declination 0º, Right ascension 133º), aligned with the HST diagram above it. As seen from Earth, these circular orbits appear foreshortened into ellipses depending on Pluto's position.
It is suspected that the Plutonian satellite system was created by a massive collision, similar to the 'Big Whack' believed to have created the Earth's moon. In both cases the high angular momenta of the moons can only be explained by such a scenario. The nearly circular orbits of the smaller moons suggests that they were also formed in this collision, rather than being captured Kuiper Belt objects. This and their near orbital resonances with Charon (see below) suggest that they formed even closer to Pluto than they are at present, and that they migrated outward as Charon achieved its current orbit. If Hydra and Nix turn out to be tidally locked, as Charon is, that will settle the issue, as tidal forces are insufficient to damp their rotations in their present orbits. Both are a Lunar grey like Charon, which is consistent with a common origin. Their difference in color from Pluto, one of the reddest bodies in the Solar system due to the effects of sunlight on the nitrogen and methane ices of its surface, may be due to a loss of such volatiles during the impact or subsequent coallescence, leaving the surfaces of the moons dominated by water ice. Such an impact would be expected to create additional debris (more moons), but these must be relatively small to have avoided detection by Hubble. It is possible that there are also undiscovered Irregular satellites, which are captured kuiper belt objects.
The Plutonian system has not been visited by spacecraft, but a flyby by the New Horizons mission is due in July 2015.
Table of Pluto's 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 in light purple. Pluto has been added for comparison, for it orbits a point outside itself.
When discovered, Hydra was somewhat brighter than Nix, and therefore thought to be larger by 20%, but follow-up observations found them to be nearly identical. It is likely that the change in brightness is due to the lightcurve of Hydra, but whether this is due to an irregular shape or to a variation in surface brightness (albedo) is unknown. The diameters of objects can be estimated from their assumed albedos; the estimates above correspond to a 35% albedo like Charon, but the moons could be as large as 130 km if they have the 4% albedo of the darkest KBOs. However, given their color and suspected chemical similarities to Charon, it is likely that their albedos are similar as well and that the diameters are closer to the lower estimates.
Nix and Hydra are very close to a 1:4:6 orbital resonance with the Charon-Pluto orbital period: Nix is within 2.7% of resonance, while Hydra is within 0.3%, though neither are in an exact resonance. It may be that these orbits originated as forced resonances when Charon was tidally boosted into its current geosynchronous orbit, and then released from resonance as Charon's orbital eccentricity was tidally damped. Today the Pluto-Charon pair continue to produce strong tidal forces, with the gravitational field at the outer moons varying by 15% peak to peak. At the lower estimated size range, Nix should have no significant precession, while Hydra should have a precession period of 15 years. However, at their maximum projected masses (assuming an albedo of 4%), the two moons may be in a 3:2 orbital resonance with each other, with libration periods of 400 to 450 days, though this may already be ruled out by the low eccentricity of Charon. Thus accurate orbital data can help resolve the sizes of these moons.
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