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Is there a significance for ~100km? Is this, roughly speaking, space -- where the atmosphere is so thin to be almost negligible? Clearly atmosphere thins gradua

Is there a significance for ~100km? Is this, roughly speaking, space -- where the atmosphere is so thin to be almost negligible? Clearly atmosphere thins gradually so how do we define where space starts? Is the significance of ~100km something to do with the effects of gravity at that altitude from the Earth's surface? Does ~100km give you weightlessness? Or is Blue Origin going up to ~100km because it's a nice round number that is roughly (whatever that means) in space. But aren't kilometres completely arbitrary? Also, can people please stop knocking Blue Origin. We get it at this point, okay? I'm a huge fan of SpaceX and Elon Musk but does Blue Origin have to lose for SpaceX to win? No. There's nothing in this post from Bezos bashing SpaceX as far as I can see. There's simply saying, look, we did it again with the same refurbished rocket. Good on them. May they do it again and again. And so may SpaceX. The next space race is on, happy days!

rory096 11y ago
100km is the Kármán line, the generally accepted boundary of space. It's the point at which aeronautics become useless, as the atmosphere is too thin to support lift greater than your weight without going faster than orbital velocity. Contrary to popular belief, in Low Earth Orbit you're still very much in reach of the Earth's gravity; it's only about 10% less than the gravity on the surface.[2] The tricky part about orbit is moving sideways fast enough that you miss the ground. So Blue Origin indeed got to space, landed and did it again. But it took a tiny fraction of the energy of the same feat for an orbital rocket (which is why SpaceX can only land its first stage). [1] https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line [2] https://what-if.xkcd.com/58/ https://what-if.xkcd.com/58/
- igravious 11y ago
  Why then do astronauts on the ISS appear weightless? Do _not_ make me Google this :) And maybe we shouldn't call them astro- (suggesting star) nauts until they get to Sol and back. I kid! I guess in some sense we are all astronauts, man, on spaceship Earth. Seriously, maybe we should call them orbiternauts?
  - thatswrong0 11y ago
    Well, astronauts are moving faster away from the Earth faster than they're falling towards it.
  - AdamTReineke 11y ago
    I love this question! Something appears to be weightless when it is falling the same speed as everything around it. If you took a kitchen scale, taped a weight to the top, and dropped them, you'd notice it appears to have a weight of zero. On the topic of falling, if you walk off a cliff, you fall to the base of the cliff. If you run, you fall farther way. The faster you go, the trajectory of your fall will be farther from the cliff. Now go really fast and you may fall miles away - but you'll be fighting air resistance and eventually slow down and hit the ground. Above the Karman Line (62mi/100km up), the air is thin enough that there barely any air resistance. If you go fast up here, the air isn't there to slow you down. Gravity still pulls you back to earth though. [Blue Origin] Now if you're above the atmosphere and going horizontally fast enough, the trajectory of your fall eventually overshoots the curvature of the earth. Follow that trajectory a quarter rotation around the earth -- you're still in space, you're still going fast, and you're still falling. You're in orbit. (SpaceX) Since you're falling forever and the space station around you is also falling, you appear to be weightless in relation to the space station. (The indie game Kerbal Space Program is a great way to play with/explore the concepts of orbital mechanics if you're intrigued.) [Edit: For reference, the different in thrust needed to go straight up and coast to the Karman line compared to going up at an angle (spending more time traveling through the dense atmosphere) and establishing a horizontal speed of about 17,500 miles per hour is very big. I believe it's at least an order of magnitude though I don't have a citation for that number.]
    - dingaling 11y ago
      > Above the Karman Line (62mi/100km up), the air is thin enough that there barely any air resistance. I On the contrary, even at the altitude of the ISS there is sufficient atmosphere to reduce its velocity by around 5 metres / hour under normal conditions. Particle density at 400 km altitude is 10E+15 particles / cubic metre ( about a billion times less than at sea level ( but much more at times of Solar flaring. The ISS makes a sonic boom which propagates to the ground, it's just incredibly weak.

rory096 11y ago

100km is the Kármán line, the generally accepted boundary of space. It's the point at which aeronautics become useless, as the atmosphere is too thin to support lift greater than your weight without going faster than orbital velocity. Contrary to popular belief, in Low Earth Orbit you're still very much in reach of the Earth's gravity; it's only about 10% less than the gravity on the surface.[2] The tricky part about orbit is moving sideways fast enough that you miss the ground. So Blue Origin indeed got to space, landed and did it again. But it took a tiny fraction of the energy of the same feat for an orbital rocket (which is why SpaceX can only land its first stage). [1] https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line [2] https://what-if.xkcd.com/58/ https://what-if.xkcd.com/58/

igravious 11y ago
Why then do astronauts on the ISS appear weightless? Do _not_ make me Google this :) And maybe we shouldn't call them astro- (suggesting star) nauts until they get to Sol and back. I kid! I guess in some sense we are all astronauts, man, on spaceship Earth. Seriously, maybe we should call them orbiternauts?
- thatswrong0 11y ago
  Well, astronauts are moving faster away from the Earth faster than they're falling towards it.
- AdamTReineke 11y ago
  I love this question! Something appears to be weightless when it is falling the same speed as everything around it. If you took a kitchen scale, taped a weight to the top, and dropped them, you'd notice it appears to have a weight of zero. On the topic of falling, if you walk off a cliff, you fall to the base of the cliff. If you run, you fall farther way. The faster you go, the trajectory of your fall will be farther from the cliff. Now go really fast and you may fall miles away - but you'll be fighting air resistance and eventually slow down and hit the ground. Above the Karman Line (62mi/100km up), the air is thin enough that there barely any air resistance. If you go fast up here, the air isn't there to slow you down. Gravity still pulls you back to earth though. [Blue Origin] Now if you're above the atmosphere and going horizontally fast enough, the trajectory of your fall eventually overshoots the curvature of the earth. Follow that trajectory a quarter rotation around the earth -- you're still in space, you're still going fast, and you're still falling. You're in orbit. (SpaceX) Since you're falling forever and the space station around you is also falling, you appear to be weightless in relation to the space station. (The indie game Kerbal Space Program is a great way to play with/explore the concepts of orbital mechanics if you're intrigued.) [Edit: For reference, the different in thrust needed to go straight up and coast to the Karman line compared to going up at an angle (spending more time traveling through the dense atmosphere) and establishing a horizontal speed of about 17,500 miles per hour is very big. I believe it's at least an order of magnitude though I don't have a citation for that number.]
  - dingaling 11y ago
    > Above the Karman Line (62mi/100km up), the air is thin enough that there barely any air resistance. I On the contrary, even at the altitude of the ISS there is sufficient atmosphere to reduce its velocity by around 5 metres / hour under normal conditions. Particle density at 400 km altitude is 10E+15 particles / cubic metre ( about a billion times less than at sea level ( but much more at times of Solar flaring. The ISS makes a sonic boom which propagates to the ground, it's just incredibly weak.