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View Diary: Energy from the Moon (165 comments)

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  •  I still prefer geosynch (none)
    You make some good arguments; let me attempt to rebut them.

    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.

    Secondly, the advantage of geosynch is that the power satellites are, from the viewpoint of the ground, stationary. That means that you can tie individual recievers to specific powersats, rather than having to hand the beam off from one reciever to the next over the course of a day. What happens when the Pacific Ocean is the part of the planet facing the moon? Do you lose the power feed for a few hours, or do you run massive (and inefficient) power lines from some Polynesian island to Japan or Australia or whereever? And, there's no particular need to build recievers at the equator; most latitudes have adequate sightlines to equatorial geosynch. Geosynch allows you to deliver power where it's needed; anything else forces you to spread the power out all across the globe and then deal with the transmission losses as you move the juice around on the ground.

    Thirdly, you rightly point out that the moon is more hospitable to humans than zero-g. But who says that the satellites (or lunar station) have to be manned? I would envision automated systems, with robots doing simple routine maintenance (a concept being developed now by NASA for maintenance on future space telescopes and the like), and the occasional visit by human repair crews.

    Construction is an issue, especially on the scale necessary to make any sort of dent in our power needs, but unless you are planning on setting up a large industrial plant on the moon, just about everything will have to come from Earth anyway, so launch costs are going to be comparable between the two.

    -dms

    •  One other thing (none)
      On the Moon, just like on the surface of the Earth, solar panels see the Sun 50% of the time. In a geosynch orbit, if I've done the math correctly, panels see the Sun roughly 98% of the time, with Earth eclipsing the Sun the remaining 2% of the time. Even ignoring inverse-square issues, that means that any given size array will generate twice as much power in orbit than on the Moon.

      -dms

      •  What about the lunar poles? (none)
        There must be some lunar terrain at the poles which gets much more illumination than 50%. Find a mountain or plateau at one of the poles and I bet you could get above 90%.

        I am become Dubya, Destroyer of Words...

        by Swampfoot on Fri Nov 11, 2005 at 12:18:32 PM PST

        [ Parent ]

    •  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.

        -dms

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