oh, Theia was liek too heavy to stay in the L5 region so it did a wobbly dance and moved forwards. As soon as it had gathered enough matter to be unstable, around 10% of earths mass, the body would do the wobbly dance. So if at 10% it was stable, and 10.1% it was unstable, i could turn it up to 11% to make it 10 times as unstable, (i hope) while not messing around too much with the mass.
@smjjames
how would you do that? if you increased the speed, the object would fly out of orbit. How would you "keep it on the same orbit"?