Halimede is about 62 kilometres in diameter (assuming an albedo of 0.04) and appears neutral (grey) in the visible light. Given the very similar colour of the moon to that of Nereid together with a high probability (41%) of their collision in the lifespan of the Solar system, it has been suggested that the moon could be a piece of Nereid.
Halimede, or Neptune IX, like many of the farther moons of Neptune, is named after one of the Nereids, the fifty daughters of Nereus and Doris. Before the announcement of its name on February 3, 2007 (IAUC 8802), Halimede was known by the designation S/2002 N 1.
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