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How satellites are oriented

Engineers can use the gravity-gradient effect to control the attitude of some satellites. This is the tendency of long thin objects to align themselves along the direction of the gravitational field (i.e. the local vertical). Satellites are made long and thin by having a large lump of brass (~2 - 3kg) on the end of a long boom (~6m) which makes the satellites look like a long dumb bell.

The gravity-gradient effect is very weak so it is not enough to give good attitude control. Engineers can activate electromagnets built into the solar panels of the satellites (X and Y facets) and the end panels (Z facets). This produces a magnetic field which reacts against the Earth's natural magnetic field and produces a torque to realign the satellite as required.

The torque produced by a magnetic field in a coil is calculated by:

where:

B is the Earth's magnetic field
A is the area
N is the number of coils
I is the current flowing in the coils.

The Earth's magnetic field decreases rapidly with height (proportional to ) but is still strong enough to provide a maximum field strength for a Low Earth Orbit satellite of about above the magnetic poles.

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