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View Diary: Energy from the Moon (165 comments)

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  •  any optical experts around? (none)
    The first argument in favor of geosynch is three words: Inverse Square Law. The moon is roughly 10x farther away than a geosynch orbit. That means a factor of 100 decrease in beam density when it reaches us; your recieving array is going to have to be that much bigger in order to pull in the same amount of power.

    Wouldn't the act of focusing the microwave beam at the ground-based antennae negate that fact?    It's not going to spread out and diffuse with distance, as the sun's unfocused rays would.

    •  It's not that simple (none)
      You do certainly focus the beam, but the inverse square law still bites you, by raising the bar of what "good focusing" means.

      Let's say, for example, that you build a reciever array 1 kilometer in diameter. Now, look at the focusing requirements so that the entire beam hits the reciever. For a geosynch emitter, distance is 33,000 km, so the angular size of the beam has to be about 0.001 degrees. Now move the emitter to the moon. It's about 10x farther, so to hit the same size target, the angular size of the beam has to be 10x better, 0.0001 degrees, much more difficult to do.

      Alternatively, say that your transmitters are good enough to achieve a 0.001 degree focus. The geosynch emitter can send all of its power to a 1 km diamter array, but the lunar emitter will need a 10 km diameter reciever, with 100 times the collecting area.

      This can be seen in the real world with a focused light beam like a flashlight. Shine it on something nearby, it makes a small spot of light. Shine it on a wall 50 feet away, and it's a big diffuse blob. Laser pointers have a tighter focus than flashlights, but aim a pointer at a target a few hundred feet away and you won't get a crisp little dot.


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