[HN Gopher] Lunar Crater Radio Telescope on the Far-Side of the ...
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Lunar Crater Radio Telescope on the Far-Side of the Moon (2020)
Author : rbanffy
Score : 188 points
Date : 2021-09-13 11:02 UTC (2 days ago)
(HTM) web link (www.nasa.gov)
(TXT) w3m dump (www.nasa.gov)
| carabiner wrote:
| Holy shit. Can some crazy tech billionaire just fund this thing
| as some branded vanity project? This looks fucking awesome.
| guenthert wrote:
| Paul Allen sadly passed already. He might have been game.
| garyfirestorm wrote:
| Did you mean can NASA do it? Yeah I think NASA should do it.
| ncmncm wrote:
| _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?
| dylan604 wrote:
| 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.
| travisporter wrote:
| Bezos could save some face by investing in something like this
| after his recent legal antics.
| Evidlo wrote:
| What's the advantage of doing this with a filled aperture vs
| using synthetic aperture with satellites?
| japanuspus wrote:
| 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.
| jvanderbot wrote:
| 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.
| adamcharnock wrote:
| 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.
| mzkply wrote:
| I'll assume Starlink will be deployed around the moon if this
| telescope ever makes it past the proposal stage.
| markbnine wrote:
| 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)._
| adamcharnock wrote:
| 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.
| mciancia wrote:
| If someone wonders, 155GB per day is 14.35Mb/s on average
| otikik wrote:
| 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.
| [deleted]
| iseewhatyousaw wrote:
| 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?
| lacker wrote:
| 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.
| garmaine wrote:
| > 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.
| rbanffy wrote:
| 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.
| garmaine wrote:
| 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.
| TeMPOraL wrote:
| 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.
| kadoban wrote:
| 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.
| tjmc wrote:
| 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.
| TeMPOraL wrote:
| 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.
| rbanffy wrote:
| Someone will get sandblasted, but that's not too bad
| because the density will fall quickly with the distance.
| versteegen wrote:
| 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."
| TeMPOraL wrote:
| 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...)
| rtkwe wrote:
| It is a small part of the basic general license that
| longer wave lengths can hug the Earth IIRC.
| punnerud wrote:
| 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.
| rtkwe wrote:
| 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.
| SiempreViernes wrote:
| Satellites overflying would be an obvious problem...
| rbanffy wrote:
| A great reason to build an array on Pluto.
| rtkwe wrote:
| 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.
| KZerda wrote:
| 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.
| femto wrote:
| 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?
| adricl wrote:
| How would you power it? A nuclear reactor?
| marcyb5st wrote:
| 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.
| phaemon wrote:
| Or a long cable...
|
| 5078dc16f05cc325c8f8804f68e20feac85bea84
| phaemon wrote:
| 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.
| lstodd wrote:
| A RITEG would do. It doesn't need much power.
| femto wrote:
| Solar would also do, as long as it has about 14 days of
| battery backup. Lunar weather is pretty consistent! :)
| LeoPanthera wrote:
| 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.
| Ginden wrote:
| 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 EUR100 million annual operating cost. If costs did
| not come down, the energy requirements had the potential to
| hobble or even sink the project.
| rtkwe wrote:
| 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.
| jl6 wrote:
| Solar+battery would work just fine. There's no dark side of the
| moon, just longer days and nights.
| eminemence wrote:
| What is going to protect this from all those meteoroids,asteroids
| and cosmic rays?
| marcyb5st wrote:
| 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 :).
| iSnow wrote:
| But mercury is a heavy metal. It must be prohibitively
| expensive to transfer the amount needed to the moon from
| earth.
| marcyb5st wrote:
| Agreed, that's why I wrote maybe impractical. I just find
| it fascinating :)
| Ginden wrote:
| > 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 degC probably won't require too much energy.
| marcyb5st wrote:
| Maybe. I honestly don't know and just read about it quite a
| while ago and can't remember the details.
| Someone wrote:
| You can do that on earth, too. See https://en.wikipedia.org/w
| iki/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.
| tiborsaas wrote:
| 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.
| Out_of_Characte wrote:
| 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.
| hutzlibu wrote:
| 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?
| ComodoHacker wrote:
| Probably the proposed wire-mesh design, it won't be a solid
| reflecting surface. Total exposed surface would be comparable
| to orbit telescopes.
| dr-detroit wrote:
| This kind of space exploration is over. You cant run a lunar
| rover on sudo apt get create react app.
| gpt5 wrote:
| 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.
| adolph wrote:
| 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 x 10-5 radians/second (sidereal, not including
| solar orbit, at equator)
|
| Moon: 0.2.66169 x 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...
| Laremere wrote:
| 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.
| sennight wrote:
| 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.
| ncmncm wrote:
| Advantage is, the rotation rate will be 28x slower.
| sennight wrote:
| 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.
| ncmncm wrote:
| 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.
| sennight wrote:
| > 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.
| [deleted]
| rtkwe wrote:
| 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.
| sennight wrote:
| > > > ...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.
| rtkwe wrote:
| 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.
| _Microft wrote:
| 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_...
| femto wrote:
| 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/
| iSnow wrote:
| 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.
| SiempreViernes wrote:
| 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.
| emilecantin wrote:
| 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?
| rhcom2 wrote:
| It won't have hurricanes and earthquakes to deal with on the
| Moon at least.
| monkeyshelli wrote:
| Moon has "moonquakes"
|
| https://science.nasa.gov/science-news/science-at-
| nasa/2006/1...
| theandrewbailey wrote:
| Neither will it need to deal with things growing on it, or
| oxygen.
| adventured wrote:
| 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.
| teslaberry wrote:
| don't buy the story, this is the death star, and when it's
| finished they will begin faking data from it to keep the charade
| going.
|
| meanwhile as they release all sorts of faked boring data about
| the cmbr observatory plans and how the 'dish' is producing
| revolutionary new discoveries, they are in fact perfecting the
| death star lazer.
|
| they will then finally turn the moon around! how? who knows. why?
| so they can then hold the earth hostage. the great reveal
| accomplished, they pick target cities, one lazer attack at a
| time. WAIT NO. !They never turn the moon around. Too implausible.
| Also too risky , they will be found out!
|
| What they really do is use the moon lazer to shoot the sun, in
| order to guide a solar X class flare directly at earth, to
| destroy earth civilization so they can rebuild it. that's wayyyyy
| more plausible, and it also means they never have to threaten
| direct attack upon earth because , earth side and orbital
| observatory wont' be able to observe a dark-side of the moon
| lazer-beam being pointed directly at the sun.
|
| never mind the problems with lazer attenuation.
|
| and finally. something crazy will happen after all that.
| yadayadayada
|
| Soho 5, , a 5th generation Sun orbiting Sun-specific telescope
| that is very far from earth starts to see what is going on with
| these anomolous observations. Why? because it's watching from a
| dramatically different angle.
|
| Rogue, iconoclastic nerdy but slightly jacked up on steroid
| scientist (EThan Hawke) almost gets fired for his persistence,
| but he knows something fishy is going on and establishes proof.
| On the way to inform the president, He nearly gets killed under
| suspicious circumstances, but he succeeds in warning the
| president of the free world ( In 2058 , this is the president of
| China of course)
|
| Spectacular reversal: Chinese Jade Yutu X mission to the dark
| side destroys the death star by stirring up excessive moon dust
| at just the perfect time (a la independence day, martyr scene ).
|
| Kaboomboom, Michael Bay Style! end of story.
|
| It's called the 'dark side' for a reason. Don't trust the
| 'science'. Use the force.
|
| Title for the movie: The Carrington Project.
| the6threplicant wrote:
| I presume that it would more feasible to do a dipole antenna
| based radio telescope like LOFAR to take advantage of the radio
| silence.
| loudmax wrote:
| 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.
| eterevsky wrote:
| > 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.
| knappe wrote:
| 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.
| aigen001 wrote:
| 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/...
| rtkwe wrote:
| 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.
| jvanderbot wrote:
| It's not a real dish (as obvious in OP), it's a metal spider
| web. That will accumulate much less dust!
| FredPret wrote:
| How can there be moon dust if there's no atmospere? Is it from
| asteroid collisions kicking up dust?
| simonh wrote:
| It's mostly regolith that's been pulverised by meteorite
| impacts. It's quite abrasive as the particles have jagged
| sharp edges.
| goodcanadian wrote:
| _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.
| credit_guy wrote:
| > 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
| Asraelite wrote:
| > 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_guy wrote:
| 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.
| dave333 wrote:
| 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.
|
| https://en.wikipedia.org/wiki/Lunar_month
| zardo wrote:
| >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.
| lmilcin wrote:
| 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.
| m3kw9 wrote:
| Is that equivalent to an extra-planetary land grab?
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