Don Williams hired me over the phone 1964 straight out of college and sight unseen. I doubt that my telephone interview impressed him much but I was probably the only person he could find back then who had taken astrodynamics coursework, programmed in FORTRAN …and didn’t cost much if he was wrong.
I joined Don’s small in-orbit control team whose other members were Murray Neufeld, Bill Snyder, Bernie Anzel, Roger Cole, Mel Richins and Marcelle Farr. Don’s team reported to Harold Rosen. Only years later did I realize how fortunate I was to “learn the ropes” from this unique group.
My first assignment was to become the team’s attitude determination specialist. Attitude (spin axis orientation) was determined using sun sensor data (a solar cell mounted behind two canted metal slits mounted on the spinning body) and “POLANG.” The dipole antenna on our early satellites emitted a linearly polarized signal. By measuring the “polarization angle” as received at a ground station, attitude data could be derived with the appropriate trigonometric calculations.
In 1965 we were keeping track of Syncom 3. Our team made attitude estimates on a regular basis. We expected attitude to remain fixed in space unless we maneuvered the spacecraft. (Newton’s first law: a body in motion….) I assumed small variations in determined attitude over time were due to estimation errors. Don Williams had a better idea.
Don noticed that Syncom’s attitude was drifting by small fractions of a degree each month in a direction perpendicular to the sun. He believed this gyroscopic precession could be due to solar radiation pressure, the force of photons from the sun “pushing” on the spacecraft. The concept of using solar sails as propulsion on interplanetary missions relies on this phenomenon.
Don walked into my office and without saying a word, wrote Einstein’s famous E = mc2 on my blackboard. In less than one minute, he said solar radiation pressure pushing on the side could change the attitude if the center of pressure was offset from the center of gravity, pointed at Einstein’s equation and walked out.
Scrambling around, I found estimates for the sun’s “E,” divided it by “c” and got solar radiation’s momentum, “mc.” The resulting force applied depends on how much radiation is reflected versus how much is absorbed. Having no idea what the reflectivity was or where the center of pressure was, I combined both unknowns into one parameter and used our attitude data to estimate the effective CG/CP offset. To no one’s surprise who knew Don, he was right.
While Syncom 3 may have been the world’s first definitive demonstration of solar radiation pressure in space, all Hughes spin stabilized satellites exhibited this slow precession in a direction perpendicular to the sun line. Both Albert Einstein and Don Williams were right!
Howard Sierer was known as “Warren” during his six years in El Segundo, but used “Howard” from 1970 forward in Denver.