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Satellites in geostationary orbit always remain above the same location on the Earth's surface, at an altitude of 35,786 kilometres (22,236 miles) above the equator. But I wonder whether they're also tidally locked, meaning a certain side of the satellite always points to the same direction relative to Earth. Imagine there's a camera on the satellite's "downside" showing Earth. Would that camera always point to and show Earth from geostationary orbit or would it rotate away or something, or are certain geostationary sats tidally locked but geostationary sats don't have to be?

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  • $\begingroup$ Do you ask about a tidally locked position or a tidally locked orientation of the satellite? $\endgroup$ – Uwe Jul 12 at 16:01
  • $\begingroup$ @Uwe Orientation. $\endgroup$ – LoveForChrist Jul 12 at 17:12
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    $\begingroup$ @Uwe What's tidally locked position? I have only heard of the orientation. $\endgroup$ – OrangeDurito Jul 13 at 10:35
  • $\begingroup$ @OrangeDurito In this case I think "position" refers to the subpoint of the smaller body on the surface of the larger body. Pluto & Charon are locked to each other, so each body remains above a relatively fixed location above the other 1, 2 This case is kind-of interesting, so I've just asked How “locked” are Pluto and Charon? How much does each librate as seen from the other? $\endgroup$ – uhoh Jul 13 at 12:54
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But I wonder whether they're also tidally locked, meaning a certain side of the satellite always points to the same direction relative to Earth

I can not write a definitive answer about satellites in GEO, but the chances are extremely low that there are spacecraft at or near geosynchronous atlitude that are using gravity gradient stabilization (a kind of tidal locking) for attitude control by design, because it is going to be an extremely weak effect that far away and these days communication spacecraft in/near GEO have no problem maintaining attitude using reaction wheels and electric thrusters.

It is possible that out there in the graveyard orbits one of them somehow happens to have attained a stable attitude with respect to the nadir by accident, but there are torques due to photon pressure and other things and so it's more likely the end up tumbling slowly and randomly.

Central forces like the gravity of a planet have a gradient; the force varies as $r^{-2}$ so it pulls slightly stronger on the parts of a spacecraft closer to the planet than its center of gravity. If the spacecraft orbits once per day and rotates around it's self once a day as well, then it's close to locking. All you need is a little bit of damping.

In the early days of artificial satellites in low Earth orbit there were many spacecraft that used gravity-gradient stabilization because it is completely passive; as long as you get a properly designed and damped spacecraft to a low enough rotation rate it will eventually fall into a nadir-facing attitude due to the damping.

You can read more about this in R. E. Fischell's Gravity Gradient Stabilization

Apparently Explorer 49 used gravity gradient stabilization around the Moon!

You can recognize these satellites because they tend to have a long axis if not a long boom with a weight at the end that points towards the body about which it rotates.

enter image description here LIDOS [JHU/APL] from here

Since answer fragmentation is the bane of Stack Exchange, instead of re-writing the same answers here one can read further about how this works and why damping is necessary in answers to the following:

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