Halley's Comet orbits the sun about every 75 years due to the gravitational force the sun provides. Compare the gravitational force between Halley's Comet and the sun when the comet is at aphelion (its greatest distance from the sun) and d is about 4.5 X 10^10m to the force at perhilion (or closet approach), where d is about 5 X 10^10m.