I have rocks orbiting moons, moons orbiting planets, planets orbiting suns, and suns orbiting black holes, and the current system could have many many layers of orbitage. the position of any object is a function of time and relative to the object it orbits. (so far so good). now I want to know for a given 2 objects(A,B), a start time and a speed, how can I work out the when and where to go. I can work out where A and B is given a time. so i just need. 1: direction to travel in from A to B(remember B is moving(not in a straight line)) 2: Time to get to b in a straight line. travel must be in a straight line with the shortest possible distance. as an extension to this question, how will i know if its better to wait, EG is it faster to stay on object A and wait for a hour when the objects may be closer, than to set off from A to B at the start. Cheers, it hurt my brain.