Faint image showing moons of Neptune.
Source: NASA, ESA, and M. Showalter (SETI Institute)
Published: February 20, 2019

This composite Hubble Space Telescope picture shows the location of tiny Hippocamp (originally designated S/2004 N 1), a moon orbiting the giant planet Neptune, nearly 3 billion miles from Earth.

The moon is so small (no more than 12 miles across) and dim, it was missed by NASA's Voyager 2 spacecraft cameras when the probe flew by Neptune in 1989. Several other moons that were discovered by Voyager appear in this 2009 image, along with a circumplanetary structure known as ring arcs.

Mark Showalter of the SETI Institute discovered Hippocamp in July 2013. He analyzed over 150 archival Neptune photographs taken by Hubble from 2004 to 2009. The same white dot appeared over and over again. He then plotted a circular orbit for the moon, which completes one revolution around Neptune every 23 hours.

The black-and-white image was taken in 2009 with Hubble's Wide Field Camera 3 in visible light. Hubble took the color inset of Neptune on August 19, 2009.

Orbit diagram showing several Neptune moons.
This diagram shows the orbits of several moons located close to the planet Neptune. All of them were discovered in 1989 by NASA's Voyager 2 spacecraft, with the exception of Hippocamp (S/2004 N 1), which was discovered in archival Hubble Space Telescope images taken from 2004 to 2009. The moons all follow prograde orbits and are nestled among Neptune's rings (not shown). The outer moon Triton was discovered in 1846 – the same year the planet itself was discovered. Triton's orbit is retrograde, suggesting it is a captured Kuiper Belt object and therefore a distant cousin of Pluto. The inner moons may have formed after Triton's capture several billion years ago. Credit: NASA, ESA, and A. Feild (STScI)


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