When the satellite was injected into its orbit, it was spin-stabilized to keep it from tumbling over and over, much as a rifle bullet is stabilized by being spun about its longitudinal axis. The Telstar satellite is roughly spherical, and it was designed to spin with the helical antenna end as its north pole and the antenna bands as its equator. On July 10, 1962, the satellite was given an initial spin of 177.7 revolutions a minute. As we expected, this rate is decreasing gradually; after two years it will only be spinning one tenth as fast. The most important reason for keeping a close watch on the satellite’s spin axis is to make sure that microwave signals are sent and received steadily. It isn’t possible to build an antenna that radiates at exactly the same power in all directions. Telstar’s antennas work very well, but they operate better in the direction of the satellite’s equator than they do towards its poles. This means that if the spin axis is constantly changing transmission will fade in and out—even at times passing through “null” where no transmission at all is possible. No single fixed orientation is perfect for the spin axis, but we decided that the best average position would be to keep it always perpendicular to the plane of the earth’s orbit. We tried to make sure that the spin axis would not vary by more than five degrees from this direction at any time—although it probably could depart as much as 15 or 20 degrees without doing serious harm. A second reason for being careful about the satellite’s spin axis is the problem of heat balance. If one end of the satellite points constantly at the sun and the other end does not, the end near the sun will get much too hot and the other will get much too cold. Therefore, we tried to fix the spin axis so that it stayed perpendicular to a line drawn from the satellite to the sun. We also wanted to get a continuing report on the effects of the magnetic field of the earth at high altitudes. We knew these would cause the spin axis to change with time, or _precess_, but we couldn’t be exactly sure what these changes would be. Since the orientation of Telstar’s spin axis was so important we installed a _torque coil_ in the satellite. This is a coil of wire in which, upon a signal from the ground, an electric current can be made to flow. The current produces a magnetic field that interacts with the earth’s magnetic field to change the position of the satellite’s spin axis. However, we could not be sure that this device would work properly—and this is another reason why we wanted to keep track of the exact position of the spin axis. Ways of Measuring the Spin Axis