4 ms·

Just paint the satellites black?

by WalterBright 2y ago

pixelesque 2y ago
Might then have a lot of overheating satellites :)
- WalterBright 2y ago
  Put the black surface an inch away from the satellite, supported by standoffs. That'll keep the heat away from the satellite. Also, the black surface only needs to be on the Earth facing side.
  - goku12 2y ago
    It isn't that simple, is it? Even if you place the black surface on a spacer (adding mass, might I add), the black surface itself is going to radiate heat back onto the satellite via blackbody radiation. This is always going to create a worse thermal situation unless you can somehow radiate away as much heat as you could with reflections. To see what I mean, check out JWST's thermal shield.
    - qingcharles 2y ago
      To complicate it further I thought that some of the military sats had rotating black panels to shield them from ground visibility?
      - goku12 2y ago
        It's likely that military satellites used a bunch of tricks to hide itself. But I don't know how useful they are these days. All major space powers have radars powerful enough to easily pick them out. They're needed for satellite collision avoidance analysis.
    - WalterBright 2y ago
      Consider a shaped charge. It seems wild to me that you can direct an explosion with various shapes of the explosive. Perhaps a similar idea can apply to the blackbody radiation - get it to radiate mostly away from the satellite. Also, the black part will be pointed at the Earth, not the Sun.
      - perihelions 2y ago
        Not sure what geometry you're thinking of. Whatever idea it is, you're stuck dealing with Kirchoff's law, which says optical emissivity and absorptivity are the same number at any given wavelength. I.e., you can't create "one-way trap doors" for heat flow—that would be a Maxwell's demon. Stacking multiple surfaces isn't a way to circumvent this. If it's just an asymmetric shape, with different material surfaces pointed in different directions, than that of course is permitted. https://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation https://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_r...
        WalterBright 2y ago
        Kirchoff's law doesn't say which direction it emits in. As shaped charges can alter the direction of where the blast wave goes, I suspect an analogous behavior of a heat shield can direct where the emissions go. I.e. as you say, an asymmetric shape. PS I haven't done thermo in nearly 50 years!
        perihelions 2y ago
        V-groove radiators are a really neat, unintuitive design, which gets strong directionality out of thermal radiation. It's the thermal design SPHEREX went with—the space telescope that went up a few weeks ago, that was all over HN. That's: a cavity that's a thin wedge in between two slightly angled plates, where the interior surface coating is partly emissive, and partly infrared-reflective—it can reflect the thermal radiation it emits. That means specular reflections, following geometric optics, so it works like a horn for light—beams it out the sides in a narrow cone. (.pdf) https://ttu-ir.tdl.org/server/api/core/bitstreams/71aee1e9-3f65-466f-a804-2737eefd2bed/content https://ttu-ir.tdl.org/server/api/core/bitstreams/71aee1e9-3...
    - tzs 2y ago
      How about something like this? | __ | |_/\_| | | | \/ |__| E 1 2 3 E is the Earth. 1 is a thin lightweight black disc to block the view of the rest of the satellite. 3 is the working parts of the satellite. 2 are mirrors to direct any emissions from 1 in the direction of 3 out to the sides and to direct any emissions from 3 in the direction of 1 out to sides.
      - goku12 2y ago
        You should check the JWST thermal shield. It addresses exactly this problem. Also keep in mind the additional cost it incurs.
smallnix 2y ago
Yes https://physicsworld.com/a/dark-coated-starlink-satellites-are-better-but-not-perfect-say-astronomers/ https://physicsworld.com/a/dark-coated-starlink-satellites-a...
xoa 2y ago
Can't tell if you're joking or not, but SpaceX has indeed been collaborating with astronomical observatories to reduce the apparent magnitude (brightness) to ground for Starlink satellites. It's not as if the potential issues weren't apparent and reported on pretty early on, ie, [0] in summer 2020 before the network entered beta. It's not perfect and the period during orbit raising before the satellites enter operational orbits is more challenging, but very significant reductions have been achieved per recs, see for example "Starlink Gen 2 Mini Satellites Photometric Characterization" [1] in 2023 and "The Brightness of Starlink Mini Satellites During Orbit-Raising" [2] in 2024: >When magnitudes are adjusted to a uniform distance of 1000 km the means are 4.58 and 7.52, respectively. The difference of 2.94 between distance-adjusted magnitudes above and below threshold implies that mitigation is 93% effective in reducing the brightness of orbit-raising spacecraft. So there has been progress, though more may be possible particularly as Starship gives them more mass to work with for less money. That may bring new possibilities to spend mass on "cosmetic" purposes to shade and further reduce magnitude even if it contributes nothing to the core functionality. Same as more mass may allow regulators to feasibly require higher levels of redundancy and more margin for deorbiting in case of issues or at EOL. Of course, that does leave older unmitigated working sats contributing to light pollution for the rest of their operational lifetimes, though worth noting that one of many major advantages for low-LEO/VLEO operation is that by design such lifetimes are much shorter, and in turn generation refresh will happen more quickly. Perhaps more importantly long term, there aren't as far as I know any actual international standards and agreements towards responsible brightness mitigation (or other issues like disposing of expended upper stages responsibly, standardized end-of-life deorbiting, etc). SpaceX has, for both PR and simple corporate self-interest reasons, been a pretty good actor so far even if they get a lot of attention for being the leading first mega constellation. But I really hope follow on efforts from other players can hit the ground running with magnitude reductions at least as good, and that SpaceX itself continues to improve (or at a bare minimum not backslide). High bandwidth fully global comms is simply too valuable a capability to really imagine going back at this point. But that's no reason not to pursue reasonable compromise mitigations, and potentially some sort of funds to ultimately create far more orbital telescopes as well as part of the package taking full advantage of what upcoming cheap megalift will make possible. ---- 0: "Impact of Satellite Constellations on Optical Astronomy and Recommendations Toward Mitigations" | https://aas.org/sites/default/files/2020-08/SATCON1-Report.pdf https://aas.org/sites/default/files/2020-08/SATCON1-Report.p... 1: https://arxiv.org/pdf/2306.06657 https://arxiv.org/pdf/2306.06657 2: https://arxiv.org/pdf/2405.12007 https://arxiv.org/pdf/2405.12007
WalterBright 2y ago
It should also be possible to de-orbit obsolete satellites faster if they can at end-of-life deploy a "sail" that will cause more drag.
- dredmorbius 2y ago
  Eddy-current-generating tethers seem a more practical option: <https://www.colorado.edu/faculty/kantha/sites/default/files/attached-files/carpenter.pdf https://www.colorado.edu/faculty/kantha/sites/default/files/...> (A plot point in Neal Stephenson's Seveneves.)
- youngtaff 2y ago
  And then we have to consider what’s the effect of all those vapourised metals on the atmosphere
  - mr_toad 2y ago
    Probably not much. There’s already an estimated 48 tons of material falling into the atmosphere every day. https://science.nasa.gov/solar-system/meteors-meteorites/facts/ https://science.nasa.gov/solar-system/meteors-meteorites/fac...
perilunar 2y ago
Even better, paint it vantablack https://en.wikipedia.org/wiki/Vantablack https://en.wikipedia.org/wiki/Vantablack
mmooss 2y ago
That would seem to remediate one problem, but they still block the view of cameras and telescopes - worse, in a way, that they might block the view without the viewer realizing they missed the occulded star.

Just paint the satellites black?

pixelesque 2y ago

Might then have a lot of overheating satellites :)

WalterBright 2y ago
Put the black surface an inch away from the satellite, supported by standoffs. That'll keep the heat away from the satellite. Also, the black surface only needs to be on the Earth facing side.
- goku12 2y ago
  It isn't that simple, is it? Even if you place the black surface on a spacer (adding mass, might I add), the black surface itself is going to radiate heat back onto the satellite via blackbody radiation. This is always going to create a worse thermal situation unless you can somehow radiate away as much heat as you could with reflections. To see what I mean, check out JWST's thermal shield.
  - qingcharles 2y ago
    To complicate it further I thought that some of the military sats had rotating black panels to shield them from ground visibility?
    - goku12 2y ago
      It's likely that military satellites used a bunch of tricks to hide itself. But I don't know how useful they are these days. All major space powers have radars powerful enough to easily pick them out. They're needed for satellite collision avoidance analysis.
  - WalterBright 2y ago
    Consider a shaped charge. It seems wild to me that you can direct an explosion with various shapes of the explosive. Perhaps a similar idea can apply to the blackbody radiation - get it to radiate mostly away from the satellite. Also, the black part will be pointed at the Earth, not the Sun.
    - perihelions 2y ago
      Not sure what geometry you're thinking of. Whatever idea it is, you're stuck dealing with Kirchoff's law, which says optical emissivity and absorptivity are the same number at any given wavelength. I.e., you can't create "one-way trap doors" for heat flow—that would be a Maxwell's demon. Stacking multiple surfaces isn't a way to circumvent this. If it's just an asymmetric shape, with different material surfaces pointed in different directions, than that of course is permitted. https://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation https://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_r...
      - WalterBright 2y ago
        Kirchoff's law doesn't say which direction it emits in. As shaped charges can alter the direction of where the blast wave goes, I suspect an analogous behavior of a heat shield can direct where the emissions go. I.e. as you say, an asymmetric shape. PS I haven't done thermo in nearly 50 years!
        perihelions 2y ago
        V-groove radiators are a really neat, unintuitive design, which gets strong directionality out of thermal radiation. It's the thermal design SPHEREX went with—the space telescope that went up a few weeks ago, that was all over HN. That's: a cavity that's a thin wedge in between two slightly angled plates, where the interior surface coating is partly emissive, and partly infrared-reflective—it can reflect the thermal radiation it emits. That means specular reflections, following geometric optics, so it works like a horn for light—beams it out the sides in a narrow cone. (.pdf) https://ttu-ir.tdl.org/server/api/core/bitstreams/71aee1e9-3f65-466f-a804-2737eefd2bed/content https://ttu-ir.tdl.org/server/api/core/bitstreams/71aee1e9-3...
  - tzs 2y ago
    How about something like this? | __ | |_/\_| | | | \/ |__| E 1 2 3 E is the Earth. 1 is a thin lightweight black disc to block the view of the rest of the satellite. 3 is the working parts of the satellite. 2 are mirrors to direct any emissions from 1 in the direction of 3 out to the sides and to direct any emissions from 3 in the direction of 1 out to sides.
    - goku12 2y ago
      You should check the JWST thermal shield. It addresses exactly this problem. Also keep in mind the additional cost it incurs.

smallnix 2y ago

Yes https://physicsworld.com/a/dark-coated-starlink-satellites-are-better-but-not-perfect-say-astronomers/ https://physicsworld.com/a/dark-coated-starlink-satellites-a...

xoa 2y ago

Can't tell if you're joking or not, but SpaceX has indeed been collaborating with astronomical observatories to reduce the apparent magnitude (brightness) to ground for Starlink satellites. It's not as if the potential issues weren't apparent and reported on pretty early on, ie, [0] in summer 2020 before the network entered beta. It's not perfect and the period during orbit raising before the satellites enter operational orbits is more challenging, but very significant reductions have been achieved per recs, see for example "Starlink Gen 2 Mini Satellites Photometric Characterization" [1] in 2023 and "The Brightness of Starlink Mini Satellites During Orbit-Raising" [2] in 2024: >When magnitudes are adjusted to a uniform distance of 1000 km the means are 4.58 and 7.52, respectively. The difference of 2.94 between distance-adjusted magnitudes above and below threshold implies that mitigation is 93% effective in reducing the brightness of orbit-raising spacecraft. So there has been progress, though more may be possible particularly as Starship gives them more mass to work with for less money. That may bring new possibilities to spend mass on "cosmetic" purposes to shade and further reduce magnitude even if it contributes nothing to the core functionality. Same as more mass may allow regulators to feasibly require higher levels of redundancy and more margin for deorbiting in case of issues or at EOL. Of course, that does leave older unmitigated working sats contributing to light pollution for the rest of their operational lifetimes, though worth noting that one of many major advantages for low-LEO/VLEO operation is that by design such lifetimes are much shorter, and in turn generation refresh will happen more quickly. Perhaps more importantly long term, there aren't as far as I know any actual international standards and agreements towards responsible brightness mitigation (or other issues like disposing of expended upper stages responsibly, standardized end-of-life deorbiting, etc). SpaceX has, for both PR and simple corporate self-interest reasons, been a pretty good actor so far even if they get a lot of attention for being the leading first mega constellation. But I really hope follow on efforts from other players can hit the ground running with magnitude reductions at least as good, and that SpaceX itself continues to improve (or at a bare minimum not backslide). High bandwidth fully global comms is simply too valuable a capability to really imagine going back at this point. But that's no reason not to pursue reasonable compromise mitigations, and potentially some sort of funds to ultimately create far more orbital telescopes as well as part of the package taking full advantage of what upcoming cheap megalift will make possible. ---- 0: "Impact of Satellite Constellations on Optical Astronomy and Recommendations Toward Mitigations" | https://aas.org/sites/default/files/2020-08/SATCON1-Report.pdf https://aas.org/sites/default/files/2020-08/SATCON1-Report.p... 1: https://arxiv.org/pdf/2306.06657 https://arxiv.org/pdf/2306.06657 2: https://arxiv.org/pdf/2405.12007 https://arxiv.org/pdf/2405.12007

WalterBright 2y ago

It should also be possible to de-orbit obsolete satellites faster if they can at end-of-life deploy a "sail" that will cause more drag.

dredmorbius 2y ago
Eddy-current-generating tethers seem a more practical option: <https://www.colorado.edu/faculty/kantha/sites/default/files/attached-files/carpenter.pdf https://www.colorado.edu/faculty/kantha/sites/default/files/...> (A plot point in Neal Stephenson's Seveneves.)
youngtaff 2y ago
And then we have to consider what’s the effect of all those vapourised metals on the atmosphere
- mr_toad 2y ago
  Probably not much. There’s already an estimated 48 tons of material falling into the atmosphere every day. https://science.nasa.gov/solar-system/meteors-meteorites/facts/ https://science.nasa.gov/solar-system/meteors-meteorites/fac...

perilunar 2y ago

Even better, paint it vantablack https://en.wikipedia.org/wiki/Vantablack https://en.wikipedia.org/wiki/Vantablack

mmooss 2y ago

That would seem to remediate one problem, but they still block the view of cameras and telescopes - worse, in a way, that they might block the view without the viewer realizing they missed the occulded star.