A team of scientists headed by Marina Brozovic at NASA’s Jet Propulsion Laboratory has found that two of Neptune’s innermost moons are protected in a never before seen orbital “dance of avoidance” that keeps them from bumping. The tiny moons Thalassa and Naiad frequently pass within 2,200 mi (3,540 km) of one another, but their orbits cooperate in such a way that they remain stable.
If you go by science textbooks, orbits seem like very plain things. The Moon rotates around the Earth more or less in a circle, the Earth revolves around the Sun in a similar circle, as do the other planets all set on about the same plane. The only outliers are the comets with their extended, elliptical loops from the outer to the inner solar system.
For sure, orbital dynamics are much more complicated than that. As many different bodies moving in very complex routes pull on one another, they can produce some surprising phenomena – moons and planets are formed, get seized by larger planets or stars, and get lodged into or thrown out of orbit.
One model that hasn’t been seen before is the Neptunian moons Thalassa and Naiad, which have orbits that are only 1,150 mi (1,850 km) away from each other. Centered on observations from ground observatories, the Hubble Space Telescope, and Voyager 2, it’s been proven that the subtle dance of these two bodies is the first fourth-order resonance orbit witnessed.
An orbital resonance happens when two bodies have orbital periods that are in just the correct ratio to either strengthen a stable orbit or throw one or more body out of its orbit. It’s akin to how even pushes on a playground swing can either cause it to move forth and back with a regular motion, slow to a stop, or go faster and faster till the rider gets tossed out.
In the situation of the little Neptunian moons, their stages and orbital tilt produce a particular resonance. Naiad makes one rotation every seven hours and Thalassa every seven and a half hours. Every time Naiad passes Thalassa four times, it moves above and below its corresponding moon in a recurring up, up, down, down zigzag pattern, steadying the orbits.
“We call this repeating pattern as a resonance,” says Brozovic. “There are many different kinds of ‘dances’ that moons, planets, and asteroids can follow, but this one is the new one.”
The oblate-shaped moons are about 60 mi (100 km) in width and are the innermost of Neptune’s 14 known moons. How they landed in their odd orbits isn’t clear, but the team thinks that it was caused by Neptune seizing its largest moon, Triton, which disturbed the original moon system and initiated the present moons and rings to develop later from the debris.
“We guess that Naiad was kicked into its slanted orbit by a previous interaction with one of Neptune’s other inner moons,” says Brozovic. “Only later, after its orbital tilt was created, could Naiad settle into this uncommon resonance with Thalassa.”
This research was first published in Icarus.