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I am new in the field of Spacecraft Attitude Dynamics and Control, and came across the terminology "Y-Thomson spin". What is it? Where can I read more about this attitude control mode?

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Here's a description of Y-Thomson spin from a thesis on the attitude control systems employed for the Drag and Atmosphere Neutral Density Explorer (DANDE), a student nanosat developed by the University of Colorado Boulder:

The y-axis spin rate is chosen such that, when a boom is deployed along the z-axis in a later phase, the y-axis spin rate coincides with the angular change of the spacecraft in orbit about the Earth. In a nearly circular orbit, this conveniently fixes the spacecraft body axes to the orbit tangent-normal-radial (TNR) reference frame. If deployed with the boom pointing to nadir, the gravity gradient restoring forces stabilize the system as well.

In this reference frame, X is the line tangential to the forward motion of the spacecraft. Z is the radial line pointing toward the center of the Earth. Y is the line 90° to the other two axes.

If you make your spacecraft spin exactly once about the Y axis during the period of its orbit, the "bottom" of the spacecraft always faces Earthward. This is convenient for attitude stabilization because, if you extend a long boom along the Z axis ("down"), you can take advantage of the difference in gravitational pull from the top of the spacecraft to the end of the boom (the gravity gradient) to keep the spacecraft stabilized in that position with relatively little stationkeeping.

I've seen it also cited as "y-Thompson." A Google search will give you several papers. TildalWave recommended this one, a small satellite using a combination of magnetorquers, reaction wheels, and thrusters for attitude control. He also dug up the original paper by Thomson:

Thomson W.T., Spin Stabilisation of Attitude against Gravity Torque, Journal of Astronautical Science, No.9, pp.31-33, 1962.

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