There have been a number of missions with spacecraft flying by planetary moons with atmospheres; there will be future missions with similar flybys. When a spacecraft such as Cassini flies by a moon with an atmosphere, the spacecraft will experience an atmospheric torque. This torque could be used to determine the density of the atmosphere. This is because the relation between the atmospheric torque vector and the atmosphere density could be established analytically using the mass properties of the spacecraft, known drag coefficient of objects in free-molecular flow, and the spacecraft velocity relative to the moon. The density estimated in this way could be used to check results measured by science instruments. Since the proposed methodology could estimate disturbance torque as small as 0.02 N-m, it could also be used to estimate disturbance torque imparted on the spacecraft during high-altitude flybys.
When the expected value of torque imparted on the spacecraft is low and within the control authority of the reaction wheel assemblies (RWAs), mission design engineers will use these RWAs to control the spacecraft attitude. Relative to thrusters, RWA can produce better pointing control and stability performance. To estimate the disturbance torque imparted on the Cassini spacecraft, the proposed methodology exploits the unique and known relation between the disturbance torque and the RWA-based attitude control error during an Enceladus or Titan flyby.
To estimate the disturbance torque imparted on the Cassini spacecraft, the unique and known relation between the disturbance torque and the attitude and attitude rate control errors during an Enceladus flyby (or a Titan flyby) on reaction wheels was used. The effectiveness of this methodology is illustrated using telemetry data obtained from the 50-km Enceladus-3 flyby. Results determined using this approach were compared with those determined using the “time rate of change of spacecraft angular momentum” approach. Results of this flyby determined that using this new approach compared very well with that estimated using the angular momentum approach. In effect, density estimates made using these two independent engineering methodologies could cross check each other. Moreover, density estimates determined using these methods could also be used to cross check science-based density estimates.
This method could be used to estimate very small torque imparted on the spacecraft. The methodology is straightforward and does not involve the use of any complex supporting ground software. This methodology uses telemetry data that are available at high telemetry frequency, and the telemetry data involved (per-axis attitude control errors and per-axis attitude rate control errors) are floating-point data with high accuracy.