Magnetorquer

A magnetorquer or magnetic torquer (also known as a torque rod) is a satellite system for attitude control, detumbling, and stabilization built from electromagnetic coils.

Magnetorquers are sets of electromagnets arranged to yield a rotationally asymmetric (anisotropic) magnetic field over an extended area.

The magnetic dipole generated by the magnetorquer is expressed by the formula where n is the number of turns of the wire, I is the current provided, and A is the vector area of the coil.

As long as current is passing through the coils and the spacecraft has not yet been stabilized in a fixed orientation with respect to the external field, the craft's spinning will continue.

Unlike thrusters, they do not require expendable propellant, so they could in theory work indefinitely as long as sufficient power is available to match the resistive load of the coils.

It is also impossible to control attitude in all three axes even if the full three coils are used, because the torque can be generated only perpendicular to the Earth's magnetic field vector.

An image of one of the Hubble Space Telescope's torque rods
The Hubble Space Telescope uses four 8 feet (2.4 m) iron torque rods as part of its pointing control system. [ 2 ]