Satellites of satellites?

[wukunlin]

Main Question or Discussion Point

so planets orbit around a star and satellites orbit around a planet,
is there any physical limitations that disallow astronomical objects rotating around satellites? or does just looks to be the case because we live in a relatively simple system?

 

[D H]

Looking in the other direction, our solar system orbits about the Milky Way, the Milky Way orbits with other galaxies that comprise the Local Group. The Local Group is a part of the Virgo Supercluster, but now we're getting so big that the expansion of space dominates over gravity. The point at which the expansion of space dominates over gravity places a limit on big side of the picture regarding how large the gravitational hierarchy can get.

If everything was a point mass, then mathematically one could (I think) come up with an infinitely deep hierarchy. The reason I said "I think" is that the N-body problem doesn't have a nice analytic solution. While a solution to the 3-body problem does exist, it is not in the elementary functions, and it has some nasty collision and escape special cases. The 4-body problem is even hairier. Beyond that, except for some specially constructed solutions, the N-body problem is pretty much intractable.

Looking away from abstract mathematics and toward the physical world, it is quite possible for a pair of stars to be orbiting one another at a close distance. Planets can orbit about the binary pair and satellites can orbit about these planets. So in a sense this is third order hierarchy. It is also possible to have a binary pair that are orbiting at some distance from one another, with each star in the pair having its own planetary system and the planets having satellites. So yet another third order hierarchy. Similarly, a pair of similarly-sized planets could be orbiting one another and have satellites orbiting about them. Combining this with the binary star would yield a four-level hierarchy.

However, I don't think that this four level hierarchy is quite what you are looking for. Restricting things to a strict hierarchy where the masses on a given level are orders of magnitude smaller than the parent level and are not co-orbiting other objects on the same level (no double stars, no double planets), the answer to your question is "probably not".

One problem is dynamic stability. Physical objects are not point masses. There is no way a body (call it X) orbiting some larger object can have sub-bodies in stable orbits about it if X's Hill sphere of lies inside X. For example, the Hill sphere of the International Space Station has a radius of about a meter. The ISS cannot have satellites.

By this measure, the Earth's moon could conceivably have satellites; the Earth-Moon L1 point is about 60,000 km from the center of the Moon. Now we have another dynamic problem.

Huge objects such as the Sun and Jupiter are extremely close to being oblate spheroids. The Earth is fairly close to being an oblate spheroid, but departures from this do exist and do have an effect on orbits. This departure is why geostationary satellites are attracted drift toward 75.3°E or 104.7°W longitude. Smaller objects such as the Moon are noticeably lumpy with regard to gravity. Even smaller just objects don't have enough mass to pull themselves into a spherical shape. Low lunar orbits are not stable because of the Moon's lumpiness. An interesting read on a pair of objects released into lunar orbit by the Apollo program: http://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/.

So, for a moon of a planet to have a satellite,

• The moon has to be quite far from the planet so as to have a reasonably sized Hill sphere,
• The moon also has to be relatively massive so as to have a reasonably sized Hill sphere,
• The hypothetical satellite's orbit has to be well inside the moon's Hill sphere, but
• The satellite's orbit can't be so small as to allow the moon's gravity anomalies to do some very weird things to the satellite's orbit.

That's a pretty narrow window. Now the question arises: How does the satellite get into that window? Some of the giant planets have moons that formed with the planet. Those giant planets are so big that they developed their own accretion disks as they formed. The moons of smaller planets such as the Earth's moon most likely did not form this way; those smaller planets are just too small for them to have had a substantial accretion disk. Satellites of planets are far too small. This route toward a moon having a satellite just doesn't exist.

The other mechanism is via capture. One way to capture a satellite is to collide with something; this is the most widely accepted hypothesis toward the formation of the Earth's moon. Another is for conditions to be just right so that a some incoming body on what appears to be a hyperbolic trajectory transfers angular momentum to the central planet and becomes a satellite. These windows are very, very small.

Bottom line: The odds are pretty much against a moon having moons.

 

[LURCH]

Of course, the Apollo vehicles did orbit the Moon (if you believe in that sort of thing). So in a way, it has happened and been observed.

 

[tony873004]

Saturn's satellite Rhea might have a ring, although I don't think that was optically confirmed. If it exists, there's a natural example.

 

[wukunlin]

So, for a moon of a planet to have a satellite,

• The moon has to be quite far from the planet so as to have a reasonably sized Hill sphere,
• The moon also has to be relatively massive so as to have a reasonably sized Hill sphere,
• The hypothetical satellite's orbit has to be well inside the moon's Hill sphere, but
• The satellite's orbit can't be so small as to allow the moon's gravity anomalies to do some very weird things to the satellite's orbit.

a lof of constraints indeed, no wonder these things aren't seen all the time

Of course, the Apollo vehicles did orbit the Moon (if you believe in that sort of thing). So in a way, it has happened and been observed.

oh, that did spring to mind :) I was thinking about something that formed naturally though

Saturn's satellite Rhea might have a ring, although I don't think that was optically confirmed. If it exists, there's a natural example.

that would be awesome

 

[D H]

Saturn's satellite Rhea might have a ring, although I don't think that was optically confirmed. If it exists, there's a natural example.

Probably not.

http://news.sciencemag.org/sciencenow/2010/06/the-moon-rings-that-never-were.html [Broken]

 

[D H]

Of course, the Apollo vehicles did orbit the Moon (if you believe in that sort of thing). So in a way, it has happened and been observed.

Those Apollo orbits were not stable orbits. Things can orbit moons for a short period of time, but astronomical time? Probably not.

In interesting link is buried in my longish first response. Here it is again:

Low lunar orbits are not stable because of the Moon's lumpiness. An interesting read on a pair of objects released into lunar orbit by the Apollo program: http://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/.

 

[tasp77]

Regarding Rhea, some Cassini pictures of the equator show 'staining' along crater rims. Hard to imagine how that happened except for the decay of a ring or belt structure around Rhea. Why a plasma affect would linger long after the supposed inducing materials are long gone is a real puzzle.

Also, curiously, the altitudes of the plasma anomalies correspond (assuming circular orbits) to periods around Rhea that are approximately 1/5, 1/6 and 1/7 of Rhea's period around Saturn.

Weird. Whatever they are.

 

[DaveC426913]

Not exactly a third order orbit, but it is kind of cool that a couple of asteroids - smaller than moons themselves - have their own moons. Eugenia has two.

Ida/Dactyl
Eugenia/Petit Prince/S2004

 

[tasp77]

There are objects like Calypso that are in a LaGrange relationship with a satellite.

Not quite a moon, but not quite independent either.

Also, another curiosity, the mass ratio of Calypso to Dione (hope that's the right moon) is not dissimilar to the mass ratio of Hyperion to Titan. (Hyperion is in a resonance, but not 1:1, with Titan)

More speculation, but consider the portion of asteroids with satellites, and then consider the number of outer satellites Jupiter and the biggees have. Some of them have to be enmooned, don't they? Many are assumed to be captured asteroids, seems like the asteroidal moon ratio would be conserved upon such a distant capture scenario.

 

[tasp77]

And then there is Cruithne, kinda sortof a moon shared by 2 planets, Venus and earth.

(If you want to look at it that way)

 

[davenn]

(If you want to look at it that way)

definately not scientifically anyway


D

 

[tasp77]

Cruithne isn't 'independent', so to speak.

And it changing from being in a resonant relationship with Venus and then to one with Earth is really interesting.

Who was it that commented we have earth and moon in a 'waltz', and then in the far corner of the ballroom, we have Cruithne moving in sync with another dancer, Venus, and then in sync with earth/moon?

I wonder if there are any orbit simulations of how a 'rock' can lock into such a relationship.