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Why Don't Moons Have Moons?

06/26/2009 10:09 AM

Ok, here's one on basic celestial mechanics.

  • The sun has planets circling it.
  • Most of the planets have moons circling them.
  • Why don't these moons have smaller moons circling them?
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#1

Re: Why don't moons have moons?

06/26/2009 10:20 AM

Because the far greater gravitational pull of the parent planet would result in the smaller moons being captured from the larger ones.

By the way, did Michelle of the resistance and her cast of idiots (e.g. Rene Artois et al) ever succeed in returning the two British airman home?

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#2
In reply to #1

Re: Why don't moons have moons?

06/26/2009 10:40 AM

OK, so why doesn't the greater gravitational pull of the sun result in the moons being captured by the sun?

As far as the other question goes, I will spook to HQ headquerters using my poloceman's radio and faind oot.

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#3

Re: Why don't moons have moons?

06/26/2009 10:53 AM

Actually they do, once you have sorted through the definitions:

See dwarf planets

Haumea has two

Makemake has one

Dwarf Planets

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#4

Re: Why don't moons have moons?

06/26/2009 12:28 PM

They could. There's a volume called Hill sphere where a moon could exist. The moon exists in the earth's Hill sphere. The moon also has a Hill sphere - there's just no moon out there. The lunar orbiter could have been considered a temporary moon.

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#5

Re: Why don't moons have moons?

06/26/2009 1:14 PM

the gravitational pull of the body the moon is circling, is greater than the gravitational pull of the orbiting moon, ie: the smaller moon would circle the larger planet not the moon

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#6

Re: Why don't moons have moons?

06/26/2009 1:27 PM

Our moon has two new moons this week. I tried to watch NASA launch them last week but the cloud cover was too heavy.

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#7

Re: Why don't moons have moons?

06/26/2009 2:24 PM

It's possible for a moon to have it's own satellite, but not likely, here's why.

In order for a moon to orbit a planet, it has to be within a certain distance from that planet (depending on that planets size). If it strays to far from the planet, it ends up orbiting the Sun instead.

In order for a moon to be able to have it's own satellite, it must be a certain distance from the planet it's orbiting, otherwise the planet captures that satellite.

So you need a moon that is far enough from a planet to keep it's own moon, yet close enough to the planet as to not fall out of orbit and become a planet instead. Such a precariously positioned moon is rare and hard to maintain with all the gravitational stress bodies experience over the course of the life of the solar system.

At least I think that's the answer.

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#8
In reply to #7

Re: Why don't moons have moons?

06/26/2009 2:27 PM

Well said!

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#9
In reply to #8

Re: Why don't moons have moons?

06/26/2009 3:36 PM

dog pile

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#22
In reply to #7

Re: Why don't moons have moons?

06/29/2009 12:02 PM

So basically it would be like balancing 2 plates with a stick :) got it!

So it is possible for a moons moons moon to have moons!! it is just very extremely almost impossible yet not totally imposible!!

I wonder if I can have moons....

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#23
In reply to #22

Re: Why don't moons have moons?

06/29/2009 12:35 PM

Stand on a street late at night in most western cities and you will get "Mooned" by the young kids in their cars.....

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#24
In reply to #23

Re: Why don't moons have moons?

06/29/2009 12:43 PM

haha. once... but twice would be very unlikely.... 3 times would be even less probable!! hahaha

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#27
In reply to #22

Re: Why don't moons have moons?

06/29/2009 10:42 PM

You are smart. (looks smart from your avatar). So you must have many she moons. Don't you have?

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#10

Re: Why Don't Moons Have Moons?

06/27/2009 12:22 AM

Moons have moons. See the Wikipedia article on Saturn's moons. I paraphrase what is written there:

"Tethys, 3rd largest of Saturn's inner moons, has two trojan moons, Telesto and Calypso. Dione, the second largest inner moon of Saturn, has two Trojan moons, known as Helene an Polydeuces."

Now, the question is, do these moons of moons have their own moons???

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#17
In reply to #10

Re: Why Don't Moons Have Moons?

06/27/2009 6:54 PM

These "moons" don't orbit Tethys as far as I can tell.

"Tethys has two co-orbital moons: Telesto and Calypso. These have been captured into Tethy's Lagrangian points. One orbits ahead of Tethys, and the other follows behind."

Link

In other words, these other moons orbit saturn and not Tethys. Think of it as several moons in a row in a single orbit around Saturn.

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#18
In reply to #17

Re: Why Don't Moons Have Moons?

06/29/2009 4:40 AM

Wow! Thanks Roger.

Lagrange points

This picture from Wikipedia shows the 5 Lagrange points of the Sun/Earth system, but any two bodies have the 5 points.

Telesto and calypso are at the two stable (L4 and L5) points which form equilateral triangles with the the two "main" bodies. Because they are both tiny (both 30 Km X 16 Km). They would not be visible from Tethys.

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#19
In reply to #18

Re: Why Don't Moons Have Moons?

06/29/2009 9:09 AM

Thanks for the great graphic.

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#40
In reply to #19

Re: Why Don't Moons Have Moons?

07/03/2009 2:51 AM

Does this mean it's theoretically possible to have a stable system which is a star and six planets or a planet and six moons arranged symmetrically so that each satellite is at the L4/L5 Lagrange points of both its neighbours?

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#20
In reply to #18

Re: Why Don't Moons Have Moons?

06/29/2009 11:32 AM

I think that must just be a generalized approximation, as it doens't appear to take into account the moons gravity, which is 1/6 of earth's.

Chris

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#30
In reply to #20

Re: Why Don't Moons Have Moons?

06/30/2009 3:35 AM

I don't know anything about this sort of thing, but, I'd have thought that (as far as the sun/earth gravitational fields is concerned) it was relatively easy to model the earth/moon system as a single body at their barycentre: the things which are going to affect the "gravity wells" at L4 and L5 are going to be the transits of other planets like Venus. If I've understood it correctly the points L4 and L5 are areas of stable equilibrium where L1, 2 and 3 are unstable, but, you can keep space stations there by constantly correcting any deviations. Also because these areas (L1, 2 and 3) have fields which effectively pull at the "top and bottom" but push in from the "sides" artificial satellites can be placed in mini orbits around the exact point.

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#32
In reply to #30

Re: Why Don't Moons Have Moons?

06/30/2009 1:47 PM

Everything is moving/orbiting/wobbling. I only meant that the shape of those field lines would change a little.. not that it would destabilize it completely, but then that would be dependent on the mass of the object at L4/L5. I agree that you would need some corrections for larger masses.

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#33
In reply to #32

Re: Why Don't Moons Have Moons?

07/01/2009 2:20 AM

Turns out that the way everything pulls on each other, our Solar System is a pretty chaotic place... Objects potentially can go out of orbit at anytime. Some scientists are betting that Mercury will be the first to wander away.

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#34
In reply to #33

Re: Why Don't Moons Have Moons?

07/01/2009 2:56 AM

Yes, vermin is trying to jump and chieve escape velocity for last so many months! He may wander any time

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#21
In reply to #18

Re: Why Don't Moons Have Moons?

06/29/2009 12:02 PM

Yes, this is a great graphic.

The Hill sphere that somebody else mentioned (I gave you a GA) is the concentric rings around the Earth in this diagram. The edge of this sphere incorporates the two La Grange points L1 & L2. Inside this sphere the gravity well of the object centered in this sphere dominates that object's motion. But all objects have a Hill sphere so a moon can have a moon. Our moon has one now that was launched by India and will have another soon from NASA. The Trojan moons of Saturn are not moons of moons, but they can happen.

Now if an object was orbiting the Earth just inside her Hill sphere, very little added energy would be required to move it out of the sphere to now be dominated by the Sun's Hill sphere. (The edge of the Sun's Hill sphere I believe is in the Oort cloud.) So a luckily timed and positioned comet could draw such an object out of Earth's influence.

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#29
In reply to #17

Re: Why Don't Moons Have Moons?

06/30/2009 3:05 AM

Tell me your middle name isn't Floyd . . .

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#31
In reply to #29

Re: Why Don't Moons Have Moons?

06/30/2009 9:11 AM

No, Floyd is my title, like Ph.D. or M.D. My middle name is Waters.

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#11

Re: Why Don't Moons Have Moons?

06/27/2009 12:24 AM

I don't know about the rest of these ass-tronomers, but moons do have moons... Ever since Pluto was downgraded to an icy, escaped moon - and it has a smaller body (Charon) orbiting it, moons now have moons!!!

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#12
In reply to #11

Re: Why Don't Moons Have Moons?

06/27/2009 1:20 AM

Boss,you wiped everybodys moons,thanks

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#13

Re: Why Don't Moons Have Moons?

06/27/2009 1:45 AM

Who says Moon doesn't have moon?

Our moon has a moon around it....

It is called "Earth"

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#14

Re: Why Don't Moons Have Moons?

06/27/2009 2:46 AM

I' leaving.

Too much "Mooning" going on here for a well brought up person like myself....

Recently I was walking through the forest and heard a young couple "Mooning" or was it "Moaning"?

I forget, one or the other......anyway its still too much whatever it was!!!

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#15
In reply to #14

Re: Why Don't Moons Have Moons?

06/27/2009 2:16 PM

Do Trojans come from trojan moons?

In high school there was a song called "The sound of lovers in Love" or some such thing. I always wondered what that sound was. I think you answered part of it. I used to do something with my tongue that may have been the other part.

There's another question I 'need' to ask: If a man talks in the forest where no woman can hear, is he still wrong?

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#16
In reply to #15

Re: Why Don't Moons Have Moons?

06/27/2009 4:16 PM

You asked:-

There's another question I 'need' to ask: If a man talks in the forest where no woman can hear, is he still wrong?

If you are married, surely you know the answer already? If not married, the answer is "YES"!

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#28
In reply to #15

Re: Why Don't Moons Have Moons?

06/29/2009 10:42 PM

"Thats no moon, its a space station!" Luke Skywalker

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#25

Re: Why Don't Moons Have Moons?

06/29/2009 5:15 PM

If a moon (moon1) had a moon (moon2) would not moon1 be a planet and not a moon?

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#26
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Re: Why Don't Moons Have Moons?

06/29/2009 10:09 PM

See #11

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#35

Re: Why Don't Moons Have Moons?

07/02/2009 10:18 AM

Moons do have moons - there are a couple of small moons in orbit around our moon right now - spacecraft.

And of course, the earth is a satellite of the sun, so in that sense also, moons have moons.

It's all a question of relative proportion, distance and gravity. The earth can support a moon because it's very far from the sun, so that the earth moon system looks like a single mass. In order for a moon of the moon to looks like a single mass, it would have to be either very small - relative to the moon, or the moon-moon system would have to be very far away from the earth.

Also, our moon can't sustain it's own moon because its gravity is all wonky. Any object captured by the moon will soon crash into it - due to the moon's uneven gravity. This is why we have no permanent (or long term) unpowered satellites around the moon as we do around the earth. You need a relatively even gravitational pull if you want to have natural satellites for the long term.

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#36

Re:

07/02/2009 7:40 PM

"Why Don't Moons Have Moons?"

Do naked pimples on the but count?

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#38
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Re:

07/03/2009 1:47 AM

Did you mean your "Butt"?

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#41
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Re:

07/03/2009 6:30 PM

Not mine! It was hypothetical. Be my guest.

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#37

Re: Why Don't Moons Have Moons?

07/02/2009 10:46 PM

Not enough green cheese!

Seriously, would someone on a moon with a moon see phases on the secondary moon like we see on ours?

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#39
In reply to #37

Re: Why Don't Moons Have Moons?

07/03/2009 2:06 AM

Good question:

Now, let us think and find out how will we see the earth, if we stand on the Moon?

Many aspects:

Size, phases, ......

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