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One of the larger moons of the planet Neptune has been through a lot. It might have started as an asteroid, and was captured by Neptune’s gravity. Or it might have started as a moon, but was hurled into a wild orbit when Neptune grabbed its largest moon. And since then, it’s been battered by impacts with other space rocks.

Nereid was discovered 75 years ago today, by Gerard Kuiper. It was only the second moon seen around the giant planet, and it’s the third-largest of Neptune’s 16 known moons.

Kuiper was observing Neptune with the 82-inch telescope at McDonald Observatory. In a pair of 40-minute exposures, the moon showed up as a tiny star near the planet. Kuiper suggested the name Nereid because, in mythology, the Nereids were daughters of Neptune.

We don’t know a lot more about the moon today than when it was discovered. It’s more than 200 miles in diameter, its gray surface probably is coated with ice and rock, and the surface is rough — perhaps the result of billions of years of impacts.

Nereid follows the most lopsided orbit of any good-sized moon in the solar system. It ranges from less than a million miles from Neptune to about six million miles. That suggests that Nereid could be the last of Neptune’s original moons. When Neptune captured its biggest moon, Triton, Triton’s gravity could have kicked out all the others, leaving only Nereid — in a wild orbit around a giant planet.

Script by Damond Benningfield

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