Apollo: The Race To The Moon

by Catherine Bly Cox

An Apollo spacecraft returning to earth from the moon did not aim directly at the earth, a passage that no heat shield could survive. Rather, it aimed at the leading edge of the earth so that, as the spacecraft sped by, it would be caught by the earth’s atmosphere and gravity. If the spacecraft was too far from the leading edge, it would continue past the earth into an elongated earth orbit. If it was too close to the leading edge, it would enter the atmosphere at a steep angle and burn up. The area that was neither too high nor too low was about a degree and a half wide—ten miles wide at the point of entry—and was called the entry corridor. From Apollo 8 onward, one of the chief functions of the Trench was to make certain that the returning spacecraft was in the middle of it.