# Lowest ISS microgravity

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.

• Interesting question! Needs an answer of the form "what does the term microgravity even mean to begin with?"
– uhoh
Commented Feb 25, 2017 at 1:26
• 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. Commented Feb 25, 2017 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
Commented Feb 25, 2017 at 16:12

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.

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:

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.

• How is the first sentence not an answer?
– Erik
Commented Feb 25, 2017 at 4:08
• The lowest point of microgravity isn't a point -- it is an arc. That arc changes depending on the attitude of the ISS.
– Erik
Commented Feb 25, 2017 at 4:26
• @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.
Commented Feb 25, 2017 at 11:44
• 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. Commented Feb 25, 2017 at 15:16
• You certainly have improved the quality of your answer, @Erik. While there are still some accessibility issues, I have retracted my downvote. Commented Feb 25, 2017 at 17:29