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General >> Discussion >> Possibility of multiple moons in Earth-Moon system
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Message started by qshadow on 01/26/15 at 06:49:33

Title: Possibility of multiple moons in Earth-Moon system
Post by qshadow on 01/26/15 at 06:49:33

Hi, I would like to check (by simulation) the possibility of Earth having more than one moon.
I have chosen a stable orbital resonance configuration of Jupiter Galilean moons which are in 1:2:4 resonance.

I have asked the question on Physorg forum first, but of course without a simulation nobody could give me a definite answer:
https://www.physicsforums.com/threads/possibility-of-multiple-moons-in-earth-moon-system.793659/

can you please help me built such a system based on the existing Solar System scenario?
I am new to this area, so it is really difficult for me to do this alone.

Thanks,
Qshadow.

Title: Re: Possibility of multiple moons in Earth-Moon sy
Post by Tony on 01/26/15 at 17:41:43

Do you have Gravity Simulator installed and running?

On Physicsforums.com I'm tony873004.  I can make you a sim similar to the one I posted in the "orbit stability" thread at PF.  But if you have Gravity Simulator installed on your Windows computer, I can help you make your own.

I'm pretty sure what you will find is that there are no stable prograde orbits external to the Moon's orbit.  Retrograde orbits can exist out to about 1/2 Hill Sphere.

I think you'll also find that interior to the Moon's orbit everything is stable out to about 280,000 km.

Title: Re: Possibility of multiple moons in Earth-Moon sy
Post by qshadow on 01/27/15 at 02:55:21

Hi Tony, I have installed the Gravity Simulator on my Windows, but i could not figure out how to add it properly, I have just started to be interested in orbital dynamics,
and I could not fill some fields in the "Create Object" tab.


Quote:
I'm pretty sure what you will find is that there are no stable prograde orbits external to the Moon's orbit. Retrograde orbits can exist out to about 1/2 Hill Sphere.
.
I think that retrograde orbits means there is no "orbital resonance" with our moon which is prograde, right?
But I was thinking that "orbital resonance" is crucial to get stable orbits, and if the bodies are "out of sync" they will destabilize, so can you please tell me what are the most important parameters for getting stable orbits for multiple moons?


Quote:
I think you'll also find that interior to the Moon's orbit everything is stable out to about 280,000 km.

How did you get this number, can I arrive at it with some calculation? Cause to get 1:2 orbital resonance with moon I got 241,000 Km.

Thanks!
Alex.

Title: Re: Possibility of multiple moons in Earth-Moon sy
Post by Tony on 01/31/15 at 18:14:53

Hi. Sorry for the slow response.
The latest alpha version of Gravity Simulator makes it easy to test this.

Download it here:
http://orbitsimulator.com/gravity/beta/setupGravitySimulatorAlpha3.exe
It works best if the original version 2.0 is already installed.

Start with any solar system simulation that includes Earth and the Moon.
Click Objects > Create Objects
Choose Number of Objects.  Let's try 20.
Choose Earth for reference object.
Press the "Distribution" button next to Semi-major axis twice, until it says below it SMA (Start, End)
(If you don't have this button, then you're running version 2.0 instead of the newer Alpha version.
Check kilometers.  The default is AU.
Type 10000, 200000.
Leave all other settings at their default.
Press "Create"
This will create you 20 objects at:
10,000
11,000
...
200,000 km from Earth in circular prograde orbits.

Experiment with different numbers.
If you want retrograde orbits, enter 180 for Inclination, and set the +/-% below this to 0.

You'll quickly find where orbits are stable around Earth.
The 280,000 km figure was just an educated guess.

Another thing to try is to first delete (or set its mass to 0) the Moon.  This will tell you whether the Sun or Moon is causing instability on your test moons.

Let me know if this helps!

Title: Re: Possibility of multiple moons in Earth-Moon sy
Post by qshadow on 02/04/15 at 10:38:01

Hi Tony,
thanks a lot for a detailed answer!
I have tried your suggestion and it worked, but I have a few problems:

1.  I have onlyplanets.gsim which doesn't have the moon and I have fullsystem with so many objects that i got lost, I also found Barycenter with Earth and moon only but it behaves VERY strange its eccentricity is so big, i mean its perigee is like 1/3 of the apogee!

2. When i use the solar system eg onlyplanets.gsim how can i view it from above, not from the side? I have tried to play with Rotating frame adjustment but it is not helping at all. I tried to set  Sun as Focus object, but it didn't help either.
But when

3. And the most important part: I wanted the moons to be aligned in perfect Resonance between themselves and the Moon, not just start arbitrarily or from the same point. How can I do it?

Regards,
Qshadow.

Title: Re: Possibility of multiple moons in Earth-Moon sy
Post by frankuitaalst on 02/09/15 at 08:56:18

Some thoughts :

qshadow wrote:
1.  I have onlyplanets.gsim which doesn't have the moon and I have fullsystem with so many objects that i got lost, I also found Barycenter with Earth and moon only but it behaves VERY strange its eccentricity is so big, i mean its perigee is like 1/3 of the apogee!
--> you kept the timestep moderate ? ( fi. 256 s is a good value )

2. When i use the solar system eg onlyplanets.gsim how can i view it from above, not from the side? I have tried to play with Rotating frame adjustment but it is not helping at all. I tried to set  Sun as Focus object, but it didn't help either.
--> have you tried the scroll bar at the right yet ?

3. And the most important part: I wanted the moons to be aligned in perfect Resonance between themselves and the Moon, not just start arbitrarily or from the same point. How can I do it?
--> The way I would do it : CALCULATE the Sma you need in order to get the desired resonance , and then use this value if you create the object ( T = 2\pi\sqrt{a^3/\mu})

I hope the info helps .

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