Word: spaceships
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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...
Deep space, far from stars or planets, is like the pond's smooth surface. An object becalmed in its emptiness floats like a galleon in the doldrums. If the object is a spaceship with propulsive power, it can cruise in any direction, meeting practically no resistance. But it must keep away from the whirlpools: the gravitational fields that surround stars and planets. If it plunges into one of them, it may end as a puff of gas in a star or a brief streak of fire in a planet's atmosphere...
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...
...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 has the earth's speed...
...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...