magnetic torquer

• Firing opposed thrusters to gain angular momentum without perturbing the orbit;
• Accelerating or decelerating a momentum wheel inside the spacecraft;
• Applying current to magnetic torquers.

A magnetic torquer is nothing else than a standard coil or solenoid. When current is fed into this coil, a torque is created until the torquer's internal magnetic field is aligned with the Earth's own magnetic field. Following Lorentz force law (without the electric term), one can find that the torque produced is:

T = âNIπr² x B

where the bold represents vector quantities, T being the torque, â being a unit vector along the axis of the torquer, N being the number of loops in the coil and r being its diameter, I being the current in the coil and B being the Earth's magnetic flux density.

Using three perpendicular magnetic torquers in a spacecraft thus enables the control of its attitude on three axis of rotation. Magnetic torquers are a cheap and easily implemented way to control the orientation of a satellite; however, there are several downsides:

• Since the Earth's magnetic field is rather tenuous, the torque produced by magnetic torquers is in general very low and as a consequence, quick adjustments of the attitude is impossible using torquers.
• The torque produced is not constant with the angular position of the spacecraft, and moreover, the Earth's magnetic field is not even constant itself, the magnetosphere being swept away by solar wind.
• Injecting current in a coil can create lots of EMIs, which is habitually bad for spacecraft applications.