# Why don't astronauts inside affect inertia of the ISS? [duplicate]

When astronauts move around inside the ISS, pushing off walls to stop and go, how does it not affect the inertia of the ISS, and send it rotating undesirably or off course, little by little. I expect the effects would accumulate quickly, unless the astronauts remained perfectly still all the time. Even though the astronauts are small, relative to the size of the ISS, I would expect their behavior to affect the motion of the ISS, the way a construction worker can affect large objects that are hanging from a crane. Also, during a spacewalk, if an astronaut interacts with a ladder, or something extended from the ISS, I would expect that effect to be magnified, since leverage is involved.﻿

It doesn't make sense to me that the ISS seems (based on video feed I've seen) to have one side constantly facing the earth, instead of rotating.

## marked as duplicate by Hobbes, peterh says reinstate Monica, Organic Marble, Nathan Tuggy, kim holderMay 20 '18 at 22:13

• "It doesn't make sense to me that the ISS seems (based on video feed I've seen) to have one side constantly facing the earth, instead of rotating." Note that the ISS must rotate once per orbit to achieve this. – Organic Marble May 20 '18 at 19:01

The ISS doesn't get to rotate because of the conservation of the angular momentum.

More detailed: if an astronaut start to rotate, yes it gets to rotate also the ISS, in the opposite direction. But if the astronaut stops the rotation, he has to do that by stopping exactly the same rotation, what he caused by starting it.

If the astronaut rotates a lot, for example, 5 times, and then stops, then yes, it causes a little change in the orientation (and not the angular moment) of the ISS. However, the ISS has gyros, doing exactly the same, exactly to handle the same reason: they are large dumb masses, capable to start and stop rotation electronically.

There are some little effects which change not only the orientation, but also the angular momentum of the ISS: the light pressure from the Sun, and the little drag with the upper atmosphere. An astronaut (inside the ISS) can't do these on the reason decribed above. But if they accumulate, there is no more way to change the angular momentum of the station, as to use the propulsion to compensate it.

P.s. the mass of an astronaut is $\approx$ 70kg, and the ISS is 400 tons. Thus the change is practically negligible, but these would solve the problem even if it wouldn't be.

• +1 What happens if the "clever" astronaut spins one way at one end, then while spinning moves to the other end and reverses her spin, then moves back to the first spot and reverses again, over and over? ;-) – uhoh May 21 '18 at 0:40
• @uhoh Thanks :-) It doesn't matter, how complex spins/movements is he doing; the angular momentum won't change. He can reach a change in the orientation of the station. But it will be compensated by the giros. – peterh says reinstate Monica May 21 '18 at 1:23
• @peterh Oh, that's right. Gratuitous cat-based science video – uhoh May 21 '18 at 1:30
• @peterh oh, I see. I think I have left a link to that video in a few different comments within this site, and it's quite popular among science geeks as it has both space science and cute cats in it. It also ranks 9th in popularity out of 193 of his videos. It's not surprising that google would recommend it to you if that's what happened. It's a real gem. – uhoh May 21 '18 at 3:43
1. ISS = 400 tons, astronaut = 80 kg, so any effects are not immediately visible.
2. Astronaut actions on the ISS are usually balanced: an astronaut pushes off from one wall, then brakes to a standstill on the opposite wall. Total momentum transfer is 0.
3. The attitude of the ISS is actively managed using Control Moment Gyroscopes.