A new inner moon of Neptune

During its 1989 flyby, the Voyager 2 spacecraft imaged six small moons of Neptune, all orbiting well interior to the large, retrograde moon Triton(1). Along with a set of nearby rings, these moons are probably younger than Neptune itself; they formed shortly after the capture of Triton and most of them have probably been fragmented multiple times by cometary impacts(1–3). Here we report observations of a seventh inner moon, Hippocamp. It is smaller than the other six, with a mean radius R ≈ 17 km. We also recover Naiad, Neptune’s innermost moon, seen for the first time since 1989. We provide new astrometry, orbit determinations, and size estimates for all the inner moons. Hippocamp orbits close to Proteus, the outermost and largest of these moons; the fractional separation is only 10 percent. Proteus has migrated outward because of tidal interactions with Neptune. We suggest that Hippocamp is probably an ancient fragment of Proteus, providing further support for the hypothesis that the inner Neptune system has been shaped by numerous impacts.