According to my prelaunch Echo manual, the S/C has black and silver kapton tape along the edges to impart spin. The documentation mentions that the earlier microsats used painted antenna elements to achieve the same result.
Given enough time, the equilibrium rotation rate will be the result of a balance of the torque from the tapes, and "drag" caused by the internal eddy currents. I am not aware of any data for AO-51, but normally spacecraft passively stabilize in a matter of days, weeks, or rarely months. Since the spin rate is falling slowly, it suggests, but does not prove, that the cause is a decrease in the applied torque from the kapton tapes. Kepton is widely used in S/C, but is subject to deterioration due to UV and particularly atomic oxygen. Going back to the Long Duration Exposure Facility and other tests in the 1980s, it was found that it and similar "soft" materials had significant changes in their optical and mechanical properties. Atomic oxygen levels have the usual exponential dependence on altitude, plus a factor of 10-1000 variation due to solar and magnetosphere conditions. Effects on kapton by atomic oxygen were of considerable interests to shuttle, ISS, and HST designers, though the reason was primarily optical and contamination. They of course have active pointing. It would be interesting to know from the Olde Tymers whether similar changes were found in much earlier LEO S/C, and how other LEO S/C using the same system as AO-51 have faired recently. Alan WA4SCA _______________________________________________ Sent via amsat...@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
