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A question from 3 years ago got an answer that would be a great answer to a different question (and concluded with a comment that more would be posted later.) Let me re-ask, making the question clearer.

Normally, ISS spins at 1 revolution per orbit, effectively one side always facing the Earth.

With two bodies in almost the same orbit, they move in an elliptical trajectory respective to each other (for example, the loose antenna cover seems to orbit ISS, even if it's not gravitationally bound to it).

The docking maneuver takes good several minutes, which comprise a good part of an orbit. If I brought Soyuz to a dead stop relative to the station 200m below the bottom port while ISS is facing Earth with it, the orbital mechanics would move me to 200m above the station within 45 minutes (half the orbital period) - or move me 45 degrees off course within some 11 minutes.

How is this dealt with? Is ISS rotation relative to Earth stopped (ISS attitude becoming constant relative to distant stars, changing relative to Earth) or does the docking ship deal with it using RCS to account for the drift? Or is this even different or I made some wrong assumptions?

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  • $\begingroup$ Interestingly, while, as per BowlOfRed's answer, the ISS doesn't turn for docking, it does turn with its bottom in prograde direction when undocking Soyuz from its bottom ports. $\endgroup$ – SF. Jan 16 '16 at 16:09
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ISS rotation is not changed for docking. The orbital drift is only a problem if you're not actively maneuvering.

Most of the approach consists of long periods of drift, with specific points where burns are planned. Orbital mechanics are well planned out so that the burns and resulting drift combine to deliver the visiting craft to the approach point at the right time.

But for final approach, you can almost ignore the orbital issues and thrust to keep the craft in the correct spot in the trajectory relative to the station. The phase is short enough, and the craft is close enough, that this won't result in a lot of fuel waste.

Another answer points to this nasa document with details about STS-135 Rendezvous. Several charts in chapter 1 show the planned approach up to MC4. After that point, the shuttle was flown manually by the pilot. The chart shows ISS in the LVLH frame, which means it keeps the Z-axis pointed at the earth (and is therefore still rotating).

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    $\begingroup$ I don't think this is correct. I can find on-orbit status reports (example) describing Free Drift attitude before final approach. ISS is typically in Torque Equilibrium Attitude (TEA) which is also mentioned in other on-orbit status reports (example) to have been re-enabled after docking or undocking. Attitude is changed with Zero Propellant Maneuvers using CMG in US segment (assuming it works, else Progress or Zarya thrusters). $\endgroup$ – TildalWave Jan 9 '16 at 5:29
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    $\begingroup$ The physical strike of the shuttle on docking sets up vibrations in the structure. To prevent enhancing them, attitude goes to free drift on contact. Active control is turned back on later. In this replay of STS-135 (youtube.com/watch?v=DvRmkfOstqM) you can hear the call confirming free drift just after docking (right after the 0:57 mark on video). $\endgroup$ – BowlOfRed Jan 9 '16 at 5:45
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    $\begingroup$ I found this answer by Robert Frost on How does the Soyuz undock from the ISS that describes change in ISS typical attitude for docking and undocking (TEA to ±YVV/LVLH). Also see page 3 in this PDF. $\endgroup$ – TildalWave Jan 9 '16 at 6:06
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    $\begingroup$ From ISS report linked in first comment: ' this morning at 9:09am EST at the MRM2 “Poisk” module, 3 min ahead of schedule and 1 min before orbital sunset. Docking hooks were driven closed at ~9:22am. At “hooks closed” signal, SM (Service Module) returned to active attitude control'. $\endgroup$ – kim holder Jan 13 '16 at 22:31

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