Lunar Crater Radio Telescope on the Far-Side of the Moon (2020)
nasa.gov
nasa.gov
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.
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.
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.
The dust also picks up charge and levitates, but I suppose that allows you to sweep it up using a charged broom.
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.
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.
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.
Moon doesn't have atmosphere, so once you've built the dish and cleaned it, it should stay clean unless a meteorite strikes nearby.
This is a reminder that he used to make sure we checked our assumptions and always considered constantly changing conditions.
>"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/...
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?
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.
With a number of landers, if they are all in line of sight of each other, optical interferometry becomes a lot easier.
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.
Worth a try, even if the agreement falls apart with the first mineral discovery on the quiet side?
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.
(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."
(Also I suppose this means I should turn in my HAM license...)
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.
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?
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).
For anyone reading, Google states that the aka band is between 26.5 GHz and 40 GHz.
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?
https://science.nasa.gov/science-news/science-at-nasa/2006/1...
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.
It might not be very environmentally friendly though, as it would be difficult to remove in the future.
The advantage is realized by putting it on the moon: Radio darkness, shielding from sunlight built in 50% of the time, nice anchoring, etc.
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.
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.
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.
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.
> > 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.
> 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.
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!"
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_...
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...
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?
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 :).
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.
The idea apparently came from Isaac Newton.
This all pales beyound mounting the cables though. If anyone does _that_ then the power issues are moot.
I suspect that returning the signal to Earth would consume more power than the actual receiver part.
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.
5078dc16f05cc325c8f8804f68e20feac85bea84
That's shorter than some transatlantic cables. I reckon it would be doable.