Lunar Lasers

Two different articles about building lasers (well, lasers and a maser perhaps) on the moon. Reuters has a story about a potential lunar power plant, creating electricity with solar panels and beaming it to Earth with microwaves. Space.com has a piece about building a sort of super-sized Star Wars program on the Moon, giant lasers set up to blast incoming space debris and not, of course, anyone here on Earth.

Comments

Re:Microwave

[mbessey]

Microwaves are easy to recieve - you simply stretch out a wire between two insulated poles, and the power just flows. That's the big advantage of microwaves over other power transmission possibilities.

Granted, given the spill-over from the "concentrated" beam of microwaves, you'll probably have to use some frequency that's not very popular for communication, but it's probably do-able.

The people who are worried about power-line emissions would probably go insane over this, though - the exposure levels would be MUCH higher.

-Mark

Two dumb ideas

[markmoss]

If you are going to do this beamed microwave thing, build it in Earth orbit, closer to the target. (distance)*(wavelength)= k*(diameter of transmitter aperture)*(diameter of beam at target), where k is a constant somewhere between 1/3 and 3. I think the moon is about 250,000 miles or 400,000 km away. So to focus a 30GHz (1 centimeter wavelength) microwave beam down to a 10 km spot on Earth takes an antenna about 400m across. Or in units the average American understands, a football-field sized antenna would put most, but not all, of the transmitted energy into a 10 mile wide spot. This whole area would have to be blanketed with receiving antennas (expensive!). And people living 20 miles away would be measuring the leaking energy and suing every time they got a cough. (Birds would be safely building nests on the antennas, but American trial lawyers never let science get in the way of a deep-pockets lawsuit.)

The best place for a solar power satellite is probably geosynchronous orbit (40,000 km). This needs a football-field sized transmitter and a mile-wide receiver; still pretty big, but maybe manageable. And the transmitter and receiver don't move relatively. A lunar array would have to keep switching between different receivers as the Earth turns. An SPS in a lower orbit would also have to keep switching receivers, but at least it would have smaller antennas.

A solar plant in orbit is in sunlight almost all the time (depending on distance from earth and orbital particulars, it might spend a few hours a year in earth-shadow). On the moon, two weeks out of every four is night.

The laser installation would also work better in a medium-height earth orbit, where it's solar panels were powered all the time and it was much closer to the targets. At least, I assume that it isn't meteroids headed for the moon that this is supposed to shoot down?

Re:This is a weapon of massless destruction

[Fenris2001]

Answers to safety questions -

1. Focus - the beam will most likely be a maser, or microwave laser. Given a reasonable size emitter in geosynch or elliptic earth orbit, the footprint on the surface of the planet is only a few kilometers wide, and has an energy density of perhaps ten to a hundred watts per square meter.

2. Guidance - the same way they keep aircraft away from anything else - tell them not to go there. Note that this isn't really a problem, as the metal skin of an aircraft would deflect the beam.

3. Of course they will coordinate with other satellite operators. Although, if some satellite DID accidentally cross the beam path, it wouldn't necessarily be harmed, for the same reasons as 2.

The proposals I've seen for this (including a gov't study in the Sixties), all addressed the safety question. The REAL question is whether or not this can be done ECONOMICALLY - it's no use if the power so produced is ten times more expensive than fossil fuels (though note that such a scheme becomes more attractive as fossil fuels become more expensive...). The most attractive source of building materials for the solar cells and support hardware is not the Earth, but asteriods that cross or come near the orbit of the Earth - they contain all the necessary elements (silicon, iron, hydrogen, carbon, etc.) to make a solar power satellite in orbit, instead of having to haul every component up from the planet.

What "light side of the moon?"

[Galvatron]

The term "dark side of the moon" refers to the side pointed AWAY FROM THE EARTH, and has nothing to do with whether or not the sun is pointing at it. The moon has a normal night/day cycle, lasting 28 days (this is also the length of one transition from full to new moon and back again. This is not a coincidence).

Didn't you ever see/read 2001? The lunar monolith being exposed to sunrise is a critical plot element.

Nobody's gotten this right yet

[Spamalamadingdong]

1. How will they focus the beam on receptor antenas?

There are some pretty simple ways of doing this. One is to send a "pilot beam" from the receiver to the transmitter, and use it as a phase reference. Using techniques of phase reversal (see this guy's bio) you can create a coherent beam at the other end of a "lumpy" medium like wavy glass (or the ionosphere, or a chicken [see the bio]).

2. How will they keep airplanes from flying across the beams?

They won't; the beam intensity isn't sufficient to be a problem. It just struck me that it would be ideal to locate airports in the middle of the receiver farms, because that will keep development from encroaching under the approach and departure paths and creating noise problems and threats to persons on the ground from crashes.

The only way for this not to harm you would be for it not to strike you. Early radar technicians learned about microwave cooking standing in front of such beams

There are easy ways to avoid it striking you (a wide-brimmed tinfoil hat might actually have usefulness against something in the real world). The best is to make sure it can't go anywhere other than where it's intended, using a technique like an encoded pilot beam. Turn off the pilot beam, the transmitter no longer has a phase reference, the various transmitter sections go incoherent, the power gets radiated all over the sky and falls to minuscule levels on Earth.