Anomalistic disturbance torques during the entry phase of the mars exploration rover missions—a telemetry and mars-surface investigation

Shortly after landing on Mars, post-flight analysis of the “Spirit” entry data suggested that the vehicle experienced large, anomalistic oscillations in angle-of-attack starting at about Mach number M = 6. Similar analysis for “Opportunity” found even larger oscillations starting immediately after maximum dynamic pressure at M = 14. Where angles-of-attack of 1–2 degrees were expected from maximum dynamic pressure to drogue deployment, the reconstructions suggested 4 to 9 degrees. The next Mars lander, 2007 Phoenix project, was concerned enough to recommend further exploration of the anomalies. Detailed analysis of Opportunity data found significant anomalies in the hypersonic aerodynamic torques. The analysis showed that these torques were essentially fixed in the spinning vehicle. Nearly a year after landing, the Opportunity rover took pictures of its aeroshell on the surface, which showed that portions of the aeroshell thermal blanket assembly still remained. This blanket assembly was supposed to burn off very early in the entry. An analysis of the aeroshell photographs led to an estimate of the aerodynamic torques that the remnants could have produced. A comparison of two estimates of the aerodynamic torque perturbations (one extracted from telemetry data and the other from Mars surface photographs) showed exceptional agreement. Trajectory simulations using a simple data derived torque perturbation model provided rigid body motions similar to that observed during the Opportunity entry. Therefore, the case of the anomalistic attitude behavior for the Opportunity EDL is now considered closed and a suggestion is put forth that a similar event occurred for the Spirit entry as well.