Re: Lunar laser weaponry...

Cyde Weys wrote:

Sayeth Mike Williams:



At the distance of the Moon, you're going to get an awful lot of laser
dispersion unless you have a ridiculously large aperture. (E.g. the
optical laser used for measuring the distance to the Moon, in
conjunction with the mirrors left there by Apollo 11 disperses to about
7 kilometres across) Cite: <http://www.lpi.usra.edu/expmoon/Apollo11/A11_Experiments_LRRR.html>



I don't think this is necessarily true for all circumstances. The situation you've given depends on two variables: the precision of the manufacture of the retro-reflector on the Moon and the coherence of the laser used. The former factor is irrelevant for the use described by the op since you're only making a one-way trip. And the latter factor can be adjusted by making a laser emitter with better coherence. It's not really even that hard -- just make the main tube (or whatever it's called) longer, the aperture smaller, and pump up the power.

Not quite. First, lasers are coherant enough that boosting the coherance lengths isn't going to help in any measurable way for focusing.

Second, making the aperture small is going the wrong way. Small apertures mean a larger beam spread due to diffraction. Tube length doesn't really matter, as long as you have a good optical quality beam coming out. Then, pipe the beam to an expanding mirror placed close to the focal point of a big-ass converging mirror (say, 100 meters across, call this the BACM). Adjust the distance of the expanding mirror so that the BACM focuses the beam to its tightest spot size right at the target. A 100 meter diffraction-limited primary mirror could focus green laser light down to a 2.5 meter spot size from the moon to the earth. The smaller your aperture (primary mirror) the larger the spot size ... 25 meter spots for 10 meter apertures, 250 meter spots for 1 meter apertures, etc.

Remember - it does not matter how much the beam is diverging when it comes out of the laser resonator - you can always use focusing optics to correct that.

Pumping up the power is always a good option, however.
.

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