Gravitational model
The first model is a simple gravitational model, which calculates the net gravitational force on each body from all other bodies, and hence the acceleration and velocity of each body, and the distance travelled after an interval of time. Given a set of bodies, each with initial {X,Y) locations and X,Y velocities, new locations are found for each body after a short interval of time. This process is repeated indefinitely.
In this applet, a planet and a string of satellites radiating from it are given an initial set of (slightly randomized) velocities. Those which have too high velocities escape from the planet. Those which move too slowly fall onto the planet (the impacts marked by flashes). The remainder orbit the planet in elliptical orbits.