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elanachan

Is the International Space Station really in outer space?

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I jsut want to butt in here and ask a quick question. If i were to orbit the moon (our lunar moon not that crazy mun in the game) at x vlocity that on paper with no other bodies would give me a perfect circular orbit of y kilometers. If we then plop earth nearby would that change our perfect circle to a slightly eliptic orbit? and be destined to have a permanent eliptic orbit as long as the earths gravity is taken into account.

Yes, that is in fact the case. Just as the gravity of the Moon pulls Earth into a slightly ellipsoid shape (to be precise, it pulls the ocean water into that shape, which we call the tides), the Earth will pull on an object in lunar orbit and disturb it, ever so gently. But the effect is so small that I'd wager no spacecraft humankind has ever built had to take it into account.

As for the other question, I can't answer that. The answer relies on too many variables. But I would blindly guess that circular equatorial orbits around the Moon are probably stable to the tune of many tens of thousands of years. A blink of an eye in the age of the solar system, but more than enough for human purposes.

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But I would blindly guess that circular equatorial orbits around the Moon are probably stable to the tune of many tens of thousands of years. A blink of an eye in the age of the solar system, but more than enough for human purposes.

Actually most low orbits of the moon are very unstable, not due to drag or earths gravity, but because of its mass concentrations.

For example the PFS-2 satellite only stayed in orbit for 35 days while it was intended to do for over a year. This was because NASA had only limited knowledge on the mass concentrations back then and underestimated the effects of it.

However, there are a few frozen orbits at 27, 50, 76 and 86 degrees at which orbits of satellites are unaffected by them.

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I'm talking about the SOI of the body being orbited, for the purposes of my analogy, anything that has gone to Venus or Mars has left Earth's SOI.

edit: The key to this whole thing is gradients, not solid lines, but at the same time, there comes a point where things even out such as the zone represented by the karman line, eventually you get high enough that you consistently cannot achieve significant lift with fixed wing aircraft. By that same token, there will be a point where you are far enough from earth that you consistently cannot maintain orbit and in effect escape the planet's influence. The SOI of any body can be drawn up in the same manor as the karman line.

Your analogy only works due to the influence of other bodies, in a situation where there is only one body of mass in an otherwise empty universe the SOI is considered to be infinite.

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Actually most low orbits of the moon are very unstable, not due to drag or earths gravity, but because of its mass concentrations. For example the PFS-2 satellite only stayed in orbit for 35 days while it was intended to do for over a year. This was because NASA had only limited knowledge on the mass concentrations back then and underestimated the effects of it. However, there are a few frozen orbits at 27, 50, 76 and 86 degrees at which orbits of satellites are unaffected by them.

Heh, just learned about that now that NASA is about to let a probe crash into the moon. I'd love to see a 'map' of the moon's gravitational field.

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Not sure where you're getting your numbers...

http://en.wikipedia.org/wiki/Lunar_distance_%28astronomy%29

"The average distance from Earth to the Moon is 384,400 km (238,900 mi)"

And what makes you think I was stating that the 10,000km mark is half way to the moon? That is one number that has been given for the edge of the exosphere, the other, which is about half way between the earth and the moon is stated to be 190,000km. There seems to be a debate on which number is the actual edge of that layer of atmosphere.

http://en.wikipedia.org/wiki/Exosphere#Upper_boundary

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And what makes you think I was stating that the 10,000km mark is half way to the moon? That is one number that has been given for the edge of the exosphere, the other, which is about half way between the earth and the moon is stated to be 190,000km. There seems to be a debate on which number is the actual edge of that layer of atmosphere.

http://en.wikipedia.org/wiki/Exosphere#Upper_boundary

The fact that you put that two sentences next to another, separated by a comma, usually means that one is an explanation or example of the other.

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...and then beyond that is the exosphere which is estimated to reach up to about 10,000km, or half the distance between the earth and the moon.
A continuation of a sentence that presents a subject which includes one of two variables where 10,000km=A and "half the distance between the earth and the moon"=B, A does not equal B.

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Dunno what's your native language, but in English, it does. This will always be taken as "10000km, in other words half the distance between Earth and Moon". You should've said "either 10000km, or half the distance between Earth and Moon". Everyday English doesn't always match with strict, mathematical language in which that sentence would work unaltered (logical "or" is true if either of the arguments is true).

Edited by Dragon01

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Wow, this has degenerated, hasn't it...

The real universe is quite a complicated, messy thing. Boundaries - as we define them - only exist to simplify complex natural things so as to better understand it, and usually only make sense in the diagrams of such natural things. As said before, atmospheric pressure versus altitude is a curve, and drawing a line somewhere on that curve and calling that space takes more into account than just "is there Earth atmosphere here?". Simply because the ISS is encountering some gas molecules of primarily terrestrial origin does not mean it isn't "in space."

Edited by Dust

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Dust: I understand the simplification of the matter, but I also question the concept that you can be both within the atmosphere and in outer space at the same time when the high school definition of outer space I remember from science class being the area free of celestial bodies and their influence, with the exception of gravitational fields. This definition is what I'm basing my question off of. Conceptually it seems like it should be possible for an object to be orbiting earth without there being any atmosphere to cause orbital decay, the most obvious example of this being earth's moon. According to what's listed on wiki, the boundary for what I'm talking about is between 10,000km and 190,000km above our planet.

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That's a perfectly reasonable definition for "outer" space elanachan, but also somewhat arbitrary. You'd probably have to be beyond the Van Allen belts to qualify by that definition.

You have to ask though, is it a useful definition? What does it classify the region below it as? Because clearly there's a significant region which is qualitatively different from the lower atmosphere. Aerodynamic flight is impossible, hard vacuum, low temperatures, etc. Any sensible definition would call this space too, since for all practical purposes it is, even though there are still some atmospheric effects.

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Dust: I understand the simplification of the matter, but I also question the concept that you can be both within the atmosphere and in outer space at the same time when the high school definition of outer space I remember from science class being the area free of celestial bodies and their influence, with the exception of gravitational fields. This definition is what I'm basing my question off of. Conceptually it seems like it should be possible for an object to be orbiting earth without there being any atmosphere to cause orbital decay, the most obvious example of this being earth's moon. According to what's listed on wiki, the boundary for what I'm talking about is between 10,000km and 190,000km above our planet.

Even then, the moon is probably bumping into a few stray particles from the atmosphere anyway. Though not enough to cause its orbit to decay of course, especially since unrelated interactions are causing it to move away from us rather than towards us.

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Boundaries - as we define them - only exist to simplify complex natural things so as to better understand it

Exactly. So the real question is "what do you mean by outer space"? The other point is that although (eg) the atmosphere has in theory infinite extent, at some point it becomes "small enough" to ignore - and a sensible point might be "when the solar wind is more significant". Some sensible definitions of outer space might be:

  • When you are closer to orbiting than flying (karman line)
  • When you can complete an orbit without compensating for atmospheric drag
  • When you can orbit for the lifetime of the mission/craft without compensating for atmospheric drag
  • When the atmosphere is thinner than the solar wind
  • When the Gravitational attraction from the body is no longer the largest (SOI)

So for the ISS you would probably get YES, YES, NO, NO, NO

Then you ask yourself, what do I care about? so for OP's question, what is important about the height of the ISS to you? Probably definition 3, in which case the answer is no. Rephrase the question more precisely and all will be clear :D

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But there is a point where it does become zero. Unless you're saying we'll detect traces of our atmosphere over Pluto, or Andromeda.

Eventually it ends because everything does. That is the one constant of our Universe. The Karman Line is a very unrealistic measure of that point, however.

The odd atom, maybe?

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Even then, the moon is probably bumping into a few stray particles from the atmosphere anyway. Though not enough to cause its orbit to decay of course, especially since unrelated interactions are causing it to move away from us rather than towards us.

The moon also complicates any possibility of using atmosphere as a basis for defining space. The moon doesn't have an atmosphere, so technically you're in space, even when you're on the moon.

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Actually, according to what I've read, the moon does indeed have an atmosphere, equivalent to our exosphere and possibly the upper thermosphere with none of the lower layers present.

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It's too thin to have any effect on spacecraft, so it's widely disregarded. It's also composed of dust, not gas...I think.

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I'm saying those gases are not part of the Earths exosphere or atmosphere. Earths gravitational influence for loose particles extends to about halfway between the Earth and the Moon. Above that, solar radiation pressure on atomic hydrogen exceeds that of the Earth’s gravitational pull.

The point is that the atmosphere seemlessly transition with those gasses in outer space. There is no line where you can say: Ok, to the left, we have mostly atmosphere, and to the right, mostly outer space gasses.

There is no border

Dust: I understand the simplification of the matter, but I also question the concept that you can be both within the atmosphere and in outer space at the same time when the high school definition of outer space I remember from science class being the area free of celestial bodies and their influence, with the exception of gravitational fields. This definition is what I'm basing my question off of. Conceptually it seems like it should be possible for an object to be orbiting earth without there being any atmosphere to cause orbital decay, the most obvious example of this being earth's moon. According to what's listed on wiki, the boundary for what I'm talking about is between 10,000km and 190,000km above our planet.

Well there is your problem: You're using high school definitions.

There is a reason we call them high school definitions. They are over simplified, so that high school kids can understand them. You can't use them in a scientific discussion

Exactly. So the real question is "what do you mean by outer space"? The other point is that although (eg) the atmosphere has in theory infinite extent, at some point it becomes "small enough" to ignore - and a sensible point might be "when the solar wind is more significant". Some sensible definitions of outer space might be:

  • When you are closer to orbiting than flying (karman line)
  • When you can complete an orbit without compensating for atmospheric drag
  • When you can orbit for the lifetime of the mission/craft without compensating for atmospheric drag
  • When the atmosphere is thinner than the solar wind
  • When the Gravitational attraction from the body is no longer the largest (SOI)

So for the ISS you would probably get YES, YES, NO, NO, NO

Then you ask yourself, what do I care about? so for OP's question, what is important about the height of the ISS to you? Probably definition 3, in which case the answer is no. Rephrase the question more precisely and all will be clear :D

Exactly why you can't put a border :P

Much like adulthood, eventually you just look back and think 'huh, we are out of the atmosphere now'

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Is the definition wrong purely because it's simplified? If so, why is it not redefined? There's a difference between lack of detail and misinformation.

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Exactly why you can't put a border :P Much like adulthood, eventually you just look back and think 'huh, we are out of the atmosphere now'

Adulthood's borders still got officially defined with legal drinking/voting/consent ages though.

It does beg the question if, in some far off future, where humans are living on one or more worlds, would space have to be defined differently for each one, since different atmospheric densities and gravitational masses would be a factor?

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