The torque experiment on Venus Express
This animation shows how the torque technique to measure the atmospheric density at the North Pole of Venus works in practice.
The technique requires the solar panels on board Venus Express to be arranged in an asymmetric configuration, positioned at an angle to each other. The effect is maximum when the angle corresponds to 90 degrees, with one of the panels perpendicular to the incoming flow and the other seeing it edge on.
Due to the asymmetry, the spacecraft experiences a torque moment: as the atmosphere causes it to turn, the internal reaction wheels compensate for the twist, keeping the spacecraft in the right attitude. The torque is recorded by the on board instrumentation and later translated into a density estimate.