# The Naked Scientists Forum

### Author Topic: Is there gravity on the ISS?  (Read 848 times)

#### thedoc

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##### Is there gravity on the ISS?
« on: 25/01/2016 22:50:02 »
Peter Culleton asked the Naked Scientists:
Hi,
Georgia (I think it was) was discussing eating food on the International Space Station as being "in zero gravity". But isn't there lots of gravity on the ISS? Like 90% of what it is at sea-level on Terra Firma? At least that's what another 5 Live Science program suggested, as I remember.
What do you think?
« Last Edit: 25/01/2016 22:50:02 by _system »

#### Space Flow

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##### Re: Is there gravity on the ISS?
« Reply #1 on: 26/01/2016 05:03:21 »
But isn't there lots of gravity on the ISS? Like 90% of what it is at sea-level on Terra Firma?
Peter, the ISS is following its Geodesic. It is traveling at a constant speed in a straight line. That means it is in free fall. Because of the angular momentum that it has (27,600 km/h; 17,100 mph), by the time it has fallen a metre towards the Earth's centre, it has travelled sideways far enough so that the curvature of the Earth makes it still be at the same distance from that centre.

Now as far as the Earth's Gravity is concerned, if an object of any description wanted to be Geosynchronous at that height, you would need to be standing on a rocket sled that kept you under constant acceleration (Hover) and then you would feel 90% of 1g.
Just like you would feel 1g acceleration if you had the same rocket sled hovering 1 metre above the ground.

Note: The above answer about the ISS is very close to being right. Technically the ISS isn't 100% geodesic. It has a very slight  acceleration backwards through its interaction with what is left of the Earth's atmosphere.
Although extremely tiny, it does have to be re-boosted because of it periodically. That would mean that there is a slight down direction towards the front. It is extremely low gravity.

#### evan_au

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##### Re: Is there gravity on the ISS?
« Reply #2 on: 26/01/2016 06:04:52 »
The technical term for the ISS is "Microgravity", ie tiny compared to what you would feel if you were on Earth.

When we "feel gravity" on Earth, what we mean is:
• We feel the floor pushing up on our feet,
• the seat holding us up,
• the blood tending to pool in our feet,
• the coffee staying in our cup while we are drinking it,
• and the teacup smashing on the ground if we let it go.
• Otherwise we would go plunging towards the center of the Earth.

Contrast this with what astronauts feel in the ISS:
• The floor does not push up on our feet,
• the seat does not hold us (unless we tie ourselves in with a seatbealt),
• the blood tends to pool more in our head,
• the coffee splashes out of an open cup, even if we drink carefully,
• and the teacup floats next to us if we let it go.
• This is because everything we see inside the ISS - the floor, the seat, the coffee/tea and cup (and even our blood) is plunging around the Earth at the same speed, so we "feel no gravity" pushing us up (or down).
So the problem is with our senses: humans cannot actually feel gravity, but only physical forces pushing against our bodies. These forces are usually ones that oppose the acceleration due to gravity.

We are so used to these forces that we ignore them most of the time - and only really notice them when they are missing, such as on the ISS or a roller-coaster.

There is one specific case where humans would be able to feel gravity directly, and that is when falling towards a stellar-mass black hole; the gravitational attraction on your feet would exceed the gravitational attraction on your head, and it would (briefly) feel like you were being stretched on a piece of medieval torture equipment. In this case we would feel the internal forces of different parts of the human body pulling against each other.

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##### Re: Is there gravity on the ISS?
« Reply #2 on: 26/01/2016 06:04:52 »