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posted by chromas on Friday October 19 2018, @01:28AM   Printer-friendly
from the Checkmate,-moon-believers! dept.

Who needs street lights? Chinese city plans fake moon

In Chengdu, there is reportedly an ambitious plan afoot for replacing the city's street lights: boosting the glow of the real moon with that of a more powerful fake one.

The capital of the southwestern Chinese province of Sichuan plans to launch an illumination satellite in 2020. According to an article in People's Daily, the artificial moon is "designed to complement the moon at night", though it would be eight times as bright. The "dusk-like glow" of the satellite would be able to light an area with a diameter of between 10 and 80km (six to 50 miles), while the precise illumination range could be controlled within tens of metres – enabling it to replace street lights.

The vision was shared by Wu Chunfeng, the chairman of the private space contractor Chengdu Aerospace Science and Technology Microelectronics System Research Institute Co (Casc), at a national mass innovation and entrepreneurship event held in Chengdu last week. Wu reportedly said testing had begun on the satellite years ago and the technology had now evolved enough to allow for launch in 2020. It is not clear whether the plan has the backing of the city of Chengdu or the Chinese government, though Casc is the main contractor for the Chinese space programme.

Also at The Guardian and Inverse.


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  • (Score: 5, Informative) by maxwell demon on Friday October 19 2018, @05:55AM (5 children)

    by maxwell demon (1608) on Friday October 19 2018, @05:55AM (#750802) Journal

    This smells like complete bullshit. Such a satellite would have to be unbelievably enormous, like building another moon, or perhaps another sun.

    It has to be enormous, but not that enormous.

    First, the moon is black. Really. Black as coal. To be more concrete, the moon has an albedo of 0.12. Which means that is reflects less than 1/8 of the light. Since a mirror reflects close to 100% of the light, this compensates the factor 8 in brightness planned.

    Second, the moon is a diffuse reflector. The mirror reflects directed. Even if we assume the mirror to be perfectly flat (I guess they will actually make it a concentrating mirror), that gives a factor 2.

    Third, they speak of 3 mirrors; if they all reflect into the same area, this gives another factor of 3.

    But the most important point is that the satellite is much closer than the moon. The moon has a distance of about 380 000 km. This satellite will probably have to be geostationary, giving a distance of 36 000 km (if not geostationary, it will be even closer). This gives a factor of roughly 10. Since the light intensity goes with the square of the distance, this gives a factor of 100.

    Now the moon has a radius of about 1700 km. Relevant for the reflected light is the cross section, which for the moon is circular, so we get approximately r² π ≈ 9.1 million km². Now we have to divide by 600 due to the considerations above, which gives a necessary area per mirror of about 15 000 km². Assuming square mirrors, this means that each mirror has to have a side length of about 120 km.

    While still enormous, it's considerably smaller than the moon, let alone the sun.

    --
    The Tao of math: The numbers you can count are not the real numbers.
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  • (Score: 0) by Anonymous Coward on Friday October 19 2018, @06:43AM

    by Anonymous Coward on Friday October 19 2018, @06:43AM (#750810)

    I'm chuckling on the inside thinking about putting something that needs to be precisely oriented, and it has a surface area of 15000 km^2, in a region of space that is not free of orbital junk. I imagine it will be something like a year before all of that turns into just more space junk.

    In all seriousness, I don't see how anyone but a complete idiot would suggest this.
    there's the circadian rythm mentioned above, there is the cloud issue mentioned above, there is a construction plus a forever maintenance cost, then there's the danger that despite forever maintenance, the gigantic object will (a) point somewhere it shouldn't and/or (b) fall on someone's head.
    I'm not sure whether the UN can forbid it, but this is the sort of thing which should be forbidden.

  • (Score: 0) by Anonymous Coward on Friday October 19 2018, @02:13PM (2 children)

    by Anonymous Coward on Friday October 19 2018, @02:13PM (#750921)

    Now we have to divide by 600 due to the considerations above, which gives a necessary area per mirror of about 15 000 km². Assuming square mirrors, this means that each mirror has to have a side length of about 120 km.

    While still enormous, it's considerably smaller than the moon, let alone the sun.

    Thanks for crunching the numbers! I think you forgot to account for the fact that TFA says this will be 8 times brighter than the moon, though ;)

    It's smaller than "The Moon" but considerably larger than many things we call "moons" in our solar system.

    The largest mirror humanity has ever sent into space is just 2.5m in diameter. This one had a mass of about 1 tonne. There are plans (with endless delays) to up humanity's game by pushing to a whopping 6.5m, with a mass of about half a tonne (and this one is not designed for reflecting visible light, although I imagine it'll do a reasonable job).

    Since those numbers assume the thing is directed... I don't even want to think about how big the reaction wheels to stabilize a mirror over a thousand times wider would be.

    Hence the funny smell!

    • (Score: 2) by maxwell demon on Friday October 19 2018, @03:15PM

      by maxwell demon (1608) on Friday October 19 2018, @03:15PM (#750950) Journal

      I think you forgot to account for the fact that TFA says this will be 8 times brighter than the moon, though ;)

      From my previous comment, emphasis added:

      First, the moon is black. Really. Black as coal. To be more concrete, the moon has an albedo of 0.12. Which means that is reflects less than 1/8 of the light. Since a mirror reflects close to 100% of the light, this compensates the factor 8 in brightness planned.

      --
      The Tao of math: The numbers you can count are not the real numbers.
    • (Score: 2) by bob_super on Friday October 19 2018, @10:01PM

      by bob_super (1357) on Friday October 19 2018, @10:01PM (#751173)

      To be fair, the biggest mirrors you mention are designed to be extremely precise.
      At the other end of the spectrum, spy sats already grew to an estimated 100m diameter, probably not as precise because of the wavelengths they address.

      Those mirrors would be closer to the spy sat structure than to JWST, especially because the chinese probably won't obsess about a few rays being reflected accidentally towards Mongolia.

  • (Score: 3, Informative) by FatPhil on Friday October 19 2018, @03:35PM

    by FatPhil (863) <reversethis-{if.fdsa} {ta} {tnelyos-cp}> on Friday October 19 2018, @03:35PM (#750960) Homepage
    Does the inverse square law apply here? The inverse square law assumes that the "source" is throwing light over a fixed solid angle, such that the target takes up ever smaller proportions of that angle as the distance increases; but this is more a set of focused beams than it is just glowing and irradiating willy-nilly.

    Thought experiment, which sensor gets more light:

    candle
    ()                                     |
    ---barrier-------------------------    | perfect mirror
                              sensor  <    |

                     ()                    |
    -----------------------------------    |
                     <                     |

                                      ()   |
    -----------------------------------    |
    <                                      |

    Distance to mirror seems to be more or less irrelevant in this highly reflective case.
    --
    Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves