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Proteus, the second largest moon of Neptune next to Triton, was discovered by the Voyager 2 spacecraft and Stephen P. Synnott in 1989 thirty three years after the discovery of a significantly smaller moon, Nereid. It was missed by ground telescope observations for a couple of reasons. One, it orbits so close to Neptune, that the reflective glare of the planet itself obscured it. Second, Proteus is the darkest object in our solar system. Reflecting only about 6% of the sunlight that strikes it, it is as dark as Saturn’s dark moon, Phoebe.

Phoebe and Proteus, given their strikingly similar dark characteristics, are thought by scientists to have formed in the same part of the solar system and are close to one another in age. After formation, it is thought that they were each captured by the gravitational pull of the two planets, Saturn and Neptune.

It has a pro-grade orbit; this means that it orbits Neptune in the same direction as the planet’s rotation. It takes Proteus 27 Earth hours to orbit Neptune, indicating its close equatorial trajectory. Its irregular shape appears to be unaltered geologically. No other moon in the solar system is as large as Proteus while still having an irregular, non-spherical shape. Astronomers say that it is as large as it can possibly be without its own gravitational pull forcing it into a spherical shape. It is heavily cratered and photos of the moon give it the appearance of the top of a head of cauliflower. Its craters are thought to have been formed by impact of asteroids or comets.

Proteus is over 400 kilometers in diameter, making it about one-ninth as wide as Earth’s own moon. In keeping with the mythological theme of nomenclature, Proteus is named for the shape shifting sea god son of Poseidon – Poseidon being the Greek god of the sea. Perhaps Proteus is named after this particular sea god because of its irregular shape. Proteus of mythology was said to have the ability to change his shape at will. One of the craters of Proteus is named Pharos – the name of the island where the mythological sea god was said to reign.

No future plans are made to send spacecraft on a voyage that will pass Neptune in the future. The data and photos gathered on the journey in 1989 is likely the last information that will be available for decades, possibly.

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