Constant orbit using physics in Unity3D

Question

I am playing around with Unity3D and adding physics to the objects in my scene. I currently have a sphere (planet) in the center of the screen, and I have another sphere (moon) positioned outside of it that is not moving. When I run the game I want the moon to orbit the planet by applying a force to it. I have been able to get the force added by calling rigidbody.AddForce() and that will move it in the specified direction but it won't orbit. I'm not sure how to add force to it so that it will continuously orbit the sphere at a constant velocity.

I've tried some examples using a ConfigurableJoint and it orbits, but it starts out bouncing a little and then starts the orbit. My goal is to have a bunch of orbiting moons orbiting at their own speed that are able to bounce off eachother but not lose their velocity.

Any ideas?

Answer 1

For the moon to orbit you would need to give the moon an initial velocity. Then have it accelerate towards the planet, that is a constant force.

gameObject.rigidbody.AddForce(1, 0, 0);
gameObject.constantForce.relativeForce = Vector3(0, 1, 0);

Answer 2

Generally speaking you will fail, eventually, because rounding errors in your integration method will slowly throw you out of orbit. You can get very close in the ways suggested, but you could consider doing something more like the Kerbal Space Program, which seems to precalculate the orbit as an ellipse and then follow that ellipse until it has a reason to believe it should stop, rather than strictly "simulating" the orbit ...

If a collision occurs, allow normal physics to resolve the collision, and then recalculate your new orbit based on the result and start following that.