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Old 11-05-2012, 01:25 AM #17
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Default Re: Hypotetical laser pointers Vs Moon

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Originally Posted by Benm View Post
The problem is mostly that it would not be effective, as educated guesses provided before demonstrate.

If you consider a somewhat more realistic figure like one million people pointing a $100 laser at the moon, there is no chance this will be visible from earth.

Interestingly, this would probably be a pretty spectacular view if you were standing on the moon looking at earth.

I suppose a more realistic experiment would be to have a large number of people point their lasers at the ISS, and receiving live feedback and footage from there. I'm not sure the space agencies would be interested in testing that, and pointing a laser towards the ISS is rather difficult to do since it (usually) cannot be seen by the naked eye, making it very hard to make people point their laser in the right direction despite the exactly known position at a given time.


Having footage from the viewpoint of what you are shining at is paramount though, the difference between that and reflection is enourmous. Consider the flash from a simple compact camera. In darkness, you can see its flash from several miles away (horizon is the limiting factor) if its directed at you, yet that flash is not of any using when trying to take a picture of an object 100 feet away holding that same camera.

ISS FLASH PROJECT - YouTube


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Old 11-06-2012, 01:44 AM #18
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Default Re: Hypotetical laser pointers Vs Moon

Sadly, the distance between the surface of the earth and the ISS isn't really that significant, pretty much negligible when compared to the moon's distance...
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Old 11-07-2012, 12:05 AM #19
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Default Re: Hypotetical laser pointers Vs Moon

Someone go the the moon NOW! In 5 days, everyone shine their laser at the moon!
GO!
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Old 11-08-2012, 12:09 AM #20
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Default Re: Hypotetical laser pointers Vs Moon

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Sadly, the distance between the surface of the earth and the ISS isn't really that significant, pretty much negligible when compared to the moon's distance...
The moon is, indeed, much further away, about 1000 times times further.

This doesnt make the experiment invalid per se though. The key to being visible is contrast. Looking down from the ISS, an area a few hundred meters in diameter would be the limit of resolution for the human eye. So to be visible at all, your laser needs to output more light towards the ISS then the rest of that area does.

Looking from the moon, this increases to an area the size of a large city.

Still, you can achieve such contrast if you were pointing your laser from a well picked area on the earth. From within a city like new york it would require great effort to make a contrast against all the city lights.

Moving to a hardly lit, large area, like the sahara desert or perhaps central australia, this changes significantly. You can still find places on earth where there are no cities within hundreds of miles.
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Old 11-08-2012, 12:16 AM #21
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Default Re: Hypotetical laser pointers Vs Moon

Nice input on this subject Benm. I find it rather entertaining, this idea.
I think to see it reflect off the moon, you would need something brighter then whats lighting up the moon, I.E. the Sun ()_(). i hope no one has a laser that powerful.
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Old 11-09-2012, 12:27 AM #22
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Default Re: Hypotetical laser pointers Vs Moon

I would only attempt to put a visible dot on the new moon, or the unlit site of a partial moon. Overwhelming the bright side of the moon to a point where you get visible contrast with the sun-lit part of the moon is another challenge entirely.

Obviously the light source would not have to be as powerful as the sun, as long as its directional like a laser. The sun puts out an equal amount of light in all directions, and the moon only captures a minute amount of that. That said, this irradiation is comparable to that on earth, in the order of 1 kW/m2. If you wanted to light up a speck of the moon, say a 1x1 km area, as brightly as the sun does, that would require a gigawatt of light.

If the smallest detail in the moon you can see without a telescope is a bit bigger then that, say 10x10km, that goes up to whopping 100 GW.

Hitting a spot that small would also require a laser with very good divergence - a good 'normal' laser with a divergence of 1 mrad would light up an area 400 km in diameter on the surface of the moon, and even with a good beam expander that figure doesnt drop under 40x40km.

Roughly adding those results, it would take the output of the entire US grid to power a laser (given its pretty efficient too) to light up a visible dot on the bright side of the moon, and this only works when its optimally focussed. Its something we could do if humanities future depended on it, but otherwise its not remotely feasible.
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