Page:The Solar System - Six Lectures - Lowell.djvu/124



meeting-point of the planet's and comet's true motions, and join any other point of it to its centre. This second radius will represent the outgoing asymptote of the relative orbit, according to the planet's pull, while the line joining its peripheral end to Jupiter will be the comet's subsequent motion in amount and direction.

From this you will perceive that the comet's subsequent career depends upon the actual speed with which, the angle under which, and the nearness to which, it approaches the planet. If it creep upon the planet from behind, it is more likely to be captured than if it meet it head on; and if it be traveling slowly, it is more likely to be caught than if it were going fast.

Any one of many things may happen. If it pass behind the planet, its actual speed is increased, and either it is sent clean out of the system, or it is at least put farther from capture than before. If it pass before the planet and in such a way that its relative speed about the planet exceeds the planet's own motion, and it is turned round through a sufficient angle, it may, from a previously direct path about the Sun, be diverted into a retrograde one. In this case, it will commonly have a small velocity after the encounter and retrograde in a small ellipse.