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Lunar Crater Radio Telescope on the Far-Side of the Moon (2020)

184 points2 daysnasa.gov
loudmax7 hours ago

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.

goodcanadian6 hours ago

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.

lmilcin5 hours ago

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.

credit_guy2 hours ago

> 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

Asraelite2 hours ago

> 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.

credit_guy38 minutes ago

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.

zardo3 hours ago

>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.

eterevsky5 hours ago

> 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.

aigen00110 minutes ago

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/...

rtkwe5 hours ago

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.

knappe4 hours ago

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.

jvanderbot1 hour ago

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

emilecantin2 hours ago

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?

rhcom22 hours ago

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

monkeyshelli59 minutes ago
theandrewbailey2 hours ago

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

adventured1 hour ago

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.

adamcharnock5 hours ago

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.

markbnine5 hours ago

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).

mciancia4 hours ago

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

adamcharnock5 hours ago

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.

mzkply2 hours ago

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

otikik43 minutes ago

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.

iseewhatyousaw2 days ago

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?

TeMPOraL11 hours ago

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.

versteegen10 hours ago

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."

rtkwe5 hours ago

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.

TeMPOraL10 hours ago

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...)

punnerud10 hours ago

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.

rtkwe5 hours ago

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

tjmc11 hours ago

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.

TeMPOraL10 hours ago

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.

rbanffy9 hours ago

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

kadoban6 hours ago

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.

SiempreViernes10 hours ago

Satellites overflying would be an obvious problem...

rtkwe5 hours ago

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.

rbanffy9 hours ago

A great reason to build an array on Pluto.

femto12 hours ago

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?

KZerda14 hours ago

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.

lacker14 hours ago

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.

garmaine12 hours ago

> 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.

rbanffy6 hours ago

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.

garmaine2 hours ago

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.

carabiner14 hours ago

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

garyfirestorm14 hours ago

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

ncmncm11 hours ago

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?

dylan6044 hours ago

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.

guenthert1 hour ago

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

travisporter6 hours ago

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

femto10 hours ago

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/

iSnow10 hours ago

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.

SiempreViernes10 hours ago

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.

Evidlo13 hours ago

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

japanuspus13 hours ago

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.

jvanderbot1 hour ago

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.

eminemence11 hours ago

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

marcyb5st11 hours ago

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 :).

Someone3 hours ago

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.

Ginden10 hours ago

> 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.

marcyb5st9 hours ago

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

iSnow9 hours ago

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

marcyb5st9 hours ago

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

tiborsaas10 hours ago

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.

ComodoHacker11 hours ago

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

Out_of_Characte11 hours ago

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.

hutzlibu10 hours ago

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?

gpt514 hours ago

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.

adolph4 hours ago

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)

https://hypertextbook.com/facts/2002/JasonAtkins.shtml

https://www.quora.com/What-is-the-angular-velocity-of-the-mo...

Laremere14 hours ago

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.

sennight13 hours ago

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.

rtkwe5 hours ago

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.

ncmncm11 hours ago

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

+1
sennight11 hours ago
_Microft13 hours ago

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.

https://en.m.wikipedia.org/wiki/Five-hundred-meter_Aperture_...

the6threplicant10 hours ago

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

m3kw93 hours ago

Is that equivalent to an extra-planetary land grab?

adricl13 hours ago

How would you power it? A nuclear reactor?

LeoPanthera12 hours ago

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.

Ginden10 hours ago

Are you sure about that?

https://physicstoday.scitation.org/do/10.1063/PT.6.2.2019013...

> 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.

rtkwe4 hours ago

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.

marcyb5st11 hours ago

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.

phaemon10 hours ago

Or a long cable...

5078dc16f05cc325c8f8804f68e20feac85bea84

phaemon3 hours ago

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.

jl612 hours ago

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

lstodd13 hours ago

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

femto12 hours ago

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

dr-detroit5 hours ago

This kind of space exploration is over. You cant run a lunar rover on sudo apt get create react app.