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At first the returning vehicle will move slowly, but eventually the pull of the earth will accelerate it to its departure speed. The pull of the sun will also make itself felt, its effect depending on the position of the moon in its orbit. In about 6½ days (with a little bit of luck), the probe will return to earth and enter the thin film of its atmosphere. The whole operation calls for precision at every point. Even a small error of speed, aiming or timing could make a probe crash against the earth, or revolve around it in wild, unpredictable swings, or even escape from its gravitation and fall into the sun. Despite many confident predictions from military and other optimists, a successful first probe of the moon is not likely.