# The Naked Scientists Forum

### Author Topic: How do astronauts move around in the vacuum of space??  (Read 4500 times)

#### thedoc

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##### How do astronauts move around in the vacuum of space??
« on: 02/08/2013 12:30:03 »
George Griffiths  asked the Naked Scientists:

Hey!

I just can't seem to get my head around it. Space is a vacuum right?

So how do astronauts move through it? How can they move through nothing? In my head, it seems if there is nothing in space then wouldn't the universe all collapse into one giant heap? Maybe i'm just not too sure what a vacuum actually is!

Also, is it true that a planets atmosphere doesn't have an end, rather it just fades out, getting thinner and thinner and thinner but with no real end?

George.

What do you think?
« Last Edit: 02/08/2013 12:30:03 by _system »

#### yor_on

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##### Re: How do astronauts move around in the vacuum of space??
« Reply #1 on: 04/08/2013 18:01:47 »
The nothing that is a vacuum is a geometrical property, containing no rest mass, but it should contain 'gravity', being the metric defining it. It seems that a very small part of the universe can be defined as 'rest mass', with a atom containing 99.99% vacuum, if I remember it right. But you can exchange accelerations (and relative motion) for relativistic mass, and you can also exchange it into 'energy', creating a equivalence to mass, as rest mass. So 'energy' can become 'mass'. Without space in between all mass would become clumped together, and you can either think of that as a universe in where we still have distances existing, even without vacuum defining that property, or as something not making sense at all. What both a vacuum and proper mass has in common is its 'metric', 'gravity'. Assuming this to define geometric distances, also assuming that those distances (gravity) gets their definition from proper mass (energy) you then find gravity to blame for the universe we measure on. But that's my view.

As for not being certain what a vacuum is, I totally am of a same mind :) It's always seem to come down to some sort of symmetry in our universe, just think of how any force has a equal force acting in the opposite direction, which also explains how you can get a motion in space just by moving (displacing) your limbs relative some defined coordinate system. F = ma, where F is the net force acting on the object, m is the mass of the object and a is the acceleration of the object. Even though correct, meaning that we find it to be so testing, it doesn't explain 'why' it must be so, but it is a symmetry to me.

#### evan_au

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##### Re: How do astronauts move around in the vacuum of space??
« Reply #2 on: 05/08/2013 09:37:13 »
Newton's law of motion state that an object will continue to move in a straight line unless acted upon by a force.

This is most clearly seen in space, where an astronaut pushing off one object will continue moving until he/she runs into another object, or the gravitational attraction of whatever object they are orbiting brings them back to their starting point.

In our solar system, the gravitational field of the Sun keeps most things in a periodic orbit. The fact that gravity has an inverse square law means that kinetic and potential energy is conserved, and so the Solar System remains at its current size without collapsing.

The atmosphere of planets does get thinner until it merges with the solar wind. The Sun's atmosphere gets thinner and slower until it merges with the intersteller medium, out where the Voyager spacecraft are now taking measurements. Presumably the galactic gas thins out until it merges with the intergalactic medium.

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##### Re: How do astronauts move around in the vacuum of space??
« Reply #2 on: 05/08/2013 09:37:13 »