Over the years, astronomers have discovered 14 moons that orbit Neptune – and that’s just 14 that we know of. Seven of them are inner moons that are relatively close to one another. In 1989, the Voyager 2 spacecraft flyby happened upon two of these inner moons, Naiad and Thalassa.
Scientists have more recently observed Naiad’s unique orbit consisting of oscillating patterns in order to avoid collision with Thalassa. They’ve labelled it a "dance of avoidance."
These moons are quite small when compared to the Earth’s moon – a pithy 60 miles or so in length to 2,159 miles in diameter. In addition, at least relatively speaking, the two moons are quite close together, orbiting only 1,150 miles apart.
But because of the shape of their orbits, they are prevented from colliding with one another. The data that confirmed this was collected by the Voyager 2 spacecraft and Hubble Space Telescope between 1981 and 2016.
"We are always excited to find these co-dependencies between moons," said co-author of the article in the journal Icarus, Mark Showalter, a planetary astronomer at the SETI Institute in Mountain View, California.
"Naiad and Thalassa have probably been locked together in this configuration for a very long time, because it makes their orbits more stable. They maintain the peace by never getting too close."
Naiad takes 7 hours to orbit Neptune while Thalassa takes 7.5 hours, so to avoid Thalassa, Naiad moves in a zigzag pattern, crossing the other’s orbit twice from above and twice from below, repeating the pattern again and again.
Naiad’s tilt and timing are impeccable each time, keeping the two from coming too close. They remain 2,200 miles or so apart even when the two orbits cross paths.
"We refer to this repeating pattern as a resonance," lead author Marina Brozović, an expert in solar system dynamics at NASA's Jet Propulsion Laboratory in Pasadena, California, said.
"There are many different types of 'dances' that planets, moons, and asteroids can follow, but this one has never been seen before."
Brozović and her colleagues think that this unique moon movement has been occurring since Neptune captured Triton, it’s the largest satellite at 1,680 miles.
"We suspect that Naiad was kicked into its tilted orbit by an earlier interaction with one of Neptune's other inner moons," said Brozović. "Only later, after its orbital tilt was established, could Naiad settle into this unusual resonance with Thalassa."