Page:Mars - Lowell.djvu/77

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the Earth to overcome it and compel the body’s return. Finally there would be reached a speed which the Earth would just be able to overcome if she took an infinite time about it. In that case the body would continue to travel away from her, at a constantly diminishing rate, but still at some rate, on and on into the depths of space, if there were no other bodies in the universe but the Earth and the molecule, till it attained infinity, at which point the truant would stop, and then reluctantly return. This velocity we may call the critical velocity. It is also known as the parabolic velocity, because it is at any point the velocity of a body moving in a parabola about the Earth, under the Earth’s attraction; the parabola being the curve of a fall from infinity. The critical velocity is the parabolic velocity, inasmuch as gravity is able to destroy on the way up just the speed it is able to impart on the way down. But, now, if the body’s departure were even hastier than this, the Earth would never be able wholly to annihilate its speed, and the body would travel out and out forever. If its speed at starting were less than twenty-seven miles a second, it would become thenceforth a satellite of the Sun; if its speed were yet greater, it would become an independent rover through space, paying brief visits only to star after star. In any case the Earth would know the vagabond no more.