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The question Why doesn't ISS start to spin if people walk inside? is a good one partly because it has attracted several good, informative answers. One of them is particularly intriguing because it contains the following statement:

In the Mission Control Center, it was possible to tell if the astronauts were awake, based on how the CMGs were operating (four of them). But the motions are very small.

The physics makes sense of course. As astronauts move (especially) along the forward/backward (roll) axis of the ISS, the movement of mass relative to the center can change the moment of inertia about the left/right (pitch) axis. Assuming angular momentum is conserved this will change the rotation rate about the pitch axis.

As pointed out in this helpful answer the ISS rotates once around its pitch axis every 93 minutes in order to keep the "bottom" facing towards the nadir.

So if everyone "ran" to the front and back ends and waited a while, the rotation rate would slow and the ISS would slowly pitch-up. If they all then "ran" to the center, this drift would reverse and it would pitch down again.

My question is, roughly how sensitive is the attitude control system to this kind of activity? Is it something that can be detected over a period of seconds, or minutes, or more like a day? How well (and how rapidly) can the ISS attitude control system detect and resolve periods of astronaut activity?

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  • $\begingroup$ Think about how much does the center of gravity shift if the astronaut moves around the station. The station has a mass that is much larger than that of the astronaut. The imparted angular momentum when shifting position inside the station reflects that. Shifting something like 1/5000th the mass of the station from the center to the outside, the maximum shift in angular momentum is likewise 1/5000th. So on one rotation of 93 minutes, the rotation would be off by about 1 second - not accounting for the inital mass distribution within the station of course, but more about the general scale. $\endgroup$
    – Adwaenyth
    May 8, 2017 at 10:37
  • $\begingroup$ @Adwaenyth You have to look at it from the perspective of moments, not masses. Much of the mass is along the port/starboard axis, where it's impact is much less. I'm pretty sure these moments of inertia are already tabulated in ISS documents on line. The other half of the problem is to look up the sensitivity of the ADCS system, or any other attitude sensors. $\endgroup$
    – uhoh
    May 8, 2017 at 11:13
  • $\begingroup$ So if everyone "ran" to the front and back ends and waited a while, the rotation rate would slow and the ISS would slowly pitch-up. I'm not sure which axis you're describing as front-back, but I believe the axis of rotation is parallel to the long central spine. To change the moment of inertia, they would have to move perpendicular to the spine. $\endgroup$
    – user687
    May 8, 2017 at 15:01
  • $\begingroup$ @BenCrowell well I am not sure which axis you are calling a central spine because the ISS is a cross of two perpendicular beams, but for sure front would be the direction it is moving in its orbit around the Earth, and back would be the place where the orbit-raising engine is, pushing it forward. Also, look again and you'll see that the previous paragraph explains: "As astronauts move (especially) along the forward/backward (roll) axis of the ISS," I think we can all agree which axis the roll axis is, except these guys :) $\endgroup$
    – uhoh
    May 8, 2017 at 15:33
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    $\begingroup$ With this I am now thinking, what is the sensor accuracy which can detect such rates and command CMGs $\endgroup$
    – zephyr0110
    Jun 7, 2019 at 17:42

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If you have linear and rotational accelerometers you can easily tell. During quiescent periods (crew sleep) the structure of the vehicle doesn't fluctuate as drastically as when the crew is moving inside and bouncing off structure.

Thus, by looking at vehicle rates, ADCO can easily figure out if the crew is awake or asleep.

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  • $\begingroup$ Thanks for your answer and Welcome to Space! The question asks: "How well can..." and I think your answer is that it "...can easily figure out..." Can you mention how you know that looking at vehicle rates easily shows this? In Stack Exchange it's important to add supporting links or references to statements used to answer questions. I have a hunch that you may be right, but it needs to be supported somehow. Thanks! $\endgroup$
    – uhoh
    Apr 4, 2019 at 23:56
  • $\begingroup$ Speaking of ADCO, you may find this interesting: What's the baseball bat for? (seen in CRS-8 berthing NASA TV broadcast) $\endgroup$
    – uhoh
    Apr 4, 2019 at 23:58

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