Space: Putting Gravity to Work

Keeping an orbiting satellite in the proper attitude is no mean trick. But there are times when antennas and cameras have to be kept pointing continually at the earth if they are to work effectively, and scientists have devised some complex systems to help them maintain position. Gyroscopes, inertial devices, gas jets and small rocket engines, working in conjunction with thousands of electrical and mechanical components, have all been tested. In an effort to reduce the weight and complexity of such systems, General Electric space specialists have taken their inspiration from the moon—a natural satellite that manages to keep the same face permanently toward the earth without the aid of any elaborate apparatus.

The secret of lunar stability is the moon’s irregular shape. On one side a bulge is pulled by the earth’s gravity; on the opposite side another bulge helps keep the moon in line by centrifugal force. G.E.’s experimental satellite employs the same principle. The “bulges” are two 11-lb. spheres on the ends of 52-ft. booms that extend from the satellite after it has been fired into orbit. One such Gravity Gradient Test Satellite (GGTS) was lofted into a 21,000-mile-high orbit in mid-June, and it is gradually but successfully stabilizing its attitude with one rod pointing toward the earth and the other away.

Earlier models of this Gravity Gradient system used on four other satellites were successful at lower altitudes. But scientists questioned whether far out, where the earth’s gravity is weak, the damping devices which kept the satellite from swinging like a pendulum in lower orbits would still be effective. G.E. experts, however, were convinced that their basic system was reliable.

They were right. Gravity and damping devices on the GGTS launched in June have already reduced the arc of its swing from a full 180° to about 30°. By mid-August, its builders hope, the oscillations will have died down to less than 8°, leaving the satellite with one face turned toward the earth.