Calculating orbits! You want your orbit to be stable, right? What's up with geostationary orbit around Mars? Speaking of Mars, astronauts travelling to Mars and then back to Earth will suffer poor health without gravity. A spinning space ship could work. How fast would it need to spin? In my spinning space ship, when I drop a wrench, how far will I have to chase it? All of this is pretty easy to calculate once you know how.