At any given time, the motion of any solar system body is affected by the gravitational pulls of all of the others. In that same paper, Halley also became the first to note the very real possibility of the collision of comets with planets, but stated that he would leave the consequences of such a contact or shock to be discussed by the Studious of Physical Matters. In the case of Shoemaker-Levy 9 we have the perfect example both of large perturbations and their possible consequences. The comet was fragmented and perturbed into an orbit where the pieces will hit Jupiter one period later. One of Newton's laws of motion states that for every action there is an equal and opposite reaction.
Comets expel dust and gas, usually from localized regions, on the sunward side of the nucleus.
This action causes a reaction by the cometary nucleus, slightly speeding it up or slowing it down. Such effects are called non-gravitational forces and are simply rocket effects, as if someone had set up one or more rocket motors on the nucleus. Only after F. L. Whipple published his icy conglomerate model of a degassing nucleus in 1950 did it all begin to make sense. The predictions for the time of perihelion passage of Comet Halley in 1986, which took into account a crude model for the reaction forces, were off by less than five hours. Much of modern physics is expressed in terms of conservation laws, laws about quantities which do not change for a given system.