Neptune’s moons locked in ‘dance of avoidance’

Posted: November 14, 2019 by oldbrew in Astrophysics, moon, research
Tags: , ,

Neptune Moon Dance: This animation illustrates how the odd orbits of Neptune’s inner moons Naiad and Thalassa enable them to avoid each other as they race around the planet. (courtesy: JPL)

Well, this is fun. Need we say more?

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, says Space Newsfeed.

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 Brozovi, 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.”

Full report here.

  1. oldbrew says:

    The two innermost satellites, Naiad and Thalassa, orbit between the Galle and LeVerrier rings. – Wikipedia

    They do about 10 orbits every 3 Earth days, and Neptune’s diameter is four times that of Earth, so they move really fast compared to our Moon.

  2. oldbrew says:

    For another type of dance see Janus and Epimetheus – moons of Saturn – whose orbits ‘cross over’.

    In a rotating frame of reference…

    Credit: Phoenix7777 @ Wikipedia

  3. johnm33 says:

    Not unlike Earth/Moon orbit then , skip to 3:05

  4. oldmanK says:

    oldbrew: the video does not show a cross over but a fast return from two points in space. Are those two point the Lagrange L4 L5 points. See and this

  5. oldbrew says:

    oldmanK – no, the J-E video is from a rotating reference frame which can be tricky to visualize – for me also 🙂
    Update: you are correct, the Lagrange points are in play, according to the theory in this paper:

    Click to access 55_2_97.pdf

    J-E orbits viewed from a fixed reference frame…

  6. oldbrew says:

    johnm33 – yes, some similarity for sure.

  7. kate gibbens says:

    Wow. Beautiful. Seems to reflect my last marriage…

  8. oldmanK says:

    This may explain it better

    How the Creator plays with marbles.

  9. BoyfromTottenham says:

    Oldbrew – that orbital dance is a beautiful and mysterious thing! Now, does anyone know how it got that way? e.g. did it start off like that, or became like that because of ….????

  10. oldmanK says:

    The question asked by Boy-T goes to the heart of the enigma so to speak. I came across this in seeking a particular answer:

    The two moons are not equal in size/mass, and this may point to the real answer. It seems that the bigger one orbits the planet, while the smaller is locked between the L4, L5 Lagrange point of its bigger brother. So both could technically have been formed from an accretion disk around the ‘mother’ planet. (But that’s just my 2c worth).

  11. oldbrew says:

    Neptune’s moons are dominated by Triton which accounts for most of their combined mass. When Triton was *captured* (according to theory) the whole system would have been drastically affected.
    – – –
    Wikipedia: Naiad is in a 73:69 orbital resonance with Thalassa in 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 maximize their separation at conjunction. However, the role of orbital inclination in maintaining this avoidance in a case where eccentricities are minimal is unusual.

    The eccentricity ratio of Naiad:Thalassa is given as 0.0047 : 0.0018 = ~Phi²:1

  12. oldbrew says:

    Orbits and resonances of the regular moons of Neptune [2019]

    Click to access 1910.13612.pdf


    We report integrated orbital fits for the inner regular moons of Neptune based on the most complete astrometric data set to date, with observations from Earth-based telescopes, Voyager 2, and the Hubble Space Telescope covering 1981-2016. We summarize the results in terms of state vectors, mean orbital elements, and orbital uncertainties. The estimated masses of the two innermost moons, Naiad and Thalassa, are GMNaiad=0.0080±0.0043 km3 s-2 and GMThalassa=0.0236±0.0064 km3 s-2, corresponding to densities of 0.80±0.48 g cm-3and 1.23±0.43 g cm-3, respectively. Our analysis shows that Naiad and Thalassa are locked in an unusual type of orbital resonance. The resonant argument 73�̇%&'(‘))’ − 69�̇-‘.’/ − 4Ω̇-‘.’/ ≈ 0 librates around 180°with an average amplitude of ~66° and a period of ~1.9 years for the nominal set of masses. This is the first fourth-order resonance discovered between the moons of the outer planets. More high precision astrometry is needed to better constrain the masses of Naiad and Thalassa, and consequently, the amplitude and the period of libration. We also report on a 13:11 near-resonance of Hippocamp and Proteus, which may lead to a mass estimate of Proteus provided that there are future observations of Hippocamp. Our fit yielded a value for Neptune’s oblateness coefficient of J2=3409.1±2.9 ´ 10-6
    [bold added]
    – – –
    They say: This is the first fourth-order resonance discovered between the moons of the outer planets.

    But we have this:
    Why Phi? – resonant moons of Uranus
    Posted: April 7, 2018

    Why Phi? – Moons of Pluto
    Posted: July 26, 2015

    And this:
    Why Phi? – the resonance of Jupiter’s Galilean moons
    Posted: November 26, 2015

    Also Titan:Hyperion 4:3 orbital resonance (Saturnian moons).