Anomalous Attitude Motion of the Polar Bear Satellite

The Polar BEAR satellite was launched successfully from Vandenberg Air Force Base, Calif., in November 1986. A Scout launch vehicle placed Polar BEAR into a circular, polar orbit at an altitude of lcxx) kIn. The satellite's four instruments are designed to yield data on RF communications, auroral displays, and magnetic fields in the earth's polar region. The Polar BEAR attitude control system is required to maintain an earth-pointing orientation for the on-board instruments. For nominal operation, Polar BEAR is stabilized rotationally to within ± 10° about any of three orthogonal axes. An 18.3-m gravity-gradient boom and a constant-speed momentum wheel, together with magnetic hysteresis rods and a boom-mounted eddy-current damper, make up Polar BEAR's passive attitude control system. Until mid-February 1987, the Polar BEAR mission proceeded as designed, and its attitude performance was nominal. However, as the satellite entered its first period of full sunlight, APL was notified of unexpectedly large attitude oscillations. The attitude motion continued to diverge from the desired operational range until Polar BEAR finally "flipped over" and was orbiting upside down.