of 0.3%. A hypothesis explaining the near-resonance suggests that the resonance originated before the outward migration of Charon after the formation of all five known moons, and is maintained by the periodic local fluctuation of 5% in the Pluto–Charon gravitational field strength. Exploration The New Horizons spacecraft visited the Pluto system and imaged Pluto and its moons during its flyby on 14 July 2015. At the time of the New Horizons flyby, Hydra was behind Pluto and was further away from New Horizons at closest approach. The larger distance of Hydra from New Horizons resulted in lower resolution images of Hydra. Before the flyby, the Long Range Reconnaissance Imager on board New Horizons performed measurements of Hydra's size, estimating Hydra to be about in diameter. Hydra's surface composition, reflectivity, and other basic physical properties were later measured by New Horizons during the flyby. The first detailed image of Hydra was downlinked, or received from the New Horizons spacecraft on 15 July 2015 after the flyby. The first detailed image of Hydra, taken from a distance of , appeared to show brightness variations and a dark circular feature across. The highest resolution images of Hydra were taken from a distance of , with an image resolution of per pixel. Derived from those images, Hydra was given the approximate size estimate of . See also List of natural satellites Notes References External links Hydra In Depth by NASA's Solar System Exploration NASA's Hubble Reveals Possible New Moons Around Pluto – Hubble press release Moons of Pluto 20050615 Moons with a prograde orbit