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# Could you see a laser  from the moon?

#### kernelpanic

##### New member
Balisong said:
They did measure an incredibly accurate distance of the moon to earth during one of the apollo missions with a laser....
No past tense. The Apollo missions(11, 14, 15) placed corner cube reflectors on the Moon's surface. These mirrors are still in use. Observatories on Earth bounce lasers off of them to determine the Moon's distance.See this- http://en.wikipedia.org/wiki/Lunar_Laser_Ranging_Experiment.

I've often mentioned this fact to people who try to tell me the lunar landings were faked... None of them had a credible response as to how we got these reflectors up there without actually ever going.

cheers,
kernelpanic

#### davidgdg

##### New member
"What was the beam divergence you used for these calculations ?"

1 mrad (standard divergence figure for 532nm)

#### davidgdg

##### New member
k-shell said:
also consider, at a divergence of 1 mrad over 20,000 miles (in a vacuum) your spot size is about 20 miles in diameter, that is a lot of energy spread out. I believe the intensity would be about 1/632282112 the power of a .125 inch spot.

imagine a laser outputting only 0.0000000015mw, that's about how bright it would be.

These calculations were thrown together, as i was leaving work. so like with everything i do when im rushing, they are prone to inaccuracy
For 20,000 miles and a 250mw pointer, I get ~ 1x10^-11 mw per mm2 (0.00000000001)
It doesn't sound like much  , but it is actually about the same flux as a dim magnitude +5 star and thus would actually be visible. It just shows how amazingly sensitive the human eye is !

#### kernelpanic

##### New member
davidgdg said:
[quote author=k-shell link=1228679479/0#12 date=1228782153]also consider, at a divergence of 1 mrad over 20,000 miles (in a vacuum) your spot size is about 20 miles in diameter, that is a lot of energy spread out. I believe the intensity would be about 1/632282112 the power of a .125 inch spot.

imagine a laser outputting only 0.0000000015mw, that's about how bright it would be.

These calculations were thrown together, as i was leaving work. so like with everything i do when im rushing, they are prone to inaccuracy
For 20,000 miles and a 250mw pointer, I get ~ 1x10^-11 mw per mm2 (0.00000000001)
It doesn't sound like much  , but it is actually about the same flux as a dim magnitude +5 star and thus would actually be visible. It just shows how amazingly sensitive the human eye is ! [/quote]

It would be possible to get a much better divergence if one used a telescope as a beam expander. The Laser lunar ranging project I linked to above achieves a beam that is only 7km (~4 miles) wide on the Moon's surface!

cheers,
kernelpanic

#### steve001

##### Well-known member
kernelpanic said:
[quote author=davidgdg link=1228679479/0#18 date=1228847970][quote author=k-shell link=1228679479/0#12 date=1228782153]also consider, at a divergence of 1 mrad over 20,000 miles (in a vacuum) your spot size is about 20 miles in diameter, that is a lot of energy spread out. I believe the intensity would be about 1/632282112 the power of a .125 inch spot.

imagine a laser outputting only 0.0000000015mw, that's about how bright it would be.

These calculations were thrown together, as i was leaving work. so like with everything i do when im rushing, they are prone to inaccuracy
For 20,000 miles and a 250mw pointer, I get ~ 1x10^-11 mw per mm2 (0.00000000001)
It doesn't sound like much  , but it is actually about the same flux as a dim magnitude +5 star and thus would actually be visible. It just shows how amazingly sensitive the human eye is ! [/quote]

It would be possible to get a much better divergence if one used a telescope as a beam expander. The Laser lunar ranging project I linked to above achieves a beam that is only 7km (~4 miles) wide on the Moon's surface!

cheers,
kernelpanic
[/quote]

It's actually less than 4 miles wide now
One arcsecond is 1/3600th of a degree, or the angular size of a quarter about five kilometers (about 3 miles) away. At the distance of the moon, this angle translates to 1.8 kilometers (just over a mile).
http://www.physics.ucsd.edu/~tmurphy/apollo/basics.html

#### davidgdg

##### New member
kernelpanic said:
[quote author=davidgdg link=1228679479/0#18 date=1228847970][quote author=k-shell link=1228679479/0#12 date=1228782153]also consider, at a divergence of 1 mrad over 20,000 miles (in a vacuum) your spot size is about 20 miles in diameter, that is a lot of energy spread out. I believe the intensity would be about 1/632282112 the power of a .125 inch spot.

imagine a laser outputting only 0.0000000015mw, that's about how bright it would be.

These calculations were thrown together, as i was leaving work. so like with everything i do when im rushing, they are prone to inaccuracy
For 20,000 miles and a 250mw pointer, I get ~ 1x10^-11 mw per mm2 (0.00000000001)
It doesn't sound like much  , but it is actually about the same flux as a dim magnitude +5 star and thus would actually be visible. It just shows how amazingly sensitive the human eye is ! [/quote]

It would be possible to get a much better divergence if one used a telescope as a beam expander. The Laser lunar ranging project I linked to above achieves a beam that is only 7km (~4 miles) wide on the Moon's surface!

cheers,
kernelpanic
[/quote]

Good point. My Viper expander reduces divergence by a factor of 10. So if a 250mw could be seen at 20,000 miles in ideal conditions, then using the same expander you would be looking at 200,000 miles. So actually we are getting very close to the moon

#### davidgdg

##### New member
Benm said:
I do see some practical problems with observing a earth based laser from the moon:

In order not to be overwhelmed by the sunlight reflecting off the earth, you would have to be looking at the totally dark half of the planet. This is possible from the moon at some points in time, though not very often so.

Additionally, you would probably have to be standing in the dark on the moon not to be overwhelmed by ambient light.

This combination gives a problem: Being both in the dark on the moon, and looking at only the dark side of the earth is only possbile during a lunar eclipse (viewed from the earth). I am not sure any astronaut has even been on the moon during one, though that might prove interesting even without lasers
Actually I don't think that would be a problem. As long as the earth is in darkness and the pupils are dark adapated, the fact that it is daylight on the moon makes no difference. Of course on Earth you could not have dark-adapted pupils in the middle of the day, but on the moon there is no atmosphere and hence no light scattering. As long as you were not seeing the moon's surface or the sun in your field of view, your eyes would become dark adapted. You could achieve this either by looking through a toilet roll or sitting in a high sided cardboard box

#### kernelpanic

##### New member
More food for thought-

The lasers used for lunar ranging aren't that powerful. Only around 2.5 Watts! And the light is passing through the atmosphere twice. Plus the divergence goes to SH** once the beam hits the corner cube reflector.

There's a very good chance that a class 3B laser (esp. green) could be seen from the Moon if collimated through a large Newtonian. With a class 4 laser, a telescope, and a computer, one just might be able to homebrew an earth based version of the NASA project.

DIY Lunar Ranging anyone? ;D

cheers,
kernelpanic

#### StrictlyBudget

##### New member
kernelpanic said:
More food for thought-

The lasers used for lunar ranging aren't that powerful. Only around 2.5 Watts! And the light is passing through the atmosphere twice. Plus the divergence goes to SH** once the beam hits the corner cube reflector.

There's a very good chance that a class 3B laser (esp. green) could be seen from the Moon if collimated through a large Newtonian. With a class 4 laser, a telescope, and a computer, one just might be able to homebrew an earth based version of the NASA project.

DIY Lunar Ranging anyone?  ;D

cheers,
kernelpanic
that's a good use of lasers. i think it would be attainable for hobby-ists. it would cost at most up to \$10,000 i guess.

#### Benm

##### Well-known member
I've read on those lunar ranging measurements - the return signal is apparently only a small (countable) number of fotons, the rest is lost by the beam being larger than the reflector and such.

But even if you could somehow make the beam more narrow, it would become more and more hard to actually hit the reflector. Analogies like shooting a dime at 100 mile range come to mind.

#### diachi

##### Well-known member
Benm said:
I've read on those lunar ranging measurements - the return signal is apparently only a small (countable) number of fotons, the rest is lost by the beam being larger than the reflector and such.

But even if you could somehow make the beam more narrow, it would become more and more hard to actually hit the reflector. Analogies like shooting a dime at 100 mile range come to mind.

it's something like 2-3 photons that the receiver gets back  :

#### k-shell

##### New member
Benm said:
I've read on those lunar ranging measurements - the return signal is apparently only a small (countable) number of fotons, the rest is lost by the beam being larger than the reflector and such.

But even if you could somehow make the beam more narrow, it would become more and more hard to actually hit the reflector. [highlight]Analogies like shooting a dime at 100 mile range come to mind[/highlight].
Tomahawk FTW

#### CoZZm0_AU

##### New member
Benm said:
I've read on those lunar ranging measurements - the return signal is apparently only a small (countable) number of fotons, the rest is lost by the beam being larger than the reflector and such.

But even if you could somehow make the beam more narrow, it would become more and more hard to actually hit the reflector. Analogies like shooting a dime at 100 mile range come to mind.
I just watched an episode of Mythbusters where they were doing moon myths and they actually went to an observatory that is equipped with a GIGAWATT power laser (obviously not CW!) to fire at the moons reflectors (to prove that man made reflectors were placed on the surface), and you're exactly right, its only a few photons that are returned and hence a very sensitive telescope is required to be able to detect them at all. Round trip is ~2.5 seconds.

#### Malak

##### New member
Re: Could you see a laser *from the moon?

The moon laser ranging wiki link says it's like shooting a moving dime at two miles, FYI.