Evolution of the Huygens Probe Spin During Parachute Descent

The anomalous spin of the Huygens probe on Titan is simulated with a simple model, which is also applied to the (also anomalous) spin history from a terrestrial parachute drop test. The model considers two parameters: a “setting angle,” which defines an equilibrium spin rate for a given descent speed, and a coefficient, which defines how quickly the probe spin changes to reach that equilibrium. The parameters required to reproduce the descent histories are compared with values derived from wind-tunnel tests and estimated by construction. Not only did both descents see spin in the opposite direction from that intended to be driven by small canted spin vanes, but both the Titan descent and the drop test require nonconstant torque parameters, requiring configuration changes and/or external torques. In both cases, unexplained short-term spin evolution occurs while under a small “stabilizer” parachute. In the Titan case, additional spin evolution anomalies appear consistent with the off-nominal deployment of an instrument boom.