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Question for you -- how do we protect enormous telescopes on the moon's surface?

andrewflnr 3y ago
Probably pretty similar to how JWST is protected, which is to say not at all, but with enough redundancy to take a couple hits.
- PaulHoule 3y ago
  Why do you mess around on the surface of the moon when something free floating can be much larger with light construction?
  - huthuthike 3y ago
    You can't just make something free floating bigger and bigger without introducing new issues. In particular, as objects get larger in zero gravity, it becomes harder to dampen oscillations. The lighter the object is the more problematic these oscillations become. You also run into issues with thermal expansion.
    - giantrobot 3y ago
      And torsion effects when trying to move structures or reorient the whole structure.
    - pyinstallwoes 3y ago
      Can you expand the oscillations part? I find that interesting.
      - chaps 3y ago
        Objects have a resonant frequency and will jiggle in strange ways. The larger the object, the larger the possible jiggles. The larger the jiggles the larger the destruction.
        pyinstallwoes 3y ago
        Are you saying any object in space will naturally be resonating with itself at some frequency or that by proximity and interaction to another object it may cause resonance on another and therefore cause it to jiggle in strange ways. Thanks!
        chaps 3y ago
        The latter. Basically [1] but not as exaggerated. Think of things like screws shaking loose over time which leads to structural failure. Plus, no easy means to release that energy like you would when you're attached to a planet. [1] https://www.youtube.com/watch?v=XggxeuFDaDU https://www.youtube.com/watch?v=XggxeuFDaDU
      - jpollock 3y ago
        Look into "Chaos Theory", "Control Theory" and "Damping". In particular, consider how you would damp an undesired movement by a satellite. A naive approach would be to apply thrust in the opposite direction. However, the control can't be exact, leading to thrust -> thrust <- over and over, eventually to the measurement limit of the thruster's control. With a large mass, it's replaced with a spring, and converted to heat. https://en.wikipedia.org/wiki/Damping https://en.wikipedia.org/wiki/Damping
        pyinstallwoes 3y ago
        Hm, sounds like a fun oscillator algorithm to try for a synthesizer!
      - adhesive_wombat 3y ago
        Basically, things that are bolted tightly onto 73 million million million tonnes of rock tend not to flop around very much. It's a near-perfect "momentum sponge". Things in virtual freefall that can flex (and everything can) do so in response to forces (e.g. thrusting, but also heat stresses, say), and will continue to do so if they start unless you take care to damp them and dump them into heat. There's nowhere for the vibration to "go" unless you design one in. Sometimes the structure of the craft itself has enough damping for practical purposes, especially when you take care to isolate large vibration sources (the ISS has a Sorbothane damper for the treadmill, for example), but when your big floppy (i.e. light) mirror surface has to stay put on a nanometre scale, it's not so simple. It's a bit like the difference between a tuning fork glued down flat to a table and one hanging from a string.
        pyinstallwoes 3y ago
        By any chance do you know the history of these solutions and what what went before understanding this or was it already calculated and known far in advance of needing to account for it? In the sense of building things that last in space.
        adhesive_wombat 3y ago
        I don't know specifically. I imagine that a lot of the concepts came from naval architecture (sloshing of fuel, water and cargo has sunk many ships through the ages, for example, as well as hull resonances called "springing") then aviation (e.g. "flutter", where the wings oscillate, has destroyed planes) and space and missiles (again with the fuel sloshing, and other modes like pogo oscillation where the vibration feeds back into the engines and self-reinforces). Some concept of it also in civil engineering: the Tacoma Narrows bridge is the canonical example. Fundamentally they're all somewhat similar in that there's a flexible and/or sloshing thing that doesn't have a huge mass that it's rigidly connected to. Spacecraft deployed in space usually have smaller forces on them (no air or water and the hard acceleration is done) but are also much flimsier due to being ultra-light. Telescopes are even worse as even a tiny vibration can ruin the usefulness of the optical paths. Vibration of telescopes is an issue on Earth as well, e.g.: https://opg.optica.org/ao/abstract.cfm?uri=ao-53-21-4651 https://opg.optica.org/ao/abstract.cfm?uri=ao-53-21-4651
      - huthuthike 3y ago
        Ok, so basically anything that has any forces on it will end up with oscillations. On Earth it tends to not matter, because those oscillations travel from the object down into the ground, which is really good at damping them because the ground is, uh, pretty big. Even for things not attached to the ground, the atmosphere inherently damps oscillations, because as objects move back and forth in a fluid, a low pressure zone is created behind the object, which pulls it away from the direction it is moving. Also, rigid objects do much better with oscillations, because the entire structure has to move (more mass moving means less movement when the same amount of force is exerted on it). Floppy objects do worse, because one section can start oscillating on its own without transferring that motion to the entire body. In space, there is nothing to damp the oscillations. They will just continue without active features of the craft to damp them. If they continue unabated a section may reach a resonant frequency, which can quickly cause failure. Even if it doesn't, those vibrations can cause cyclic loading failures, or just affect the stationkeeping of the craft or it's usefulness in gathering scientific data. To make a spacecraft resistant to oscillations requires devices like gyroscopes or friction dampers, or long weighted booms which decrease the magnitude of oscillations. Making the craft rigid helps, but the larger it is, the less rigid it will be. And to make it more rigid, or to include more anti-oscillation devices, means more weight. That's an important limiting factor when you need to get the object up into orbit. One misunderstanding some people have is to think that there are no external forces on free-floating structures. This is untrue. Most importantly, they are all affected by the solar wind, which is a generally constant pressure pushing the object away from the sun. Of course they will also be affected by gravity, and if close enough to the earth they will interact with the atmosphere. (There's not really a clean cutoff to where our atmosphere ends and space begins.) As a result, spacecraft have to perform some amount of stationkeeping maneuvers, which involves applying a force on one section of the craft. That itself will cause further oscillations, because the force can never be transferred perfectly to the entire body. (Imagine pushing a piece of paper in the air with a single finger. Yes, you can get the paper to move in a direction, but you cannot get all segments of the paper to move in exactly the same manner when exerting force at only one point.) So forces on spacecraft are inherently unavoidable, and oscillations happen any time a force is applied. Oscillations are challenging to control in a free-floating vacuum environment, and become more problematic the larger a craft is. This results in fundamental issues with operating very large spacecraft. That's not to say it is impossible. But in space it's not a simple solution to say "just build it bigger."
  - skykooler 3y ago
    Because you have to get the materials into orbit to make something free floating. A telescope on the surface of the moon can be built from locally sourced materials - the moon's surface is largely aluminum oxide, and aluminum is an excellent material for mirrors in an environment where there is no air to form an oxide layer to dull it - so instead of having to send up the entire telescope, you just need to send the equipment to build the telescope (which for big telescopes can be quite a lot smaller and lighter).
  - andrewflnr 3y ago
    I don't know why you're asking me, dude. I'm only talking about protection from space hazards, but I know "light construction" doesn't help with that.
dividedbyzero 3y ago
Make it very modular and have little robots scurry over with a replacement module if one gets hit by a micrometeorite or otherwise fails.
- LASR 3y ago
  Do you play Factorio? I’m not sure how much of that is actually feasible.
caymanjim 3y ago
Protect them from what? We have enormous telescopes in space already. The moon is safer than where Hubble and JWST are now.
- dwaltrip 3y ago
  Wouldn’t the moon’s gravity pull in more material, putting a telescope at higher risk?
  - DrBazza 3y ago
    Correct. A large hill/small mountain's worth of debris hits the Earth every year. That includes micrometeorites that without an atmosphere, would pepper the Earth's surface. It's also the reason why any Moon base would be buried under layers of regolith to protect it. Glass domes on the moon are sci-fi fantasy.
    - OkayPhysicist 3y ago
      Since the moon is tidally locked, the side facing Earth and its far more massive gravity well would experience vastly less debris. It'd be a boring place to build your glass domed moon base, but I don't think it would be completely infeasible. Especially if it was built in a depression in the landscape, the approach angles would be greatly limited.
    - ceejayoz 3y ago
      The ISS gets hit with micrometeorites pretty regularly, without any regolith to protect it. Micrometeorites make correspondingly micro holes, which in said glass dome are patchable. A 2mm hole in a Soyuz docked to the station was fixed with a bit of Kapton tape and some epoxy, and was detected by a very small pressure drop in the crew spaces. Larger rocks are more of a problem, but quite rare. Your dome is much more likely to get smashed by someone mixing up the pedals in a rover. https://www.livescience.com/how-many-moon-meteorites https://www.livescience.com/how-many-moon-meteorites > "if you pick a square kilometer patch of ground, it will be hit by one of those pingpong-sized meteoroids once every thousand years or so" We've landed in visibly quite smooth areas; for example: https://www.flickr.com/photos/nasa2explore/48299974871 https://www.flickr.com/photos/nasa2explore/48299974871

Probably pretty similar to how JWST is protected, which is to say not at all, but with enough redundancy to take a couple hits.

PaulHoule 3y ago
Why do you mess around on the surface of the moon when something free floating can be much larger with light construction?
- huthuthike 3y ago
  You can't just make something free floating bigger and bigger without introducing new issues. In particular, as objects get larger in zero gravity, it becomes harder to dampen oscillations. The lighter the object is the more problematic these oscillations become. You also run into issues with thermal expansion.
  - giantrobot 3y ago
    And torsion effects when trying to move structures or reorient the whole structure.
  - pyinstallwoes 3y ago
    Can you expand the oscillations part? I find that interesting.
    - chaps 3y ago
      Objects have a resonant frequency and will jiggle in strange ways. The larger the object, the larger the possible jiggles. The larger the jiggles the larger the destruction.
      - pyinstallwoes 3y ago
        Are you saying any object in space will naturally be resonating with itself at some frequency or that by proximity and interaction to another object it may cause resonance on another and therefore cause it to jiggle in strange ways. Thanks!
        chaps 3y ago
        The latter. Basically [1] but not as exaggerated. Think of things like screws shaking loose over time which leads to structural failure. Plus, no easy means to release that energy like you would when you're attached to a planet. [1] https://www.youtube.com/watch?v=XggxeuFDaDU https://www.youtube.com/watch?v=XggxeuFDaDU
    - jpollock 3y ago
      Look into "Chaos Theory", "Control Theory" and "Damping". In particular, consider how you would damp an undesired movement by a satellite. A naive approach would be to apply thrust in the opposite direction. However, the control can't be exact, leading to thrust -> thrust <- over and over, eventually to the measurement limit of the thruster's control. With a large mass, it's replaced with a spring, and converted to heat. https://en.wikipedia.org/wiki/Damping https://en.wikipedia.org/wiki/Damping
      - pyinstallwoes 3y ago
        Hm, sounds like a fun oscillator algorithm to try for a synthesizer!
    - adhesive_wombat 3y ago
      Basically, things that are bolted tightly onto 73 million million million tonnes of rock tend not to flop around very much. It's a near-perfect "momentum sponge". Things in virtual freefall that can flex (and everything can) do so in response to forces (e.g. thrusting, but also heat stresses, say), and will continue to do so if they start unless you take care to damp them and dump them into heat. There's nowhere for the vibration to "go" unless you design one in. Sometimes the structure of the craft itself has enough damping for practical purposes, especially when you take care to isolate large vibration sources (the ISS has a Sorbothane damper for the treadmill, for example), but when your big floppy (i.e. light) mirror surface has to stay put on a nanometre scale, it's not so simple. It's a bit like the difference between a tuning fork glued down flat to a table and one hanging from a string.
      - pyinstallwoes 3y ago
        By any chance do you know the history of these solutions and what what went before understanding this or was it already calculated and known far in advance of needing to account for it? In the sense of building things that last in space.
        adhesive_wombat 3y ago
        I don't know specifically. I imagine that a lot of the concepts came from naval architecture (sloshing of fuel, water and cargo has sunk many ships through the ages, for example, as well as hull resonances called "springing") then aviation (e.g. "flutter", where the wings oscillate, has destroyed planes) and space and missiles (again with the fuel sloshing, and other modes like pogo oscillation where the vibration feeds back into the engines and self-reinforces). Some concept of it also in civil engineering: the Tacoma Narrows bridge is the canonical example. Fundamentally they're all somewhat similar in that there's a flexible and/or sloshing thing that doesn't have a huge mass that it's rigidly connected to. Spacecraft deployed in space usually have smaller forces on them (no air or water and the hard acceleration is done) but are also much flimsier due to being ultra-light. Telescopes are even worse as even a tiny vibration can ruin the usefulness of the optical paths. Vibration of telescopes is an issue on Earth as well, e.g.: https://opg.optica.org/ao/abstract.cfm?uri=ao-53-21-4651 https://opg.optica.org/ao/abstract.cfm?uri=ao-53-21-4651
    - huthuthike 3y ago
      Ok, so basically anything that has any forces on it will end up with oscillations. On Earth it tends to not matter, because those oscillations travel from the object down into the ground, which is really good at damping them because the ground is, uh, pretty big. Even for things not attached to the ground, the atmosphere inherently damps oscillations, because as objects move back and forth in a fluid, a low pressure zone is created behind the object, which pulls it away from the direction it is moving. Also, rigid objects do much better with oscillations, because the entire structure has to move (more mass moving means less movement when the same amount of force is exerted on it). Floppy objects do worse, because one section can start oscillating on its own without transferring that motion to the entire body. In space, there is nothing to damp the oscillations. They will just continue without active features of the craft to damp them. If they continue unabated a section may reach a resonant frequency, which can quickly cause failure. Even if it doesn't, those vibrations can cause cyclic loading failures, or just affect the stationkeeping of the craft or it's usefulness in gathering scientific data. To make a spacecraft resistant to oscillations requires devices like gyroscopes or friction dampers, or long weighted booms which decrease the magnitude of oscillations. Making the craft rigid helps, but the larger it is, the less rigid it will be. And to make it more rigid, or to include more anti-oscillation devices, means more weight. That's an important limiting factor when you need to get the object up into orbit. One misunderstanding some people have is to think that there are no external forces on free-floating structures. This is untrue. Most importantly, they are all affected by the solar wind, which is a generally constant pressure pushing the object away from the sun. Of course they will also be affected by gravity, and if close enough to the earth they will interact with the atmosphere. (There's not really a clean cutoff to where our atmosphere ends and space begins.) As a result, spacecraft have to perform some amount of stationkeeping maneuvers, which involves applying a force on one section of the craft. That itself will cause further oscillations, because the force can never be transferred perfectly to the entire body. (Imagine pushing a piece of paper in the air with a single finger. Yes, you can get the paper to move in a direction, but you cannot get all segments of the paper to move in exactly the same manner when exerting force at only one point.) So forces on spacecraft are inherently unavoidable, and oscillations happen any time a force is applied. Oscillations are challenging to control in a free-floating vacuum environment, and become more problematic the larger a craft is. This results in fundamental issues with operating very large spacecraft. That's not to say it is impossible. But in space it's not a simple solution to say "just build it bigger."
- skykooler 3y ago
  Because you have to get the materials into orbit to make something free floating. A telescope on the surface of the moon can be built from locally sourced materials - the moon's surface is largely aluminum oxide, and aluminum is an excellent material for mirrors in an environment where there is no air to form an oxide layer to dull it - so instead of having to send up the entire telescope, you just need to send the equipment to build the telescope (which for big telescopes can be quite a lot smaller and lighter).
- andrewflnr 3y ago
  I don't know why you're asking me, dude. I'm only talking about protection from space hazards, but I know "light construction" doesn't help with that.

dividedbyzero 3y ago

Make it very modular and have little robots scurry over with a replacement module if one gets hit by a micrometeorite or otherwise fails.

LASR 3y ago
Do you play Factorio? I’m not sure how much of that is actually feasible.

caymanjim 3y ago

Protect them from what? We have enormous telescopes in space already. The moon is safer than where Hubble and JWST are now.

dwaltrip 3y ago
Wouldn’t the moon’s gravity pull in more material, putting a telescope at higher risk?
- DrBazza 3y ago
  Correct. A large hill/small mountain's worth of debris hits the Earth every year. That includes micrometeorites that without an atmosphere, would pepper the Earth's surface. It's also the reason why any Moon base would be buried under layers of regolith to protect it. Glass domes on the moon are sci-fi fantasy.
  - OkayPhysicist 3y ago
    Since the moon is tidally locked, the side facing Earth and its far more massive gravity well would experience vastly less debris. It'd be a boring place to build your glass domed moon base, but I don't think it would be completely infeasible. Especially if it was built in a depression in the landscape, the approach angles would be greatly limited.
  - ceejayoz 3y ago
    The ISS gets hit with micrometeorites pretty regularly, without any regolith to protect it. Micrometeorites make correspondingly micro holes, which in said glass dome are patchable. A 2mm hole in a Soyuz docked to the station was fixed with a bit of Kapton tape and some epoxy, and was detected by a very small pressure drop in the crew spaces. Larger rocks are more of a problem, but quite rare. Your dome is much more likely to get smashed by someone mixing up the pedals in a rover. https://www.livescience.com/how-many-moon-meteorites https://www.livescience.com/how-many-moon-meteorites > "if you pick a square kilometer patch of ground, it will be hit by one of those pingpong-sized meteoroids once every thousand years or so" We've landed in visibly quite smooth areas; for example: https://www.flickr.com/photos/nasa2explore/48299974871 https://www.flickr.com/photos/nasa2explore/48299974871