Researchers introduce the smallest of Neptune's moon

Researchers introduce the smallest of Neptune's moon

When Voyager II flew past Neptune in August of 1989 it revealed a complex system of 13 moons, six of which had never been seen before. This brought Neptune's total moon count up to 14. Now named Hippocamp, after the mythical sea monster, it's only about 35km across and orbits near the outer edge of this cluster of moons.

Since then, however, improvements in ground-based optics and the existence of the Hubble Space Telescope have enabled us to find a few small bodies that had been missed by the Voyagers, as well as other small objects elsewhere in the Solar System, such as the Kuiper Belt object. This image was captured by the Hubble Space Telescope in the year 2004. The moon is visible inside the red box, with an enlarged version inset at upper right.

A team of astronomers, led by Mark Showalter of the SETI Institute, have used the NASA/ESA Hubble Space Telescope to study the origin of the smallest known moon orbiting the planet Neptune, discovered in 2013. "With Hubble, now we know that a little piece of Proteus got left behind and we see it today as Hippocamp". "You can't normally do that because the moons would smear out, but we essentially took the orbital motion out of the images.

There's an terrible lot of good science to be done at Uranus and Neptune", concluded Showalter. As he was looking for the so-called arcs in Neptune's rings, the researcher made a decision to increase the image's field of view from around 70,000 km out from Neptune to 200,000 km. And there it was. And it has an Australian link, the name choice was inspired by a visit to the sea horses of the Great Barrier Reef by Mark Showalter, the moon's discoverer.

"The discovery of Hippocamp, orbiting so close to the much larger moon Proteus, provides a particularly dramatic illustration of the Neptune system's battered history", Showalter said in an email. If Hippocamp is a product of this impact, as Showalter speculates, then it's just a tiny piece of the total wreckage, about 2% of the total mass excavated from Proteus during the shattering collision.

Since the discovery of the small inner moons, scientists have believed that they are younger than Neptune and that they formed after Neptune captured its largest moon, Triton. Proteus has a large crater, Pharos, where an impact blasted away 50 times the volume of Hippocamp, suggesting that there was more than enough debris around to form the tiny moon.

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Hippocamp is one of the seven "inner moons" - six of which were discovered 30 years ago during Voyager 2's fly-by.

"We came up with a technique to shift around the pixels in one image so that the image matches the geometry of another image taken at a different time", explained Showalter.

You might be wondering where the name comes from. The 21-mile-diameter moon, Hippocamp (named after a mythological seahorse) is believed to have formed from debris created billions of years ago when a comet slammed into Proteus, the largest of Neptune's inner moons. It was the ideal name, and the IAU accepted it.

A faint and frigid little moon doesn't have to go by "Neptune XIV" anymore. And who knows what else will be discovered.

"Until some day we send an orbiter to Neptune", he says. Further study of Neptune and Uranus could offer insight into those even more alien worlds.

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