English: With the position of Neptune's moon Thalassa held steady (the yellow dot at center), the orbital motions of the next inner moon Naiad, on a more inclined orbit, are shown in red.

Naiad is in a 73:69 orbital resonance with Thalassa, a "dance of avoidance". As it orbits Neptune, the more inclined Naiad successively passes Thalassa twice from above and then twice from below, in a cycle that repeats every ~21.5 Earth days. The two moons are about 3540 km apart when they pass each other. Although their orbital radii differ by only 1850 km, Naiad swings ~2800 km above or below Thalassa's orbital plane at closest approach. Thus this resonance, like many such orbital correlations, serves to stabilize the orbits by maximizing separation at conjunction. However, the role of orbital inclination in maintaining this avoidance in a case where eccentricities are minimal is unusual.

Original caption published with media:

Even by the wild standards of the outer solar system, the strange orbits that carry Neptune's two innermost moons are unprecedented, according to newly published research.

Orbital dynamics experts are calling it a "dance of avoidance" performed by the tiny moons Naiad and Thalassa. The two are true partners, orbiting only about 1,150 miles (1,850 kilometers) apart. But they never get that close to each other; Naiad's orbit is tilted and perfectly timed. Every time it passes the slower-moving Thalassa, the two are about 2,200 miles (3,540 kilometers) apart.

In this perpetual choreography, 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.

Although the dance may appear odd, it keeps the orbits stable, researchers said.

"We refer to this repeating pattern as a resonance," said Marina Brozovic, an expert in solar system dynamics at NASA's Jet Propulsion Laboratory in Pasadena, California, and the lead author of the new paper, which was published Nov. 13 in Icarus. "There are many different types of 'dances' that planets, moons and asteroids can follow, but this one has never been seen before."

Far from the pull of the Sun, the giant planets of the outer solar system are the dominant sources of gravity, and collectively, they boast dozens upon dozens of moons. Some of those moons formed alongside their planets and never went anywhere; others were captured later, then locked into orbits dictated by their planets. Some orbit in the opposite direction their planets rotate; others swap orbits with each other as if to avoid collision.

Neptune has 14 confirmed moons. Neso, the farthest-flung of them, orbits in a wildly elliptical loop that carries it nearly 46 million miles (74 million kilometers) away from the planet and takes 27 years to complete.

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.

So how did they end up together - but apart? It's thought that the original satellite system was disrupted when Neptune captured its giant moon, Triton, and that these inner moons and rings formed from the leftover debris.

"We suspect that Naiad was kicked into its tilted orbit by an earlier interaction with one of Neptune's other inner moons," Brozovic said. "Only later, after its orbital tilt was established, could Naiad settle into this unusual resonance with Thalassa."

Brozovic and her colleagues discovered the unusual orbital pattern using analysis of observations by NASA's Hubble Space Telescope. The work also provides the first hint about the internal composition of Neptune's inner moons. Researchers used the observations to compute their mass and, thus, their densities - which were close to that of water ice.

"We are always excited to find these co-dependencies between moons," said Mark Showalter, a planetary astronomer at the SETI Institute in Mountain View, California, and a co-author of the new paper. "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."