Question about things moving through outer space

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Apparently, if I have this down correctly, even the vacuum of outer space has a density, and thus matter in it. With that, I have a few questions: I think I know what happens when something moves in a vacuum at high speeds, namely around and at light speed. Now, for much slower speeds, I must ask this: would an object, such as a rock thrown by a human (or astronaut's) hand moving at slow speeds like 3 meters per second or 15 meters per second eventually slow down with the amount of matter in the vacuum of outer space, even if the matter in space slows down the object by very, very small amounts? Also, would a very small object with a very small mass (and by extension, inertia) come to a stop sooner than heavier objects in a vacuum even with how little matter is in the vacuum, assuming the small object in question is moving at either of the two speeds mentioned above?
 
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  • #2
Drakkith
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Now, for much slower speeds, I must ask this: would an object, such as a rock thrown by a human (or astronaut's) hand moving at slow speeds like 3 meters per second or 15 meters per second eventually slow down with the amount of matter in the vacuum of outer space, even if the matter in space slows down the object by very, very small amounts?
The quick answer is yes, the object will eventually slow down through collisions with particles in the near-vacuum. The not so quick answer is that whether the object 'slows down' is greatly complicated by real world factors like orbits, radiation pressure, multi-body gravitational interactions, and more.

Also, would a very small object with a very small mass (and by extension, inertia) come to a stop sooner in a vacuum even with how little matter is in the vacuum, assuming the small object in question is moving at either of the two speeds mentioned above?
In an ideal scenario where we don't have to worry about the complicating factors I mentioned above, yes, the object would eventually come to a stop.

As an example of a complicating factor, consider that if we were to somehow suddenly cut the Earth's orbital velocity around the Sun by 90%, it would actually begin speeding up as it fell towards the Sun, eventually swinging around the Sun in a very close perihelion at an absurdly high speed. It then would begin to slow down, just like a ball thrown upwards does when thrown between two people, eventually reaching aphelion where it would again be at its 90% slowed down speed.
 
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  • #3
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The quick answer is yes, the object will eventually slow down through collisions with particles in the near-vacuum. The not so quick answer is that whether the object 'slows down' is greatly complicated by real world factors like orbits, radiation pressure, multi-body gravitational interactions, and more.



In an ideal scenario where we don't have to worry about the complicating factors I mentioned above, yes, the object would eventually come to a stop.

As an example of a complicating factor, consider that if we were to somehow suddenly cut the Earth's orbital velocity around the Sun by 90%, it would actually begin speeding up as it fell towards the Sun, eventually swinging around the Sun in a very close perihelion at an absurdly high speed. It then would begin to slow down, just like a ball thrown upwards does when thrown between two people, eventually reaching aphelion where it would again be at its 90% slowed down speed.
Yeah, I completely forgot about those. Thanks for the answer, though.

Still, for my second question, not regarding any of those other factors (unless you want to bring them in), would the smaller object with smaller mass stop sooner than an object that is any heavier than the smaller one in a near vacuum?
 
  • #4
Drakkith
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I think it depends more on density than anything else. The larger object might be twice as massive, yet have four times the surface area, leading to much more drag and a quicker stop.
 
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I think it depends more on density than anything else. The larger object might be twice as massive, yet have four times the surface area, leading to much more drag and a quicker stop.
Oh, there is that. Again, thank you for the answer.
 
  • #6
even the vacuum of outer space has a density
Excuse me, as you can guess, I'm new here and many things aren't clear to me. If we are speaking about the vacuum, that is absence of mass, density and matter; how can we speak about friction? If there isn't the mass, there isn't the force too, and we have no element that can stop the run of our object. Moreover, absence of matter brings us to the absence of amount of matter, and we don't have any matter that can deny our object to go. At least this is my thought.
 
  • #7
Ibix
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If we are speaking about the vacuum, that is absence of mass, density and matter; how can we speak about friction?
Space isn't quite a vacuum. There's a hydrogen ion per cubic meter or so even out in deep space.
 
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  • #8
russ_watters
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Space isn't quite a vacuum. There's a hydrogen ion per cubic meter or so even out in deep space.
Can we translate that into drag? Say, for a ship traveling at a third the speed of light? Minus the relativity corrections...?

Let's say the ship has a cross sectional area of 10 cubic meters. That means it encounters 109 hydrogen atoms per second. That's 1-15 grams. Or a force (momentum flux) of 1-10 Newtons (if we absorb all the momentum).

Someone check my math, because that's a lot of exponents and chemistry conversions I barely remember.
 
  • #9
DrClaude
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Let's say the ship has a cross sectional area of 10 cubic meters.
You have a problem with units here :smile:
 
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  • #10
jbriggs444
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Can we translate that into drag? Say, for a ship traveling at a third the speed of light? Minus the relativity corrections...?

Let's say the ship has a cross sectional area of 10 cubic meters. That means it encounters 109 hydrogen atoms per second. That's 1-15 grams. Or a force (momentum flux) of 1-10 Newtons (if we absorb all the momentum).

Someone check my math, because that's a lot of exponents and chemistry conversions I barely remember.
One should be able to extend this calculation to estimate the time it would take for a particular object travelling in the interstellar medium to reach 1/2 of its initial speed. We could call that a half-fast metric. :-)
 
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  • #11
Space isn't quite a vacuum. There's a hydrogen ion per cubic meter or so even out in deep space.
Alright now I understand. So, correct me if I wrong, there is a small mass so a small friction and the bodies in moviment are destined to stop their run in the universe. Thus I think: if all the objects in the universe are destined to stop their run, all the universe in a very far future, is destined to stop. Obviously I mean all the objects that made the whole universe...
 
  • #12
Ibix
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Thus I think: if all the objects in the universe are destined to stop their run, all the universe in a very far future, is destined to stop.
Well, that's a lot more complex issue. As noted upthread there's an awful lot more going on than just friction. And on the truly large scale, local definitions of "stopped" with respect to nearby matter still allow distant objects to be receding. It's also worth noting that in order to stop even a small mass it has to collide with a lot of molecules, which are now moving around with higher velocity.

Normally you just say friction slows something down and causes something to heat up, and then the heat radiates away somewhere. When you are talking about the whole universe, though, where could it radiate to? It's not completely clear to me what the final state would be. I think that's probably far beyond the scope of the original question, however.
 
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  • #13
phinds
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Alright now I understand. So, correct me if I wrong, there is a small mass so a small friction and the bodies in moviment are destined to stop their run in the universe. Thus I think: if all the objects in the universe are destined to stop their run, all the universe in a very far future, is destined to stop. Obviously I mean all the objects that made the whole universe...
Keep in mind that, right or wrong, you are talking about proper motion. This has nothing to do with recession so galactic clusters and other larger objects will continue to recede from each other.
 
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  • #14
sophiecentaur
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Also, would a very small object with a very small mass (and by extension, inertia) come to a stop
It depends on what you mean by "stop". That has to mean 'acquire the same mean velocity of the gas it passes through' because Momentum will be conserved. Every particle it encounters will acquire some of the object's momentum and that particle will share it with other particles in its path. There will be a swathe of gas, moving in the direction of the recently passing rock, which will get wider and wider and (relatively) slower and slower. There could even be vortices on a huge scale where multiple collisions have occurred.

How, actually, to specify the velocity of the gas particles? it's all relative. They will have a mean velocity of zero, relative to the other 'nearby' particles but, relative to the nearest large object (say a galaxy, one million light years away) it could be enormous. Also, whatever the temperature of that region of space, the particles will be jostling amongst themselves at low velocities.
bodies in moviment are destined to stop their run in the universe.
They could also run into a region of higher density (like a galaxy). If you look in virtually any direction, you will see a galaxy and the rock would only need to fly close enough to the galaxy to encounter a more dense region of gas and that could cause it to slow down and be 'captured'. This could take billions of years, though.
 
  • #15
phinds
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They could also run into a region of higher density (like a galaxy). If you look in virtually any direction, you will see a galaxy and the rock would only need to fly close enough to the galaxy to encounter a more dense region of gas and that could cause it to slow down and be 'captured'. This could take billions of years, though.
That's a good point and @Sundown444 be aware that if that happens, there could be two results. First, it could impact a galactic object, in which case it would become part of that object (let's skip "glancing blow" type encounters) in which case it would take on the velocity of that object after the impact. Second, it could go into orbit around a galactic body in which case its motion through space would not stop and in fact would become complicated instead of linear.
 
  • #16
sophiecentaur
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Second, it could go into orbit around a galactic body
That has to involve two other significant bodies for actual capture. There was an extra galactic comet spotted recently, iirc. (I do love the way that observations can actually tell us that; obvious once you understand the system but impressive nonetheless).
 
  • #17
phinds
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That has to involve two other significant bodies for actual capture. There was an extra galactic comet spotted recently, iirc. (I do love the way that observations can actually tell us that; obvious once you understand the system but impressive nonetheless).
That's interesting. I recall nothing of orbital mechanics. Why two bodies?
 
  • #18
sophiecentaur
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That's interesting. I recall nothing of orbital mechanics. Why two bodies?
If one body goes past a star / planet, assuming there's no actual contact, you have an 'orbit'. No net energy is lost so the small object will carry on out without being captured. (It's already got enough energy to escape back out)

If a (big enough) second star /planet is there, in the right place and with the right velocity then it can temporarily take some momentum from the incoming object by deflecting it and 'pass it on' to the main star / planet in an orbit that ends up as closed. The relative velocities of the object and first star / planet will be lower so the orbital energy is less - a non-escape situation. All three have their orbits altered but the small object is the only one affected significantly.

Before the incoming object could be influenced by the 'extra' star / planet again, the motion of the object and its star / planet will carry it away from th extra body. This is the reverse of the Slingshot technique. The only way that the incoming object will be thrown back out again is if there happens to be a resonance between two planets and if the extra planet arrives back at a suitable place relative to the main planet's orbit. So the orbital energy has been transferred (almost) permanently away from the incoming object.

That's all a bit long winded but I think I have described it as I see it.
 
  • #19
Ibix
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That's interesting. I recall nothing of orbital mechanics. Why two bodies?
It's the reverse of a slingshot manoeuvre. Instead of a probe passing Jupiter and stealing some of its orbital angular momentum (note orbital, Jupiter needs to be orbiting something), it passes by and gives Jupiter some orbital angular momentum, letting itself be captured.
 
  • #20
phinds
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It's the reverse of a slingshot manoeuvre. Instead of a probe passing Jupiter and stealing some of its orbital angular momentum (note orbital, Jupiter needs to be orbiting something), it passes by and gives Jupiter some orbital angular momentum, letting itself be captured.
Well, that's what I imagined except I'm not sure I'm understanding or sophiecentaur. I imagined an the object just coming close enough to a large body to be captured. Granted this would require a very precise angle but is it not possible?
 
  • #21
DaveC426913
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Well, that's what I imagined except I'm not sure I'm understanding or sophiecentaur. I imagined an the object just coming close enough to a large body to be captured. Granted this would require a very precise angle but is it not possible?
No.
Consider the time reversed scenario:

The object is in a stable orbit, and spontaneously (without assistance) leaves that orbit to go hurtling into empty space.
 
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  • #22
Ibix
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I imagined an the object just coming close enough to a large body to be captured.
Unless it crashes, no, as Dave's time-reverse argument shows.
 
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  • #23
DaveC426913
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Unless it crashes,
Also known as "litho-braking". :oldbiggrin:
 
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  • #24
Ibix
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Also known as "litho-braking". :oldbiggrin:
That's up there with "high impedance air gap prevented operation" (I forgot to plug it in).
 
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  • #25
phinds
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OK, I can see that the time reversal of a stable orbit, absent other forces, is not going to result in its opposite so that argument makes sense. Thanks.
 

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