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.