Chinese biz baron wants to shove his artificial moon where the sun doesn't shine – literally A Chinese businessman has announced plans to light Chengdu at night by launching an artificial "moon" to direct the out-of-sight Sun's rays down onto the city's streets. The idea is the brainchild of Wu Chunfeng, chairman of Chengdu Aerospace Science and Technology Microelectronics System Research Institute. He said …
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> However, Wu is convinced it'll be enough to save the city a fortune in electricity bills for street lighting.
What's the "plan B" for when it's cloudy? Which judging by the 10 day weather forecast happens quite a lot
messing with the day/night cycle - unintended consequences follow
Lighting streets with lamps is one thing. Shining the sunlight onto an area 7/24 is not the same thing. You can turn a street lamp OFF. The satellite is a "one size fits all" kind of thing. You can't control it. It's ON whether you want it or not.
But I think it'd be a nice target for ground-based 'satellite killer' laser testing - mirror? Oh CRAP!
(oh and for this to work properly, the mirror would have to be the size of a city, something about energy density, etc.)
A city having no electricity bill for its street lighting sounds like return to me.
Trying to get some numbers to go with that notion didn't yield much, but a BBC News article on the City of Westminster running their street lighting at 75% on average mentioned it would save UKP 420.000 on their leccy bill. Westminster is very very roughly 1% of Chengdu in area, but it's probably lit more brightly than Chengdu on average; let's say four times as bright, for ease of calculation. So taking that amount saved and multiplying by the difference in area we get about UKP 42 million, without correcting for energy price differences between UK and China. This would not quite pay for a single Space-X launch, but it's in the ballpark. I expect you'd need at least a few of them to get the materials plus an assembly robot up and into geostationary orbit.
If this guy has designed a really lightweight mirror it miiiight work.
The atmospheric drag up there at 36,000 KM is negligible, but such a large mirror might start acting like a solar sail. Compensation for that should be possible by careful orientation, which can be handled by a couple of gyroscopes. And the power for the gyroscopes can be generated by solar panels. Just make sure those gyroscopes are of a high enough quality (no China Export) or the satelite will be as usefull as Chandhra currently is.
but such a large mirror might start acting like a solar sail.
Never mind that, you actually have to continually adjust the mirror's orientation to keep the solar reflection aimed at Chengdu anyway. If it's geostationary it will have a fixed position w.r.t. its target but the angle towards the sun will change with the time of night, and if he's putting the mirror in one of the appropriate Lagrange points the target will move relative to the mirror. Plus it will be more distant, hence appear smaller, and he'd need a way bigger mirror (so more materials and thus rocket launches) if he's to get anywhere near enough light on his city's streets.
A bunch of solar panels and a stonking big battery looks way more feasible.
Those Lagrange points are more than ten times the distance from a geostationary orbit (roughly 385,000 vs 36,000 KM for L3, L4 and L5, L1 a bit less and L2 a bit more but completely useless behind the moon), so it will supply less than 1% of the light with the same size of the mirror. Besides that, the mirror will see Chengdu only about half the time and half of that while it is in daylight. For some reason I don't really think that would be a good idea.
Problem: you need to move a parabolic mirror that is pointed/focused at a city from geosync orbit.
a) the mirror is large enough to reflect enough sunlight to light an entire city;
b) the energy density of the sun that lights it up in the day is 'n'
c) you want to reflect 'n * m' energy for night-time visibility, where 'm' represents the fraction of daylight brightness you want at night
d) the surface area of your mirror must be 'city area' x 'n' x 'm', with extra factors added in for reflectivity and atmospheric losses.
So, HOW BIG does that mirror need to be?
a) the mirror is large enough to reflect enough sunlight to light an entire city;
b) the energy density of the sun that lights it up in the day is 'n'
c) you want to reflect 'n * m' energy for night-time visibility, where 'm' represents the fraction of daylight brightness you want at night
d) the surface area of your mirror must be 'city area' x 'n' x 'm', with extra factors added in for reflectivity and atmospheric losses.
Right idea, but "n" doesn't appear in the final equation. If it's transferring all incident sunlight then the incident power density simply cancels. Atmospheric loses are partly factored in already too, the sun's light has to come through it, angle obviously varies, but it does this during the day, leaving city area * m as the important bit. The fact sensitivity is approximately logarithmic should help a lot though, 1/100th the light power wont run a solar farm, but incident solar irradiation is usually taken at roughly 1kW/m2, yet a 20W LED bulb can light a reasonable sized room. On the negative side, area is of course square with dimension, so 1/100th the incident power requires 1/10th the diameter of the area you're trying to illuminate.
Still sceptical, it has a pipe-dream feel to it, but not physically impossible.
"So, HOW BIG does that mirror need to be?"
They say 8 times as bright as the moon. Moonlight is under 1 lux, so call it 8 lux. Full sunlight is around 100,000 lux. So you need a surface area of around 8/100,000ths the area of the city you intend to illuminate. If I did my quick maths right, You need a reflector of 48x48ft for every square mile of city you wish to illuminate.
My back of the envelope calculation runs thus.
Sun is about 400,000 times brighter than the moon.
Aiming at 8 times brighter than the moon, so looking for an area about 1/50,000 the area of Chengdu - which is big! (14,000 km²)
So about 600m diameter mirror.
Big, but not impossible.
The gyroscopes can maintain the angular orientation of the satellite. However, this (giant) mirror will still act as a solar sail - radiation pressure from reflection - https://en.wikipedia.org/wiki/Radiation_pressure
So what happens to that radiation pressure? In order for the satellite to remain in its orbit, something has to counteract it. The maximum radiation pressure would presumably be 9.08N (see link above) per square kilometer of mirror, although in reality it would presumably be somewhat less depending on the angle of the mirror to the sun. Not an insignificant amount of force to counter, particularly given its continuous nature, and it will have to be countered.
"So what happens to that radiation pressure? In order for the satellite to remain in its orbit, something has to counteract it. "
Not necessarily. It could be arranged that the pressure during one half of the orbit is exactly the same but opposite direction to the pressure in the other half of the orbit. In fact, adjusting the mirror "sails" during China's daylight period could be used to adjust the satellite's orbit, thus reducing or eliminating the need for it to have any orbital engines.
Over its far smaller apparent size compared to the Moon, I assume? I don't see how this could possibly light things up enough that you wouldn't want street lights. It isn't as if street lights turn off when there's a full Moon, and there's no way this could come anywhere near the amount of light you get from a full Moon.
Are we sure this isn't Elon Musk in disguise? It sounds like one of his harebrained ideas.
"the moon is not very reflective. It has an albedo of around 10-15%"
The moon is actually about as reflective as your average tarmacked (asphalt for our American cousins) road . Which sounds like nonsense until you think of a road illuminated in bright noon summer sunlight with no clouds (or indeed atmosphere). Also, of course, you mainly see the moon at night when your eyes have adjusted to the darkness, so it appears brighter.
Here's a picture of the Earth and Moon taken together which gives a better idea of it's colour and relative brightness.
Here's a picture of the Earth and Moon taken together which gives a better idea of it's colour and relative brightness.
Yes, but that's the dark side of the moon!
(Astonishing to think that genuinely is the dark side of the moon, only seen directly by 27 people to date. The Chinese should become the first people to actually land something there this year.)
Like most Chinese cities, this place is really busy and has a ton of lights inside buildings. They could probably turn a lot of streetlamps down or off that are near big buildings already pouring a ton of light out and save some money there. Meanwhile, it's going to be more cost-effective to just get more efficient systems for generating power and turning it into light rather than sending even more unneeded junk into orbit.
"Here's a picture of the Earth and Moon taken together which gives a better idea of it's colour and relative brightness"
Wrong picture mate. Here is the correct one
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