# Could the ISS stop rotating wrt the stars for a few days, then start again?

Suppose for some reason it was particularly advantageous to stop the ISS from rotating once per orbit, so that it maintained a constant attitude with respect to the stars (or the Sun) instead of with respect to the Earth. Maybe some kind of emergency procedure.

Could the ISS de-spin itself in say a half-day - or better yet a few hours - remain that way for a few days, and then spin up again for normal operation?

I'm interested in the attitude control - does propulsion exist to do this, and could the attitude control system either handle this or be put into a safe state and allow the maneuver to be managed and monitored by the crew? I didn't say ground because I am not sure if comms link could be managed in this state without advanced planning.

Any structural issues caused by all the torques, stresses and tensions involved?

This is not a full answer, but some numbers:

What are the ISS moments of inertia around design axes? The total moment of inertia of the station is about $M = 55\cdot 10^6 \rm kg m^2$

How often must the ISS desaturate its control moment gyros? The reaction wheels are desaturated when they reach $13000 \rm ft lbf sec$ which is $L = 17 \rm kJs = 17 \rm kgm^2s^{-1}$.

Dividing the two gives us $L/M = 1/3200~\rm rad/s = 1/20,000 Hz$. This means, if we just spin up a reaction wheel to full power, the station will start to rotate once every 5.5 hours.

In its usual orbit, the station rotates once every 90 minutes, about three and a half times as fast. That means, reaction wheels are not sufficient and thrusters needs to be used. As desaturation of reaction wheels takes place every few months, we can conclude that this is well within reach of the capabilities of ISS. I can not give an estimate how fast the change in rotation can be done with respect to stability of the station.

• Thanks for the numerical help! - Is that $M_{X X}$ or $M_{YY}$? Isn't the second much smaller than the first? Can you add a link to the source? – uhoh Jan 9 '17 at 21:18
• Source is the 3rd PDF linked in the only answer on the moments of inertia, page 86. Mxx is about a third of Myy, but I have to adjust my number by some 50%, misread the table. Also, it is just the main truss, but all other parts seem to have much smaller numbers. – asdfex Jan 9 '17 at 21:32
• OK got them now. I wrote that BC (before coffee) - thanks! – uhoh Jan 9 '17 at 22:04
• @uhoh I know Im not supposed to make conversational comments but BC is just too good. Im totally stealing that joke. – Magic Octopus Urn Jul 30 at 22:12

I don't think it's happened in a long time, but in the early stages of assembly, the ISS sometimes flew in a "XPH" attitude when the beta angle* was between 10 and 75 degrees.

The only public info I could find on this is from a rather annoying flash animation NASA page; it shows some animations of the orbits, here are two frames from the XPH example showing that it's at least quasi-inertial.

In this stage, the proto-ISS would fly in XVV (X axis to velocity vector), the above mentioned XPH, or YVV (Y axis to velocity vector), for various values of the beta angle, maneuvering between them as required.

*Note: the beta angle is defined as the angle between the Sun vector and the ISS orbital plane. High values of the beta angle imply the ISS is in sunlight longer.