During its 1989 flyby, the Voyager 2 spacecraft imaged six small moons of Neptune, all with orbits well interior to that of the large, retrograde moon Triton1. 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 impacts1,2,3. Here we report Hubble Space Telescope observations of a seventh inner moon, Hippocamp. It is smaller than the other six, with a mean radius of about 17 kilometres. We also observe Naiad, Neptune’s innermost moon, which was last seen in 1989, and provide astrometry, orbit determinations and size estimates for all the inner moons, using an analysis technique that involves distorting consecutive images to compensate for each moon’s orbital motion and that is potentially applicable to searches for other moons and exoplanets. Hippocamp orbits close to Proteus, the outermost and largest of these moons, and the orbital semimajor axes of the two moons differ by only ten per cent. Proteus has migrated outwards because of tidal interactions with Neptune. Our results 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.
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All source data used in this study are in the public domain and may be obtained from the STScI archive at http://archive.stsci.edu/hst/search.php. The Voyager images referenced in this paper can be retrieved from NASA’s Planetary Data System at https://pds-rings.seti.org/viewmaster/volumes/VGISS_8xxx/VGISS_8207. Data files for every image analysed in this investigation, at nearly every intermediate step in the analysis, are permanently archived at http://dmp.seti.org/mshowalter/neptune_xiv.
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Support for this work was provided by NASA through grant numbers HST-GO-10398, -11656 and -14217 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. Additional support for M.R.S. and R.S.F. was provided by NASA’s Outer Planets Program through grant NNX14AO40G. We thank A. Roman of the Space Telescope Science Institute for extensive support during the planning of the HST observations. M. Brozovic of the Jet Propulsion Laboratory provided numerical integrations to help us identify detections of Naiad.
Nature thanks T. Becker and the other anonymous reviewer(s) for their contribution to the peer review of this work.