Word: earth
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Dates: during 1950-1959
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...Mars, whose orbit is outside the earth's, the spaceship must climb up the side of the sun's gravitational pit-by speeding up. To reach Venus it must climb down-by slowing down...
...voyage to Mars the space navigator takes his departure from earth in the same direction that the earth is moving around its orbit (see chart). His ship must have a speed of only 870 m.p.h. over escape velocity. The excess speed is added to the earth's orbital speed (66,600 m.p.h.) that the spaceship had before it was launched. This is enough to offset the sun's gravitational pull, allows the ship to swing outward in an ellipse. If the timing is right, it makes a rendezvous with Mars on its orbit...
...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...
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...