changing orbital elements by hand

[nchoksi4456]

Hi all,

Hopefully this is a quick question: when I change the orbital elements of a particle e.g. sim.integrate(integrate_a_little) sim.particles[1].e += delta_e # NC: kick the eccentricity sim.integrate(integrate_further)

Is REBOUND self-consistently updating the particle's velocity vector to satisfy the new eccentricity? I think the answer is yes. But wanted to confirm I'm not missing something subtle.

Thanks, Nick

[dtamayo]

No, that doesn't work. There are too many options (are you changing e keeping all other parameters fixed? are you changing e but keeping the position of the body the same?). You could check modify_orbits_direct in REBOUNDx for how to do this by hand yourself

[nchoksi4456]

Thanks Dan. What exactly is REBOUND doing then, when one applies the line sim.particles[1].e += delta_e? Because REBOUND does not throw an error. My impression based on the outputs was that it was changing eccentricity while keeping the position of the body the same.

[dtamayo]

Sorry, looks like Hanno did update this so that this will work! It's updating e while keeping all other orbital elements the same

[dtamayo]

which I think implies that you're changing the body's position

[nchoksi4456]

Thanks. Could you point me to where in the REBOUND source code these changes are applied?

[dtamayo]

https://github.com/hannorein/rebound/blob/main/rebound/particle.py#L720

[hannorein]

That's correct. I think this implementation makes sense in some case, or is at least self consistent. But use it with caution. There are many potential conceptual pitfalls. Jacobi versus heliocentric, etc...