Other comets have periods of several millions of years with orbits that take them far beyond the orbit of Pluto. Comets, like the planets, travel in regular orbits. However, some comets occasionally speed up or slow down in their orbits.
Solar radiation causes ice to evaporate on the sunward side of the nucleus.
Molecules released by the evaporation stream away from the comet and generate a jet-type reaction that pushes the comet away from the Sun and slows it down. If the nucleus is rotating, the force may be in another direction and cause it to speed up. It is thought that most comets originate in a vast cloud of ice and dust that surrounds the Solar System. The Oort Cloud , as it is called, extends several thousand times farther from the Sun than Pluto, the outermost planet. A star passing near the solar system may have disturbed the motions of some comets in the Oort Cloud, sending them into the Solar System.