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I remember a cartoon about some spy guy. In that cartoon they did mass media by projecting from the surface of the earth to the moon in the night sky.

Would that technique be possible? Could an earth-like moon be used as a "blackboard" or is this scientifically not possible? How big would such a device / facility be?

I would like to use this as a one-way channel of podcast without the consumer need of a receiver.

Assuming clouds are not present of course.

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    $\begingroup$ I would recommend taking a look at this for shooting lasers at the moon from earth. what-if.xkcd.com/13 $\endgroup$ – Mourdos Oct 2 '14 at 14:22
  • $\begingroup$ We do project laser beams on the Moon for measurement purposes, which is why we've installed mirrors there. but it feels like a very wide and very strong beam would be required to project clear images, even monochrome, on the surface. $\endgroup$ – mechalynx Oct 2 '14 at 14:23
  • $\begingroup$ By the way, is projecting an image the only case you're interested in? Would, for example, having an array of panels on the Moon be a possibility to consider? $\endgroup$ – mechalynx Oct 2 '14 at 14:37
  • $\begingroup$ @Mourdos I was just about to link that $\endgroup$ – user329 Oct 2 '14 at 14:48
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    $\begingroup$ I really like your question, Fulli. They are almost all very interesting. $\endgroup$ – DonyorM Oct 2 '14 at 15:19
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Projection is too expensive

Advertising on the moon using projection would be so costly that it is unlikely anyone would pay for the advertising space. However, there are other methods of advertising on the moon. The convenient fact that the moon is tidally locked means that if you put a physical advert (rather than a projected one) on the moon's surface it would stay visible (at least during phases of the moon when the advert is lit by the sun).

A physical advert

Setting this up would still be hugely expensive, but if the advert could be changed simply by sending a signal from Earth, then running costs could be minimal. If an organisation were to provide the initial huge investment, over time it could be paid off provided that the design of the system required minimal maintenance.

For example, a vast army of simple robots that can display black or white could be spread out across the facing surface of the moon. They require no power apart from when a signal arrives to tell them to change colour for the next advertisement. That minimal power requirement can be covered by solar cells and a small battery. Ideally the robots would be mobile so that they can move around to cover any dead robots, so that a small number of failures does not significantly diminish the image quality. This would also mean they could be sent in large batches and spread out autonomously rather than needing to be placed.

The moon is black

Although the moon has a white appearance due to being only visible when brighter than the sky, it actually has a low albedo of about 0.12 (dark grey). This means that an area of the moon's surface covered with something white would appear from Earth as an area much brighter than the rest of the moon. Adverts on the moon would not just be pictures, but would be shining out brilliantly and difficult to miss.

Colour and animation

The same principle could be used with robots able to display red, green, blue or any combination. This would allow for full colour photographs to be displayed on the moon. Since the image would only be visible when that part of the moon's surface is in sunlight, the robots would have power and could change colour periodically provided they were provided with a timer which could be synchronised when the next advert is transmitted. This would allow for cycling through the colours required for an animation, so full colour high definition video could be played on the moon. The frame rate would depend on the efficiency of colour change and the power provided by the solar cells, but the resolution could be as high as the number of available robots would allow. High definition doesn't take more power, just more robots.

Difficulties

I've made this seem very easy (just expensive) but of course it is not quite so simple. The moon's surface is irregular and there may be parts that the robots cannot reach or regions too steep for them to settle on. The images may have gaps or lines where rough terrain or mountain ridges prevent colour being applied. Initial adverts may be coarse, low definition proofs of concept, with resolution being increased and gaps being filled gradually over time as investors are convinced by the influx of advertising revenue and start to invest more.

Of course covering the moon's surface would take a vast amount of material, but due to the relative brightness of white material (or red, green or blue material) against the dark grey of the moon's surface, leaving large gaps between the robots would just result in a less bright image. The colour would still be reasonably accurate as the moon's surface visible between the panels would contribute relatively little to the perceived image.

Scientific funding

Part of the cost may be absorbed by combining the purpose of the robot army. There are plenty of scientific uses for a huge array of robots that can each point a sensor in a variable direction. A small robot with a large colour panel and a few small scientific instruments would appear exactly the same from Earth as if it was just a colour panel. Scientific funding may be provided to set up the robot array in return for control over what is measured, or sufficient investment may be gained based on the returns expected from selling both advertising space and time on the scientific array.

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  • $\begingroup$ While this is great, isn't it much easier to just create an artificial enormous satellite, and use that for advertising? Sure, it's not "the Moon", which has some added charisma. $\endgroup$ – o0'. Oct 2 '14 at 15:48
  • $\begingroup$ @Lohoris, it wouldn't even need to be that large. A simple mylar sheet a mile across in low orbit would appear to be the size of the Moon. $\endgroup$ – Mark Oct 2 '14 at 19:09
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    $\begingroup$ I'm not sure I'd be happy about a mile wide accident waiting to happen in low orbit... $\endgroup$ – trichoplax Oct 2 '14 at 19:38
  • $\begingroup$ The nice thing about the moon is that impacts are less common on the facing side and even when they happen they just take out some of the robots, and others can take their place. With a self supporting satellite it would be harder to avoid total collapse if part of it is struck. Still possible, but perhaps harder to sell to the investors. $\endgroup$ – trichoplax Oct 2 '14 at 19:40
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    $\begingroup$ @githubphagocyte true, however I expect it to cost orders of magnitude less than doing it on the Moon, so you can afford to just build it again multiple times, in case you had the budget to do it on the Moon anyway. $\endgroup$ – o0'. Oct 3 '14 at 7:50
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In general it would be cost prohibitive. while theoretically it could be done with enough power, (the moon is a reflective surface) the power needed would be extreme. Projecting when the moon is visible you'll need enough power to overpower the sun, since that is what you are seeing. Remember that even though the moon looks like a fairly small object in the sky you would need to broadcast against a screen bigger than the whole of North America. Because of diffusion, to have any chance of a clear picture you will have to use lasers just to get a clear enough picture to reflect back.

When the moon face is dark (earth eclipse etc) it would be slightly easier because you wouldn't be fighting against the sun, but the power out put would be immense.

According to Wiki "The intensity of moonlight varies greatly depending on the lunar cycle but even the full moon typically provides only about 0.2 lux illumination"

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    $\begingroup$ When the dark side is facing earth, the moon is only "visible" from the bright side of the earth (that is, during day). While you would not need to fight against sunlight reflected from the moon, you would have to fight against sun light diffused by the air (the bright blue sky). $\endgroup$ – celtschk Oct 2 '14 at 15:04
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    $\begingroup$ Mostly true, the earth eclipses the moon frequently... You also have the quarter moon where the majority is dark. $\endgroup$ – bowlturner Oct 2 '14 at 15:08
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    $\begingroup$ The "dark side" of the moon never faces earth, that's the whole point of the term: it refers to the side that, due to tidal locking, always faces away, and thus, until space travel, was completely unknown (and even now, is known far less well than the near, or "light" side). $\endgroup$ – Matthew Najmon Oct 2 '14 at 17:48
  • $\begingroup$ @MatthewNajmon OK, I'll rephase it to when the face of the moon is dark. $\endgroup$ – bowlturner Oct 2 '14 at 17:54
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    $\begingroup$ The "dark" side of the Moon is the part of the Moon which is not illuminated by the Sun. The "far" side of the Moon is that which does not face the Earth. Since the Earth and the Sun are in different positions, depending on the relative angle to each the two are not necessarily the same. In fact, about the only time when the two would be (essentially) equivalent is when the Earth is almost directly in the line between the Sun and the Moon, such that the same side of the Moon that faces the Sun also faces the Earth. For a fuller answer I'd recommend Astronomy, maybe Space Exploration. @Matthew $\endgroup$ – a CVn Oct 2 '14 at 20:04
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Off course thats possible, just a bit expensive. The beamer kind of "projection" from earth is a bit difficult because of the energy consuming light source (especialy since light bulbs exceeding 60W are prohibited now, at least in Europe), but from a light intensity point of view, a similar idea is quite well worked out here http://what-if.xkcd.com/13/ (if you have this kind of questions more often i also reccomend reading other topics there). So its possible although you need quite some windmills to power it all up, but then there is also the problem of diffusion trought earths atmosphere if you project from here, it would blur your image a bit, but some lowres logo, would definatly be recognizable. And as you already mentioned there is the problem of clouds every now an then (so dont put your projector lights in Netherlands).

The "easier" approach would be to make some shaddows, do you know how a dia projector works? If you manage to get a space ship at a strategic position between the sun and the moon, i think it should work, quite crisp. The only problem is the size of the "dia picture". Unlike the dia projector the lamp "our sun" is bigger then the projection screen (the moon), so getting closer to the sun, is not going to reduce the size of the necessary dia picture, since most of the light will go around the "dia picture", fading the effect of the dia picture. But if you would make a funny shaped kind of baloon, lets say the size of the earth (think about lunar eclipses), and you could manage to control its position, with respect to the sun, and the moon, you can definatly get your add up there.

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    $\begingroup$ "light bulbs exceeding 60W are prohibited now, at least in Europe" Only incandescents, which, all things considered, have a lousy power to luminance ratio (about 10 lm/W). You can get the same light output from a LED lightbulb that draws on the order of a tenth of the power (more like 100 lm/W) and there is no regulation against that. Technology hasn't quite caught up with reasonably priced high-lumiosity LED lightbulbs, but manufacturers have certainly come a long way compared to even just a few years ago. $\endgroup$ – a CVn Dec 9 '14 at 20:33
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If I may quote a (largish) bit of Heinlein:

Harriman was shown into the office of the president of the Moka-Coka Company ("Only a Moke is truly a coke" — "Drink the Cola drink with the Lift"). He paused at the door, some twenty feet from the president's desk and quickly pinned a two-inch wide button to his lapel.

Patterson Griggs looked up. "Well, this is really an honor, D.D. Do come in and—" The soft-drink executive stopped suddenly, his expression changed. "What are you doing wearing that?" he snapped. "Trying to annoy me?"

"That" was the two-inch disc; Harriman unpinned it and put it in his pocket. It was a celluloid advertising pin, in plain yellow; printed on it in black, almost covering it, was a simple 6+, the trademark of Moka-Coka's only serious rival.

"No," answered Harriman, "though I don't blame you for being irritated. I see half the school kids in the country wearing these silly buttons. But I came to give you a friendly tip, not to annoy you."

"What do you mean?"

"When I paused at your door that pin on my lapel was just the size-to you, standing at your desk-as the full Moon looks when you are standing in your garden, looking up at it. You didn't have any trouble reading what was on the pin, did you? I know you didn't; you yelled at me before either one of us stirred."

"What about it?"

"How would you feel—and what would the effect be on your sales—if there was 'six-plus' written across the face of the Moon instead of just on a school kid's sweater?"

Griggs thought about it, then said, "D.D., don't make poor jokes. I've had a bad day."

"I'm not joking. As you have probably heard around the street, I'm behind this Moon trip venture. Between ourselves, Pat, it's quite an expensive undertaking, even for me. A few days ago a man came to me—you'll pardon me if I don't mention names? You can figure it out. Anyhow, this man represented a client who wanted to buy the advertising concession for the Moon. He knew we weren't sure of success; but he said his client would take the risk.

"At first I couldn't figure out what he was talking about; he set me straight. Then I thought he was kidding. Then I was shocked. Look at this—" Harriman took out a large sheet of paper and spread it on Griggs' desk. "You see the equipment is set up anywhere near the center of the Moon, as we see it. Eighteen pyrotechnics rockets shoot out in eighteen directions, like the spokes of a wheel, but to carefully calculated distances. They hit and the bombs they carry go off, spreading finely divided carbon black for calculated distances. There's no air on the Moon, you know, Pat—a fine powder will throw just as easily as a javelin. Here's your result." He turned the paper over; on the back there was a picture of the Moon, printed lightly. Overlaying it, in black, heavy print was: 6+

The Man Who Sold the Moon by R.A. Heinlein

So while projecting an image on the moon might not be possible, there are other means of displaying an image on it.

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As already linked, Randall Munroe discussed illuminating the moon from earth – it's not feasible.

trichopax' proposal to cover significant parts of the moon in white (or even red / green / blue) is perhaps less energy-hungry, but I definitely wouldn't it consider realistic, either. The moon may not actually be as bright as it seems to us, but it's not that dark either. You'd need to cover a fraction of the surface that's comparable to the albedo. That requires at least something like $500000\ \mathrm{m^3}$ of paint. Going by the price of 16M \$ to launch a ton to GTO, even the cost of getting that mass up there would exceed 7 trillion dollars. (I'm only establishing lower bounds here; the real cost would likely be much, much higher).

Both ideas share essentially the same problem: most of the light (99.97%) would never hit earth, but scatter – lost in space.

That needn't be though! If you could directly beam from the moon straight to earth, you'd gain a lot. Doing this with lasers would again raise supply problems, but there's a much simpler solution: set up lots of mirrors to reflect sunlight. Because you're on the moon, these could be very lightweight – no wind and little gravity to keep against.
The challenge: the mirrors would need to be continually adjusted, to track earth. Every single one would need an actuator. But that would of course also mean you can change the advertised message very quickly.

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