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In what position is the lowest point of microgravity on the ISS?

Thought the answer was everywhere on the ISS but was told this was not right.

Nathan Tuggy
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Ken Beestin
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    Interesting question! Needs an answer of the form "what does the term microgravity even mean to begin with?" – uhoh Feb 25 '17 at 01:26
  • There are two ways of interpreting this: 1. Net acceleration at different points, which seems like microgravity but isn't caused by gravitational forces 2. Net gravitational forces from the structure itself, as the gravity of Earth is balanced by orbital motion - but note that there are many other gravitational forces that are relevant on that scale, since the force exerted by the ISS is so small. – kim holder Feb 25 '17 at 16:03
  • "Microgravity" is a standard term used in the industry and follows your first definition. Probably a bad name, but it wouldn't be the first. There is (was?) even a Microgravity Office in the Space Station Program at NASA/JSC back in the 90s. – Erik Feb 25 '17 at 16:12

1 Answers1

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Assuming you are mounted on the ISS in some relatively rigid way, the lowest microgravity lies along a "sweet line" (really a very shallow arc) of microgravity which passes through the center of mass in the direction of the ISS' velocity vector. Isobars of microgravity are ellipses in cross-section.

Keplerian effects refer to the acceleration of any point in a rigid body due to its distance from the center of mass of the body. Every point in an orbiting rigid body, taken as a point mass, wants to travel in a slightly different orbit. Structural rigidity prevents this from occuring, resulting in a contamination of the microgravity environment. Constant acceleration surfaces are elliptical tubes aligned along the body s velocity vector as shown in Figure 2.1.3. Figure 2.1.4 shows the gravity gradient structure, where the body's velocity vector is perpendicular to the page. These figures show that an orbiting body has a "sweet line" of microgravity which passes through the center of mass in the direction of the body's velocity vector.

Microgravity Envelope

From "Space Station Based Microacceleration Platform", which also provides some definition of microgravity.

Here is another view of the microgravity isobars depicted above superimposed on a model of the ISS:

enter image description here

enter image description here

From here. Of course, the position of these isobars change relative to the ISS as the ISS' attitude and center of mass changes.

Other effects are important as well -- crew-induced vibration, atmospheric drag (which creates 0.3 micro-gs alone), etc. This is why microgravity free flyers that depart from the ISS and periodically return have been proposed to support microgravity experiments. These free flyers could minimize some of these effects.

Erik
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  • This is not an answer to the question. It's a comment, suggesting the answer might be found in a linked document. A picture of the planet Earth with a cartoon-like unphysical representation of an orbit is not a guide to the distribution of microgravity within the ISS. This is actually an interesting question and needs a good answer. Check out Figure 2.1.4, Appendix 1, and think about where the precise center of mass of the ISS is really located - in a crewed area, or within the truss! – uhoh Feb 25 '17 at 04:07
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    How is the first sentence not an answer? – Erik Feb 25 '17 at 04:08
  • Where is the center of mass of the ISS? It's more of a definition than a proper stackexchange answer. It's just a sentence that is not incorrect. A good answer might for example have an arrow pointing to some place in here https://i.stack.imgur.com/2Ehyj.png (if indeed it is in there) with caveats about dockings and other things that can have small impact on the location of the center of mass within the ISS's structure. – uhoh Feb 25 '17 at 04:12
  • Still looking for the center of mass... I believe it's in here https://i.stack.imgur.com/2Ehyj.png but I'd like to find some authoritative verification. Then a plot like this https://i.stack.imgur.com/pgHlX.png can be added, with the proper placement of the origin at the ISS center of mass. – uhoh Feb 25 '17 at 04:25
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    The lowest point of microgravity isn't a point -- it is an arc. That arc changes depending on the attitude of the ISS. – Erik Feb 25 '17 at 04:26
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    @uhoh: your question is a purely academic (moot) though. Whole ISS is slowed due to atmospheric drag, this force completely overshadowing whatever differences are there from the difference in orbital attitude/inclination from placement within ISS; and that in order, is overshadowed by forced air motion from ISS air circulation systems. There is no practical purpose determining this lowest gravity arc as the effect is completely obscured by "noise" - accelerations/forces of other origin. – SF. Feb 25 '17 at 11:44
  • ...never mind it's not constant - it shifts as center of mass shifts, with docking craft, with supplies moved, service module driving along the rails, resources used up, astronauts moving between work stations and so on. – SF. Feb 25 '17 at 11:49
  • @SF. Let's wait until (if) I post an answer. Too much angst and misquoting going on here. But I'll definitely check the deceleration due to drag - never thought of that! – uhoh Feb 25 '17 at 12:53
  • Downvoted for two reasons. One, it is wrong, and two, it is not accessible. I'd downvote twice if I could. – David Hammen Feb 25 '17 at 15:02
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    Accessibility is a point of personal interest. Perhaps the smartest person I work with (and I work with a lot of very smart people) couldn't make heads or tails out of this answer because he cannot see. The bulk of the answer is in the form of an image, for which the alt text is "Microgravity Envelope". That "Microgravity Envelope" needs to be expanded to a hundred words or more ("an image is worth a thousand words") before I retract my downvote. – David Hammen Feb 25 '17 at 15:16
  • The issue seems to be whether to interpret microgravity in terms of net acceleration expressed in units of g, or in terms of strictly gravitational forces within the structure. Clarification by the asker would seem necessary. The text of the document in the top screenshot can actually be copied and pasted, that would be preferable, and not only for the accessibility issue David raises. – kim holder Feb 25 '17 at 15:48
  • Totally agree with you on accessibility DavidHammen -- as I require it as well. Not sure what you see as wrong -- perhaps you can elaborate. – Erik Feb 25 '17 at 16:04
  • "For reasons that are discussed later, there are many advantages to performing scientific experiments under conditions where the apparent weight of the experiment system is reduced. The name given to such a research environment is microgravity." From: https://www.nasa.gov/pdf/62474main_Microgravity_Teachers_Guide.pdf – Erik Feb 25 '17 at 16:31
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    You certainly have improved the quality of your answer, @Erik. While there are still some accessibility issues, I have retracted my downvote. – David Hammen Feb 25 '17 at 17:29