Hippocamp (moon)

Lore & Background

Hippocamp was discovered by astronomer Mark Showalter of the SETI Institute on 1 July 2013. Showalter found it by analyzing archived Hubble Space Telescope images of Neptune taken between 2004 and 2009, using a technique similar to panning to stack multiple short-exposure images. The moon was so faint that it eluded detection during the Voyager 2 flyby in 1989. Its provisional designation was S/2004 N 1, and it was officially named Hippocamp in February 2019 after the mythological sea-horse symbolizing Poseidon. The name was proposed by Showalter, partly due to his passion for scuba diving and the seahorse genus Hippocampus.

The moon orbits Neptune in about 23 hours, just under one Earth day. It is about 35 km in diameter and is the second-smallest known moon of Neptune. Its orbit lies unusually close to that of Proteus, Neptune's largest inner moon. Based on this proximity and the fact that Proteus bears a large impact crater named Pharos (about 250 km in diameter), it has been hypothesized that Hippocamp may have accreted from material ejected by that impact several billion years ago. Hippocamp is thought to have been repeatedly disrupted by comet impacts and re-accreted, which explains its circular orbit.

Reader's Guide

Hippocamp's significance lies in its origin story, which is tightly linked to the violent history of Neptune's satellite system. The article describes the Neptunian moon system as the most lopsided among the giant planets, with Triton making up nearly all the mass. After Triton's capture, many original inner moons were destroyed, and some reformed from rubble. Proteus, the largest reformed inner moon, bears a huge impact crater, and Hippocamp is hypothesized to be a fragment ejected from that event. This makes Hippocamp a potential third-generation satellite, formed from debris of a moon that itself re-accreted after Triton's capture. The moon's small size and close orbit to Proteus support this scenario, though Hippocamp accounts for only two percent of the missing volume from the Pharos impact. Its repeated disruption and re-accretion over billions of years further shaped its orbit and mass. Hippocamp's discovery also highlights the value of archival data: Showalter found it by re-examining old Hubble images, a technique that could have been used by anyone since the data were public.

Did You Know?

• Hippocamp was discovered by analyzing archived Hubble Space Telescope images taken between 2004 and 2009.
• The moon is so dim that it was not detected during the Voyager 2 flyby of Neptune in 1989.
• Hippocamp is hypothesized to have formed from debris ejected by an impact on the larger moon Proteus.