# Laser Pointers paint the moon

1. Jul 29, 2009

### DavidSnider

I ran across this site from 2001:
http://www.afineline.org/projects/paint.html

Apparently it didn't work very well. Anybody care to guess what kind of equipment it would take to make a red splotch appear on the moon to the naked eye from earth?

2. Jul 29, 2009

### Pengwuino

You probably should check this out: http://www.lpi.usra.edu/lunar/missions/apollo/apollo_11/experiments/lrr/

There was a documentary on during the anniversary of the landing where they visited the McDonald Observatory and discussed it. The laser they used was extremely high powered (it was either a 10MW or 10GW laser) that sent off pulses to the site and returned back only a few photons at a time.

3. Jul 29, 2009

### mgb_phys

1, What surface brightness would you need on the moon to see a spot?
Lets say a point on the shadowed half of the moon needs to be 1% as bright as the illuminated side. That means a surface Irradiance of 1% of the sun.
The sun's irradiance is around 1.7 W/m^2/nm at the top of the atmosphere in the visible and that's integrated over around 300nm of visible band so something like 500W/m^2.
The laser wins here because it's brightness only needs to be in a single wavelength.

2, How big is your laser spot going to be?
A good laser diode has a beam divergence of around 1mRad so at a distance of 400 million metres it will be a spot size of 400,000m.

3, How much power
If we are putting 1mW of power into a circle of 400,000m diameter for each laser and to get the 1% brightness of the sun we need 5W in each of those square metres.

We need
5W/m^2 * pi*200,000^2 m^2 = 600Bn Watts which at 0.001W each is 600 Trillion laser diodes.

4. Jul 29, 2009

### turin

Well, in "The Idea" section, a claim is made that, "In theory, even a single laser pointer could reach the Moon." That's an ambiguous statement. If the author means that the light from a single Earth-based $10 laser pointer can be made visible on the Moon (say to an astronaut standing on the Moon), then that is simply false. One thing that most people don't realize is that a laser beam most certainly spreads. This is an unavoidable consequence of Huygen's principle, and it is escpecially pronounced in the small laser pointer devices. Combine that with the fact that the output power is on the order of milliWatts, and the distance to the Moon is over 100 thousand miles away - the intensity on the Moon will not be visible. As a very liberal calculation: consider the$100 15 mW laser pointer, by Wicked Lasers, which has a beam divergence of 1 mrad. The intensity of the beamspot on the Moon would be roughly 30 μW/m2. Cheaper (i.e. reasonably priced) laser pointers produce much worse results. That is, the intensity would be like a million times dimmer on the surface of the Moon.

Now, suppose that a million people, for some reason, all decided to spend $100 on a laser pointer so that they could accomplish this goal. There remain logistic issues: they would not much improve the situation of a single laser pointer unless their beamspots overlap in both surface and time. Even supposing that this overlap were accomplished, the intensity would be less than the output intensity of a single laser pointer. Combine that with the diffuse reflection from the powdery surface of the Moon, and only a small fraction of that light will make it back to Earth. As already argued, even if all of the light made it back to Earth, the intensity at the Earth would just not be visible. I would guess that you'd need to multiply the number of lasers at least 10's of thousands of times, and that's assuming good quality laser pointers in perfect coordination, so, a minimum of 10 billion$100 laser pointers.

edit: I see that mgb_phys got to this argument first.

Last edited: Jul 29, 2009