Page:The Solar System - Six Lectures - Lowell.djvu/121



For an original elliptic orbit, this is not necessarily the case. A comet pursuing such a path may have its velocity increased and yet not pass out of the system. In many cases, however, it would so result, and we can thus perceive how comets might come to us from other systems from purely internal forces there.

The maximum effect in retarding the comet's motion occurs when the comet approaches Jupiter in such a direction and with such a relative speed as to be turned back upon the planet's path, and to leave the planet in the direction of the planet's quit, with a relative speed equaling the planet's own. It is then left stock-still to fall into the Sun.

Jupiter can do more than this. Though to leave a comet stock-still to drop into the Sun, thus shortening the major axis to one half its own, is its maximum effect in the way of contracting the orbit, its power over the comet exceeds such limit. The planet can actually prevent a comet bound round the Sun from attaining its object. It can cause the comet to make itself in place of the Sun the goal of its pilgrimage, and sweeping round the planet, to go back into space without visiting the Sun at all.

Consider the hyperbola the planet causes the comet to describe. What the planet does is to