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>why not just move an comet into earth orbit This is probably substantially more difficult than establishing a moon colony and mining there. Comets have lots o

>why not just move an comet into earth orbit This is probably substantially more difficult than establishing a moon colony and mining there. Comets have lots of orbital speed you have to overcome, and are generally pretty big. Edit: >PS: If you really want rail guns, you can also get free delta-v from solar sails and use them maintain a launch platform's orbit. Solar sails cannot maintain the orbit of an orbital rail gun launch platform -_- All of your ideas are more of a moonshot than an actual moonshot.

Retric 9y ago
Solar sails provide free delta v if you are already in orbit and willing to wait. It's not useful for moving people around the solar system, but a rock that takes 100 years to get somewhere is not a big deal. On that timescale they can move literal mountains.
- Sir_Cmpwn 9y ago
  Free delta-V, yes, but not much - certainly not enough to maintain the orbit of a mass driver.
  - Retric 9y ago
    As you just want station keeping you can ignore the mass of the sail and mass driver, all of the delta-v from the sail get's transfers to what the mass driver launches. Thus you can actually use a fairly thick sail without issue. The problem is because the station is staying still you are effectively limited to the square root of a sails acceleration. Still ~7 newtons per km^2 adds up and you avoid the delta v deficit from being on the moon in the first place.
    - Sir_Cmpwn 9y ago
      You don't just need station keeping. Every launch takes energy away from the orbit of the launch platform. Consecutive launches basically deorbit the launch platform. To maintain the orbit, the solar sails must generate an equal amount of orbital energy as the launch platform imparts upon vehicles launched. In other words, you could just launch stuff with its own solar sail to the same effect. This happens on the moon, too, but the moon is basically a limitless supply of orbital energy.
      - Retric 9y ago
        I think you misunderstood what I meant by station keeping. The sail is used to return the platform to it's orbit after launch. You probably want something else for normal station keeping. Anyway, I was wrong about the square root thing. It's 1:1. A 1km2 sail represents 7m/s/s for 1 kg of stuff. Normally the sail eats into that and you quickly get far from the sun, but that's not the case here. Fire a 100kg object at 20,000 m/s and a 10km/10km sail can return the station to orbit in around 50 minutes. Granted if you are in earth orbit it's a little more complex. But you probably want to be in a fairly high orbit anyway to maximize delta v.
        deleted 9y ago
        [deleted]

Retric 9y ago

Solar sails provide free delta v if you are already in orbit and willing to wait. It's not useful for moving people around the solar system, but a rock that takes 100 years to get somewhere is not a big deal. On that timescale they can move literal mountains.

Sir_Cmpwn 9y ago
Free delta-V, yes, but not much - certainly not enough to maintain the orbit of a mass driver.
- Retric 9y ago
  As you just want station keeping you can ignore the mass of the sail and mass driver, all of the delta-v from the sail get's transfers to what the mass driver launches. Thus you can actually use a fairly thick sail without issue. The problem is because the station is staying still you are effectively limited to the square root of a sails acceleration. Still ~7 newtons per km^2 adds up and you avoid the delta v deficit from being on the moon in the first place.
  - Sir_Cmpwn 9y ago
    You don't just need station keeping. Every launch takes energy away from the orbit of the launch platform. Consecutive launches basically deorbit the launch platform. To maintain the orbit, the solar sails must generate an equal amount of orbital energy as the launch platform imparts upon vehicles launched. In other words, you could just launch stuff with its own solar sail to the same effect. This happens on the moon, too, but the moon is basically a limitless supply of orbital energy.
    - Retric 9y ago
      I think you misunderstood what I meant by station keeping. The sail is used to return the platform to it's orbit after launch. You probably want something else for normal station keeping. Anyway, I was wrong about the square root thing. It's 1:1. A 1km2 sail represents 7m/s/s for 1 kg of stuff. Normally the sail eats into that and you quickly get far from the sun, but that's not the case here. Fire a 100kg object at 20,000 m/s and a 10km/10km sail can return the station to orbit in around 50 minutes. Granted if you are in earth orbit it's a little more complex. But you probably want to be in a fairly high orbit anyway to maximize delta v.
      - deleted 9y ago
        [deleted]