Word: space
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Dates: during 1950-1959
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Looking at it from the other end. a spaceship that starts its voyage on the surface of a planet has a hard time climbing out of its gravitational pit. Once it has reached untroubled space, it can coast for millions of miles on its unopposed momentum...
...fight free of the earth, the space navigator must reach a speed called escape velocity. Figured at the surface of the earth, this is 25,000 m.p.h. But rockets do not start suddenly. They accelerate gradually, keeping their speed fairly low while still in the atmosphere, then spurting quickly. If a rocket is moving 24,000 m.p.h. when it is 300 miles above the surface, it will escape from the earth's gravitation. When the Russian Lunik launchers, watching their bird with Doppler (speed-measuring) radios, saw it pass the critical speed, they knew it would never return...
Solar Orbit. The earth and moon, whirling around each other, are not alone in space. They also orbit around the sun, and so do the other planets. A gravity chart of the solar system shows an enormously deep pit, the sun's, with much smaller pits in its slope, one for each planet. When a spaceship has climbed out of the earth's gravitational pit, it is still deep in the sun's pit. This does not mean that it will fall into the sun. Besides the comparatively small speed contributed by its own engine, it also...
...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...