# The Naked Scientists Forum

### Author Topic: How can you step outside the ISS at 18000 mph?  (Read 1907 times)

#### Harri

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##### How can you step outside the ISS at 18000 mph?
« on: 16/01/2016 20:54:32 »
I have just watched 2 guys floating outside the ISS which as far as I know is traveling around 18000 mph? If I stepped outside of a train doing 100mph I am going to be in trouble! So how can you step outside of the ISS traveling at speeds in the 1000's?

Inside the train or ISS I am traveling at speed but standing still I understand that. An onlooker looking at me through the window will see me traveling at speed, I understand that.

But stepping outside the ISS at 18000mph!?

#### Bored chemist

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##### Re: How can you step outside the ISS at 18000 mph?
« Reply #1 on: 16/01/2016 21:41:48 »
There's nothing to hit: not even air.

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#### Harri

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##### Re: How can you step outside the ISS at 18000 mph?
« Reply #2 on: 16/01/2016 21:49:34 »
But ... the ISS is being propelled along at 18000mph. You step outside, what is propelling you along?

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#### Space Flow

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##### Re: How can you step outside the ISS at 18000 mph?
« Reply #3 on: 17/01/2016 04:17:50 »
But ... the ISS is being propelled along at 18000mph. You step outside, what is propelling you along?
The ISS is not being propelled along at all unless it is in the middle of making an orbital adjustment, which is the only times it is under any acceleration. The rest of the time there is no force acting on it. It is what is called Geodesic. It will continue being Geodesic and as such in free fall and so will you whether you are floating on the inside of it's walls or the outside. The station and you are at rest relative to each other. You are both following a Geodesic that has the same angular momentum.
It would take an input of energy to change the parameters of that Geodesic for either the ISS or you or both.

Stepping out of a train at 100 mph, you immediately encounter an atmosphere that is not moving with you or the train. Your 100 miles an hour even without that atmosphere, is not even a very small fraction of the speed you would have to have to maintain your height above the ground, so your Geodesic intersects the ground not far from your stepping out point. The Ground from your and the trains perspective is traveling at 100 mph towards the back of the train. The combination of all those things leeds me to suggest that you should not step our of this train, and although outside the ISS is not the safest situation to be in, it is many orders of magnitude safer than stepping off a moving train.

Hope that helps.

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#### evan_au

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##### Re: How can you step outside the ISS at 18000 mph?
« Reply #4 on: 17/01/2016 09:49:24 »
Quote
How can you step outside the ISS at 18000 mph?
Because you and the ISS are both traveling in the same direction at the same 18000 mph, if you float inside, you will be stationary relative to the ISS.

For the same reason, if you float outside, you will be stationary relative to the ISS.
Quote from: Bored Chemist
There's nothing to hit: not even air.
I would rather pedantically say there's hardly any air outside the ISS.

While 100km altitude is officially the start of "Space", there's still enough air at their normal altitude of 330-430km that the ISS orbit slowly decays, and they need to boost the altitude every month or so. During the 45-minute "night" of each orbit, they orient the solar panels so as to minimize air resistance; this reduces use of expensive rocket fuel.

For the astronaut, the main resistance comes from the fact that he is inside an inflated balloon, which tends to adopt the maximum volume. Every movement takes it away from this maximum volume, and that takes real effort; even wiggling your fingers takes effort.

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#### Harri

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##### Re: How can you step outside the ISS at 18000 mph?
« Reply #5 on: 17/01/2016 10:51:37 »
Well ... those replies do help a lot.

I'm not a scientist but when I'm told a guy is going to step out of a machine orbiting the Earth at around 18000 mph I have to question what on earth, or what 'above' earth, is going on here? Most people I guess just watch and take it at face value.

If this has to do with the theory of relativity then I've always struggled trying to understand what the heck it all means! I've read simplifications and I've watched youtube vid's and still I don't seem to grasp it! Your answers regarding my question does go some way to making things a little clearer though, so thanks.

#### Colin2B

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##### Re: How can you step outside the ISS at 18000 mph?
« Reply #6 on: 17/01/2016 11:03:47 »

If this has to do with the theory of relativity then I've always struggled trying to understand what the heck it all means!
If you mean Einstein's relativity, then no it has nothing to do with that.
But it does have to do with a slightly different relativity as explained by the others.
The train is travelling at 100mph but relative to it, that is from the point of view of the train, the air is rushing past at 100mph, so is the ground. If you step out the air resistance will slow you down very quickly, so will the ground.
As Evan pointed out at the altitude of the space station there is little air resistance and the ground is a long way away. However, if the space walker were outside and the spacecraft made a course or altitude correction then the walker would be in trouble because they would continue on their path and the spacecraft would move away.

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#### Harri

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##### Re: How can you step outside the ISS at 18000 mph?
« Reply #7 on: 17/01/2016 22:13:38 »
oh boy ...
Quote
Because you and the ISS are both traveling in the same direction at the same 18000 mph, if you float inside, you will be stationary relative to the ISS.

For the same reason, if you float outside, you will be stationary relative to the ISS.

I understand outside the ISS I will be stationary relative to it. Can I just clarify that, floating outside the ISS, I AM also traveling at 18000mph as is the ISS?

#### Space Flow

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##### Re: How can you step outside the ISS at 18000 mph?
« Reply #8 on: 18/01/2016 00:11:44 »
I understand outside the ISS I will be stationary relative to it. Can I just clarify that, floating outside the ISS, I AM also traveling at 18000mph as is the ISS?
Exactly.
While outside and if not attached you and the ISS can be considered as two separate satellites having the exact same orbital characteristics.
You do have to be careful of space junk with different relative velocities to yourself though.
I believe there's a lot out there and the ISS regularly gets it's outer skin punctured. The forward facing part of the ISS has been designed to absorb most of those punctures without affecting the inside.
A spacesuit offers no such protection.

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#### The Naked Scientists Forum

##### Re: How can you step outside the ISS at 18000 mph?
« Reply #8 on: 18/01/2016 00:11:44 »