Step '4": Through trial and error, determine the correct initial velocity (to the tenths place) that will allow the planet to orbit the sun in a circular orbit. If the initial speed is too high or too low, the orbit will be elliptical. 1What speed is just right to allow a circular orbit? It may be a bit tricky to determine if the orbit is exactly circular without a grid, but the readings for potential energy, kinetic energy, and rotational momentum will help. Since a circular orbit is a constant distance from the star and has a constant speed, none of these values will change. If the orbit is circular, the energy graphs will be straight lines and the rotational momentum value lrp) will not change