Moons of Pluto - Characteristics

Characteristics

The Plutonian system is highly compact: the five 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 or dwarf 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 recalculations taking into account older images, the orbits of the moons are confirmed to be circular and coplanar, with inclinations differing less than 0.4° and eccentricities less than 0.005. The diagram on the left shows the view from the axis of the moons' orbits (declination 0°, right ascension 133°), aligned with the HST diagram above it. As seen from Earth, these circular orbits appear foreshortened into ellipses depending on Pluto's position.

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 light curve 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.