<blockquote>
  <p>Neither of these seem correct to me.</p>
</blockquote>

<p>You are correct. The Apollo lunar lander was 4 meters across, or about 2 milliarcseconds at a distance of 385000 kilometers. The Hubble has a resolution of 50 milliarcseconds. Even the Hubble can't see the tracks and artifacts that the Apollo astronauts left on the Moon, let alone a backyard astronomer.</p>

<p>Regarding a backyard astronomer "seeing" the retroreflectors left on the Moon, that too is incorrect. "Seeing" those retroreflectors requires an extremely powerful pulsed laser, a rather large telescope, and rather expensive photodetectors. The final item is why I wrote "seeing" (in quotes) rather than seeing (without quotes).</p>

<p>The professional astronomers who use those retroreflectors to measure the distance between the Earth and the Moon receive one reflected photon for every 10<sup>17</sup> photons sent toward the Moon. They are looking for individual photons. Using pulsed lasers achieves two ends. First and foremost, it lets the astronomers measure distance. A continuous laser wouldn't let them do that. Secondly, it enables them to distinguish those individual photons from background noise. The lasers used in the lunar laser ranging experiments have a peak power in the gigawatts.</p>

<p>An amateur backyard astronomer who a few million dollar or so to spare might be able to build a system that "sees" the retroreflectors left on the Moon. An amateur backyard astronomer cannot see the artifacts left on the Moon.</p>

CC BY-SA 3.0
2015-11-13T16:06:49.693Z
2015-11-13T16:06:49.693Z
David Hammen
<ol>
<li><p>You can point a laser at the moon, but you won't be able to perceive any photons reflecting back.</p>

<p>The beam spreads out due to diffraction. The rest of the illuminated side of the moon would far overwhelm your eyes, even at a slim crescent, to perceive any reflected photons from your laser.</p>

<p><a href="https://what-if.xkcd.com/13/" rel="nofollow">xkcd's What If? #13</a> shows what would happen if you got more people and more lasers.</p>

<p>The retroreflectors left by Apollo astronauts won't help. You need a lot of <a href="http://physics.ucsd.edu/~tmurphy/apollo/basics.html" rel="nofollow">complicated equipment</a> to get even one photon back that you can distinguish.</p></li>
<li><p>Phil Plait, author of the Bad Astronomy blog, <a href="http://physics.ucsd.edu/~tmurphy/apollo/basics.html" rel="nofollow">covered</a> why we can't use telescopes, even the Hubble, to resolve artifacts on the moon. He walks through the calculations, and manmade objects on the moon are far smaller than the resolving power of any telescope.</p></li>
</ol>

CC BY-SA 3.0
2015-11-13T16:01:59.830Z
2015-11-14T18:23:34.497Z
2015-11-14T18:23:34.497Z
JustinW80
<p>The video is hilariously wrong.</p>

<p>However, the principle of laser ranging is more or less right, and it does require the reflectors left behind by the astronauts on the Moon. It's just that the physics and technology involved are far beyond just pointing a toy laser at the Moon.</p>

<p>Project APOLLO (Apache Point Observatory Lunar Laser-ranging Operation) is actually doing this.</p>

<p><a href="http://physics.ucsd.edu/~tmurphy/apollo/basics.html" rel="noreferrer">http://physics.ucsd.edu/~tmurphy/apollo/basics.html</a></p>

<p>You need a fairly large telescope to start with - both for collimating the outgoing light pulse, and for receiving as much of the reflection as possible. APOLLO uses a 3.5 meter telescope at the <a href="http://www.apo.nmsu.edu/" rel="noreferrer">Apache Point observatory</a>.</p>

<p>You need a laser that can generate a high-energy light pulse that is very short. The pulse is injected into the telescope's optical train and sent to the Moon. This is not a laser pointer; it's a high power Q-switched laser for research, <a href="http://physics.ucsd.edu/~tmurphy/apollo/onscope.html" rel="noreferrer">a device the size of a refrigerator</a>.</p>

<p>On the receiving end, you need a very good detector also plugged into the telescope. Of the many, many photons sent to the Moon in the pulse, only between 1 and 5 photons make it back down to the detector. You need a detector that can tell the extra 1 to 5 photons from the background noise of light coming from the Moon.</p>

<p><a href="https://i.sstatic.net/jHEhu.gif" rel="noreferrer"><img src="https://i.sstatic.net/jHEhu.gif" alt="enter image description here"></a></p>

<p>Using this technique, the distance from Earth to Moon can be measured with very high precision.</p>

<p>This is the APOLLO system in action:</p>

<p><a href="https://i.sstatic.net/LYFQv.gif" rel="noreferrer"><img src="https://i.sstatic.net/LYFQv.gif" alt="enter image description here"></a></p>

<hr>

<p>Regarding observing human artifacts on the Moon with terrestrial telescopes, I wish it was doable, but it's not. Again the YouTube video is wrong.</p>

<p>The biggest telescope accessible to amateurs has an aperture (diameter) in the range of 1 meter or a little bit larger (the aperture of the largest amateur-owned telescopes currently). The resolving power of a telescope (the size of finest details discernible) depends on aperture - if aperture is measured in mm and the resolving power in measured in arcseconds, then the formula is:</p>

<p>resolving power = 100 / aperture</p>

<p>So a 1 meter telescope has a resolving power of 0.1 arcseconds.</p>

<p>The distance from Earth to Moon is 384000 km (3.84 * 10^8 meters). With a resolving power of 0.1 arcseconds, the size of the smallest detail discernible on the Moon is:</p>

<p>detail size = distance_from_Earth_to_Moon * arctan(resolving power)</p>

<p>or</p>

<p>detail size = <a href="http://www.wolframalpha.com/input/?i=3.84%20*%2010%5E8%20*%20arctan%280.1%20arcsec%29" rel="noreferrer">3.84 * 10^8 * arctan(0.1 arcsec)</a> = 186 meters</p>

<p>Anything smaller than 186 meters would be blurred into a single dot. There's nothing we've done on the Moon that's as big as that. It's not possible to see traces of human activity with amateur-level telescopes, even with extremely large meter-class dobsonian telescopes. Even with professional telescopes, we just don't have the aperture yet to resolve that kind of detail.</p>

<p>However, satellites in orbit around the Moon, such as the LRO, were able to image traces of the Apollo missions. That's because they are a lot closer to the site.</p>

<p><a href="http://www.nasa.gov/mission_pages/LRO/news/apollo-sites.html" rel="noreferrer">http://www.nasa.gov/mission_pages/LRO/news/apollo-sites.html</a></p>

<p><a href="https://i.sstatic.net/S5Eiz.jpg" rel="noreferrer"><img src="https://i.sstatic.net/S5Eiz.jpg" alt="enter image description here"></a></p>

CC BY-SA 3.0
2015-11-15T00:35:39.803Z
Argon gas laser, or have they gone to a solid state diode type now? I don't know whether anyone makes a diode with the required output yet.
Wayfaring Stranger
CC BY-SA 3.0
2015-11-15T08:57:37.540Z
Energy per pulse is one factor. Also, I don't think you can make pulses that short with diodes. But I could be wrong.
Florin Andrei
CC BY-SA 3.0
2015-11-15T19:30:24.120Z
“The video is hilariously wrong.”  Thanks.  I posted a comment on the video pointing back here.  Those who think such admonishment is worth spreading, you might try upvoting and replying to that comment or posting another such comment to the video, so they get seen among the sea of "typical" YouTube comments.
JDługosz
CC BY-SA 3.0
2016-07-18T15:37:03.427Z
Thanks for the great answer!  Say, could the Hubble at 2.4 m and no atmosphere, image anything of ours on the moon?  Is that close, or no chance??  (Ah actually David answered that below: No.  50 versus 2 milliarcseconds!)
Fattie
CC BY-SA 3.0
2015-11-13T19:33:56.473Z
2015-11-16T03:20:07.113Z
2015-11-16T03:20:07.113Z
Florin Andrei
<p>In <a href="http://youtu.be/UHU226kWZIA">this video</a> the host of Test Tube Plus states that</p>

<ul>
<li>you can go out a buy a laser, point it at the moon, and see the retroreflector left by the astronauts.  When you point at the right spot you'll see a reflection, elsewhere not.  He reiterates that this is not just a hypothetical use of 2nd person for effect, but that <em>you</em> the viewer can go out and try this yourself, and anyone paying any attention to moonlanding conspiracy theories ought to try it</li>
<li>tracks and artefacts at the landing site can be seen with amateur telescopes, and again this is approachable to anyone who cares to look</li>
</ul>

<p>Neither of these seem correct to me.  Could someone with real numbers weigh in on this?</p>

<p>I seem to recall that seeing artefacts is only possible in the latest high-res images taken from lunar orbit, and the telescope that uses the reflector gets back single photons on some trials and is a much more powerful laser than you could buy!  But, a continuous beam may be different? How big of a laser would you need to see a reflection with a viewing instrument that doesn't cost more than the laser?</p>

<amateur-observing><moonlanding>
Visibility of human activity on the moon
JDługosz
# Visibility of human activity on the moon

(Asked by: username_0 on 2015-11-13)

In [this video](http://youtu.be/UHU226kWZIA) the host of Test Tube Plus states that

* you can go out a buy a laser, point it at the moon, and see the retroreflector left by the astronauts. When you point at the right spot you'll see a reflection, elsewhere not. He reiterates that this is not just a hypothetical use of 2nd person for effect, but that *you* the viewer can go out and try this yourself, and anyone paying any attention to moonlanding conspiracy theories ought to try it
* tracks and artefacts at the landing site can be seen with amateur telescopes, and again this is approachable to anyone who cares to look

Neither of these seem correct to me. Could someone with real numbers weigh in on this?

I seem to recall that seeing artefacts is only possible in the latest high-res images taken from lunar orbit, and the telescope that uses the reflector gets back single photons on some trials and is a much more powerful laser than you could buy! But, a continuous beam may be different? How big of a laser would you need to see a reflection with a viewing instrument that doesn't cost more than the laser?

**Tags:** amateur-observing, moonlanding

---

## 3 answers

### Answer by username_3 on 2015-11-13. Score: 17 | Accepted answer

The video is hilariously wrong.

However, the principle of laser ranging is more or less right, and it does require the reflectors left behind by the astronauts on the Moon. It's just that the physics and technology involved are far beyond just pointing a toy laser at the Moon.

Project APOLLO (Apache Point Observatory Lunar Laser-ranging Operation) is actually doing this.

<http://physics.ucsd.edu/~tmurphy/apollo/basics.html>

You need a fairly large telescope to start with - both for collimating the outgoing light pulse, and for receiving as much of the reflection as possible. APOLLO uses a 3.5 meter telescope at the [Apache Point observatory](http://www.apo.nmsu.edu/).

You need a laser that can generate a high-energy light pulse that is very short. The pulse is injected into the telescope's optical train and sent to the Moon. This is not a laser pointer; it's a high power Q-switched laser for research, [a device the size of a refrigerator](http://physics.ucsd.edu/~tmurphy/apollo/onscope.html).

On the receiving end, you need a very good detector also plugged into the telescope. Of the many, many photons sent to the Moon in the pulse, only between 1 and 5 photons make it back down to the detector. You need a detector that can tell the extra 1 to 5 photons from the background noise of light coming from the Moon.

[![enter image description here](https://i.sstatic.net/jHEhu.gif)](https://i.sstatic.net/jHEhu.gif)

Using this technique, the distance from Earth to Moon can be measured with very high precision.

This is the APOLLO system in action:

[![enter image description here](https://i.sstatic.net/LYFQv.gif)](https://i.sstatic.net/LYFQv.gif)

---

Regarding observing human artifacts on the Moon with terrestrial telescopes, I wish it was doable, but it's not. Again the YouTube video is wrong.

The biggest telescope accessible to amateurs has an aperture (diameter) in the range of 1 meter or a little bit larger (the aperture of the largest amateur-owned telescopes currently). The resolving power of a telescope (the size of finest details discernible) depends on aperture - if aperture is measured in mm and the resolving power in measured in arcseconds, then the formula is:

resolving power = 100 / aperture

So a 1 meter telescope has a resolving power of 0.1 arcseconds.

The distance from Earth to Moon is 384000 km (3.84 \* 10^8 meters). With a resolving power of 0.1 arcseconds, the size of the smallest detail discernible on the Moon is:

detail size = distance\_from\_Earth\_to\_Moon \* arctan(resolving power)

or

detail size = [3.84 \* 10^8 \* arctan(0.1 arcsec)](http://www.wolframalpha.com/input/?i=3.84%20*%2010%5E8%20*%20arctan%280.1%20arcsec%29) = 186 meters

Anything smaller than 186 meters would be blurred into a single dot. There's nothing we've done on the Moon that's as big as that. It's not possible to see traces of human activity with amateur-level telescopes, even with extremely large meter-class dobsonian telescopes. Even with professional telescopes, we just don't have the aperture yet to resolve that kind of detail.

However, satellites in orbit around the Moon, such as the LRO, were able to image traces of the Apollo missions. That's because they are a lot closer to the site.

<http://www.nasa.gov/mission_pages/LRO/news/apollo-sites.html>

[![enter image description here](https://i.sstatic.net/S5Eiz.jpg)](https://i.sstatic.net/S5Eiz.jpg)

> **Comments:**
> **username_4**: Argon gas laser, or have they gone to a solid state diode type now? I don't know whether anyone makes a diode with the required output yet.  
> **username_3**: Energy per pulse is one factor. Also, I don't think you can make pulses that short with diodes. But I could be wrong.  
> **username_0**: “The video is hilariously wrong.”  Thanks.  I posted a comment on the video pointing back here.  Those who think such admonishment is worth spreading, you might try upvoting and replying to that comment or posting another such comment to the video, so they get seen among the sea of "typical" YouTube comments.  
> **username_5**: Thanks for the great answer!  Say, could the Hubble at 2.4 m and no atmosphere, image anything of ours on the moon?  Is that close, or no chance??  (Ah actually David answered that below: No.  50 versus 2 milliarcseconds!)  

---

### Answer by username_1 on 2015-11-13. Score: 7

> Neither of these seem correct to me.

You are correct. The Apollo lunar lander was 4 meters across, or about 2 milliarcseconds at a distance of 385000 kilometers. The Hubble has a resolution of 50 milliarcseconds. Even the Hubble can't see the tracks and artifacts that the Apollo astronauts left on the Moon, let alone a backyard astronomer.

Regarding a backyard astronomer "seeing" the retroreflectors left on the Moon, that too is incorrect. "Seeing" those retroreflectors requires an extremely powerful pulsed laser, a rather large telescope, and rather expensive photodetectors. The final item is why I wrote "seeing" (in quotes) rather than seeing (without quotes).

The professional astronomers who use those retroreflectors to measure the distance between the Earth and the Moon receive one reflected photon for every 1017 photons sent toward the Moon. They are looking for individual photons. Using pulsed lasers achieves two ends. First and foremost, it lets the astronomers measure distance. A continuous laser wouldn't let them do that. Secondly, it enables them to distinguish those individual photons from background noise. The lasers used in the lunar laser ranging experiments have a peak power in the gigawatts.

An amateur backyard astronomer who a few million dollar or so to spare might be able to build a system that "sees" the retroreflectors left on the Moon. An amateur backyard astronomer cannot see the artifacts left on the Moon.


---

### Answer by username_2 on 2015-11-13. Score: 5

1. You can point a laser at the moon, but you won't be able to perceive any photons reflecting back.

   The beam spreads out due to diffraction. The rest of the illuminated side of the moon would far overwhelm your eyes, even at a slim crescent, to perceive any reflected photons from your laser.

   [xkcd's What If? #13](https://what-if.xkcd.com/13/) shows what would happen if you got more people and more lasers.

   The retroreflectors left by Apollo astronauts won't help. You need a lot of [complicated equipment](http://physics.ucsd.edu/~tmurphy/apollo/basics.html) to get even one photon back that you can distinguish.
2. Phil Plait, author of the Bad Astronomy blog, [covered](http://physics.ucsd.edu/~tmurphy/apollo/basics.html) why we can't use telescopes, even the Hubble, to resolve artifacts on the moon. He walks through the calculations, and manmade objects on the moon are far smaller than the resolving power of any telescope.


---

