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Message started by thejames on 05/22/06 at 11:39:44

Title: Initial Settings
Post by thejames on 05/22/06 at 11:39:44

I've generated a random star system and loaded the masses, sizes and orbit radius' for the primary star, 2 planets and each of their moons.  I've added the eccentricity and inclination for the two planets only.  The problem I'm having is that it seems the moons start flying apart.

Now I know the system I've generated is totally sci-fi, but is there a bug or something that would cause the moons to start going their own way?

The time interval I'm using is 1.
All planets and moons are revolving around the appropriate parent / barycenter.

Could someone plug this data in to see if they get the same fly-apart scenario that I am?  For all other settings I just used the default / blank value when creating the objects.

Mass: 1.07 sol
Size: 1.050078139 sol

Mass: 0.277 earth
Size: 8312 km
SMA: 0.8 au
ecc: 0.003
inc: 3

MOON 1a:
Mass: 0.007901 earth
Size: 3415 km
SMA: 581000 km

MOON 1b:
Mass: 0.048103 earth
Size: 6249 km
SMA: 1246000 km

MOON 1c:
Mass: 0.058004 earth
Size: 4914 km
SMA: 664000 km

Mass: 1.206 earth
Size: 13571 km
SMA: 1.2 au
ecc: 0.008
inc: 2

MOON 2a:
Mass: 0.098484 earth
Size: 6098 km
SMA: 54000 km

MOON 2b:
Mass: 0.0211324
Size: 3781 km
SMA: 746000 km

Title: Re: Initial Settings
Post by Tony on 05/22/06 at 17:23:23

I tried your simulation.  Here's a link:

You're right.  Planet 1's moons wander away from the planet without completing even a single orbit, even at a slow time step.

The problem is that they exist outside the planet's Hill sphere.  The formula for Hill Sphere is
So for your system

a moon can not possibly orbit farther than 760,000 km from the planet before the star's gravity is overwhelming and strips the planet away.

Although moon1a and moon 1c are within the Hill Sphere, they are in prograde orbits.  Object near the edge of the Hill Sphere need to be in retrograde orbits to take full advantage of the Hill Sphere.

For circular stable orbits, you moons should be within 1/3 of a Hill Sphere.  That's why they're flying away.

Title: Re: Initial Settings
Post by James on 05/22/06 at 20:42:21

Hi Tony,

Thanks for the quick reply! :)

I hadn't considered the Hill Sphere value for this and I'm guessing the star generator I'm using doesn't really consider it either.

I was looking at the formula and came up with a different result...  am I calculating this wrong?

m = 1.65369 * 10^24 kg (0.277 earth)
M = 2.1293 * 10^30 kg (1.07 sol)
a = 120,000,000 km (0.8 au)

3M = 6.3879 * 10^30 kg
m/3M = 7.76636 * 10^-7
cube root = 0.009633225
r = a * cuberoot result = 1,155,986.97 km

Definitely not the same as your result.

Question about pro/retrograde orbits...  is there a way to set the type of orbit for the object when it is created?

I totally love this program... I've been playing around with it all weekend... much to my wife's chagrin. :)



Title: Re: Initial Settings
Post by Tony on 05/22/06 at 22:07:45

Hmmm... I get
(1.65369 * 10^24 )/ (6.3879 * 10^30) = 2.58878504672897E-07  This differs from your answer.


( 2.58878504672897E-07)^(1/3) = 0.00637331421516372


0.00637331421516372 * 120000000 = 764797.705819646

A retrograde orbit is a prograde orbit with an inclination of 180 degrees.  Try it.  It will orbit in plane, but backwards from an object with 0 inclination.  And it will remain stable out to th edge of the Hill Sphere.  Interesting to note:  Earth's Moon is almost at a distance of 1/3 of a Hill Sphere from Earth.  And its orbit is starting to show signs of instability.  The process that made its orbit expand should have completely circularized its orbit.  And it probably did, until it reached its current orbit, where the solar pertabution is strong.

I'm glad you like the program.  Now go pay some attention to your wife  :o

Title: Re: Initial Settings
Post by thejames on 05/23/06 at 13:09:26

You're right...  I probably entered it in wrong in the spreadsheet initially...  I just ran it here at work and got the same result as you.

Thanks for the tips... I appreciate the quick reply. :)



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