A measure of the orbital motion of a comet or asteroid with respect to a planet, usually Jupiter (T_J). The Tisserand parameter takes into account the semimajor axis, eccentricity, and inclination of the small body’s orbit, and remains broadly constant during the small body’s lifetime. It is a form of the restricted three-body problem and is useful in identifying small bodies observed before and after encounters with planets, as its numerical value remains largely unchanged by the encounter. The Tisserand parameter can also be used to classify planet-crossing bodies. For example, members of the Jupiter comet family have T_J between 2 and 3, whereas for most asteroids T_J is larger than 3. If T_J is just less than 3, a comet can experience very strong encounters with Jupiter. The parameter is named after the French astronomer François Félix Tisserand (1845–96).