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Comment by iseewhatyousaw | Hacker News Reader

With the Artemis Program hopefully establishing a permanent lunar presence, it'll be interesting to see what massive radio telescope arrays will be planned and built in the decades to come.

The one thing I do wonder about is if building such telescopes will somehow restrict the amount/type of human activity that can take place on the far-side of the moon without it also impacting radio astronomy?

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If my understanding of physics is correct (and if it isn't, I'd appreciate a correction because this would be some fundamental problem), if they stuck the antennas in a crater that's opaque to most radio frequencies, and make a wall around the edges - making sure no point in the (extended) inner surface can see anything else other than the inner surface and the sky, then there shouldn't be a problem with radio interference.

Radio waves are light, and to a good approximation, radio emitters are like lightbulbs. On Earth, we have a problem because atmosphere scatters radiation. On the Moon, if you can't see the radio source directly or through a set of reflections, its signal won't get to you, period. So if you stick the antennas at the bottom of a well, they should not get any interference even if there's plenty of human activity nearby.

One problem I see is that human activity near the telescopes could create dust clouds, and those would definitely scatter radiation - and in low gravity, it could take some time for them to settle. I imagine it would make sense to prohibit rocket launches and construction work involving explosives in the vicinity of the telescopes.

EDIT: I'm looking at the picture in the TFA:

https://www.nasa.gov/sites/default/files/thumbnails/image/ni...

The crater shown there is already the kind of well I'm describing - its edges go above the nearby surface, and at least on the diagram, at no point the inner edge can see the rest of the Moon's surface.

Despite the low gravity, the complete lack of atmosphere would allow the dust to settle as fast as if you dropped a brick from the same height.

Right. I'm mostly worried about dust reaching near-escape velocities, allowing it to take its sweet time as it falls back down. I'd have to do some math to see if this is an actual problem - it might be that it's very hard to create such a cloud.

Someone will get sandblasted, but that’s not too bad because the density will fall quickly with the distance.

Not quite, radio waves can diffract around obstacles/edges, and lower-frequency waves apparently do this more efficiently. https://en.wikipedia.org/wiki/Ground_wave#Radio_propagation

(IANARE)

Edit: It seems that diffraction around edges and electromagnetic ground waves are two quite different phenomena. (A third separate effect being a refractive index vertical gradient in the atmosphere causing diffraction, acting as a waveguide.) EM ground waves require that the ground is partially conductive, which the Earth is, but I suspect the Moon isn't particularly because it's dry. Still, diffraction will occur.

Edit2: A better link for diffraction: https://en.wikipedia.org/wiki/Radio_propagation#Diffraction

"However, the angle cannot be too sharp or the signal will not diffract. ... Lower frequencies diffract around large smooth obstacles such as hills more easily."

Thank you! That's the glaring hole in my knowledge I was hoping someone would point out!

(Also I suppose this means I should turn in my HAM license...)

HAM license is the minimum knowledge so you don’t damage for everyone else, now starts the real learning.

So this is the opposite, you are eager to learn and are not afraid that people point out holes in the knowledge. Keep it up.

It is a small part of the basic general license that longer wave lengths can hug the Earth IIRC.

Fortunately we can control our intentional emissions really well and could test for unintentional ones, most intentional communications will be in the higher bands anyways because it's higher bandwidth, the antennas are more manageable, and it's the standard currently anyways.

Satellites overflying would be an obvious problem...

A great reason to build an array on Pluto.

Those satellites would be operating in higher frequency bands than most radio telescopes care about. If it does turn out to be an issue they can also be programmed to stop transmitting when they’re ‘in view’ of the telescope.

I believe that even despite the low gravity of the moon, dust settles out quite quickly. There's just nothing to stop it from free-fall, nothing to push against or mix with.

Depends on the radio frequencies used. This dish is designed to look at relatively low frequencies -- 6mhz to 30mhz. So I imagine that any potential new far-moon missions would probably just avoid those frequencies. These frequencies wouldn't be as useful as they are on earth anyways, because there's relatively low bandwidth, and there's no useful ionosphere to bounce signals are for long-range propogation like there is on earth.

Edit:

However, on the other hand, there could potentially be a lot of unintended low-frequency broadcasters -- microchips, etc. So I imagine there would still be a need for hold-out zones, but they'd probably not be as impactful as there are on earth.

Maybe the convention should be that all permanent settlements go on the side of the Moon closest to Earth? That way Moon bases get 24/7 communications with Earth and are sited on the side of the Moon that gets all the electromagnetic noise from Earth. The quiet side of the Moon could be an "EM sanctuary" reserved for research.

Worth a try, even if the agreement falls apart with the first mineral discovery on the quiet side?

You wouldn't want to put up cell towers near the radio telescopes. Green Bank has a 10 mile zone where radio activity is restricted, and on the moon you could probably get much more space dedicated to telescopes for a long time. But even if one day there starts to be some radio interference due to activity on the moon, the lack of atmosphere will probably always cause there to be orders of magnitude less interference, and there are some wavelengths that simply can't be observed from the earth because of atmospheric interference.

> and there are some wavelengths that simply can't be observed from the earth because of atmospheric interference.

Of particular relevance is the frequency associated with the transition from opaque plasma to neutral matter, which made interstellar space transparent. This coincides with the first stars, and is as close to the Big Bang as we can observe. Cosmologists are very interested in generating a detailed cosmic map at these frequencies, but they are unfortunately blocked by (1) the ionosphere of the Earth's atmosphere, and (2) subject to tons of radio interference. It's like right smack in the middle of the most commonly used frequency bands. Lunar far-side observatories are pretty much the best path towards making these measurements.

How large would a lander need to be to still be able to make useful observations from a point on the far side? Or even the near side, as the noise would come from the Earth and a directional antenna can always look the other way. A 3m dish would fit on an LM-sized lander and that’s “relatively simple” tech.

Pizza-box sized omnidirectional antennas was the plan, I think. A couple of them spread out over a large area like a crater floor. And yes, they were looking at robotic deployment.

I was looking more for the simplest-possible deployment. A dish on top of a single lander is probably easy, but now that I'm thinking of it, the arrangement only makes sense if we have a human settlement nearby to maintain and upgrade it - if not, just having a larger dish and never landing anywhere is a much better idea.

With a number of landers, if they are all in line of sight of each other, optical interferometry becomes a lot easier.

Look at the DARE mission proposal.

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