This is a binary system. The velocities of the two objects are initially equal-and-opposite (object 1's velocity directed down and object 2's directed up). You might expect them to have repeating orbits, but can you explain why there is often a drift of the orbits
in one direction or another? And when would you expect no drift?

Note that occasionally the objects get a bit too close and the simulation does not handle the physics with enough accuracy to really reproduce the orbits accurately.

Simulation written by Andrew Duffy, and first posted on 5-12-2018.