Newton's Laws of Motion include the statement that all bodies will move in a straight line when not acted upon by an external force. A moving body is carried on its path through its momentum. In the presence of a gravitational force, however, the body will instantaneously travel along a very short straight line, however this straight line will have its orientation in space redirected from moment to moment so that the patch of the body is bent into an elliptical path. The momentum of the body is conserved, and once it was started into motion, it will continue to move by its own inertia. The gravitational force of the Sun causes the speed of the body go get accelerated and the net effect is that the body moves in an elliptical path so long as the forces it feels at each part of its orbit remain the same as in the previous orbit. Because Jupiter and the other planets can get close to the comet at various times, the orbit is generally not exactly the same each time. These perturbations can be large or small. Some comets have orbited around the Sun for hundreds of years, and then suddenly have their orbits radically altered when Jupiter happened to be close to the comet during one of its loops around the Sun. The orbit is altered, and in some instances, the comet can get ejected from the inner solar system never to return.