Why is microgravity experienced on the ISS despite the presence of gravity?

[fog37]

Hello Forum,

It is said that astronauts on the International Space Station (ISS) experience microgravity and not complete zero gravity. The acceleration of gravity at the ISS altitude is still pretty significant (~8.7 m/s^2), far from zero.

However the free fall motion of the ISS and its passengers produces the sensation of zero weight (weightlessness), i.e. the apparent weight becomes zero but the actual weight is not zero. The sensation of our weight disappears.

Why is it called microgravity? Where does the tiny gravity effect come from? Shouldn't the free fall motion provide that complete zero gravity sensation and feeling?

thanks
fog37

 

[fresh_42]

I assume that ≈350 km height isn't enough to be in a 'good' vacuum. ISS loses height over time, it has to be lifted from time to time.
Therefore it's not a perfect free fall. (Maybe also the moon's gravity comes into play.)

 

[fog37]

I just read that because the ISS is big, as big as a football field, there is a variation in gravitational acceleration much as 14 micro-g between the bottom and the top of the ISS, i.e. there is a gravity gradient.

I am not clear why this gravity gradient would cause microgravity. Aren't both the top and the bottom of the ISS free falling? at the same time? What does this micro gradient cause on an object that is inside the ISS?

Also, I would say that the microgravity could be caused by the mutual attraction between the various objects inside the space stations (astronaut to astronaut, space station to astronaut, etc.)

thanks

 

[fog37]

Thanks fresh_42. I guess you mean that because it slows down it start getting out of orbit (constant distance from the Earth surface) so it needs to be lifted back at the right altitude and speed and that involves forces which break that zero gravity/free fall balance...

 

[russ_watters]

Aren't both the top and the bottom of the ISS free falling? at the same time?

But not at the same rate nor at the same gravitational acceleration. The top is further from Earth than the bottom, so it is being accelerated less by gravity. And it is moving faster, so it is being accelerated more by centripetal acceleration.

Just a little.

What does this micro gradient cause on an object that is inside the ISS?

I'll give you a hint: it's in the title of the thread and it also contains the word "micro"...

 

[fog37]

thanks russ_watters.

The bottom part of the ISS feels a downward acceleration of gravity g_bottom and the top part an acceleration g_top.

g_bottom > g_top

The ISS moves downward as a rigid body. The role of the centripetal force is played by the force of gravity mg itself. The bottom part seems to require a larger centripetal force than the top part of the ISS because it need to follow a more curved path than the top part. However, the top part travels at a faster tangential speed so it would seem to require a larger centripetal force...I am a little confused on which part (top or bottom) experiences the largest centripetal force...

And how would this create a microgravity environment on the objects inside the ISS?

 

[fog37]

In the meantime, I found this

http://www.spaceflight.esa.int/impress/text/education/Microgravity/Question_Microgravity_008.html

I will study it and report back

 

[russ_watters]

And how would this create a microgravity environment on the objects inside the ISS?

The ISS is rigid, so it is under tension due to these forces. Two people floating on opposite sides of the station are not rigidly connected: so they drift apart.

 

[fog37]

Thanks. I didn't know that (i.e. two people could drift apart).

So if an experiment is carried out inside the ISS, it should be carried as close as possible to the center of mass of the ISS where the microgravity is the least. Otherwise, the components of the experiment will feel these gravitational type of forces.

The point about experiments in microgravity is to eliminate all possible forces acting on the experiment (force of gravity included, as much as possible)

 

[ZapperZ]

Thanks. I didn't know that (i.e. two people could drift apart).

So if an experiment is carried out inside the ISS, it should be carried as close as possible to the center of mass of the ISS where the microgravity is the least. Otherwise, the components of the experiment will feel these gravitational type of forces.

The point about experiments in microgravity is to eliminate all possible forces acting on the experiment (force of gravity included, as much as possible)

But if everything is "falling" at the same acceleration, there is no difference between that, and being in zero g.

Zz.

 

[fog37]

Thanks ZapperZ.

I wasn't clear. I guess I would say we can simulate the absence of the force of gravity by using free fall, i.e. making everything fall with the same acceleration (~8.7 m/s^2)

What interested me in this topic was the fact that it is not a perfectly zero gravity environment and wanted to know the causes of that...

 

[ZapperZ]

Thanks ZapperZ.

I wasn't clear. I guess I would say we can simulate the absence of the force of gravity by using free fall, i.e. making everything fall with the same acceleration (~8.7 m/s^2)

What interested me in this topic was the fact that it is not a perfectly zero gravity environment and wanted to know the causes of that...

But I still don't understand your problem.

You DO know that in a uniform circular motion, the object making that circular motion is in a constant "free fall" towards the center, don't you?

If ISS is in "zero g", then it will NOT move in a circular orbit! The fact that it is and not flying off to some random direction means that it is still tethered to the Earth's gravitational field. Yet, I claim that this is the same as being "weightless", and that this weightlessness is no different than being in "zero g".

Zz.

 

[fog37]

I agree with what you are saying:

weightlessness is a little of a misnomer since the force of gravity is there providing the centripetal force for the ISS to move into its orbit. What there is absence of are the effect of gravity: regardless of the presence of gravity we are able to simulate an environment where the effects of gravity are not present (only in small part)

 

[ZapperZ]

I agree with what you are saying:

weightlessness is a little of a misnomer since the force of gravity is there providing the centripetal force for the ISS to move into its orbit. What there is absence of are the effect of gravity: regardless of the presence of gravity we are able to simulate an environment where the effects of gravity are not present (only in small part)

Well, I disagree with what you're saying. "Weightlessness" is the more accurate term than "zero g", because g isn't zero in this case. Weightlessness refers to the fact that there is no "normal reaction force" that we teach students in intro physics when they have to draw a free-body diagram. So the object "sense no weight", and thus, weightlessness.

But if you do a proper treatment of this right out of intro physics, "zero g" environment and "weightlessness" is no different, the same way you can't tell if you're moving with constant velocity or stationary. So that is why I do not understand the problem here.

Zz.

 

[fog37]

Ok, let me try to be more clear:

1) The ISS experience a force of gravity downward equal to (M_iss)*(8.7 m/s). This force is far from being zero.
2) The force (M_iss)*(8.7 m/s) plays the role of the centripetal force
3) Weightlessness is the lack of the perception of weight, i.e. the lack of the sensation of this force (M_iss)*(8.7 m/s)
4) We produce weigthlessness, i.e. the sensation of weight and its effects too, by free falling around our planet
5) the point of making experiments on the iss is to produce conditions un which the objet being investigated does not feel the effects of gravity

 

[ZapperZ]

5) the point of making experiments on the iss is to produce conditions un which the objet being investigated does not feel the effects of gravity

But you haven't shown the difference between "zero g" and "weightlessness", and HOW, in terms of mechanics, that those two would be any different! If you are inside a closed box, can you construct an experiment to distinguish between the two, i.e. can you determine if you are really in "zero g" or just "free falling"?

Zz.

 

[A.T.]

3) Weightlessness is the lack of the perception of weight, i.e. the lack of the sensation of this force (M_iss)*(8.7 m/s)

Weightlessness is the lack of contact forces that support you against gravity. Gravity itself is acting approx. uniformly on your body, so it doesn't cause any "sensation".

 

[fog37]

Ok,

I guess I should say that the feeling of zero gravity is due to the absence of a contact support force (normal force) on our body. The contact may be there but not the force itself...

 

[ZapperZ]

Ok,

I guess I should say that the feeling of zero gravity is due to the absence of a contact support force (normal force) on our body. The contact may be there but not the force itself...

You haven't answered my question. Can you device an experiment to distinguish the two?

Zz.

 

[fog37]

Well, the typical experiment is an elevator in free fall. We are inside the elevator with a scale under our feet. The scale reads zero. The"apparent weight" is zero even if our actual weight is still mg...

But that apparent weight being zero has real effects: our body parts don't feel the same type of compression it would if the elevator was not in free fall.

any better?

 

[ZapperZ]

Well, the typical experiment is an elevator in free fall. We are inside the elevator with a scale under our feet. The scale reads zero. The"apparent weight" is zero even if our actual weight is still mg...

But that apparent weight being zero has real effects: our body parts don't feel the same type of compression it would if the elevator was not in free fall.

any better?

No, because if I put that elevator in space with zero g, you will NOT be able to detect any difference!

Zz.

 

[fog37]

Well, first of all we need to assume we are inside a gravitational field that exerts a downward attractive force.

As you mentioned, if everything is "falling" at the same acceleration, there is no difference between that, and being in zero g.

I am genuinely not seeing your question...

 

[ZapperZ]

Well, first of all we need to assume we are inside a gravitational field that exerts a downward attractive force.

As you mentioned, if everything is "falling" at the same acceleration, there is no difference between that, and being in zero g.

I am genuinely not seeing your question...

And I'm genuinely not see why you don't understand what I asked.

This is the foundation of Relativity, and General Relativity, that you can't tell the difference between "accelerating upwards" and being in a gravitational field with the same g, that there is no experiment that you can construct to distinguish between the two! So if you can't tell if you are accelerating, or in a g-field, how can you tell if you are "free falling" versus "in zero g"? No experiment that you can do will get you to distinguish that!

So therefore, why would any experiment that you do on ISS matter that you are not in "zero g", as long as everything is free falling?

Zz.

 

[russ_watters]

ZZ, my read of the OP's question is that he was confused about the difference between "zero" and "micro": that he didn't realize that the prefix "micro" is used because the acceleration of an object in the space station with respect to the space station actually isn't necessarily quite zero.

I think your post #11 misses that point and implies a disagreement/error where none exists: he was correct that the acceleration of an object with respect to the space station is lowest at the center of mass.

 

[fog37]

"...So therefore, why would any experiment that you do on ISS matter that you are not in "zero g", as long as everything is free falling?..."

Well, let me think

--Everything inside the ISS (ISS included) is falling downward
-- There is no possible experiment that can tell us if we are in free fall or in a zero g environment
-- Isn't an experiment conducted on ISS in "microgravity" purposely conducted in that setting so the effect of that gravity would cause are not included?
-- From my reading, tidal forces, small attractive forces between other objects having mass inside the ISS, and other effects cause the environment to not be completely gravity free.

Did you have a chance to click on that link I found last night?

http://www.spaceflight.esa.int/impress/text/education/Microgravity/Question_Microgravity_008.html

that is what sparked my very initial question...

 

[ZapperZ]

ZZ, my read of the OP's question is that he was confused about the difference between "zero" and "micro": that he didn't realize that the prefix "micro" is used because the acceleration of an object in the space station with respect to the space station actually isn't necessarily quite zero.

I think your post #11 misses that point and implies a disagreement/error where none exists: he was correct that the acceleration of an object with respect to the space station is lowest at the center of mass.

Sorry, but I disagree. I think you overestimated what the OP understood. Based on Post #21, it appears that the OP does not fully understand the simplest idea to start with. And that was what I was trying to established FIRST.

Zz.

 

[fog37]

I would like to review, for my own sake, some basic ideas:

The ISS is about 100 meter wide and it is at an altitude of about 400 meter. The ISS is in continuous free fall towards Earth but manages to remain at a fixed distance away from the Earth surface. This is possible because the ISS has a specific speed (4.76 miles/s). If the ISS was at a higher altitude the required speed would be lower.
The force that allows the ISS to remain in its orbit is the attractive gravitational force F=G (M_earth)* (M_ISS)/(R_earth+400)^2. This force is the centripetal force (M_ISS)*v^2/(R_earth+400) where v turns out to be the 4.76 miles/s

this discussion assumes that the attractive gravitational force F=G (M_earth)* (M_ISS)/(R_earth+400)^2 is applied to the center of gravity of the ISS. Those parts of the ISS that are either higher or lower than the center of gravity of the ISS experience a different attractive gravitational force because g is slightly different at those locations. The ISS is a rigid body so everything moves together. So the parts that are lower and higher than the center of mass feel some nonzero contact force.
For instance, the higher part requires a centripetal force that is larger than what the single gravitational force mg can offer. So a contact force contributes to provide the needed centripetal force.

Astronauts or other objects that are not linked to the ISS will possibly drift around inside the ISS due to the fact that their individual gravitational force may not match the required centripetal force. Everything and everybody is still in free fall toward planet earth. But an astronaut above the center of mass of the ISS will drift toward the top part of the ISS while an astronaut below the ISS center of mass will drift toward the lower part of the ISS...

right or wrong? :)

 

[jbriggs444]

I read #21 as indicating that one can distinguish between an elevator going down the shaft in free fall and an elevator stopped on the second floor at Macy's. (quickly stand on a scale and look at the reading). It's not wrong. It's just not responsive to a question about how to distinguish between "weightless" and "zero g".

 

[russ_watters]

Sorry, but I disagree. I think you overestimated what the OP understood. Based on Post #21, it appears that the OP does not fully understand the simplest idea to start with. And that was what I was trying to established FIRST.

I think you misread post 11 - read something that isn't there - and have continued your error with post #21.

Your response to post 11 implies you think post 11 is saying that a person in the ISS may accelerate at 9 m/s^2 with respect to the ISS. That isn't what he's saying/what the discussion is about.

 

[russ_watters]

I read #21 as indicating that one can distinguish between an elevator going down the shaft in free fall and an elevator stopped on the second floor at Macy's. (quickly stand on a scale and look at the reading). It's not wrong. It's just not responsive to a question about how to distinguish between "weightless" and "zero g".

Agreed. So my point was that for the OP's question, there is no need to make that distinction, nor is there an incorrect understanding implied. I'm quite certain he knows that the elevator and person are both accelerating toward the Earth at g.
Edit: He even provided the value for g at the ISS in the OP.