Starship Asterisk*

Chris Peterson wrote:

geckzilla wrote:
I could think of some circumstances that would allow for moons to have moons. There's definitely nothing about physics that makes it impossible once you're at some nth level of of orbiting bodies.

Not impossible, no. But whenever you have more than two bodies orbiting, you have an unstable situation. No doubt, planetary moons occasionally capture bodies which become their own moons. But the odds of everything working out so that the resulting orbit is stable over millions of years is very small. So over our narrow temporal window of observation, it's not surprising we don't see them.

Parent Body 	Tidally-locked Satellites
---------------------------------------------------------
Earth 	        Moon

Mars            Phobos · Deimos

Jupiter 	     Metis · Adrastea · Amalthea · Thebe · Io · Europa · Ganymede · Callisto

Saturn 	      Pan · Atlas · Prometheus · Pandora · Epimetheus · Janus · Mimas 
                Enceladus · Telesto · Tethys · Calypso · Dione · Rhea · Titan · Iapetus
                              
Uranus 	      Miranda · Ariel · Umbriel · Titania · Oberon

Neptune 	     Proteus · Triton

Pluto 	       Charon 

https://en.wikipedia.org/wiki/Neso(moon) wrote:
<<Neso (for Greek Nereid: Νησώ) is the outermost natural satellite of Neptune. It is an Irregular moon discovered by Holman, Gladman, et al. on August 14, 2002. Neso orbits Neptune at a distance of more than 48 Gm (= 3220 Neptune radii!), making it the most distant known moon of any planet. At apocenter, the satellite is more than 72 Gm from Neptune. This distance is of such an order that it exceeds Mercury's aphelion, which is approximately 70 Gm from the Sun. Neso is also the moon with the longest orbital period, 26.67 years. Neso is about 60 km in diameter based on an assumed albedo.>>

https://en.wikipedia.org/wiki/Hill_sphere wrote:
<<An astronomical body's Hill sphere is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sphere of its own. Any object within that distance would tend to become a satellite of the moon, rather than of the planet itself.

The radius R_H of the Hill sphere for a moon is, approximately equal to (1-e) (where e is the moon's eccentricity) times the moon's semi-major axis a_m divided by the planet's radius (a_m/R_p) times the moon's radius R_m.

Hill sphere radius R_H ~ (1-e) x (a_m/R_p) x R_m

An astronaut could not orbit the Space Shuttle (with mass of 104 tonnes), where the orbit is 300 km above the Earth, because its Hill sphere is only 120 cm in radius, much smaller than the shuttle itself. A sphere of this size and mass would be denser than lead. In fact, in any low Earth orbit, a spherical body must be more dense than lead in order to fit inside its own Hill sphere, or else it will be incapable of supporting an orbit. A spherical geostationary satellite, however, would only need to be more than 6% of the density of water to support satellites of its own.

Within the Solar System, the planet with the largest Hill radius (and thus capable of the most distant moons like Neso) is Neptune, with 116 million km, or 0.775 au; its great distance from the Sun amply compensates for its small mass relative to Jupiter (whose own Hill radius measures 53 million km). An asteroid from the asteroid belt will have a Hill sphere that can reach 220 000 km (for 1 Ceres), diminishing rapidly with decreasing mass. The Hill sphere of (66391) 1999 KW4, a Mercury-crosser asteroid that has a moon (S/2001 (66391) 1), measures 22 km in radius.

A typical extrasolar "hot Jupiter", HD 209458 b, has a Hill sphere radius of 593,000 km, about 8 times its physical radius of approx 71,000 km. Even the smallest close-in extrasolar planet, CoRoT-7b, still has a Hill sphere radius (61,000 km) six times its physical radius (approx 10,000 km). Therefore, these planets could have small moons close in, although not within their respective Roche limits.>>

hamilton1 wrote:I think that these types of photos raise more questions then they answer, 40,000 km is simply too far.

geckzilla wrote:I could think of some circumstances that would allow for moons to have moons. There's definitely nothing about physics that makes it impossible once you're at some nth level of of orbiting bodies.

Pandora is irregularly shaped and very small so that would make it difficult to orbit, though. "Pandora's" in this case is clearly referring to the mythical character.

geckzilla wrote:
I could think of some circumstances that would allow for moons to have moons. There's definitely nothing about physics that makes it impossible once you're at some nth level of of orbiting bodies. You just need a large enough object with a smooth enough gravitational field to orbit and then that object must be distant enough from whatever object it is orbiting so that they don't disturb one another. Sure, in reality it does stop at a certain point, but I think it has more to do with formation history than with the nature of physics itself.

Pandora is irregularly shaped and very small so that would make it difficult to orbit, though. "Pandora's" in this case is clearly referring to the mythical character.

• One needs to think outside the(/Pandora's?) box:

https://en.wikipedia.org/wiki/Co-orbital_configuration#Trojan_moons wrote:

[b][color=#0000FF]Rotating-frame depiction of the horseshoe exchange orbits of Janus & Epimetheus[/color][/b]

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<<The Saturnian system contains two sets of trojan moons. Both Tethys and Dione have two trojan moons, Telesto and Calypso in Tethys's L4 and L5 respectively, and Helene and Polydeuces in Dione's L4 and L5 respectively. Polydeuces is noticeable for its wide libration: it wanders as far as ±30° from its Lagrangian point and ±2% from its mean orbital radius, along a tadpole orbit in 790 days (288 times its orbital period around Saturn, the same as Dione's).

The Saturnian moons Janus and Epimetheus share their orbits, the difference in semi-major axes being less than either's mean diameter. This means the moon with the smaller semi-major axis will slowly catch up with the other. As it does this, the moons gravitationally tug at each other, increasing the semi-major axis of the moon that has caught up and decreasing that of the other. This reverses their relative positions proportionally to their masses and causes this process to begin anew with the moons' roles reversed. In other words, they effectively swap orbits, ultimately oscillating both about their mass-weighted mean orbit.>>