Amagnetorquer ormagnetic torquer (also known as atorque rod) is asatellite system forattitude control,detumbling, and stabilization built fromelectromagnetic coils. The magnetorquer creates a magnetic dipole that interfaces with an ambient magnetic field, usuallyEarth's, so that the counter-forces produced provide usefultorque.

Magnetorquers are sets of electromagnets arranged to yield a rotationally asymmetric (anisotropic) magnetic field over an extended area. That field is controlled by switchingcurrent flow through the coils on or off, usually under computerizedfeedbackcontrol. The magnets themselves are mechanically anchored to the craft, so that any magnetic force they exert on the surrounding magnetic field will lead to a magnetic reverse force and result in mechanical torque about the vessel'scenter of gravity. This makes it possible to freely pivot the craft around in a known localgradient of the magnetic field by only using electrical energy.

The magnetic dipole generated by the magnetorquer is expressed by the formula

${\displaystyle \mathbf{m}=nI\mathbf{A},}$

wheren is the number of turns of the wire,I is the current provided, andA is thevector area of the coil. The dipole interacts with the magnetic field generating a torque

${\displaystyle \mathit{\tau }=\mathbf{m}\times \mathbf{B},}$

wherem is the magnetic dipole vector,B the magnetic field vector (for a spacecraft it is the Earth magnetic field vector), andτ is the generated torque vector.

The construction of a magnetorquer is based on the realization of a coil with a defined area and number of turns according to the required performances. However, there are different ways to obtain the coil; thus, depending on the construction strategy, it is possible to find three types of magnetorquer, apparently very different from each other but based on the same concept:^[1]

Air-core magnetorquer

This comprises the basic concept of magnetorquer, a conductive wire wrapped around a non-conductive support anchored to the satellite. This kind of magnetorquer can provide a consistent magnetic dipole with an acceptable mass and encumbrance.

Embedded coil

This is constructed creating a spiral trace inside thePCBs of solar panels which generates the effect of the coil. This solution is the one with the least impact on the satellite as it is entirely contained within the solar panels. However, due to the physical limit in the board thickness and the presence of other circuits and electronic components, it is not possible to reach a high value of the magnetic dipole.

Torque rod

This is the most efficient solution. A conductive wire is wrapped around a ferromagnetic core which is magnetized when excited by the coil, thus generating a dipole considerably higher than the other solutions. However, the disadvantage is the presence of a residual magnetic dipole that remains even when the coil is turned off because of thehysteresis in the magnetization curve of the core. It is therefore necessary to demagnetize the core with a proper demagnetizing procedure. Normally, the presence of the core (generally consisting of heavy metal) increases the mass of the system.

Typically three coils are used, although reduced configurations of two or even one magnet can suffice where full attitude control is not needed or external forces like asymmetricdrag allowunderactuated control. The three coil assembly usually takes the form of three perpendicular coils, because this setup equalizes therotational symmetry of the fields which can be generated; no matter how the external field and the craft are placed with respect to each other, approximately the same torque can always be generated simply by using different amounts of current on the three different coils.

As long ascurrent 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.^{[citation needed]}

Magnetorquers are lightweight, reliable, and energy-efficient. Unlikethrusters, they do not require expendablepropellant, so they could in theory work indefinitely as long as sufficientpower is available to match theresistive load of the coils. In Earth orbit, sunlight is one such practically inexhaustible energy source, usingsolar panels.

Another advantage overmomentum wheels andcontrol moment gyroscopes is the absence ofmoving parts, hence significantly higher reliability.

The main disadvantage of magnetorquers is that very highmagnetic flux densities are needed if large craft have to be turned quickly. This either necessitates a very highcurrent in the coils, or much higher ambient flux densities than are available inEarth orbit. Consequently, the torques provided are very limited and only serve to accelerate or decelerate the change in a spacecraft's attitude by small amounts. Over time, active control can produce fast spinning even on Earth, but for accurate attitude control and stabilization the torques provided are often insufficient. To overcome this, magnetorquer are often combined withreaction wheels.

A broader disadvantage is the dependence on Earth's magnetic field strength, making this approach unsuitable for deep space missions, and also more suitable forlow Earth orbits as opposed to higher ones such asgeosynchronous. The dependence on the highly variable intensity of Earth's magnetic field is problematic because then the attitude control problem becomes highlynonlinear. 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.^[3][4]

Any spinning satellite made of a conductive material will lose rotational momentum in Earth's magnetic field due to generation ofeddy currents in its body and the corresponding braking force proportional to its spin rate.^[5]Aerodynamicfriction losses can also play a part. This means that the magnetorquer will have to be continuously operated, and at a power level which is enough to counter the resistive forces present. This is not always possible within the energy constraints of the vessel.

The Michigan Exploration Laboratory (MXL) suspects that theM-CubedCubeSat, a joint project run by MXL andJPL, became magnetically conjoined toExplorer-1 Prime, a second CubeSat released at the same time, via strong onboard magnets used for passive attitude control, after deploying on October 28, 2011.^[6]This is the first non-destructive latching of two satellites.^[7]

• Gravity-gradient stabilization, a passive stabilization technique

• Spacecraft attitude control
• Spacecraft propulsion
• Spacecraft components

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