Shining light on the moon

Is it possible to shine a visible light (laser I guess) on the moon and see it reflected with the naked eye from earth? If you could, and enough people complained about it, could anything be done to stop you?

I guess if it were possible we’d have a moon crammed full of corporate logos…?

They did this, you know, back in the days of Apollo – they placed a corner-cube array at one of the landing sites. Corner cubes are retroreflectors that efficiently send light back the way it came (like reflectors on strweet signs and jogger’s vests). It worked, too – they got the retroreflection.

It was, I think, very weak. Even laser beams spreasd out rapidly, once you get past their Rayleigh Range, so , Goldfinger’s assertions to the contrary, you don’t “project a spot of light on the Moon!” – you get a really big spot that covers a huge area, and it’s not going to be visible from Earth. So Chairface Chippendale can’t carve a “CHA” into the moon, and we don’t need to worry about advertisers doing that.
On the other hand, there are other ways for advertisers to put messages on the Moon. See Heinlein’s The Man Whio Sold the Moon, Asimov’s Buy Jupiter, or Clarke’s short story in (I think) Tales of Ten Worlds or Tales of the White Harte.

It’s possible, in the sense that physics doesn’t forbid it, but in order to project an earth-visible image on a new moon (i.e. when most of the visible side is in relative darkness), you’d need an array of lasers so powerful that you’d have lots of other problems to worry about; here are a few:

-Powering the damn thing - the bigger the image, the more spread out is the light, so you have to increase the overall intensity to compensate - someone else can do the calcs, but I suspect we’re talking about a ridiculously large power budget - probably comparable to the power requirement of whole nations.
-Atmospheric effects - if you use a comparatively small number of lasers, their individual output must be very high; you’re going to ionise a lot of atmosphere on the way up there (possibly to a dangerous extent). If you use a much more widely-distributed array of comparatively lower-powered (but in fact probably still phenomenally powerful) lasers, then they are all subject to individual variances due to atmospheric distortion and your moon image will be smudgy.

In short, yes, it can be done and no, we can’t do it.

This might not be all that big a problem, as a large area on the moon will only appear as a small spot from here on Earth. The problem is delivering enough energy to that area - i.e. the intensity of the beam.

True – which is why I noted that the retroflected beam from tyhe Apollo array wasn’t visible.

My gut feeling was that the spot size would be absurdly large, but some quick calculations show that’s not the case – even only a moderately large beam waist will make an acceptably small spot on the moon, without any clever focusing. But it’ll still spread out, and ebven with high reflectivity on the surface you’re not going to detect it without sophisticated gear.

Here is some info. The retroreflectors are still there and still used to measure the distance.

As for illuminating it bright enough to see with the naked eye - let’s say you have a laser system that can “paint” a pattern on a mountain from 10 miles away. To do that to the moon would take 1 billion times more energy. Actually even that isn’t enough because you can’t do it in complete darkness. If we can see the moon (i.e. it’s not new moon), part of the sun is lit by the sun, so the laser has to be of comparable brightness to be visible right next to it. It’s like doing a slide show in broad daylight with an open window right next to the screen.


In your sigfile, you spell the word “machanics”. Is that a typo, or something clever I don’t get?

Fixed it. Thanks.

They damned well better be! If those moon people mess with our stuff again, I swear there’s going to be an ass-beating.

Or at least we’ll moon them something fierce. :smiley:

Did you mean, “Part of the moon is lit by the sun”?

Let me get that nit for you, <pluck!> Street signs and jogger’s vests use glass spheres coated on the back side with reflective material, not cube corners. The fabric and tape are sold under the brand name Scotchlite and the street signs are made with a multi step process. That is you can’t buy reflective paint in a can. Tail lights and markers cars and bicycle reflectors are cube corners.
I think we need to set a national priority to put up an geostationary space station with a 500 terawatt nuclear power station to power the lasers for a light show on the moon.

<puts Floyd on the stereo, cues up Zep> Hey, how about The Alan Parsons Project? :smiley:

But they are retroreflectors, i.e. reflectors that reflect light back in the direction it came from. The light is focused by the glass beads and reflected back. Try looking at one from all directions, and you’ll see it’s significantly brighter when you are standing near the light source (or between the light source and the retroreflector).

It’s the same reason why you get “red eye” when you use an on-camera flash. The eye acts like a glass bead retro-reflector, reflecting light back towards the flash (which is very close to the camera lens).

I saw a neat demonstration of retro-reflectors the other day when I was on an airliner. The sun was low in the sky, and I could see the shadow of my own plane on a nearby suburb. I saw dozens of bright lights near the shadow, most of them red. The lights seem to turn on when they came near the shadow, and go off afterwards. They are all retro-reflectors, probably car taillight covers and stop signs, reflecting sunlight back towards the sun.

As scr4 correctly noted, I was referring here to the concept of retroreflectors, not saying that street signs worked this way.

Although I do note that many applications that used to use the sort of s[pheres you refer to now use inexpensive plastic corner cubes. They’re now much easier to fabricate into flexible plastic pieces that are more efficient than the older types.
Heck, I wrote an article about all of this in Optics and Photonics News over a decade ago.

The first thing I thought of when I read the thread title was the fellow who wanted to organize people with laser pointers to illuminate the moon.

I’m not convinced that your intuitions are correct here. Remember that lasers radiate in a very narrow wavelength, so pretty much all the power goes towards visible (or at least detectable) light. (A five milliwatt green laser produces a visible illumination a couple of miles away.) So you get a lot more illumination for your watt than you would with, say, an incandescent bulb. For example, these guys are seriously contemplating powering a lunar-based voltaic cell from an Earth-based laser array. Sounds nuts to me, but they claim that a relatively small 2 MW laser array could provide 200KW of illumination and 50 KW of usable power on the moon. Now I don’t have the knowhow to figure out how bright a reflected light would have to be to be visible from the lunar surface, but a dot 100m across radiating at 200KW might do the trick.

If you’re content with a pulsed display, you could get away with even lower power requirements.

So I’m thinking that creating a visible dot on the moon is possible, even at our level of technology.

I’ve also seen this effect on dew-soaked grass - the droplets around the shadow of your head reflect (more or less) straight back and your shadow looks like it has a halo. (in practice, the halo effect is also enhanced by the fact that the shadows of the individual blades of grass are hidden behind the blades themselves).

That’s about the size and brightness of an illuminated high school football field. It’s not even close to being visible from the moon.

The problem isn’t how to get the light from earth to the moon; the problem is how to generate so much power that the illuminated surface is visible from 240,000 miles away, and bright enough to compete with the sunlit part of the moon. Look at this photo of the crescent earth. Do you see any lights on the dark side? No, because even our brightest cities are nowhere near the brightness of the sunlit part.

Nuclear powered satellite laser of some sort?
Eliminates the pesky atmosphere issue. The power useage I’m not so sure of.

Could a high orbit sat deliver enough of a laser punch to reflect off the moon and be seen from earth??

Atmosphere isn’t an issue here. If you want to see the spot from the ground, there’s no point in making it smaller than is allowed by our atmosphere. The issue is generating and delivering so much light that a significant size patch of ground (many square miles, perhaps hundreds) is lit up as bright as day, literally. We don’t even do it here on the ground - think how dim the lighting is in sports stadiums and offices, compared to full sunlight.