This annotated image made available by researcher Mark R. Showalter in February 2019 shows the planet Neptune and its moons, captured by the Hubble Space Telescope in 2004. The recently discovered moon, Hippocamp, is indicated by a red box and an enlarged version is inset at upper right. (Mark R. Showalter via AP)

Neptune's newest, tiniest moon likely fragment of bigger one

February 20, 2019 - 12:15 pm

CAPE CANAVERAL, Fla. (AP) — Neptune's newest and tiniest moon is probably an ancient fragment of a much larger moon orbiting unusually close.

In the journal Nature on Wednesday, California astronomers shine a light on the 21-mile-wide (34-kilometer-wide) moon Hippocamp, named after the mythological sea horse.

SETI Institute's Mark Showalter discovered Neptune's 14th moon in 2013. Showalter and his research team theorize Hippocamp was formed from debris created when a comet slammed into Proteus, the largest of Neptune's inner moons. The two moons orbit just 7,500 miles (12,000 kilometers) apart and were likely even closer in the past.

Scientists have long believed Neptune's inner moons were repeatedly smashed by comets. Showalter says finding little Hippocamp so close to big Proteus provides "a particularly dramatic illustration of the Neptune system's battered history."

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