Does the ISS Rotate to Stay Earth-Facing?

[DaveC426913]

TL;DR

Does the ISS maintain orientation with the Earth, or with the stars?

Does the ISS remain oriented with the Earth? I.e. it rotates 360 degrees every 24 hours, or is it fixed with the stars?

If the latter, there should be a time when it's visible end-on.

I looked it up but did not find the deets.

I guess that what the gyros are for.

 

[Baluncore]

TL;DR Summary: Does the ISS maintain orientation with the Earth, or with the stars?

I looked it up but did not find the deets.

I would expect the body to roll forwards once per orbit, so the Earth remains in the same view direction and position. Orbiting end-on minimises body atmospheric drag. The solar panels independently track the Sun, or for the 46 minute night, lie flat to reduce atmospheric drag.
You can check it by watching the following simulation for 93 minutes.
See: https://www.heavens-above.com/ISS_3D.aspx

 

[Filip Larsen]

I believe ISS during normal operation maintain its orientation relative to the gravity-gradient (or local horizon), even if its principal axes are not in the most stable orientation. Maintaining this orientation is relevant for the levels of micro-gravity at different places in ISS and important during rendezvous where different approaches has a "coupling" to where the docking ports are.

 

[berkeman]

How is the International Space Station’s attitude and altitude controlled and can any current functions be replaced or upgraded?

All International Space Station propulsion is provided by the Russian Segment and Russian cargo spacecraft.  Propulsion is used for station reboost, attitude control, debris avoidance maneuvers and eventual deorbit operations are handled by the Russian Segment and Progress cargo craft. The U.S. gyroscopes provide day-to-day attitude control or controlling the orientation of the station. Russian thrusters are used for attitude control during dynamic events like spacecraft dockings and provide attitude control recovery when the gyroscopes reach their control limits.

Northrop Grumman’s Cygnus is the only U.S. commercial spacecraft currently in testing to provide limited capability for future reboosts. This capability relies on the Russian Segment for attitude control during the small reboost. It does not currently have the capability to replace attitude control functions for the space station or carry adequate propellant for long-term sustained operations.

Attitude control and propulsive reboost capability is a continuous requirement, which means the space station needs a continuous and steady supply of propulsion spacecraft. Changes to the current propulsion scheme would take considerable new hardware/software development, and significant time and funding to enact.

https://www.nasa.gov/international-space-station-frequently-asked-questions/

 

[DaveC426913]

...even if its principal axes are not in the most stable orientation.

Yeh. An extended object like that, left to its own devices, will tend to align itself across the gravitational gradient - i.e. pointing up/down.

https://www.nasa.gov/international-space-station-frequently-asked-questions/

Yup. Read that. But it didn't specify what that orientation is.

 

[mfb]

Yeh. An extended object like that, left to its own devices, will tend to align itself across the gravitational gradient - i.e. pointing up/down.

Indeed. The ISS is mostly "flat", so it needs active stabilization. A vertical arrangement with passive stability was considered but it would have needed a larger structure.

The ISS has many Earth-facing instruments, so it has to rotate once per orbit (~90 min) to keep them pointing at Earth.