Moons and mini-moons

[robina_williams]

Do any moons have mini-moons of their own orbiting them?

 

[Kalstang]

Not in this solar system...at least none that I know of....unless you want to count astroids as mini moons also...in which case theres quite a few <img src="/images/icons/wink.gif" /> <img src="/images/icons/smile.gif" /> <img src="/images/icons/laugh.gif" /> <div class="Discussion_UserSignature"> <font color="#ffff00"><p><font color="#3366ff">I have an answer for everything...you may not like the answer or it may not satisfy your curiosity..but it will still be an answer.</font> <br /><font color="#ff0000">"Imagination is more important then Knowledge" ~Albert Einstien~</font> <br /><font color="#cc99ff">Guns dont kill people. People kill people</font>.</p></font><p><font color="#ff6600">Solar System</font></p> </div>

 

[mikeemmert]

Hi, robina_williams <img src="/images/icons/smile.gif" /><br /><br />Wikipedia turns out to be a good source for these kinds of investigations, usually. I know it has a deserved bad reputation for topics having to do with politics, but the math and science articles are usually pretty good.<br /><br />Anyway, here is a link on the Hill Sphere which can answer your question, at least as far as the theoretical aspects of it is concerned.<br /><br />No sub-moons have been observed by astronomers, as far as I know. However, Saturn's moon Dione has two smaller Saturnian moons, Helene and Polydeuces, orbiting in Dione's Lagrange points L4 and L5. Similarly, Tethys, another Saturn Moon, has two smaller Saturnian moons, Telesto and Calypso, orbiting in it's Lagrange points.<br /><br />Whether Telesto, Calypso, Helene, and Polydeuces are "moons" of Tethys and Dione is subject to interpretation at this point. The IAU has not yet "ruled" on Lagrangians. I consider them to be in a completely separate class than a conventional moon. It's kind of like if you consider Pluto a planet. I don't.<br /><br />I think it's an escaped Lagrangian of Neptune <img src="/images/icons/laugh.gif" /> .

 

[vogon13]

There are dynamical reasons to not expect existing moons of satellites. (I have been trying to work out consistent nomenclature for all this stuff, with little succes)<br /><br />However, it may be possible for satellites to have formed with subsidiary bodies of their own. Let's 'play' with Dione a little bit. Upon completion of Dione's accretion from the proto nebula, we can expect Dione to have been rotating on it's axis rather faster than the current ~66 hours. It may have rotated in the 5 to 15 hour range. (over time, tidal effects from Saturn would have arrested this rapid rotation) Let's say Dione rotated in 8 hours. For moons around Dione below an altitude where they would have revolved about Dione in less than 8 hours, they would have experienced tidal effects from <i>Dione</i> that would have tended to lower their orbits. Obviously, this effect cannot continue for too long, as the moon will contact the surface and be lost.<br /><br />For satellites above the 8 hour orbital altitude, tidal effects from Dione will tend to accelerate the moon away from Dione.<br /><br />There is also an additional effect, from Saturn, that will tend to de-circularize the orbits of objects orbiting Dione. Keep in mind, if a moon around Dione has it's orbital eccentricity pumped up too much, it will either be lost to Dione, or it will contact the surface of Dione.<br /><br />We have three fairly effective mechanisms here tending to 'erase' moons of satellites.<br /><br /><br />Now for 'captured' objects in the outer reaches of a planets Hill sphere, satellites may be acccompanied by moons, but their formation history will be more in line of the parent population of the captured bodies, rather than the Saturn gestated satellites much closer in.<br /><br /><br />It is possible, perhaps, that objects 'spun off' long ago from Dione (and other satellites) are still around today, just waiting for us to recognize them as such.<br /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[robina_williams]

Fascinating. Thanks for all the info.

 

[vogon13]

Telesto, Calypso, Helene, and Polydeuces and possibly Hyperion may be such objects, spawned round other satellites, and liberated to nearby parking slots.<br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[search]

As refered above there are no "moons of moons" (as far as I know). The discussion regarding the cases above mentioned is about co-orbits (two or more celestial bodies that share, or very nearly share, the same orbit) and not of a moon orbiting another moon.<br /><br />However the tought (without excluding the possibility) is interesting so here is an artist view on such (Moon of a Moon'- Moon, with an Atmosphere of Argon, in Orbit of another Moon- Digital Art ©2003):<br />http://www.thomashouck.com/argon_atmosphere.jpg

 

[raccoon]

Though I may be mistaken, I believe I've read about two asteroids the literally orbit each other.

 

[mithridates]

No, you're not mistaken. Those two are pretty much clumps of rubble, with one of them spinning so fast that its mountains are lower than its lowlands.<br /><br />I just read about those two but I can't recall their names. <div class="Discussion_UserSignature"> <p>----- </p><p>http://mithridates.blogspot.com</p> </div>

 

[mithridates]

Found it!<br /><br />http://www.spaceref.com/news/viewpr.html?pid=21049 <div class="Discussion_UserSignature"> <p>----- </p><p>http://mithridates.blogspot.com</p> </div>

 

[raccoon]

Well all I can say is that if two Asteroids orbit each other, Then I'm sure there is somewhere out there in space where a moon has a smaller moon of it's own too.

 

[vogon13]

I have been outputting some speculation on this topic mostly because all the great Cassini pictures seem to beg for careful consideration of all the possibilities.<br /><br />Polydueces, Calypso, etc., are fascinating objects. How did they wind up in their respective locations?<br /><br />We seem to have some planetarily associated similar objects to them, Cruithne, Patroclus, etc., do all these objects have similar gestational histories? Or do several mechanisms put objects in these Trojan relationships with their hosts?<br /><br />Hyperion is definitely a 'weirdy' type moon. And it's mass ratio to Titan is quite similar to that of Dione to it's little Trojanettes. Coincidence? Or a clue as to some formational mechanism that relates all these objects?<br /><br />The mass ratio of earth to Cruithne is orders of magnitude different from the Saturn satellite ratios.<br /><br />Additionally, the mass ratio of Jupiter to its' Galilean satellites (orbital periods and orbital radii too) are scaled to Uranus and its' 4 largest satellites. Weird coincidence? Or a characteristic of a particular formation history of a class of gas giant planets?<br /><br />Why isn't Saturn like Jupiter and Uranus in this regard?<br /><br /><br />{ain't this fun?!?!?!?!?!}<br /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[robina_williams]

The Thomas Houck picture is very interesting -- thank you for posting it.

 

[Kalstang]

You know i've always wondered how a planet could be tipped on its side like Uranus. I can understand a planet going backwards from all the other planets around it. But a planet thats tipped on its side? And why is it that Uranus is the only planet to be doing this? <div class="Discussion_UserSignature"> <font color="#ffff00"><p><font color="#3366ff">I have an answer for everything...you may not like the answer or it may not satisfy your curiosity..but it will still be an answer.</font> <br /><font color="#ff0000">"Imagination is more important then Knowledge" ~Albert Einstien~</font> <br /><font color="#cc99ff">Guns dont kill people. People kill people</font>.</p></font><p><font color="#ff6600">Solar System</font></p> </div>

 

[search]

Hello Racoon<br />Yes I remember that also and mithridates later post references the case. <br /><br />I believe however the discussion and question, and somebody correct me if I am mistaken, is about 3 body systems in which one is a planet with a moon (statellite) orbiting that planet and another moon (satellite or some other name?) orbiting that planets moon (statellite). <br /><br />This case I do not know in our solar system and as far as I know not in any extra solar systems discovered so far (but these are just starting to be observed). <br /><br />I guess that since now there is a standard definition of planet (either we like or not) it will be easier to define other sky bodies... <br /><br />Reminder about moons (satellites):<br />"For a body that is large enough (massive enough) to satisfy the definition of “planet”, an object in orbit around the planet is called a “satellite” of the planet if the point that represents their common centre of gravity (called the “barycentre”) is located inside the surface of the planet.<br />The Moon is a satellite of the Earth. The reason the Moon is called a “satellite” instead of a “planet” is because the common centre of gravity between the Earth and Moon (called the “barycentre”) resides below the surface of the Earth."

 

[alokmohan]

Moon is moon.Right from ancient times.Moon is the first astronomical object which astonished human beings and they had yard stick to see moving stars and fixed star.Moon being the first cause of astronomy,we may not compare it to other objects in space.Any way there is no moon of moon is a correct stand.

 

[robina_williams]

You're absolutely right, Search. It was a 3-body system I had in mind when I asked the question: planet, moon, moonlet orbiting the planet's moon.

 

[search]

Moon is the name of Earth natural satellite.<br /><br />Other planets satellites are "commonly" called moons.<br /><br />From second site above:<br />"Satellites of satellites<br />No "moons of moons" (natural satellites that orbit the natural satelite of another body) are known. It is uncertain whether such objects can be stable in the long term. In most cases, the tidal effects of their primaries make such a system unstable; the gravity from other nearby objects (most notably the primary) would perturb the orbit of the moon's moon until it broke away or impacted its primary. In theory, a secondary satellite could exist in a primary satellite's Hill sphere, outside of which it would be lost due to the greater gravitational pull of the planet (or other object) that the primary satellite orbits. For example, the Moon orbits the Earth because the Moon is 370,000 km from Earth, well within Earth's Hill sphere, which has a radius of 1.5 million km (0.01 AU or 235 Earth radii). If a Moon-sized object were to orbit the Earth outside its Hill sphere, it would soon be captured by the Sun and become a dwarf planet in a near-Earth orbit."<br /><br />I also doubt that the mechanics of such system would allow the definition I mentioned in the post above to remain valid for the "moon" of the "moon" concerning the barycenter. <br /><br />Again Satellite (moon):<br />"For a body that is large enough (massive enough) to satisfy the definition of “planet”, an object in orbit around the planet is called a “satellite” of the planet if the point that represents their common centre of gravity (called the “barycentre”) is located inside the surface of the planet". <br /><br />P.S.<br />Such systems may have existed in the distant past but the mechanics of the solar system end up repositioning these sky bodies originating the actual solar system.

 

[robina_williams]

Could you explain your last sentence, please, for a non-astronomer:<br />Such systems may have existed in the distant past but the mechanics of the solar system end up repositioning these sky bodies originating the actual solar system. <br />

 

[search]

I have nothing to fundament the note that is why it was a P.S. (post scriptum, a Latin expression meaning "after writing" and abbreviated P.S. is a sentence, paragraph, or occasionally many paragraphs added, often hastily and incidentally). <br /><br />It was just food for tought considering that the stability of orbits was different in the past and may change in the future.

 

[MeteorWayne]

I believe it is this. While an object may exist for a time in such situations, over a small part of the life of the solar system it is unstable. Hence it will be ejected from it's system, or impact the larger object <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[search]

Exactly

 

[vogon13]

Yes, I concur.<br /><br />There are dynamical reasons to not expect existing moons of satellites. (I have been trying to work out consistent nomenclature for all this stuff, with little succes) <br /><br />However, it may be possible for satellites to have formed with subsidiary bodies of their own. Let's 'play' with Dione a little bit. Upon completion of Dione's accretion from the proto nebula, we can expect Dione to have been rotating on it's axis rather faster than the current ~66 hours. It may have rotated in the 5 to 15 hour range. (over time, tidal effects from Saturn would have arrested this rapid rotation) Let's say Dione rotated in 8 hours. For moons around Dione below an altitude where they would have revolved about Dione in less than 8 hours, they would have experienced tidal effects from Dione that would have tended to lower their orbits. Obviously, this effect cannot continue for too long, as the moon will contact the surface and be lost. <br /><br />For satellites above the 8 hour orbital altitude, tidal effects from Dione will tend to accelerate the moon away from Dione. <br /><br />There is also an additional effect, from Saturn, that will tend to de-circularize the orbits of objects orbiting Dione. Keep in mind, if a moon around Dione has it's orbital eccentricity pumped up too much, it will either be lost to Dione, or it will contact the surface of Dione. <br /><br />We have three fairly effective mechanisms here tending to 'erase' moons of satellites. <br /><br /><br />Now for 'captured' objects in the outer reaches of a planets Hill sphere, satellites may be acccompanied by moons, but their formation history will be more in line of the parent population of the captured bodies, rather than the Saturn gestated satellites much closer in. <br /><br /><br />It is possible, perhaps, that objects 'spun off' long ago from Dione (and other satellites) are still around today, just waiting for us to recognize them as such. <br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[robina_williams]

Thank you, MeteorWayne and Vogon13. I hadn't realized that the 'moonlet' situation I had in mind might be temporary; I had assumed an orbiting moonlet would stay fixed in its orbit around its moon.

 

[silylene old]

<font color="yellow">Thank you, MeteorWayne and Vogon13. I hadn't realized that the 'moonlet' situation I had in mind might be temporary; I had assumed an orbiting moonlet would stay fixed in its orbit around its moon. </font><br /><br />I would think that the most stable potential moonlet system in our solar system should be if a moonlet orbited our own Moon.<br /><br />Why?<br />- Our moon is relatively massive<br />- Our moon is rather distant from the primary planet (Earth)<br />- Our moon is not significantly subject to complex dynamic multi-bodygravitational interactions as occurs in Jovian and Saturnian systems (for example). Thus I suspect a moonlet orbiting the Moon would have a greater lifetime before being perturbed away.<br /><br />+++<br /><br />There was a humorous SF short story some time ago about the discovery of a moonlet system, where the moonlet was quite small (meter sized) and orbiting very close to the surface of an airless moon, and that the existance of the moonlet was quite a surprise to astronauts when they first landed there. I also remember this story had many bad puns within it. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>