Space: The Moon in Their Grasp

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A Delicate Balance. On the day of the successful rendezvous, however, the fog that had shrouded Cape Kennedy during the night—and the cloud that had hovered over Gemini 6 even longer —suddenly blew away. "For the third time, go," exulted Schirra just before the Titan II left the pad in a launch that was as close to perfect as any in all the Cape's history.

Calculating with split-second exactitude, NASA scientists had determined the proper time of Gemini 6's launch by the position and orbit of Gemini 7. Because the earth rotates on its axis once every 24 hours, while Gemini 7 was circling around it once every 96 minutes, there were only one or two brief periods a day when the launch pad for Gemini 6 was located approximately under Gemini 7's orbit and when the orbiting ship was close by—the proper launch "window" for a rendezvous attempt. For Wednesday, ideal launch time had been calculated at 26 seconds after 8:37 a.m. And in an impressive display of launch-pad precision, Gemini 6 lifted off—on schedule to the second. Rendezvous with Gemini 7 would be possible on the fourth orbit.

A powerful Titan II rocket swiftly shoved Gemini 6 into an elliptical orbit that dipped as close to earth as 100 miles (perigee) and swung as far away as 161 miles (apogee). The average velocity was 17,535 m.p.h., only 8 m.p.h. slower than planned. Even more important, a maneuver of Gemini 6's second-stage launch rocket had placed the capsule in an orbital plane that nearly coincided with Gemini 7's; its path was almost directly below that of Gemini 7, slanting away at an angle of less than one-tenth of 1°.

Some 1,200 miles ahead, Gemini 7 sailed along at 17,290 m.p.h. The strategy of rendezvous, painstakingly plotted by NASA scientists and computers, called for Gemini 6 to catch up by taking advantage of orbital mechanics—the physical laws that govern the motion of orbiting bodies. Those laws state that an orbital path is determined by a delicate balance between gravity, which tries to pull a satellite down, and centrifugal force, which is proportional to the satellite's speed and tends to shove it farther away from the earth. A satellite orbiting close to earth, where the pull of gravity is strong, needs a high velocity to keep itself aloft. At higher altitudes, where the strength of gravity has decreased, a lower velocity will maintain an orbit. In last week's rendezvous mission, Gemini 6 was inserted into a lower orbit than Gemini 7; thus it was moving at a higher speed and would eventually overtake its target.