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NASA and others seem careful to talk about microgravity instead of zero gravity. Why is for example the ISS not in zero gravity? Is it because of movements onboard? Because of variations in center of mass caused by the inhomogeneity of Earth and the direction to the Moon? Because of the eccentricity of the orbit? Or is microgravity a more fundamental intrinsic property of orbiting?

I've seen the term "picogravity". Are there some established limits for micro, pico and zero gravity? Are there terms for the surface gravity of the large moons (say 0.10 to 0.17 g) and for larger asteroids (such as 0.001 g to 0.028g)?

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The reason the Space Station is called a micro-g environment rather than a zero g environment is because the Space Station is rotating, because it's in low Earth orbit, and because it's big (for a spacecraft).

The Space Station nominally rotates at the orbital rate so as to keep the nadir-pointing windows pointing downward. This alone means an accelerometer attached to the Space Station at a distance of 7.7 meters from the ISS center of mass will register an acceleration of one micro-g. Every additional 7.7 meters from the ISS center of mass adds an additional micro-g to this sensed acceleration.

On top of that, the Space Station has a rather high drag coefficient compared to other spacecraft thanks to those huge solar arrays. That drag becomes measurable (i.e., micro-g level), particularly during periods of high solar activity (e.g., now, and presumably 11 years from now) and/or when the Space Station altitude is low.

One last factor is gravity gradient. The ISS is big. The variation in gravitational acceleration over a span of 100 meters is as much as 14 micro-g at ISS altitude.

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Great answer! Are rotation and drag the most important casuses of gravity on the ISS? I listed some other proposed causes in my question: inhomogenity of Earth mass, the position of the Moon, eccentricity of ISS' orbit, movements of the ISS. –  LocalFluff Mar 6 at 13:52
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I missed one factor, gravity gradient, which I'm adding to my answer. The inhomogenity of Earth mass, the position of the Moon, eccentricity of ISS' orbit: They don't contribute, or not much. You can't feel gravity. The movement of the ISS: I assume you're asking about when the ISS raises its altitude. The accelerations during those ISS reboosts are well beyond the micro-g range. Sensitive experiments have to be shut down while the ISS is undergoing a reboost. Those reboosts are planned and of relatively short duration. –  David Hammen Mar 6 at 14:28
    
The solar panels are so big and cause so much drag, that when the station's orbit takes it to the dark side of Earth, the panels are rotated to be in-line with the station in order to reduce the drag. –  Nickolai Mar 6 at 20:27
    
For an account of extreme gravity gradient (i.e. tides) see Larry Niven's short story Neutron Star –  Jim Garrison Mar 7 at 7:46

Gravity is the force of attraction between objects. When you are in orbit, there is still that force of attraction (gravity) pulling you and the earth together. The reason you don't crash into the earth is not because you have "zero gravity", but because you are moving. In fact, you are falling. Gravity is pulling you straight down towards the center of the earth. The thing is, you're moving so fast that you are always missing the earth. If it weren't for the earth's gravitational pull, you would continue off into space in a straight line. Gravity keeps you in orbit, thus you are not in "zero" gravity. And if it weren't for your forward motion, you'd fall directly towards the earth.

The effect of gravity is always there. Think of it this way - the moon feels the earth's gravity, right? That's why it orbits the earth. It, too, is just continually missing the earth because of its motion. And the earth feels the moon's gravity. That is why we have tides. The water on the earth, and in fact everything on earth, feels the pull of the moon's gravity. It's so small (microgravity) that you don't notice it on your body, but it's strong enough to cause the tides.

And similarly, when you are in orbit in the space station, everything in your body (your blood, skin, hair, etc.) are still pulled towards the earth - sort of a tidal force affecting your bodily fluids. Thus you are not in "zero" gravity - your blood is not truly floating wherever it wants to go - it is constantly being pulled towards the earth. So being oriented with your head nearer the earth than your toes is like standing on your head on the surface of the earth, it's just that the forces are much weaker. In true zero gravity - out in deep space so far away from any galaxies that gravitational pull is at a minimum (it's never truly zero), your blood would act differently than it does in low earth orbit where it feels the earth's gravitational pull.

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The first 2/3 of your answer aren't really relevant, and the last 1/3 of your answer is inaccurate. –  Sparr Mar 6 at 20:27

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