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Chinese biz baron wants to shove his artificial moon where the sun doesn't shine – literally

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The silver lining

> 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

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Re: The silver lining

Oh, that's when the hidden multi-terrawatt laser beam will cut through the clouds to provide light and tranquillity for anyone standing below...

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Flame

Re: The silver lining

Didn't I see something like this in a Bond movie?

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Re: The silver lining

I reckon they could rent out the nightlight to other parts of the world on cloudy days which may help recoup some of the investment.

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Meh

Re: The silver lining

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.)

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Re: The silver lining

Does this man own a private island and an army of goons??

He should probably be stopped.

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Re: The silver lining

" multi-terrawatt laser" ... that will be useful, just walk outside with a cigarette, no need to worry about matches - it will just light itself.

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Paris Hilton

Re: The silver lining

because the planet is not getting warmer fast enough -- let's heat it at night, too

nice microclimate it will create locally.

also -- what about micrometeoroids?

last but not least -- how many hours of beauty sleep would chengdu Misses Hiltons miss?

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Re: The silver lining

The Chinese read Heinlein - 'The Man Who Sold The Moon' - except in the book, the power satellite explodes ;)

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Re: The silver lining

And in at least two Simpsons' episodes?

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Re: The silver lining

Maybe it has remote controlled obsurators or blinds for when it's not needed. Anything is possible, it's Chinese.

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Headmaster

Re: The silver lining

What's a terrawatt?

https://en.wikipedia.org/wiki/Watt#Terawatt

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Re: The silver lining

What's a terrawatt?

It's a Terrahawk crossed with a Scots engineer, as any fule kno.

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Re: The silver lining

Hey now. Goons? That's a bit harsh.

They're either Henchmen or Minions.

Hench 4 Life!!

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Re: The silver lining

> Hench 4 Life!!

Henchmen never get laid, just paid to be bullet magnets/early alarm systems - need to be a motivated evil lieutenant or better.

Just saying...

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Re: What's a terrawatt?

A terrawatt is a watt of power generated in space but consumed or dissipated on Earth of course.

Tsk! Watt are they teaching them in school these days?

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Coat

Re: Minions?

Would those be miniature onions?

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Re: Minions?

aha the old focus-sun-for-night-light rather tha focus-sun-for-death-ray switcharoo. cunning.

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return

>prohibitively expensive with little or no return

A city having no electricity bill for its street lighting sounds like return to me.

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Re: return

Compared to the initial outlay, it's negligible.

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Boffin

Re: return

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.

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Re: return

I agree, the project is not that far fetched, its a fairly simple usage of technology, the trick is keeping the mirror in a stable orbit. large mirror = drag, so careful calculations will be need or lots of fuel

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Drag

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.

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Re: Drag

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.

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Re: Lagrange point

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.

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Boffin

Re: Drag

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?

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Size of the mirror

Totally irrelevant as the mirror will be eclipsed at the time of greatest need anyway, unless it is in a very complex (and inherently unstable) orbit.

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Re: Size of the mirror

actually, if the orbit were not perfectly along the equator, the eclipsing problem would be solved [but of course the satellite would appear to move north and south from the earth's perspective]. So the earth 'eclipsing' problem is real, but solvable.

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Angel

Re: Drag

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.

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Re: Drag

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.

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Re: Drag

"So, HOW BIG does that mirror need to be?"

Very, very big. But that is far from impractical, because the mirror can be made of plastic just a few molecules thick which is unfurled in space to be many square kilometres in area.

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Re: Drag

"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.

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Re: Drag

"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.

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Re: "So, HOW BIG does that mirror need to be?"

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.

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Re: "So, HOW BIG does that mirror need to be?"

and the chinese military would love to use it to help light up other areas in a more focussed manner (to a few micron for example).

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Eight times brighter than the Moon?

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.

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jzl

Re: Eight times brighter than the Moon?

Not that I disagree with you in principle about the scale of this thing, it's worth pointing out that the moon is not very reflective. It has an albedo of around 10-15%. A mirror would be closer to 100%.

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Anonymous Coward

Re: Eight times brighter than the Moon?

Objects in the mirror are closer than they appear.

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jzl

Re: Eight times brighter than the Moon?

Also worth pointing out that this isn't a uniform reflector like the moon. It'll be a shaped mirror focussing on a relatively small area.

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Re: Eight times brighter than the Moon?

And have big teeth!

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Boffin

Re: Eight times brighter than the Moon?

"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.

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Devil

Re: Eight times brighter than the Moon?

"It'll be a shaped mirror focussing on a relatively small area."

a city isn't all that "small" but yeah, not like the moon which is competely UNfocused reflection.

A space-mirror would have solar energy reflected from it based on the total surface area of the mirror. if the mirror were exactly the size of the city, perfectly focused, etc. it would light it up like daylight. A fraction of the surface area of the city means an equivalent fraction of the light level, and so HOW bright did you want it? HOW big does that mirror need to be? 1/10 the surface area of a city with a population THAT large? Oh let's just ballpark it at 100 SQUARE MILES!!!

(article said 10 to 80 mile area - is that radius, diameter, or square mileage? if square miles, then make the mirror about 1/10 of that - but 10 square miles is only ~3 miles or so on a side, maybe the general area around his house but that's about it)

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Boffin

Re: Eight times brighter than the Moon?

Yes, it might be bright, but it is a single point source for light. The thing about street lights is they can be set up to only illuminate areas that need it whereas this is going to annoy some (e.g. lighting their bedrooms where now they are not) and under-service others by not lighting areas properly (buildings cast shadows after all). Simple computer modelling would reveal this and I bet it has been done and suppressed. As mentioned above, it will not help much with cloudy nights, which means the current infrastructure still has to be maintained. The point about fauna in the article sounds like a paid expert giving testimony. Missing from the article is whether this might interfere with any telescopes. Given the way the national government rearranged people's lives in order to make way for their large radio observatory, what would be the result of this thing interfering with another government project? Would the people of Chengdu be treated to a fireworks show?

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Re: Eight times brighter than the Moon?

Here's a picture of the Earth and Moon taken together

Is that actually taken together? Who or what took the picture?

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Facepalm

Re: Eight times brighter than the Moon?

Deep Space Climate Observatory (DSCOVR). Do try to keep up.

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Joke

Re: Eight times brighter than the Moon?

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.)

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Re: Eight times brighter than the Moon?

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.

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Coat

Re: Eight times brighter than the Moon?

"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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Pint

Re: David

"Is that actually taken together? Who or what took the picture?"

Is this the culprit?

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