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Lunar Crater Radio Telescope on the Far-Side of the Moon (2020) (nasa.gov)
192 points by rbanffy 13 days ago | hide | past | favorite | 101 comments

Youtube channel Launch Pad Astronomy discussed some of the challenges for a lunar telescope: https://www.youtube.com/watch?v=QKJY7gH2n9I

One of the problems is moon dust. There's a lot of it. If you build a big parabolic dish on the moon, you'll need some way to either remove dust or prevent it from accumulating in the first place.

Another issue is the change in temperature. Without an atmosphere stabilize the temperature, day times become extremely hot and night times are extremely cold. This means that you need to engineer something that can withstand very significant swings in temperature over the course of the lunar day.

Challenges like these highlight the benefit of placing James Webb space telescope at Lagrange Point L2. The temperature remains constant and there's far less interplanetary dust than moon dust.

That isn't to say that we shouldn't build a lunar telescope, but we should have a clear understanding of the difficulties.

One of the problems is moon dust.

Well, yes, but at radio wavelengths, it would take an awful lot of dust to significantly impact the performance of the telescope. My gut says centuries worth if not millennia. And the article is proposing a wire mesh primary reflector, so dust will mostly just fall through.

Another issue is the change in temperature.

Similar answer to the above. The wire mesh reflector would expand and contract. I suspect the main impact would be to change the location of the focal point. We can handle that by moving the receiver up and down similar to what we do with telescopes on Earth.

I think the wire mesh expansion and contraction is not such a huge issue for a number of reasons.

First of all, you will probably want to make observations when not illuminated by the Sun, so that the receiver is shielded by the mass of the Moon from the Sun interference.

Second, on the Moon there is no weather. You basically have only two conditions, it either is illuminated by the full sun or it is not (with some time while it is partly illuminated, but most of the time is just basically two temperatures). During night the temperature will be rather predictable for the entire length of the night.

So I think the simplest solution would be to just use the telescope for about 50% of the time when its geometry is stable and is shielded from the Sun.

It is not such a huge issue, the basic fact of this kind of telescope is that you can't steer it so you still rely on orbital mechanics of the body on which you place to move the part of the sky which it listens to.

In this case even restricting to 50% of the time does not change much as it still requires 1 whole year to make observation of entire part of the sky it can observe.

> Another issue is the change in temperature.

The changes in temperature are exaggerated. If you cover the telescope during day with some type of cover (like space blanket [1]), then everything that's not in direct light will stay at the same temperature as during the lunar night. For the simple reason that there's no air on the moon to transfer heat, and the regolith is a very good insulator.

So you could end up using the telescope 14 days out of 28 and be done with it.

But what's more, you can set up the space blanket to only protect the part of the telescope that's in direct sunlight. For the rest of the telescope it will appear like it's perfect night, because there's not atmosphere to disperse the rays of the sun. So you can end up using the telescope during the lunar day as well, although not at full exposure.

[1] https://en.wikipedia.org/wiki/Space_blanket

> 14 days out of 28

Out of 29 and a half actually.

I'm curious where the misconception that a lunar month is 28 days comes from. I believed it too but I can't find any good reason for doing so.

You are right, and I was just lazy. 28 is an even number, and it felt nicer to say 14 days out of 28, rather than 14.75 out of 29.5.

Separately though, the misconception comes from the fact that we have 4 phases of the Moon (new, first quarter, full, last quarter), and it's easy to conceptualize that the interval between two is 7 days, or one week, rather than 7.38264725.

Sidereal month (relative to fixed stars) is 27.32 days. Synodic month relative to Earth is 29.53. There are other months that correct sidereal for motion around the sun, eccentricity of the lunar orbit, and tilt of lunar orbit, but the difference is tiny.


>it would take an awful lot of dust to significantly impact the performance of the telescope.

The dust also picks up charge and levitates, but I suppose that allows you to sweep it up using a charged broom.

> One of the problems is moon dust. There's a lot of it. If you build a big parabolic dish on the moon, you'll need some way to either remove dust or prevent it from accumulating in the first place.

Moon doesn't have atmosphere, so once you've built the dish and cleaned it, it should stay clean unless a meteorite strikes nearby.

Solar winds can also kick up moon dust. It would be a long slow process though and I think the reflector would still work under a thin layer of dust. By the time it’s a problem we’ll hopefully be there in person to either dust it off or build a bigger better one.

Minor correction but the moon does have an atmosphere although it is largely negligible when compared to earth's.

>"At sea level on Earth, we breathe in an atmosphere where each cubic centimeter contains 10,000,000,000,000,000,000 molecules; by comparison the lunar atmosphere has less than 1,000,000 molecules in the same volume."

>"We think that there are several sources for gases in the moon's atmosphere. These include high energy photons and solar wind particles knocking atoms from the lunar surface, chemical reactions between the solar wind and lunar surface material, evaporation of surface material, material released from the impacts of comets and meteoroids, and out-gassing from the moon's interior."

Source: https://www.nasa.gov/mission_pages/LADEE/news/lunar-atmosphe...

Also interesting, there is a thin film of electrostatically charged dust (regolith) that is visible across the moon's horizon during lunar sunrises and sunsets. The Apollo 17 crew drew a sketch to depict this phenomenon https://en.wikipedia.org/wiki/Atmosphere_of_the_Moon#/media/...

One my favorite quotes from one of my astronomy professors is: "Everything has an atmosphere, no matter how tenuous."

This is a reminder that he used to make sure we checked our assumptions and always considered constantly changing conditions.

It's not a real dish (as obvious in OP), it's a metal spider web. That will accumulate much less dust!

How can there be moon dust if there’s no atmospere? Is it from asteroid collisions kicking up dust?

It’s mostly regolith that’s been pulverised by meteorite impacts. It’s quite abrasive as the particles have jagged sharp edges.

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?

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:


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.

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


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."

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.

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.

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.

Satellites overflying would be an obvious problem...

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.

A great reason to build an array on Pluto.

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.


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.

Holy shit. Can some crazy tech billionaire just fund this thing as some branded vanity project? This looks fucking awesome.

Did you mean can NASA do it? Yeah I think NASA should do it.

If it's un-crewed, it should be brilliant.

Another pair of telescopes should be sited in Shackleton Crater, 21 km across, at the exact south pole of the moon. https://apod.nasa.gov/apod/ap110423.html

One of them would be a conventional radio telescope. The other, an infrared telescope.

The temperature within the crater is a nearly-constant 90K. You could power it with solar panels rotating on vertical axes posted at strategic points on the crater rim, that are almost always in sunlight.

The crater is so big that, for the infra scope, you could build it out of optically flat mirrors placed around the circumference.

The crater always points to the same spot in the sky, so you could get really, really long exposures. That you can't point it is OK, because there is so much to see if you are looking far enough away. At some distance and red-shift it would be an X-ray telescope, others an ultraviolet scope. Maybe a gamma-ray scope, at the extremum?

I'm all about having the SR Hadden Lunar Radio if that's what it takes to get the thing. If NASA builds it, if ESA builds it, if JAXA builds it, I don't care. Private funding is not a new thing for getting telescopes built, but the private funders are not the ones doing the building. They just write the checks.

Bezos could save some face by investing in something like this after his recent legal antics.

Paul Allen sadly passed already. He might have been game.

AFAIK, these telescopes produce a lot of data. Do we send this data back to Earth, or keep it on the moon and work on it remotely? Do we have the bandwidth to send it back to Earth? I’m really interested to know how much bandwidth can be achieved with an Earth-Moon link.

LROC, which may be the closest we have in volume to a lunar telescope, sends its data back to Earth. http://lroc.sese.asu.edu/about

LROC is one of seven instruments on board LRO. Together, these instruments have a downlink allocation of 310 Gbits per Ka band pass and up to 4 passes per day. That translates into 155 GBytes per day of data or 56,575 GBytes per year (55 TBytes). These data are processed by each respective instrument's Science Operation Center (SOC) with the final products being delivered to the NASA Planetary Data System (PDS).

If someone wonders, 155GB per day is 14.35Mb/s on average

That’s a great answer, thank you. And that’s actually a lot more link capacity than I expected.

For anyone reading, Google states that the aka band is between 26.5 GHz and 40 GHz.

I'll assume Starlink will be deployed around the moon if this telescope ever makes it past the proposal stage.

This looks cool, but...

We couldn't even maintain Arecibo enough to prevent its collapse, and it was here on earth. What's going to happen to a telescope like this?

It won't have hurricanes and earthquakes to deal with on the Moon at least.

Neither will it need to deal with things growing on it, or oxygen.

Arecibo lasted for half a century.

It would be more than acceptable if we built a telescope on the Moon and it lasted half a century before we moved on to replacing it and building the next interesting thing.

Given the lack of weather and other disturbances, I wonder if it would work to spray a conductive layer directly onto the crater's interior surface, maybe using a hovering rocket with the conductive material injected into its exhaust? I gather craters are hyperbolic [1, 2], but maybe the middle portion is close enough to a parabola to focus waves onto a receiver?

It might not be very environmentally friendly though, as it would be difficult to remove in the future.

[1] https://pubmed.ncbi.nlm.nih.gov/29755255/

[2] https://pubmed.ncbi.nlm.nih.gov/17994980/

If I read this right, they want to do the whole wiremesh/DuAxi robot dance because the craters are not parabolic enough. They use them because they allow to build a parabola without ferrying thousands of tons of material to the moon but they are not suitable on their own.

I expect craters are not very smooth surfaces at cm-scales, and just spraying something on top will not make it smooth on it's own.

What's the advantage of doing this with a filled aperture vs using synthetic aperture with satellites?

Signal strength. Synthetic aperture might match the spatial resolution of filled aperture, but signal strength still depends on collected area. For radars you can compensate by upping the transmitted power, not so for passive listening.

It's not really "filled". It's a metal spider web.

The advantage is realized by putting it on the moon: Radio darkness, shielding from sunlight built in 50% of the time, nice anchoring, etc.

Is there any convention where saying "the Far-Side" is necessary as opposed to "the Far Side"? Either in a style guide or industry jargon? I've noticed a tendency for this kind of unnecessary and excessive hyphenation to happen in some circumstances and I wonder about it. It seems similar to things like "make sure your dependencies are up-to-date", which there is no reason to prefer over the already correct English "up to date". If it was being used as an adjective, I would understand more, e.g. "ensure up-to-date dependencies".

How limiting is the fact that the telescope can’t be aimed, but is moving with the moons rotation?

Naively I’d assume that beyond the inability to aim it as a specific spot, it would also mean that we can’t do a long exposure of anything, as the image would get smeared with the moon’s movement.

This is a limitation known to several radio telescopes. For example, the Arecibo telescope couldn't aim much until they added the arm in 1997. They could aim the telescope not by moving the dish, but by moving the focal point/receiver. This only works with spherical dishes, but it is an option.

Arecibo was distance limited by the Earth's rotation speed, since radar returns had to make it back before the target shifted out of the telescopes steerability window. As the Moon is tidally locked - that long stare limitation remains.

Advantage is, the rotation rate will be 28x slower.

If you want to take a radar measurement for something once every 28 days, its great - except for the whole needing to be on the Moon part. Even if you were to build a network of radio telescopes on the dark side of the Moon, you'd still have an unavoidable revisit period - and building on the Earth facing side to overcome that defeats the purpose of putting radio telescopes on the Moon. The possibilities for optical telescopes are pretty interesting though.

You don't need to be there yourself.

And, you can build a dozen of them, half-way around the far side, and get a lot more opportunities to point at anything.

Build two close together, and range things hours away.

> You don't need to be there yourself.

lol, dunno if you are aware or not - but the Arecibo telescope catastrophically collapsed on itself. Serviceability is the concern. The maintenance logs are long for scientific instruments, there is no such thing as a 'set it and forget it' telescope.

Hubble has been visited I think twice in 3 decades.

That is an interesting choice for a counter example, because when I think of Hubble I think of failed gyroscopes. Hubble has required 5 servicing missions and always seems to be at risk of moving from a state of crippled-but-still-useful to flat-out-busted-forever. Look at the program's weekly observation logs, keep an eye on the file size - the interesting ones are smaller than average. Gyros and safemode. If you suddenly feel the urge to blurt out "Bbut funding and politics!", then consider my point proven - a Moon based telescope would be far more vulnerable to such externalities.


A telescope on the moon will not need any gyroscopes. The first Hubble visit was to fix a manufacturing/testing flaw impossible in a radio telescope.

Arecibo's failure would also be impossible, as there is no oxidation and no weather on the moon. The only likely failure mode would be from metal fatigue as a consequence of temperature swings, but they are 100% predictable.

Maybe you are not aware of the very large difference between an optical telescope and a radio telescope? Optical telescopes always have many precision moving parts. Radio telescopes often have none at all.

Anyway, in ten years, given promised SpaceX Starship progress, a visit will be much cheaper than a single Shuttle visit to ISS. Probably such progress will end up turning on thousands of heat-shield tiles not each needing a diaper change after each flight, as the current design seems to require.

That's just if you're doing active radar ranging to objects instead of passive radio observations which is most of what radio telescopes do anyways.

> > > ...can’t do a long exposure of anything...

> > They could aim the telescope not by moving the dish, but by moving the focal point/receiver.

> As the Moon is tidally locked - that long stare limitation remains.

See the point now? Passive radio observation doesn't change the fact that the Moon orbits the Earth - and has an even longer delay between revisits to the same patch of sky.

Yeah it's not going to be able to point for days at a single target but that will be helped by the relative silence of where it is.

> Arecibo was distance limited by the Earth's rotation speed, since radar returns had to make it back before the target shifted out of the telescopes steerability window

You were talking about active radar ranging a lot though which is not at all what these are meant for. It's like complaining a car can't tow a trailer, it's technically true but it's orthogonal to it's intended purpose.

> ...it's technically true but it's orthogonal to it's intended purpose.

Well that is the more verbose and less helpful way of saying the same thing: can't do long stare for the same reason this recognizable thing couldn't - rotation. Sticking with your car analogy: Upon hearing somebody else answer the question of "Can this car tow a trailer?", you swing in on a chandelier and declare "Actually it can, if the trailer is imaginary!"

The „Five-hundred Meter Aperture Spherical Telescope“ (FAST) can deform its wiremesh mirror to target different directions and to remove the spherical aberration issue by turning the surface shape into a paraboloid.

If that’s possible for a robotically constructed and maintained structure is more than questionable.


Earth rotates at 1670 kilometers/hour, the Moon travels at 3,683 kilometers per hour, tidally locked to Earth so the telescope travels at that speed. However, the Moon has a lot further to travel so a more meaningful measure is change in angle over time. For the purpose of a "long exposure" the Moon would have a lot less smear.

Earth: 7.2921159 × 10−5 radians/second (sidereal, not including solar orbit, at equator)

Moon: 0.2.66169 × 10−5 radians/second (on average for its eliptical orbit)



What is going to protect this from all those meteoroids,asteroids and cosmic rays?

For this approach I don't know. It's a wire mesh so I think it can resist some damage.

I read about other approaches that make me marvel at mankind's genius. For instance, on the moon you could slowly spin a pool of liquid Mercury to obtain a radio telescope that is basically immune from microimpacts. Not sure how it would work once the mercury freezes solid due to lack of sunlight, but I think it's such a beautiful (but maybe impractical) idea :).

You can do that on earth, too. See https://en.wikipedia.org/wiki/NASA_Orbital_Debris_Observator..., https://en.wikipedia.org/wiki/Large_Zenith_Telescope for (decommissioned) examples (both found via https://en.wikipedia.org/wiki/Liquid-mirror_telescope)

The idea apparently came from Isaac Newton.

> Not sure how it would work once the mercury freezes solid due to lack of sunlight, but I think it's such a beautiful (but maybe impractical) idea :).

Can't you heat it through induction heating? Vacuum is good isolation, so it won't lose heat too fast. Keeping whole pool above -30 °C probably won't require too much energy.

Maybe. I honestly don't know and just read about it quite a while ago and can't remember the details.

But mercury is a heavy metal. It must be prohibitively expensive to transfer the amount needed to the moon from earth.

Agreed, that's why I wrote maybe impractical. I just find it fascinating :)

Statistics :) The chance of a wire is hit by something is probably extremely low. Electronics working in space is also an understood problem so I don't think it's an issue.

Probably the proposed wire-mesh design, it won't be a solid reflecting surface. Total exposed surface would be comparable to orbit telescopes.

Cosmic rays are a solved problem. at most you will run out of backup computers. asteroids, you just hope you'll not win the cosmic lottery of collisions.

Like we do on earth with the big ones.

But since there is no atmosphere stopping the small ones on the moon, who also can do damage: this likely will be a problem, because there are a lot of them over time.

So a laser shield might sound science fiction, but will maybe be necessary, for longtime operation?

Or is it possible to build some protective sphere, that does not hinder transmission too much?

It occurs to me that the opposite of this would be a "Lunar Crater Laser Weapon on the Near-Side of the Moon" used to terrorize the population.

I presume that it would more feasible to do a dipole antenna based radio telescope like LOFAR to take advantage of the radio silence.

How would you power it? A nuclear reactor?

Radio receivers are not power hungry. Even a small solar cell and battery would probably suffice.

I suspect that returning the signal to Earth would consume more power than the actual receiver part.

Are you sure about that?


> In a breakaway session, a telescope engineer told colleagues that power consumption alone could cost tens of millions of euros each year, a sizable chunk of the array’s projected €100 million annual operating cost. If costs did not come down, the energy requirements had the potential to hobble or even sink the project.

You're comparing two vastly different types of radio observatories. That's for an array where you have dozens to hundreds of amps, signal processors and antennas that require power. This would be much closer to the Arecibo telescope or the Five-hundred-meter Aperture Spherical radio Telescope in power requirements.

As others mentioned solar is just fine. I believe the majority of the energy will be needed to keep the batteries warm enough to work (like the drone that is currently on Mars). Bouncing the signal back to earth will be more problematic as you need a satellite orbiting the moon.

Or a long cable...


Actually, it wouldn't need to be that long. The circumference of the moon is 40k kilometres, so assuming the site was in the center of the far side (though there's no reason it needs to be), that would put it at ~10,000 km from sight of the earth.

That's shorter than some transatlantic cables. I reckon it would be doable.

Solar+battery would work just fine. There’s no dark side of the moon, just longer days and nights.

A RITEG would do. It doesn't need much power.

Solar would also do, as long as it has about 14 days of battery backup. Lunar weather is pretty consistent! :)

Solar+batteries sort of can do it, but that wasn't ever attempted on the Moon, while ritegs are a very well known and proven tech.

This all pales beyound mounting the cables though. If anyone does _that_ then the power issues are moot.

Is that equivalent to an extra-planetary land grab?

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