It may be that these orbits originated as forced resonances when Charon was tidally boosted into its current synchronous orbit, and then released from resonance as Charon's orbital eccentricity was tidally damped. The Pluto–Charon pair creates strong tidal forces, with the gravitational field at the outer moons varying by 15% peak to peak. However, it was calculated that a resonance with Charon could boost either Nix or Hydra into its current orbit, but not both: boosting Hydra would have required a near-zero Charonian eccentricity of 0.024, whereas boosting Nix would have required a larger eccentricity of at least 0.05. This suggests that Nix and Hydra were instead captured material, formed around Pluto–Charon, and migrated inward until they were trapped in resonance with Charon. The existence of Kerberos and Styx may support this idea. Rotation Prior to the New Horizons mission, Nix, Hydra, Styx, and Kerberos were predicted to rotate chaotically or tumble. However, New Horizons imaging found that they had not tidally spun down to near a spin synchronous state where chaotic rotation or tumbling would be expected. New Horizons imaging found that all 4 moons were at high obliquity. Either they were born that way, or they were tipped by a spin precession resonance. Styx may be experiencing intermittent and chaotic obliquity variations. Mark R. Showalter had speculated that, "Nix can flip its entire pole. It could actually be possible to spend a day on Nix in which the sun rises in the east and sets in the north. It is almost random-looking in the way it rotates." Only one other moon, Saturn's moon Hyperion, is known to tumble, though it is likely that Haumea's moons do so as well. Origin It is suspected that Pluto's satellite system was created by a massive collision, similar to the "big whack" thought to have created the 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 closer to Pluto than they are at present and migrated outward as Charon reached its current orbit. Their grey color is different from that of Pluto, one of the reddest bodies in the Solar System. This is thought