I understand that when someone is in low earth orbit, the "pull" of their inertia is equal to the pull of gravity. However, the force of gravity is still acting on them. However, if they are in outer space there the pull in any direction is miniscule, they should feel different that the way they feel in orbit, because you feel forces on your body and in outer space there is no net force while in orbit there is. Am I correct?

  • $\begingroup$ As far as physiology is concerned, the only reasonable experiment-backed answer is "We don't know", since no human has ever been farther than Moon's orbit. $\endgroup$ – Deer Hunter May 29 '13 at 19:27
  • $\begingroup$ However, the difference between micro-gravity and outer space zero-gravity seems to be much less significant than the difference between normal gravity and micro-gravity. Expect nausea, cramps, bloated face, muscular deconditioning, bone trouble. Awkward showers and WCs. $\endgroup$ – Deer Hunter May 29 '13 at 19:31
  • $\begingroup$ A 60kg person would just feel 2N of force at 100,000km away from the earth, and the moon is about 300,000km away $\endgroup$ – Ovi May 29 '13 at 19:37

They should feel the same. You only feel forces in orbit if there is something causing sensations, and nothing does in either case. Even on earth, you don't feel the "force" of gravity; you feel the force of the floor pushing you up so that you don't start falling under gravity's influence. In orbit, there is no floor, so you don't feel gravity.

You might imagine a force pushing on your spacesuit which then pushes against you. But in orbit, the force on your spacesuit causes an acceleration of the spacesuit that's exactly the acceleration that your body experiences, so there will be no relative difference in the motion of your body and your spacesuit, so you won't feel anything.

Or you might imagine your left arm feeling a different force than your right, and so feeling pulled apart. But the accelerations they experience in orbit will be the same, so their relative positions won't change, and you won't feel any difference.

(Technically, there might be minuscule differences referred to as tidal forces. These should be measurable by extremely sensitive instruments, but not by humans orbiting anything humans are likely to orbit any time soon.)

  • $\begingroup$ I understand that you and your spacesuit have the same relative motion, maybe on earth you just got used to the force of gravity pulling on you since you were a baby, so when you go into microgravity (not orbit) you would feel a difference? $\endgroup$ – Ovi May 29 '13 at 19:36
  • $\begingroup$ Sure, you would feel a difference between earth and microgravity. Normally, your liver lies on top of your stomach a little, so your stomach is constantly pushing up on the liver. In microgravity, the push would stop, so you'd feel funny. Things like this are presumably what cause that funny sensation in your stomach on a roller coaster ride. But the difference between orbit and deep space would not be noticeable. $\endgroup$ – Mike May 29 '13 at 19:42
  • $\begingroup$ well why? I understand that in orbit your organs have the same relative motion to the other parts of your body, but there is still a net force on them. If there is a net force on your stomach (for example someone punching you) you would feel it, so you feel net forces. $\endgroup$ – Ovi May 29 '13 at 19:52
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    $\begingroup$ No, that's the point. You don't feel net forces, you feel relative accelerations. If a pin just above my finger is accelerating toward my finger, but my finger is accelerating just as fast, I don't feel anything. There's no relative acceleration, and no sensation. $\endgroup$ – Mike May 29 '13 at 20:09
  • $\begingroup$ In the case of someone punching you, your stomach is accelerating relative to the rest of your organs. What you feel, is the difference in acceleration of the front of your stomach relative to the back of your stomach, or relative to your liver, or whatever. There is a net force, but it is causing a relative acceleration. $\endgroup$ – Mike May 29 '13 at 20:15

The sensation with respect to gravity will be almost identical, to be precise: undetectable by your body. While freely falling (in an earth orbit or towards some other body in deep space) only the gravitational tidal forces could be sensed and these vanish quickly (even faster than the square law) with distance from the planet or other body.

Can you feel the different pull of the earth on your feet versus your head? No. In earth orbit, it's even less tidal force. In deep space even lesser. So for all intents and purposes, you wouldn't be able to tell the difference.

BTW, the Apollo astronauts, on their way to and from the moon, pass a point where the gravitation of earth and moon cancel. They jokingly said that they could feel a bump there to make fun of ignorant news reporters, but in fact that moment is unnoticeable, even though the vehicle changes from a decelerated motion to an accelerated one towards earth.


Remarkably, it is the case that as long as one is freely falling in a uniform gravitational field, one cannot "feel" the difference between an arbitrarily strong field and an arbitrarily weak one; one "feels" weightless in any case.

However, in reality, there are tidal forces (no uniform gravitational fields) which, for extended bodies, tend to stretch one in the radial direction and compress one in the others.

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So, for example, if one were in orbit around a compact object such that the tidal forces were significant, one could certainly "feel" the difference between orbital free fall and outer space.

For low-Earth orbit, the tidal forces are there but not significant enough for one to "feel" them.

  • $\begingroup$ Why was this downvoted? As a layman, I'd like to know what the downvoter sees wrong with it. $\endgroup$ – mowwwalker May 30 '13 at 2:08

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