An effect that accelerates or decelerates a comet's motion, changing its orbital period. Such forces are caused by jets of gas emerging from active regions on the surface of the nucleus, giving a rocket-like effect. Non-gravitational forces are most marked when the nucleus is highly active, close to perihelion, leading to uncertainties in subsequent returns for some periodic comets. They are responsible for systematic changes in the perihelion time of Comet Encke, and the apparently delayed 1992 return of Comet Swift–Tuttle.
Subjects: Astronomy and Astrophysics.