5

We all know ISS is orbiting the earth so it has translational motion, but, does it rotate on its own axis as well? Thanks

max busch
  • 53
  • 1
  • 5

2 Answers2

8

Yep! Since the same side of the ISS is always facing down towards the earth, every time it orbits once around the Earth, it rotates once around its axis.

Actually, every time an ocean liner travels around the Earth once, it has also rotated around it's port-stern starboard axis. A better example would be a plane, it slowly rotates around the axis through it's wings as it travels around the Earth.

There are some nice images in some of these Quora answers.

Actually I will repost one of them here. The ISS is traveling in the direction of it's own $X_{LVLH}$ axis, and so it rotates about it's $Y_{LVLH}$ axis once per orbit. LVLH stands for "Local Vertical, Local Horizontal."

enter image description here

enter image description here

above: ISS spacecraft directions, from NASA.

uhoh
  • 148,791
  • 53
  • 476
  • 1,473
  • 1
    You could add that it rotates around the orange axis – Innovine Jan 08 '17 at 17:26
  • 1
    Interesting. The maximum acceleration in the pressurized areas (at 36 m from the axis) is about 5x10^-5 m/s2. So I guess that would be one limit on how good the microgravity is, unless you do experiments right at the axis. –  Jan 08 '17 at 18:07
  • 3
    I think you mean port-starboard axis, stern=rear end – Austin Jan 08 '17 at 18:59
  • @Austin all I know is when I used to go fishing there was an oar on the left and an oar on the right, and sometimes an engine in the back. Thanks for the heads-up, I've made an edit. – uhoh Jan 09 '17 at 00:48
  • @Innovine since some people perceive and distinguish colors differently, I try not to use them as much as I used to. Actually some people here in SXSE leave beautiful answers that are mostly text and equations. I haven't evolved to that level yet. – uhoh Jan 09 '17 at 00:50
  • For whatever reason, in the ops community the purple axis is usually referred to as zenith/nadir. – Organic Marble Jan 09 '17 at 02:03
  • @OrganicMarble Now that you mention it, the ISS doesn't have a deck, does it! If you can recommend a better graphic please do, or go ahead and just edit the answer and make some improvements. I only had aa few minutes, so I grabbed the first thing I could find that looked easy to understand. – uhoh Jan 09 '17 at 02:18
  • I actually like that picture, it's a lot more up to date than a lot of them you see that still have the Solar Power Platform pictured. Just odd that they labeled the purple axis that away. If you look in the ISS Reference Coordinate Systems manual (www.asi.org/adb/04/02/00/iss-coordinate-systems.pdf) you will find many references to zenith and nadir, but none to deck and overhead. ("Overhead" is in the document once, but it's referring to the cupola window of that name). – Organic Marble Jan 09 '17 at 02:27
  • @uhoh the majority of the answers involving equations are not so helpful, more like a chance for the author to show off. But they get a lot of upvotes because complicated pages of equations looks authorative. It's a trend I really dislike. – Innovine Jan 09 '17 at 06:53
  • 2
    @Innovine there are over 10,000 users here. How do you know which answers are helpful to which users? For me, equations are a second form of communication. Equations can be explainers ('oh ya, it's quadratic in x, now I get it") or even reminders or mnemonics to recall distant memories of what we once learned in school. Don't knock the (correct) use of equations, and don't worry if people get points you don't think they "deserve". It's a big world and there's room for (almost) everybody. – uhoh Jan 09 '17 at 06:59
  • @OrganicMarble Ya I've used nadir as recently as yesterday but that refers to the direction from a given point toward the center of the earth. It might happen to coincide with the spacecraft's -Z axis on average, but that's only because of attitude control. I'm pretty sure that (at least when a "sky" can be defined) nadir is usually just the opposite direction to zenith, and isn't related to your attitude (standing or laying down). – uhoh Jan 09 '17 at 07:10
  • @OrganicMarble I've asked about nadir here. – uhoh Jan 09 '17 at 07:58
3

One thing should be emphasized: unlike an airplane, the rotation of the ISS is intentionally set at one rotation per revolution, so its solar panels are facing the Sun most of the time. Both the rotation and the revolution are usually adjusted every time it has to dock with another spacecraft.

Branko Stojkovic
  • 31
  • 1
  • 1
    Note that the solar arrays and the core rotate independently (rotary joints at S3 and P3 modules), so the core is 1 rot per rev, but the solar panels can be 0 rot per rev for max power, or: sun slicer is 1 rot per rev for min drag, night glider uses the low drag orientation only when in Earth-shade, and a max drag orientation can be used to lower the orbit. – amI Jan 19 '20 at 03:08