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One thing that I've always wondered is why space colonization focuses so much on planets. It seems like orbital platforms would both fix the low-gravity health

One thing that I've always wondered is why space colonization focuses so much on planets. It seems like orbital platforms would both fix the low-gravity health issue (spin the platform) and provide easier access to lucrative resources (you can hit asteroids without dealing with getting in/out of gravity wells).

tonmoy 10y ago
1. It is much easier to land on/get into the orbit of a planet. For example, transfer from low earth orbit to Mars requires a delta V of about 4.3 km/s. Now to get into Mars' orbit, this needs to be reduced to about 3 km/s (rough napkin-top calculation, take with a grain of salt). However if you want to "stop" at a massless orbital platform/asteroid, you would have to reduce all 4.3 km/s (without any aerobrake assist). 2. If we assume an orbital platform is like a ring (similar to Halo), and if the diameter of this thing is 2km, then it would need to spin very fast (70m/s). This is okay, until you realize, that so much speed means a lot of difference in gravity from the floor to ceiling. For example, a man who is 2m tall, would feel 1g on his feet and about 0.95g = 9.3m/s^2 on his head. The difference in g would give rise to different health issues. You could make the station bigger, but then it would be probably better to stick to a natural body that is similar size instead
- abecedarius 10y ago
  I thought delta-v from LEO to Mars was over 6km/s? There are near-Earth asteroids at much lower delta-v. https://www.planetaryresources.com/2015/08/how-we-choose-our-asteroid-targets/ https://www.planetaryresources.com/2015/08/how-we-choose-our... gets into some of this. At the 1km radius you give, a 2m human would have head at 0.2% lesser acceleration than feet. Am I miscalculating? Note that 1g is just one place in the design space, though an attractive one for a big enough station.
  - tonmoy 10y ago
    I am probably wrong with the 4.3 number (compiled the first result from google without double checking). And it won't be 0.2%, the head's speed would be 0.2% of the feet, but since g=v^2/r, the gravity difference would be about 3%
    - abecedarius 10y ago
      No, g also equals omega^2 r. The angular velocity omega is the same for head and feet, leaving only the change to r. https://en.wikipedia.org/wiki/Circular_motion#Formulas https://en.wikipedia.org/wiki/Circular_motion#Formulas (So what's wrong with g = v^2/r? Nothing -- both v and r change in that change to g. To first order, they cancel out in the change to v/r, leaving only a linear factor.)
      - tonmoy 10y ago
        You are right, I was had made a very ugly mistake I guess
        abecedarius 10y ago
        No worries! It's a natural mistake.
- mrfusion 10y ago
  You could spin the station at half that speed and still have half a g and mitigate those other issues.
  - tonmoy 10y ago
    At half the speed, it would be quarter the gravity. And 2km diameter is huge, it would be easier to make a smaller base camp on a planet where there already is gravity present
- Pinckney 10y ago
  I believe you're mistaken about the gravity. Position is as Re^(iv/rt), so |acceleration| is as Rv^2/r^2 -- right? For a fixed station of floor radius r, acceleration is directly proportional to distance from the center, R, so with r=1000m, a 2m astronaut will feel 99.8% of normal gravity at their head. Assuming 5% variation from head to toe is tolerable, a 40m radius habitat would suffice, which could be rotated at 20m/s. There has actually been research done on how humans tolerate rotational gravity (although not long term, AFAIK.) A 30% gradient at 0.5g is reportedly "disturbing". But perhaps long term one could become acclimated, just as one does at sea. http://www.artificial-gravity.com/Dissertation/2_2.htm http://www.artificial-gravity.com/Dissertation/2_2.htm
elsonrodriguez 10y ago
When lots of people have lots of babies on Earth, we just sprawl out. Having lots of people with lots of babies in a floating tin can is not a problem we can easily solve with our current level of technology. Within my lifetime, I have much more faith that we'll be able to construct habitats out of dirt on planets with an atmosphere than modules out of asteroids in the vacuum of space, and that's the appeal of planets in general, and Mars in particular.
- M_Grey 10y ago
  Realistically, "Colonies" will not be colonies for a long time, just the equivalent of barracks; people will live there for a while, but their main lives will not be in space.
M_Grey 10y ago
Definitely, and there are some hefty asteroids out there we could probably bubble-form into habitats, live on the interior surface and have low and high -g sectors. Hell, even moons makes more sense than planets, until lift ceases to be an issue.

tonmoy 10y ago

1. It is much easier to land on/get into the orbit of a planet. For example, transfer from low earth orbit to Mars requires a delta V of about 4.3 km/s. Now to get into Mars' orbit, this needs to be reduced to about 3 km/s (rough napkin-top calculation, take with a grain of salt). However if you want to "stop" at a massless orbital platform/asteroid, you would have to reduce all 4.3 km/s (without any aerobrake assist). 2. If we assume an orbital platform is like a ring (similar to Halo), and if the diameter of this thing is 2km, then it would need to spin very fast (70m/s). This is okay, until you realize, that so much speed means a lot of difference in gravity from the floor to ceiling. For example, a man who is 2m tall, would feel 1g on his feet and about 0.95g = 9.3m/s^2 on his head. The difference in g would give rise to different health issues. You could make the station bigger, but then it would be probably better to stick to a natural body that is similar size instead

abecedarius 10y ago
I thought delta-v from LEO to Mars was over 6km/s? There are near-Earth asteroids at much lower delta-v. https://www.planetaryresources.com/2015/08/how-we-choose-our-asteroid-targets/ https://www.planetaryresources.com/2015/08/how-we-choose-our... gets into some of this. At the 1km radius you give, a 2m human would have head at 0.2% lesser acceleration than feet. Am I miscalculating? Note that 1g is just one place in the design space, though an attractive one for a big enough station.
- tonmoy 10y ago
  I am probably wrong with the 4.3 number (compiled the first result from google without double checking). And it won't be 0.2%, the head's speed would be 0.2% of the feet, but since g=v^2/r, the gravity difference would be about 3%
  - abecedarius 10y ago
    No, g also equals omega^2 r. The angular velocity omega is the same for head and feet, leaving only the change to r. https://en.wikipedia.org/wiki/Circular_motion#Formulas https://en.wikipedia.org/wiki/Circular_motion#Formulas (So what's wrong with g = v^2/r? Nothing -- both v and r change in that change to g. To first order, they cancel out in the change to v/r, leaving only a linear factor.)
    - tonmoy 10y ago
      You are right, I was had made a very ugly mistake I guess
      - abecedarius 10y ago
        No worries! It's a natural mistake.
mrfusion 10y ago
You could spin the station at half that speed and still have half a g and mitigate those other issues.
- tonmoy 10y ago
  At half the speed, it would be quarter the gravity. And 2km diameter is huge, it would be easier to make a smaller base camp on a planet where there already is gravity present
Pinckney 10y ago
I believe you're mistaken about the gravity. Position is as Re^(iv/rt), so |acceleration| is as Rv^2/r^2 -- right? For a fixed station of floor radius r, acceleration is directly proportional to distance from the center, R, so with r=1000m, a 2m astronaut will feel 99.8% of normal gravity at their head. Assuming 5% variation from head to toe is tolerable, a 40m radius habitat would suffice, which could be rotated at 20m/s. There has actually been research done on how humans tolerate rotational gravity (although not long term, AFAIK.) A 30% gradient at 0.5g is reportedly "disturbing". But perhaps long term one could become acclimated, just as one does at sea. http://www.artificial-gravity.com/Dissertation/2_2.htm http://www.artificial-gravity.com/Dissertation/2_2.htm

elsonrodriguez 10y ago

When lots of people have lots of babies on Earth, we just sprawl out. Having lots of people with lots of babies in a floating tin can is not a problem we can easily solve with our current level of technology. Within my lifetime, I have much more faith that we'll be able to construct habitats out of dirt on planets with an atmosphere than modules out of asteroids in the vacuum of space, and that's the appeal of planets in general, and Mars in particular.

M_Grey 10y ago
Realistically, "Colonies" will not be colonies for a long time, just the equivalent of barracks; people will live there for a while, but their main lives will not be in space.

M_Grey 10y ago

Definitely, and there are some hefty asteroids out there we could probably bubble-form into habitats, live on the interior surface and have low and high -g sectors. Hell, even moons makes more sense than planets, until lift ceases to be an issue.