Gravity Simulator http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl General >> Discussion >> Bound trajectories check http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1411323250 Message started by frankuitaalst on 09/21/14 at 11:14:10

 Title: Bound trajectories check Post by frankuitaalst on 09/21/14 at 11:14:10 It is known that in a two body problem a planet is bound if the total energy is smaller than zero ( elliptical orbit ) . In this case we have : E= Ep+Ek <0 :  mv²/2 - GmM/r <0 . This means the velocity should not be too big . I now have a system of a planet orbiting  a binary. And want to have a quick check if the planet is bound to the binary. Any idea's how I should proceed ? mv²/2 -GmMtot <0  (vrel to center of mass Mtot ) or : m Vrel1²/2+ mVrel2²/2 - GmM1/r1 -GmM2/r2 <0   ( two bodies considered )

 Title: Re: Bound trajectories check Post by Tony on 09/22/14 at 06:38:58 Compute the center of mass of the binary.  Pretend all the mass was there, and treat it as a single object.  Now see if energy is negative, or check if ecc < 1 or sma is positive.

Title: Re: Bound trajectories check
Post by frankuitaalst on 09/23/14 at 13:21:43

Tony wrote:
 Compute the center of mass of the binary.  Pretend all the mass was there, and treat it as a single object.  Now see if energy is negative, or check if ecc < 1 or sma is positive.

Thanks Tony , you think treating the binary at its COG gives better results than taking account of the two seperate masses ?
I think in general yes it is but :

If COG is used one  does not take into account a change in energy due to a close encounter of one part of the binary .
I'm not very sure in this case .
Is there a way to test  in GravSim the result if I would test the second approach ?

 Title: Re: Bound trajectories check Post by frankuitaalst on 09/24/14 at 22:41:33 I gave it another thought . Of couse you are right Tony , at long distances this is indeed the criterium . I just wondered If at close distances the separate approach could be appropriate . I had the impression that although a body which is in eliptical orbit around a binary can be trown out after a "close " approach to one part of the binary .

 Title: Re: Bound trajectories check Post by Tony on 09/25/14 at 18:26:47 I think the secondary bodies have to be about 5x as far as the aphelion of the primary stars for stable orbits.Gravity Simulator can do this.File > NewCreate another star around the Sun.File > NewCreate a low-mass body around the Sun and choose "barycenter" in the focus object.There's also a View > View Orbital Energy option, but I think I coded it wrong.  It's not doing what I expect.  You could always output data to view in Excel.  Energy of the system is the sum of U + KE between every body in the system (-GMm/d^2 + 0.5 * m * v^2).  It should be constant.