Date: 21 Jul 88 18:40:30 GMT
From: ulysses!thumper!karn@ucbvax.berkeley.edu  (Phil R. Karn)
Subject: Re: Solar Sails

> [...] My recollection is that
> people who run orbiting satellites have to take it into consideration as
> a minor source of orbit perturbations.

The effect of solar radiation pressure on a satellite's orbit is usually
negligible. Of course, it can become significant for an unusually large
and light satellite like Echo, or a satellite with a solar sail.

The effect of solar radiation pressure on a satellite's ATTITUDE, on the
other hand, is major -- in most cases it is the single most significant
perturbing force. Voyager was able to save a considerable amount of
attitude control fuel early in its mission by slowly rolling around its
antenna boresight axis during the cruise phase. This had the effect of
averaging out the effect of the solar radiation torque on the
magnetometer boom, greatly reducing the angular momentum imparted to the
spacecraft that had to be removed with the attitude control thrusters. I
am hoping that we can use a similar trick with AMSAT Phase IV, since the
antennas present an assymetrical cross section to the sun.

On AMSAT-Oscar-7, each of the 145/432 MHz turnstile antenna elements
(made out of ordinary metal carpenter's rule from the local hardware
store!) was painted white on one side and black on the other.  The
radiation pressure of light on a reflecting surface is twice that of
light on an absorbing surface, so the resulting torque produced a nice
slow spin.  A permanent bar magnet along the spin axis kept the spin
axis in line with the earth's magnetic field, and the eddy current drag
of the earth's magnetic field cutting across the metallic spacecraft
kept the spin rate from ramping up too high. Note that the direction of
rotation is OPPOSITE that of the toy "radiometers" one can find in
science museum gift shops. They contain air and work by the reaction of
the heated air on the black surface; this force exceeds the imbalance in
photon pressure.

Phil