These two moons orbit only about 1,150 miles (1,850 kilometers) apart. But they never get that close to each other. The moons are found to be locked in ‘dance of avoidance.’ Naiad swirls around the ice giant every seven hours, while Thalassa, on the outside track, takes seven and a half hours. An observer sitting on Thalassa would see Naiad in an orbit that varies wildly in a zigzag pattern, passing by twice from above and then twice from below. This up, up, down, down pattern, repeats every time Naiad gains four laps on Thalassa.
Marina Brozovic, an expert in solar system dynamics at NASA’s Jet Propulsion Laboratory in Pasadena, California, said, “We refer to this repeating pattern as a resonance. There are many different types of ‘dances’ that planets, moons, and asteroids can follow, but this one has never been seen before.”
Naiad and Thalassa are small and shaped like Tic Tacs, spanning only about 60 miles (100 kilometers) in length. They are two of Neptune’s seven inner moons, part of a closely packed system that is interwoven with faint rings.
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