<blockquote>
  <p>Could large craters on the moon be used as reflective lenses for radio
  signals?</p>
</blockquote>

<p>You'd have to line the surface with something reflective to microwaves, like a metallic mesh, or similar materials.</p>

<p>Secondly, the shape of the crater is probably not quite ideal, so it would have to be adjusted a little, carved up a bit in various places. But it's a good start, and definitely better than starting with a flat ground.</p>

<p>There is also the question of stability - you need to make sure that whatever changes you make (carving a different shape, lining it with mesh) do not affect the stability of the crater, or else various parts may slide or collapse. This is an engineering problem.</p>

<blockquote>
  <p>Acting like a large radio telescope reflecting radio waves to a
  satellite positioned over the crater.</p>
</blockquote>

<p>Not possible unless the crater is exactly on the equator, and even then it would be tricky.</p>

<p>But a crater like the one in your picture is so strongly curved, the focal length is about the same as the diameter. In other words, if the diameter of the hole is X, the altitude of the receiver is pretty close to X - give or take something like 50% or so, depending on the exact curvature. It might be easier to just build a giant arch over the crater. Again, this is a matter of engineering.</p>

CC BY-SA 3.0
2014-05-06T00:13:57.220Z
2014-05-06T00:13:57.220Z
Florin Andrei
<p>If it works for Arecibo it could be made to work on the Moon, Ceres, and maybe even Mars. The point about sending a prefab is a good one. And using two radio telescopes in tandem separated in space would allow for astronomers to take advantage of the improved angular resolution this affords provided they were arranged so as to allow them to simultaneously focus on the same point. That would be a neat problem in astrophysics!  Radio telescopes aimed at the same point simultaneously from the Earth and Mars while they are on opposite sides of their orbits would have incredible resolution.  </p>

CC BY-SA 4.0
2015-09-20T02:41:52.030Z
2018-12-17T10:39:06.723Z
2018-12-17T10:39:06.723Z
user9250
<p>Yes, Frank Drake looked into this - see page 91 here
<a href="http://www.lpi.usra.edu/lunar/strategies/objectives/ast_observations_moon.pdf" rel="nofollow">http://www.lpi.usra.edu/lunar/strategies/objectives/ast_observations_moon.pdf</a></p>

<p>He calculated that you could make a telescope with diameter 30 km using steel, and up to 60 to 90 km with stronger materials. And no problem with wind loading on the Moon.</p>

CC BY-SA 3.0
2016-04-03T16:43:18.667Z
2016-04-03T16:43:18.667Z
Robert Walker
<p>I agree that the crater needs to be lined, but you also have the problem of maintaining a satellite in stationary orbit above the crater. Nearly impossible unless the crater is on the equatorial plane. Also, a stationary satellite around the moon would be influenced by the Earth, so you would need to burn fuel to keep the satellite in position.</p>

CC BY-SA 3.0
2014-05-06T00:16:07.950Z
"a stationary satellite around the moon would be influenced by the Earth" - if the orbit is close enough to the Moon, the orbit would be stable. Look up the notion of Hill sphere. Orbits inside the Hill sphere tend to be stable.
Florin Andrei
CC BY-SA 3.0
2014-05-06T01:53:50.610Z
@FlorinAndrei The moon only rotates once every 29 days, that's a pretty high orbit. I think it would easily be outside the hill sphere.
LDC3
CC BY-SA 3.0
2014-05-07T06:03:27.773Z
A calculation on [Wolfram|Alpha](http://www.wolframalpha.com/input/?i=%28%28gravitational+constant%29*%28moon+mass%29*%2829+days%2F%282pi%29%29%5E2%29%5E%281%2F3%29) says that selenosynchronous orbit would be 92 040 km from the Moon's centre. This seems to be about the same as the size of the [Moon's Hill sphere](http://www.wolframalpha.com/input/?i=%28earth-moon+distance%29*%28%28moon+mass%29%2F%28earth+mass%29%29%5E%281%2F3%29) (if I've understood [Wikipedia's formula](https://en.wikipedia.org/wiki/Hill_sphere) correctly).
Warrick
CC BY-SA 3.0
2014-05-08T01:35:21.543Z
@Warrick I get the Hill Sphere to be 61530 km and the stationary orbit at 88460 km. That means the satellite would be in an unstable position.
LDC3
CC BY-SA 3.0
2014-05-08T05:40:12.383Z
@LDC3: I'm not familiar with Hill spheres so I might be misapplying the formula. There seems to be a factor of $(1/3)^{1/3}$ in the first formula but not the last. Either way, I'm on your side that selenosynchronous orbits would need effort to maintain.
Warrick
CC BY-SA 3.0
2014-05-14T08:28:35.043Z
@FlorinAndrei a lunar-stationary orbit, if it's even possible given the proximity of the moon to the earth, would not be close to the surface.
jwenting
CC BY-SA 3.0
2014-05-05T03:09:16.633Z
2014-05-05T03:09:16.633Z
LDC3
<p>Interesting idea. I think the answer is both yes and no -- yes with a manufactured dish but no in the crater's raw state.</p>

<p>The Arecibo telescope sits in a natural crater, but adds a dish which has a couple of important things required by a radio dish:</p>

<ol>
<li>a radio-reflective surface</li>
<li>a specific curvature, classically parabolic, but also shaped</li>
<li>low surface roughness typically in the order of mm to µm for radio frequencies we're interested in. See <a href="http://en.wikipedia.org/wiki/Ruze%27s_Equation" rel="noreferrer">http://en.wikipedia.org/wiki/Ruze%27s_Equation</a></li>
<li>a receiver close to the reflective surface (focal length), a satellite would probably be beyond the focal point</li>
<li>probably some some other things, but that's probably enough.</li>
</ol>

<p>Edit: Added #4</p>

CC BY-SA 4.0
2018-12-16T20:20:57.300Z
Arecibo does not sit in a crater, it's a sink hole, and irrelevant to the question, which specifically mentions [impact] craters on the moon, and "their" suitablility.
tahwos
CC BY-SA 3.0
2014-05-05T16:57:26.357Z
Could rovers feasibly resurface a naturally nearly spherical Lunar crater to an Arecibo type telescope, by moving the Lunar soil around a bit and then burning it to a precision glassy surface by focusing Solar light on it at incredible heat? Does that sound as a possible technology this century, or is it conceptually physically just wrong? Does it make more sense to just fold out or build our own parabolic dishes from scratch over there?
LocalFluff
CC BY-SA 3.0
2014-05-06T22:15:06.587Z
Actually the Arecibo dish is spherical, not parabolic, because it's steered by moving the detector, not the dish. Reference: https://en.wikipedia.org/wiki/Arecibo_Observatory
Keith Thompson
CC BY-SA 3.0
2014-05-09T00:05:53.593Z
LocalFluff -- another interesting idea, but it may be easier to just install a pre-fab surface. Have a look at Spektr-R.
user1563
CC BY-SA 3.0
2015-09-20T07:55:53.507Z
A satellite couldn't be used. There are no Luna Stationary orbits. There are the Earth-Moon Lagrange points, but all are much to high, and inconveniently positioned. The receiver would be part of the telescope, just as at Arecibo. A satellite could be used as a relay, to transmit data back to Earth.
James K
CC BY-SA 3.0
2014-05-05T03:04:20.457Z
2014-05-09T00:08:08.773Z
2014-05-09T00:08:08.773Z
user1563
<p>Could large craters on the moon be used as reflective lenses for radio signals?</p>

<p>Acting like a large radio telescope reflecting radio waves to a satellite positioned over the crater.</p>

<p><img src="https://i.sstatic.net/xAMMj.jpg" alt="enter image description here"></p>

<the-moon><radio-astronomy>
Can impact craters on the moon act like giant radio telescopes?
Reactgular
# Can impact craters on the moon act like giant radio telescopes?

(Asked by: username_0 on 2014-05-04)

Could large craters on the moon be used as reflective lenses for radio signals?

Acting like a large radio telescope reflecting radio waves to a satellite positioned over the crater.

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

**Tags:** the-moon, radio-astronomy

---

## 5 answers

### Answer by username_7 on 2014-05-05. Score: 14 | Accepted answer

Interesting idea. I think the answer is both yes and no -- yes with a manufactured dish but no in the crater's raw state.

The Arecibo telescope sits in a natural crater, but adds a dish which has a couple of important things required by a radio dish:

1. a radio-reflective surface
2. a specific curvature, classically parabolic, but also shaped
3. low surface roughness typically in the order of mm to µm for radio frequencies we're interested in. See <http://en.wikipedia.org/wiki/Ruze%27s_Equation>
4. a receiver close to the reflective surface (focal length), a satellite would probably be beyond the focal point
5. probably some some other things, but that's probably enough.

Edit: Added #4

> **Comments:**
> **username_9**: Could rovers feasibly resurface a naturally nearly spherical Lunar crater to an Arecibo type telescope, by moving the Lunar soil around a bit and then burning it to a precision glassy surface by focusing Solar light on it at incredible heat? Does that sound as a possible technology this century, or is it conceptually physically just wrong? Does it make more sense to just fold out or build our own parabolic dishes from scratch over there?  
> **username_10**: Actually the Arecibo dish is spherical, not parabolic, because it's steered by moving the detector, not the dish. Reference: https://en.wikipedia.org/wiki/Arecibo_Observatory  
> **username_7**: LocalFluff -- another interesting idea, but it may be easier to just install a pre-fab surface. Have a look at Spektr-R.  
> **username_11**: A satellite couldn't be used. There are no Luna Stationary orbits. There are the Earth-Moon Lagrange points, but all are much to high, and inconveniently positioned. The receiver would be part of the telescope, just as at Arecibo. A satellite could be used as a relay, to transmit data back to Earth.  
> **username_8**: Arecibo does not sit in a crater, it's a sink hole, and irrelevant to the question, which specifically mentions [impact] craters on the moon, and "their" suitablility.  

---

### Answer by username_1 on 2014-05-06. Score: 5

> Could large craters on the moon be used as reflective lenses for radio
> signals?

You'd have to line the surface with something reflective to microwaves, like a metallic mesh, or similar materials.

Secondly, the shape of the crater is probably not quite ideal, so it would have to be adjusted a little, carved up a bit in various places. But it's a good start, and definitely better than starting with a flat ground.

There is also the question of stability - you need to make sure that whatever changes you make (carving a different shape, lining it with mesh) do not affect the stability of the crater, or else various parts may slide or collapse. This is an engineering problem.

> Acting like a large radio telescope reflecting radio waves to a
> satellite positioned over the crater.

Not possible unless the crater is exactly on the equator, and even then it would be tricky.

But a crater like the one in your picture is so strongly curved, the focal length is about the same as the diameter. In other words, if the diameter of the hole is X, the altitude of the receiver is pretty close to X - give or take something like 50% or so, depending on the exact curvature. It might be easier to just build a giant arch over the crater. Again, this is a matter of engineering.


---

### Answer by username_2 on 2015-09-20. Score: 4

If it works for Arecibo it could be made to work on the Moon, Ceres, and maybe even Mars. The point about sending a prefab is a good one. And using two radio telescopes in tandem separated in space would allow for astronomers to take advantage of the improved angular resolution this affords provided they were arranged so as to allow them to simultaneously focus on the same point. That would be a neat problem in astrophysics! Radio telescopes aimed at the same point simultaneously from the Earth and Mars while they are on opposite sides of their orbits would have incredible resolution.


---

### Answer by username_3 on 2016-04-03. Score: 3

Yes, Frank Drake looked into this - see page 91 here
<http://www.lpi.usra.edu/lunar/strategies/objectives/ast_observations_moon.pdf>

He calculated that you could make a telescope with diameter 30 km using steel, and up to 60 to 90 km with stronger materials. And no problem with wind loading on the Moon.


---

### Answer by username_4 on 2014-05-05. Score: 2

I agree that the crater needs to be lined, but you also have the problem of maintaining a satellite in stationary orbit above the crater. Nearly impossible unless the crater is on the equatorial plane. Also, a stationary satellite around the moon would be influenced by the Earth, so you would need to burn fuel to keep the satellite in position.

> **Comments:**
> **username_1**: "a stationary satellite around the moon would be influenced by the Earth" - if the orbit is close enough to the Moon, the orbit would be stable. Look up the notion of Hill sphere. Orbits inside the Hill sphere tend to be stable.  
> **username_4**: @FlorinAndrei The moon only rotates once every 29 days, that's a pretty high orbit. I think it would easily be outside the hill sphere.  
> **username_5**: A calculation on [Wolfram|Alpha](http://www.wolframalpha.com/input/?i=%28%28gravitational+constant%29*%28moon+mass%29*%2829+days%2F%282pi%29%29%5E2%29%5E%281%2F3%29) says that selenosynchronous orbit would be 92 040 km from the Moon's centre. This seems to be about the same as the size of the [Moon's Hill sphere](http://www.wolframalpha.com/input/?i=%28earth-moon+distance%29*%28%28moon+mass%29%2F%28earth+mass%29%29%5E%281%2F3%29) (if I've understood [Wikipedia's formula](https://en.wikipedia.org/wiki/Hill_sphere) correctly).  
> **username_4**: @Warrick I get the Hill Sphere to be 61530 km and the stationary orbit at 88460 km. That means the satellite would be in an unstable position.  
> **username_5**: @LDC3: I'm not familiar with Hill spheres so I might be misapplying the formula. There seems to be a factor of $(1/3)^{1/3}$ in the first formula but not the last. Either way, I'm on your side that selenosynchronous orbits would need effort to maintain.  
> **username_6**: @FlorinAndrei a lunar-stationary orbit, if it's even possible given the proximity of the moon to the earth, would not be close to the surface.  

---

