Although first recorded by Hubble in 2004, subsequent imagery shows that this tiny, innermost moon of Neptune likely had had a violent history.
An article published in the February 21st issue of Nature, authored by Mark Showalter (SETI Institute) and three colleagues, is titled "The seventh inner moon of Neptune." From the title alone, you might conclude that these observers have announced the discovery of a tiny new body circling Planet Eight.
Not so, however. The International Astronomical Union actually announced the discovery back in July 2013, when this moon carried the temporary designation S/2004 N 1 (because it was first recorded on images taken in 2004, shown at upper right). You can read all about it here.
What's new in Nature is the body's now-official name, the novel "stacking" technique used by the team in its discovery, and what this asteroid-size object is telling us about the history of the Neptune's system.
First, the name: Showalter has chosen Hippocamp, the name of a mythical sea creature typically portrayed with the head and torso of a horse (sometimes with wings) and the tail of a fish. It nicely fits the IAU's naming scheme for Neptunian moons, involving Greek and Roman mythology of the seas, as Hippocampus is the genus for seahorses. "I'm a scuba diver and am very fond of seahorses," he tells Sky & Telescope. "That's the real reason I liked the name."
Showalter has a knack for tracking down faint outer-planet bodies — he'd already chalked up weirdly shaped Pan (30 km) within Saturn's A ring, Mab (25 km) and Cupid (18 km) around Uranus, and the elongated moonlets Kerberos (19 × 10) and Styx (16 × 9) around Pluto. His key to a success is a technique familiar to amateur astrophotographers.
You might recall that Voyager 2 discovered six small moons inside the orbit of Triton when it flew past Neptune in 1989. But Hippocamp was too small and escaped detection. Showalter searched for more inner moons with dedicated Hubble Space Telescope runs in 2004 and 2005. Orbital motion within about 200,000 km of the planet is so rapid that the HST images had to be no longer than about 5 minutes to avoid excessive smear. But these short shots wouldn't be sensitive enough to record something as small as Hippocamp.
But anything orbiting very close to Neptune surely would be in a circular orbit in the planet's equatorial plane, with easily predictable motion from frame to frame. So Showalter digitally offset each Hubble image to match the predicted motion and "stacked" them. Doing this, Hippocamp repeatedly popped into view. He used this same trick, likewise yielding positive detections, with follow-up Hubble runs in 2009 and 2016. (Curious, his team ran the orbital motion of Hippocamp back in time to see if, perhaps, this little body might have been lurking unseen in a Voyager image. It wasn't.)
An extra benefit of these searches is refined orbits for Neptune's close-in moons. For example, 75-km-wide Naiad hadn't been seen (at least with certainty) since Voyager 2's flyby. Showlater's team did spot it — but in a spot diametrically opposite where it should have been, based on the presumed orbit. Thalassa (92 km) proved less challenging but was still displaced 19° from its predicted orbital longitude. The team also reports that no other moons larger than 24 km likely lie within 200,000 km of Neptune or larger than 20 km farther out.
Hippocamp's Violent History
Curiously, Hippocamp has an orbit situated just 12,000 km inside that of Proteus, the largest and most massive of Voyager 2's six discoveries with an estimated diameter of 407 km. Tidal interactions with Neptune are slowly driving Proteus outward, meaning that it was much closer to Hippocamp in the past. So are these two related in some way?
Given the prevalence of orbital resonances among outer-planet bodies, dynamicists might have expected to see such relationships among Neptune's tightly spaced inner moons. However, Showalter and his team have used the Hubble data to refine the orbits of all seven — and no resonances of any kind have turned up
Proteus bears a large crater called Pharos. It's at least 230 km across — so huge, compared to the moon itself, that its formation must have come close to shattering Proteus. Hippocamp, by comparison, represents about 2% of the mass that would have been ejected during the Pharos impact. So quite plausibly Hippocamp assembled from fragments of Proteus. In fact, Showalter's team calculates that impacts with interloping comets should have shattered Hippocamp nine times over the past 4 billion years, only to pull itself back together each time. (These multiple reassemblies also neatly explain why Hippocamp's orbit is neither eccentric nor inclined, despite the significant gravitational influence of Proteus.)
Given all the "missing mass" from the Pharos impact, maybe Hippocamp started out larger and has gradually been whittled down to its present size. Even so, its continued survival seems remarkable — maybe, if the IAU had allowed it, Showalter might have considered the name "Phoenix" instead!