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For a voyage to Venus, which revolves nearer the sun, the space navigator starts his ship in the direction opposite to the earth's orbital motion. Its net departure speed above escape velocity is subtracted from the orbital speed. This makes it move too slowly to stay on the earth's orbit, so the sun's gravitation curves it inward to Venus.
Perhaps the most striking thing about space navigation is the ease of longdistance travel after successful launching. Mars never comes closer to the earth than 34.5 million miles, Venus never closer than 25 million miles. To cover these great distances, it takes more time (146 days to Venus, 260 days to Mars), but only slightly more speed than is needed to go to the moon, which is only 230,000 miles away. This is because space between the planets is comparatively smooth. It is only slightly affected by planetary gravitation, and the great pull of the sun is countered by the orbital speed that a spaceship inherits from its home planet.
Interstellar Escape. Full escape from the gravitational pull of the sun would be tougher. Starting from the earth's surface, a ship would need 36,800 m.p.h. Soaring past Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, it would reach the outer limits of the solar system with almost no speed left. Then, like a chip on a glassy lake, it could drift for millions of years before it approached the nearest star, Proxima Centauri, which is 25 trillion miles away from the sun. Man's spaceships can probably reach interstellar escape velocity in a generation, but there will be little profit in interstellar voyages. They will take too long. The barrier that protects the stars and their planetary systems from human invasion is not space but time, and the shortness of man's life.
How close is interplanetary voyaging? The great weight (2,925 lbs. of instrumented payload) of Sputnik III proved to the space-wise that the Russians had practically licked the initial problems of interplanetary flight. U.S. scientists reckon that the Soviets' Lunik, with only a little more speed, would have swooped past Mars and soared out toward the asteroids. George Paul Sutton, professor of aeronautical engineering at M.I.T., believes that present propulsion systems with a little refinement can send a space vehicle as far as Jupiter or even to Saturn, 750 million miles from the earth.
Astronomers can hardly wait for the day when these first space scouts are launched. For oddly enough, they know less in many ways about the planets, the earth's neighbors, than they do about far-distant stars. The reason is that stars shine in their own light, revealing much about themselves to astronomers' spectroscopes. The solar system's planets are visible only in the reflected light of the sun. Their spectra carry little firm information, and the details that can be seen on their surfaces are clear enough to excite but too vague to satisfy human curiosity.
Controversial Moon. The moon is an exception. It is so close that it shows a wealth of detail that astronomers have studied for centuries. They have also argued bitterly over many questions presented by its serene face, e.g.: Are the ring-shaped craters the result of volcanic activity or meteor impacts?