Push into Space

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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.

To 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 to earth. A lesser speed than escape velocity sets a satellite revolving around the earth just free of the atmosphere. A satellite can be compared to a chip or leaf circling around the sides of a whirlpool without escaping from it or immediately being swallowed.

Near the rim of the earth's gravitational pit is a much smaller pit belonging to the moon. An object shot away from the earth at 24,800 m.p.h. will reach the boundary, about 34,000 miles short of the moon, where the moon's pull is as strong as the earth's. If it reaches this point with a small velocity, it will fall on the moon. If it crosses the line at good speed, it will shoot past the moon, its course merely deflected. This is what happened to the Lunik.

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 has the earth's speed in its travel on its orbit. If the ship has only barely escape velocity, it will circle around the sun indefinitely on an orbit close to the earth's—just as bombs, in the newsreel pictures of a decade ago, seemed to hover in space just below the plane that released them.

To go to 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.

For a 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.