# Why does the ISS rotate exactly once per orbit?

Looking at the HDEV experiment, one can see that the orientation of the ISS is always the same with respect to the Earth. This means that the ISS must rotate exactly once per orbit (like the Moon tidally locked in orbit around the Earth); otherwise one would not see always the 'same' surface area of the Earth filling the video.

Why is this the case? Because of communication purposes? And how is this locked rotation being achieved?

• – BowlOfRed Oct 16 '15 at 5:17

## 1 Answer

ISS orbits most of its time in what is called a Torque Equilibrium Attitude (TEA). Since gravitational acceleration varies depending on distance from the Earth, non-symmetric objects of any appreciable size have a net torque acting on them that is attitude-dependent, relative to the local vertical/local horizontal. For the most part, the ISS's attitude is controlled through the use of Control-Moment Gyroscopes (CMG), with chemical thrusters used as backup when attitude demands exceed CMG authority or to desaturate CMGs. Flying in TEA minimizes the loads on the CMGs.

Maintaining a constant attitude relative to Local Vertical Local Horizontal (LVLH) also has the benefit of making thermal and debris constraints more easily met as well, as these environments are relatively constant with respect to the LVLH frame.

• I failed to find the exact phrase Torque-Equivalent Attitude in a google search. But I did find torque equilibrium attitude. Is this what you are referring to? – Ruslan Oct 16 '15 at 6:43
• That's a fair clarification. – Tristan Oct 16 '15 at 16:25