# Why doesn't the ISS start to spin if people walk inside?

I have read that prior to launch all people and tools are fastened down to prevent a rocket from losing the balance. Why doesn't the ISS begin to spin as astronauts are walking inside of it? Or is there some compensation mechanism for it to maintain its orientation?

• Walking (linear momentum) doesn't even induce spins, pirouettes (angular momentum) do. – Rainer P. May 7 '17 at 14:14
• Walking in the ISS is not possible anyway without gravity. – Uwe May 7 '17 at 17:22
• One of the rare space questions where Kerbal Space Program makes understanding worse :D Unlike in KSP, in reality angular momentum is conserved. While the movements of astronauts and equipment do require adjustments from the on-board balancing systems (gyroscopes, thrusters), they cannot create angular momentum out of thin air. Walking is impossible, but imagine a person "climbing" a ladder going all the way around a tube - there will be a change in rotation whenever he pulls himself forward, and the exact opposite change as he stops himself. – Luaan May 7 '17 at 23:28
• @Uwe maybe not practical, but if you really wanted to walk in zero gee you could invest in some velcro. From the SF archives: youtu.be/0iiXUeil5fQ and also youtu.be/muPNlnm_i44?t=116 – uhoh May 8 '17 at 0:43
• Actually, to the extent that one can "walk" in the ISS, it does spin due to people walking around. But two things prevent this from being much more than noise: 1) The ISS is much more massive than a single human, and 2) the human has to stop "walking" eventually, or at least turn around and walk the opposite direction. To set the ISS spinning one would have to continuously walk a cylindrical path -- as soon as the walking stopped the spinning would stop (ignoring all sorts of messy details, of course). – Dan May 8 '17 at 1:15

Consider the differences in mass. A person is say 100 kg to keep it round.

The ISS masses 420,000kg right now. (It changes of course as modules are added, removed, changed, payload arrives, departs).

That is a very low effect that any one person can have. There are only 6 crew on the ISS at any one time, so their torquing effect is quite low.

Regardless there are 6 (?) control gyros that manage the orientation of the station. Like the gyros in the Hubble, they seem prone to failure and several have had to be replaced over the years.

• Is the effect really negligible? An astronaut weighs about 1/5000 of the space station. If an astronaut spins like Tim Peak did, making a rotation a second, the ISS would make one rotation every hour and a half. But I suppose he couldn't stop spinning without interacting with the space station so that its rotation is halted. Does the air even things out? – LocalFluff May 7 '17 at 12:28
• @DenisKulagin, see en.wikipedia.org/wiki/Reaction_wheel and en.wikipedia.org/wiki/Control_moment_gyroscope – Hobbes May 7 '17 at 13:21
• @LocalFluff Angular moment of inertia (which is the only relevant quantity here) is mass times lever arm squared. The astronaut to station ratio is far less than 1/5000. – Rainer P. May 7 '17 at 14:33
• @DenisKulagin No, CMGs do not require propulsion and they do not shift mass around the station. They work on the physical principle of the conservation of angular momentum. – Adam Wuerl May 7 '17 at 15:26
• A person is say 100 kilos to keep it round. Spherical person? In a vacuum? – dotancohen May 7 '17 at 17:46

Why doesn't ISS start to spin if people walk inside?

The ISS is already spinning. Its rotational period has been set to 93 minutes in order to match its orbital period and keep one side of the station permanently pointing at the earth. Moving astronauts will not make it start to spin, they will alter its spin.

We can consider two different effects that an astronaut could have on the station: (1) a permanent change in its state of rotation, or (2) a temporary change.

An example of a permanent change would be if an astronaut grabs a massive piece of equipment from a portion of the station that is far from the axis, brings it to a point on the axis, and leaves it there. This reduces the station's moment of inertia, and therefore permanently increases its rate of rotation due to conservation of angular momentum. This change will persist until counteracted by a compensating redistribution of mass away from the axis or possibly a use of CMGs or thrusters.

An example of a temporary change would be an astronaut moving their own body closer to the axis and then back out. Because the change is temporary, it only ends up changing the phase of the rotation. That's undesirable, because they want to maintain the same orientation relative to the earth.

It's possible in principle for an astronaut to do something like moving rapidly and continuously in a circle at a fixed distance from the axis, so as steal or donate a significant amount of angular momentum to the station. The effect would be temporary, and the geometry of the space station doesn't seem very helpful in trying to perform such a circuit.

Astronauts' motions could also cause the station's rotation to precess or wobble. This would presumably be corrected by the CMGs.

• This is a good answer, it's important to remember that man LEO spacecraft rotate to maintain a certain attitude relationship with respect to the nadir. I've linked to your answer in this follow-up question. – uhoh May 8 '17 at 0:34
• At the risk of being pedantic, "The ISS is already spinning" is a potentially misleading statement. Everything in the universe is both spinning and not spinning, depending on your reference frame. From a velocity-fixed reference frame, the ISS is not spinning. From an ECI frame, it is. – Arthur Dent May 8 '17 at 13:58
• @ArthurDent: At the risk of being even more pedantic, there is an objective distinction between inertial and noninertial frames of reference. It's not a matter of opinion whether the earth is spinning about its axis, since that causes it to have an ellipsoidal shape. – Ben Crowell May 8 '17 at 14:49

The CMGs (Control Moment Gyros) do have to compensate for astronaut motion. 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 CMGs don't require any fuel for general operation, but periodically they get "saturated" and then a desaturation burn is required. The CMGs can each be pointed independently. So, initially they are pointed in different directions. If a rotation is required, they are moved to have a net angular momentum opposite to that, causing the vehicle to rotate (or stop rotating). Over time they will end up all aligned in the same direction which is "saturation".

During a launch, everything is fastened because a rocket accelerates really fast (3G or more), so any loose object "falls" really quickly and hits hard, potentially damaging important equipment.
Changes in the center of gravity similarly have huge effects during launch because of this large acceleration.

In orbit, forces are much smaller by comparison. Because everything is in 0 gravity, you only need small forces to move around. Also, forces are generally balanced. You move around the station by pushing off in one spot, then flying to the next wall where you stop by applying the same force in the opposite direction.

Despite this, the attitude of the ISS is controlled using gyroscopes (and thrusters, if necessary).