Scenarios for Manned Space Development
Section 2.
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Exposure to Laser Signal Beams from Close Range

It has been proposed to have a laser on the moon point at Earth to send messages in the visible or invisible spectrum, or simply to mark the location of and draw attention to the moonbase.

Such a laser, by its very nature, is pointed upward at the angle above the horizon necessary to intersect the Earth. This angle varies a little, but for our purposes can be considered constant. Now, given that it has enough energy to be detectable as a red dot in a telescope placed somewhere on the disk of the earth, then it is obvious that there is some distance less than d = 400,000 km at which the energy deposited on a retina is damaging.

Now, what are the chances of such an action occurring? It is not from LEO or GEO because for our purposes they are nearly the d above. There is nowhere on the lunar surface that such is possible, so it is not possible for d close to 0; that leaves a cone from the beacon out through near lunar space. What might be in that cone? A low lunar satellite might pass through it a few times a year, if that often, because the dispersion will still be so low that it will not be highly likely that any given orbit will pass through it. And even then, the transit speed will be so high that the integrated energy deposition will be very low.

The third case is a ship in transit. It is conceivable that some orbital paths might intersect the beam at sufficient distances that they could remain in the beam for seconds or minutes. Let us make a truly gross assumption, that it happens on every trip out and back. An outward-travelling craft rendezvousing with the moon does not follow a radius out from the Earth - in fact it isn't even close. So it would only be at predictable times and intervals during the Earth-Moon transit. Low-G/High-ISP craft will spiral outwards and might actually pass through it several times, depending on the orbital plane. Spacecraft like Apollo only come close once per trip.

Now the orbit planes can't be ignored, and are in fact the reason why there are windows for lunar trips, particular if they leave from a LEO station. One must travel such that the lines of nodes line up at arrival, otherwise you will pass well above or below the moon when you reach 400,000 km. What this means is that even if your craft passes between the Earth and Moon at a dangerous distance, it is highly likely that your orbital plane will be well above or below the radius line and associated conic beam.

And if one does pass through the beam, stewards can easily make sure that all the passengers they are responsible for are kept away from the windows at the time of possible intersection.

This leaves local aerial transport. If a lunar "hopper" were to do a suborbital hop such that it went directly over the beacon early in the flight, it is very unlikely there would be someone looking directly downward for the fraction of second that it passed through the pencil thin beam, especially if it was known the hopper's trajectory would intersect the beam.

For all intents and purposes, the danger of a message laser being damaging is negligible. The low probability of intersection and the low exposure times to vehicles in the path of the laser makes the problem of accidental exposure to the laser a very minor one.

Scenarios for Manned Space Development

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