# Is it actually possible to hit the moon?



## LaserFreak (Jun 14, 2005)

Of course, taking into account beam divergence and all that other stuff, could a laser actually be made powerful enough to escape the earth's atmosphere and hit the moon? Once light gets into space it goes on infinitely, right? Would the beam still get wider as it continues on through space?

I know some of you tech-heads could probably come up with some real good answers......


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## Ivar (Jun 14, 2005)

During the lunar landings, they acctually did place mirrors on the moon that were set there with the specific reason of reflecting laser light. Today with very powerfull lasers it is possible for astronomers to measure the earth-moon distance accurate to centimeters by sending out a laser beam and measuring the time it takes for it to come back again. Even with really powerful lasers however, only a very weak light is coming back because of the beam divergence. The beam is many kilometers thick when it reaches the moon


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## LaserFreak (Jun 14, 2005)

So I take it beam divergence isn't affected by atmospheric conditions, it's soley dependant on the laser itself?


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## The_LED_Museum (Jun 14, 2005)

Yes, **some** of your laser photons will reach the moon, but don't expect to receive many of them back via reflection from the moon's surface itself or from the retroreflectors that were placed on the moon for that purpose.

If your laser was powerful enough (probably pulsed with a short pulse width in the megawatt range), and you had the appropriate filters and CCD pickup at the end of a large enough telescope, then you should be able to pick up your laser's reflection from the moon.


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## LaserFreak (Jun 14, 2005)

So, if I'm understanding you correctly, LED Museum, you would basically have to have a laser that's at least 1MW to even see a minor reflection with a telescope? Wow.


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## TurtleDude (Jun 14, 2005)

last time i heard, the most colimated laser beam that we have been able to send to the moon was about 2 miles across when it reached the surface.


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## LaserFreak (Jun 14, 2005)

That's a huge beam! I could imagine how dim that would be.


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## comozo (Jun 14, 2005)

Atmospheric conditions like temperature does affect beam divergence, so does beam stability. Believe it or not the photons from a 5mw laser can make it out of the atmosphere.
Link 1 <a href=" http://www.sfgate.com/cgi-bin/article.cgi?
file=/chronicle/archive/2002/01/17/MN81755.DTL]link1[/url]
link 2 http://science.nasa.gov/headlines/y2004/06may_lunarranging.htm


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## paulr (Jun 14, 2005)

Yes you need a pretty powerful laser but it's been done before.


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## TurtleDude (Jun 15, 2005)

oh of course its been done, some highly scientific uber accurate clocks are calibrated by measuring the distance to the moon and back via laser, and in theory, if you have a clear lign of sight (clear as in vacuum and not air) the laser should in fact go on forever


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## LaserFreak (Jun 15, 2005)

[ QUOTE ]
*TurtleDude said:*
oh of course its been done, some highly scientific uber accurate clocks are calibrated by measuring the distance to the moon and back via laser, and in theory, if you have a clear lign of sight (clear as in vacuum and not air) the laser should in fact go on forever 

[/ QUOTE ]

Ah, that's what I thought. Space is theoretically a vaccum, as far as light , and sound is concerned, right? I would imagine (althogh I am certainly no science pro) that light travels infinitely throughout space. Otherwise, how would we be able to see a star that's thousands, if not millions, of miles away with the naked eye? (under the right conditions, of course)

I mean, wouldn't anything that emminates or otherwise reflects light have a "beam" that would grow wider with added viewing distance in an atmosphere similar to ours? It would seem that our atmosphere is in a non-vaccum light state (please excuse the crudeness of this response) 

In a way, it actually makes sense. The light from the sun travels millions of miles to get to Earth, but only takes appoximately 8 minutes to reach the surface of the Earth. Technically, wouldn't that mean that the light divergence of the sun doesn't grow until it reaches the atmosphere of the Earth? If that is the case, once a laser beam exits the atmosphere, the beam shouldn't grow any wider, am I right or way off here?

This is a very interesting topic to me. I'm glad that you guys have some intelligent responses to this.


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## paulr (Jun 15, 2005)

No I mean someone aimed a laser at the moon, so you could see it blinking thru a telescope with the naked eye, as opposed to with some real slow integrating photometer. I put a link in another thread but am not sure where it is now.


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## Ivar (Jun 15, 2005)

The other thing is how you define "hitting" the moon. A few photons will probably hit the moon with a standard green laser, but nowhere near enough for you to see. Even with super powerful lasers used for measuring the distance, the reflected light is extremely dim, so special filters are needed on their telescopes that can measure the direction and frequency of the light coming in


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## comozo (Jun 15, 2005)

Lasernerd this statement is way off. "Technically, wouldn't that mean that the light divergence of the sun doesn't grow until it reaches the atmosphere of the Earth? If that is the case, once a laser beam exits the atmosphere, the beam shouldn't grow any wider, am I right or way off here?"
Atmosphere has only a little to do with divergence. I suggest you take time to learn about the wave nature of light. Here is something for you to ponder, light can never have zero divergence but if it did imagine what the Sun would look like and feel like or for that matter any celestial body.


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## TurtleDude (Jun 15, 2005)

and also to put that in perspective, millions of miles is also not very accurate, there is no star within a million miles of us, you need to use AU (astronomical units) and light years (the distance a beam of light will travel in a year, to our sun from here is about 8 light minutes)


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## The_LED_Museum (Jun 15, 2005)

One AU is approximately 93,000,000 miles - the distance between the Earth and sun.


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## LaserFreak (Jun 15, 2005)

LOL..The name's LaserFreak, not Lasernerd, but that's ok /ubbthreads/images/graemlins/grin.gif
Now that I think about it, if all light did have zero divergence in space than we probably wouldn't exist.

I am really learning a lot here. I suspected that I might be using the wrong terminology for what I was talking about.

Thanks for your input, guys. It really seems like there are a lot of factors to consider here.


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## trident (Jun 15, 2005)

What if a satellite (far outside our atmosphere) had a bright laser pointed at the moon... would it be possible for someone's naked eye to see the dot it produces? Can we fly satellites close enough to the moon such that beam divergence isn't as huge an issue?


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## LaserFreak (Jun 15, 2005)

[ QUOTE ]
*trident said:*
What if a satellite (far outside our atmosphere) had a bright laser pointed at the moon... would it be possible for someone's naked eye to see the dot it produces? Can we fly satellites close enough to the moon such that beam divergence isn't as huge an issue? 

[/ QUOTE ]

Hehe...it's YOUR avatar that got me started on this! /ubbthreads/images/graemlins/grin.gif

LOL..good question..I'm almost inclined to think after reading everyone elses responses that even if we had a satellite close to the moon shining a laser at it, we here on earth probably still wouldn't be able to see it without special optics and telescopes..divergence would still be a factor...


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## trident (Jun 15, 2005)

the mad scientist in me wants to launch a satellite with a huge grid array of RGB lasers, so that I can project commercial advertisements onto the moon for everyone to enjoy.


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## loalight (Jun 15, 2005)

Unless you were shining (from a satellite or from earth) onto a *dark* part of the moon, you would also be competing with some very bright sunlight unfiltered by any atmosphere.


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## prescottrecorder (Jun 15, 2005)

And by the way, "space" has huge clouds of material that can absorb light but is tranparent to radio frequencies. So radio astronomers can "see" further. There are even naturally occuring lasers in space (very,very big of course) where light is amplified by the stimulated emission of radiation; the same mechanism used in any manufactured laser.


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## JonSidneyB (Jun 15, 2005)

I think it will be at what ever angle to laser is at when it leaves the atomosphere. If it is growing by 5degrees, that is what it will continue to grow but but there is more.

Space is not empty. The most abundent element in the universe is hydrogen. I think it is something like 96 percent of the known mass. Thats why there is the idea of space travel using a Bussard collector, I big scoop to gather the hydrogen for fuel.


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## enLIGHTenment (Jun 15, 2005)

You're not going to get a detectable spot on the moon from any hand held laser. Handhelds just aren't powerful enough and don't have large enough optics.

Research ranging measurements are made using a 0.80 meter telescope as the objective optic. The beam on the moon is still far too weak to be naked eye visible from the ground.

http://www.as.utexas.edu/mcdonald/facilities/laser.html


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## paulr (Jun 15, 2005)

Here's the link I mentioned earlier:

http://www.k3pgp.org/viseme.htm


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## loalight (Jun 15, 2005)

nice.

I wonder how big the retroreflector is. I have one, but it's only 1".


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## enLIGHTenment (Jun 15, 2005)

[ QUOTE ]
*loalight said:*
I wonder how big the retroreflector is. I have one, but it's only 1". 

[/ QUOTE ]

The largest lunar retroreflector is a 105cm x 64cm array of small reflector half cubes.


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## LaserFreak (Jun 16, 2005)

Those are great links guys! Now THAT'S a laser!


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## loalight (Jun 16, 2005)

Ah, of course it's an array. 

just like reflective paint on signs and roads! (I think retroreflectors are neat.)


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## Raccoon (Jun 17, 2005)

Sorry I didn't see this thread until now, but I have a most difinitive answer. Yes.

I was just at the Apache Point Observatory in New Mexico (next to SunSpot and East of White Sands and Holloman Air Force Base) which beams a 2.0 Watt 532nm green laser through a 3.5 meter telescope directed at the moon. They receive only a few nano-watts of laser light back which is undetectable to the naked eye.

The beam is reflected off of retro-reflectors that were mounted all around the Apallo landers, and returns the light to earth scattered within 10's of meters radius of the origin telescope.

It's really nifty to know that such a low power laser, at only 2 Watts, can calculate the moon's distance to an exact millimeter. They said specifically 1mm when I asked. This is due to the splended seeing conditions at Apache Point, at 9700ft.

I believe the reflected laser light may be visible if it weren't for the reflected light of the sun being so much brighter.


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## trident (Jun 17, 2005)

so, suppose we were to blast it with lasers during new moon, when there isn't any sun hitting it... do you guys think it would be slightly visible?


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## LaserFreak (Jun 17, 2005)

Well, I'd imagine if there weren't any sun hitting it it might be a little hard to find, at least with the naked eye. Maybe with a telescope.....Sounds to me like you'd still need the 2 watts of output power too...


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## sniper (Jun 17, 2005)

Yeah, three guys did that in about 1970 or so. /ubbthreads/images/graemlins/hahaha.gif


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## TurtleDude (Jun 17, 2005)

maybe someone could set up a large array of lasers where several people bring in a few high powered modules (a few watts each) and have the beams combined and fired at the moon durring a lunar eclipse, you would probably see a little green twinkle on the surface


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## Catdaddy (Jun 18, 2005)

Well, let's start this thing out and see what we can all do working together from all over the country at the same time! The...Green Moon Project" /ubbthreads/images/graemlins/smile.gif



I am in with 200mw in Charleston SC.


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## The_LED_Museum (Jun 18, 2005)

I'm in with ~80mW in Seattle WA. /ubbthreads/images/graemlins/smile.gif


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## Raccoon (Jun 18, 2005)

Being retro-reflected light, it really does come right back at you. Someone in Redmond would not see (or detect) light emitted from Seattle.


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## TurtleDude (Jun 18, 2005)

so? dont spoil our fun /ubbthreads/images/graemlins/smile.gif

it would still be cool to see how many photons we canget to fly at the moon at the same time.


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## Changots (Jun 18, 2005)

here's what i think...if you can't see your laser hit the moon, then it doesn't matter if does or not /ubbthreads/images/graemlins/grouphug.gif


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## LaserFreak (Jun 18, 2005)

I'm in, providing I get my 150mW in time! /ubbthreads/images/graemlins/grin.gif


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## Changots (Jun 18, 2005)

LOL ROFL LMAO BTW LOL K THNX...........


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## tvodrd (Jun 19, 2005)

Radio amateurs (hams) have communicated via "moon bounce" for decades. It requires directional antennas with tracking capability and ~1KW effective radiated power to communicate with a similar station. The antenna arrays have a great deal of gain when transmitting or receiving, so actual transmitter power may only need to be ~100W or so. It was/is typically done in CW (morse code) due to signal/noise considerations, though some guys have now done it with voice I think. I was never in that aspect of ham radio but remember reading about it.

Larry


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## LaserFreak (Jun 19, 2005)

Umm...ok...not quite on topic..... /ubbthreads/images/graemlins/grin.gif


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## tvodrd (Jun 19, 2005)

Radio waves are _photons._ Their photon management just requires different techniques, and _Masers_ predate Lasers. /ubbthreads/images/graemlins/grin.gif

Larry


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## Draco_Americanus (Jun 19, 2005)

masers! lasers! and phasers! Oh My!!


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## Ivar (Jun 20, 2005)

I have also heard people use Radar to measure the distace, and although they can travel much further, it's not as acurate due to the huge wavelength of radio waves.


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## Ivar (Jun 23, 2005)

Are there any Rasers? using radio waves


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## Raccoon (Jun 24, 2005)

*L*ight *A*mplification by the *S*timulated *E*mission of *R*adiation.

While you can achieve line-of-sight microwave beam transmission at near-infrared frequencies, I hardly think it would qualify under the same acronym.


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## enLIGHTenment (Jun 24, 2005)

[ QUOTE ]
*Ivar said:*
Are there any Rasers? using radio waves 

[/ QUOTE ]

Yes, but they operate in the microwave region and are called MASERs.


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## deezdrama05 (Jun 24, 2005)

I think it would be a cool experiment for everyone on this forum who is interested to coordinate a certain 5 minute window of time where we all shine our green lasers at a new moon, just to see what happens, it might not conclude results but it would be awesome to just try and see what happens, I have a telescope that uses a 35mw green laser for a finderscope, ebay has some 4.5 inch newtonian reflectors that are very inexpensive- around 75 dollars shipped -these scopes are not good for deep space objects - but work extremely well on the moon and planets, it would probably be more difficult but some of us with scopes could pick a certain crater or feature on the moon to point at together, and with astrophotagraphy with elapsed photos- images(light) can be photographed that are invisible to the naked eye. This would be a tuff experiment to pull off but would be fun and interesting as to see what results would show up.


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## deezdrama05 (Jun 24, 2005)

and if several of us picked and shined our lasers into a certain crater i bet with a time lapsed astrophoto you could see a crater illuminated green! /ubbthreads/images/graemlins/huh.gif


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## Ivar (Jun 25, 2005)

We just have to figure out the time and date, and get a petition going. I can't join because I don't live in the US.


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## senecaripple (Jun 26, 2005)

i cant join either, i dont want to live in "the big house"


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## Draco_Americanus (Jun 26, 2005)

senecaripple,
To my knowlage, there is nothing illegal about a person aiming a laser into the night sky, it's when morons aim them at air craft that becomes a legal concern. People that do that must not relize that from the piolot's view there is a nice green line going right to the source.
I would also assume the same would happen if you aimed a MaxaBeam at an aircraft too


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## Changots (Jun 26, 2005)

it's illegal if it's over the 5mW legal output


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## senecaripple (Jun 27, 2005)

i think you also need a backstop!, and with my luck, i'll miss the moon and hit one of the u.s. spy satellites with my fugly face holding a laser pointer, then i get NSA come a knocking on my door, accusing me of blindingg one of their agents. then i'll be sharing a cell block with that guy from n.j.


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## yaesumofo (Jun 27, 2005)

I know this is a bit off topic. but I does talk about the moon and electromagnetic radiation. So it is close.

You guys really want to bounce somthing off the moon? Try radio waves. We amateur radio operators have been doing it for years. It is cool.
Yaesumofo


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## LaserFreak (Jun 27, 2005)

Wow...I had no idea how popular this topic would be...

It is a bit off topic...but cool nonetheless...I'm sure you can get better, farther signals from the moon than just working the skip in the atmosphere (I was a big CB'er a few years ago and shot skip frequently).

I don't quite understand how RF is equal to lasers......but whatever...someone mentioned it in this thread earlier...maybe I should go back and read more /ubbthreads/images/graemlins/grin.gif


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## comozo (Jun 27, 2005)

I really do wish people would take time to read the sticky post called "laws,Regulations" before making false declarative statements. Here is a copy of a previous post.

"I remember reading this posting on alt.lasers within the last year. An excerpt from SAMS LASER FAQ. 

"CDRH Clarification of CDRH Regulations 
Question: How are laser demonstrations - especially in classrooms - handled? Are they subject to CDRH rules? 
Answer: No. The CDRH deals with commercial products only: either laser systems that are assembled to be sold to the public, or laser shows that are created (designed) to be "sold" as a "product" to an audience. Laser shows that are not part of a commercial endeavor are NOT subject to CDRH rules, no matter what power levels are involved. This was AMAZING to hear as it runs counter to just about everything I've read here and elsewhere, so I questioned Jerry at length about it." 
The rest of the article can be read here. 

page link


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