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Chinese City 'Plans To Launch Artificial Moon To Replace Streetlights' (theguardian.com)
The south-western Chinese city of Chengdu is planning to launch an illumination satellite in 2020 that 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 10-80km, while the precise illumination range could be controlled within tens of meters -- enabling it to replace streetlights," reports The Guardian. From the report: 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 program.
The People's Daily was quick to reassure those concerned about the fake moon's impact on night-time wildlife. It cited Kang Weimin, director of the Institute of Optics, School of Aerospace, Harbin Institute of Technology, who "explained that the light of the satellite is similar to a dusk-like glow, so it should not affect animals' routines."
The People's Daily was quick to reassure those concerned about the fake moon's impact on night-time wildlife. It cited Kang Weimin, director of the Institute of Optics, School of Aerospace, Harbin Institute of Technology, who "explained that the light of the satellite is similar to a dusk-like glow, so it should not affect animals' routines."
In-city viewing is less than ideal, of course, but it's not impossible to find darker areas, even within a metropolis city limits.
I can't imagine that hobbyist astronomers are going to be very thrilled about this.
File under 'M' for 'Manic ranting'
It never dawned on me that someone lighting up the earth with satellites would ever make sense. Man will run this earth, without looking back, until it bites him in the ass.
--
Walking with a friend in the dark is better than walking alone in the light - Helen Keller
That this won't see the light of day.
Starships were meant to fly, Hands up and touch the sky - Nicky Minaj
Just how did this make it from story idea someplace in Chengdu to the pages of the Gaurdian or was that the Onion and finally to Slashdot without anyone asking just how the hell it's supposed to work ?
Just what orbit are they going to put it in where it can cast a 10-80 km diameter circle of illumination and be able to illuminate the city for a significant fraction of the night ?
"...similar to a dusk-like glow, so it should not affect animals' routines."
Is s/he a fool or does s/he think we are?
Dusk all night long, no, that wouldn't upset anything.
Why not a "Natural, noon-like glare" ? Shouldn't upset anything either.
They can always amp-up the emitters if they have a lawless uprising in the city.
Don't pretend for even a nanosecond that they didn't think of that, regardless of engineering feasibility.
I can just picture the initial testing being like ants under a magnifying glass. Oooops, still trying to get that dialed in guys.
Chengdu weather: Of the 8 days shown, 5 are cloudy. No illumination from space then.
They plan on using mirrors, and I assume a geo orbit that doesn't get eclipsed by the Earth.
Even if they did get it up there, I doubt it would last long.
Hubble has had all its gyros replaced already and the replacements are failing again.
This is something that needs precise control, every night, continuously. It doesn't have to just point itself somewhere and stay still for a long period of time, it needs to constantly move the mirror to keep the desired spot illuminated as it orbits. That's going to require the gyros and reaction wheels to be on 24/7.
The nice thing about the moon is the cycle - when it's full it's nice illumination and the world looks interesting, but a new moon is almost as great with a fairly starry sky even in a city if the air is really clear.
What they could do to help this though, is to turn off the light very late - say midnight or 1am. That way you still get the safety benefits of good illumination, but you also can still stay up late to see stars if you like.
I wonder how safe this will be compared to real street-lights though - one of the issues with moonlight even when it is bright, is that the light is so flat with poor shadowing that depth perception is hampered. Maybe since this is brighter it will be a lot better.
"There is more worth loving than we have strength to love." - Brian Jay Stanley
No, a bigger problem: Chengdu smog is as bad as Beijing. They rarely see the moon, and often you can't tell where the sun is.
Will the fake moonlight be able to penetrate thick smog?
Nope, GEO wobbles. Satellites up there require station keeping and direction pointing thrusters. (There are some clever ways to limit the directional thrusters, but in GEO there is no way to avoid needing position thrusters and for precise pointing like this you'll need pointing thrusters.)
while (sig==sig) sig=!sig;
Let's assume, for the moment, that it is a "good idea". How should it be implemented?
If there is to be only one satellite, then there are only two orbits that can work: GEO and a polar Molniya that has the satellite overhead between dusk and dawn. With more satellites more orbits can work, but let's assume one for now.
Let's figure out how big a mirror is needed. First, it will need to have at least some degree of focus to keep the spot size bounded. That is, a flat plane won't do. Let's compute the total light needed over a 10 km x 80 km ellipse, an area of about 2500 km^2. Given that 1 lux = 1 lumen / m^2, the lumens we need will be lux * area. Let's assume a partial moon (0.1 lux) is the minimum needed. Multiply those numbers, and that's 250,000,000 lumens. Let's say naked sunlight (allowing for atmospheric losses) is roughly 100,000 lux. Area = lumens/lux, yielding a mirror area of 2500 m^2.
And that's for a *perfect* mirror: No losses, with perfect focus. Assuming a circular mirror, that's a minimum diameter of about 60 m. That million-to-one illumination ratio really rocks. So, at a first glance, the optics alone says it's doable, at least near the minimums I've used. VERY doable!
A mirror in a Molniya orbit will have to deal with a rapidly/continuously varying sun angle every night, but there may be rotational tricks to deal with that, *if* we can manage to rapidly change the mirror shape. But it would rapidly complicate things, so let's set the Molniya orbit aside.
A mirror in GEO need only cope with seasonal variations. While we're at it, let's increase the mirror area by a factor of 10, so we'll be sure to have abundant margin to play with; let's say a nice, round 200 m diameter.
We already have communication satellites at GEO that stay aimed with precision for 15 years or more. Of course, they're aiming tiny antennas that are a millionth the size of our mirror, so we still must consider the aiming problem.
Satellites use aiming actuators for the antennas, and thrusters and gyros (torquers) for the satellite body, which combined yield good pointing precision. Our reflector won't have that luxury: The antenna will dwarf any satellite body, and will also have minimal rigidity: It really can't be aimed much at all, and even then not quickly.
To get some rigidity we can spin the mirror, which should also help with its shape, though that may not be needed. The total range of motion needed to track the sun matches the 23.5 degree tilt axis over a year, which is roughly 0.1 degree per day. (Well, OK, the total wobble is 23.5 degrees in each direction, but we only need to split the difference to bounce the sun toward the ground, which is 11.75 degrees, doubled, which gets us back to 23.5 degrees).
Next is the issue of aiming/pointing. Using gas or chemical thrusters alone may be a non-starter, including ion thrusters. But with a spinning disk, we really should be able to use precession. But precess against what?
Given the slow rate of angular motion, I suspect a 100-300 km long gravity tether should be able to provide enough restoring force to make the job manageable. The satellite body will then need only to slightly pivot the mirror relative to the tether, an extremely low-energy operation.
The satellite will still need an ion thruster to stay on-station (very gentle thrust). But even with that, our reflector satellite will be vastly simpler than a communications satellite, and perhaps about the same mass for an equivalent mission duration.
Oh, almost forgot: Let's put a small hole in the center of the mirror so light can reach some solar cells to power the satellite! No reactors or RTGs here.
I think that about does it, as an "educated guess". Total mass isn't really an issue: Some member of the Long March family will be able to loft it.
(Yes, yes, I know I haven't calculated either the atmospheric dispersion or the "f-number" of the optics. But I made the mirror 10x larger so I wouldn't have to!)
With a few alternations, you could make it project ads across the entire city! Alternatively, they could make it look like a giant eyeball, so everyone knew they were being watched.
Back in the late 70s, the city of Chicago replaced all the mercury streetlamps with sodium vapor lights. It was sold as a way to improve visibility at night and make driving safer and everything safer.
They didn't expect it to completely alter the behavior of birds in the city, messing up their diurnal cycles and screwing with their reproduction. Also, it changed the city's character. Night time used to be this magical silvery place in Chicago. It was just beautiful and romantic. The streetlights were the color of a winter moon, and you could still see stars. After the sodium vapor lamps, it was like this phony daylight all the time and yellow and it's ugly as hell. Also, no more stars.
You can really fuck up artificial lighting. Maybe it's just because it was what I was used to, but if you see any old color photos of Chicago neighborhoods at night, you can really see the difference.
You are welcome on my lawn.
How do you get a mirror that big into geostationary orbit?
Easy. Ask the Russians. They experiomented with this sort of thing in the 1990s.
The mirror deployed successfully, and, when illuminated, produced a 5 km wide bright spot, which traversed Europe from southern France to western Russia at a speed of 8 km/s.[1] The bright spot had a luminosity equivalent to approximately that of a full moon.
https://en.wikipedia.org/wiki/...
Regarding focusing: even with a perfectly shaped mirror, you can't circumvent the fact that the sun is not a point source. Sun diameter D=1.4e+9 m, sun distance L=1.5e+11 m, geostationary orbit distance R=3.6e+7 m. The spot at the earth surface will have a minimum diameter D R/L=340 km, for ideal optics. The claimed diameter of 10 to 80 km is physically impossible from geostationary orbit. They would need to use low-earth orbit, about 1000 km altitude, which would require multiple satellites to illuminate a single town throughout the night.
By the way, the difference between an ideal focusing mirror and a flat mirror is negligible for illumination purposes. For a 200 m diameter mirror, it would add another 0.2 km to the spot size if it's flat rather than paraboloid.
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I'd bet dollars to donuts that this is a mirror.
According to TFA, it is based on mirrors. It will be in GEO, with is at 42,000 km, so it would have to be aimed with extreme accuracy, and to light an entire metropolitan area, it would have to be enormous.
And it is supposed to be ready-to-go in 2020. That is wildly implausible.
From the story, China's Clean Air Challenge: $3 Billion Air Pollution from Transport in Chengdu
Quotes:
1) "Economic loss from transport-related air pollution in Chengdu, China, was $3 billion in 2013, and that number is on the rise."
2) "Chengdu, the fourth largest city in China, has over 15 million residents and 4 million vehicles, with 27 percent of PM2.5 emissions coming from transport, increasing risks to public health, the environment and the economy."
Yay, more sunlight to heat the Earth! What could possibly go wrong?
Escher was the first MC and Giger invented the HR department.
Or the way many Americans think that all of Mexico is like Tijuana. Or the way many Americans think that Asian = oriental. Or the way many Americans think that there aren't any other free countries in the world (heaven forbid if you should point out some countries that enjoy more freedom than the USA).
In general, Americans are ignorant, uncultured and untravelled rubes.
It says southwestern city not all cities have the smog issue but then you are likely American and geography is your greatest enemy.
I've spent time in Chengdu. Seen the pandas, climbed the scared mountain. Not much of a view any more.
Most of the populated regions of China have terrible air .
You may be thinking of Kunming, further south in Yunnan, which is now the only major city in China with breathable air and blue sky.
And don't call me a Meiguoren.
In general, Americans are ignorant, uncultured and untravelled rubes.
In general, human beings are ignorant, uncultured and untravelled rubes... and the hilarious thing is that you'd already know this if you weren't as well.
Now stay off our American site, rube.
Come now, as a fellow American, I have to call you out. Our culture is is immature, nieve, rough, stobborn unnecessarily pigheaded, and ultimately hypocritical. Because we are a child nation with guns and a thirst for blood, compared to the rest of the world. It's hardly fair to scapegoat all of Humanity for the limitations of Americans
The funny thing is, you don't need anywhere near 1% daytime solar radiation. The eye has crazy dynamic range.
The sun is ~100000 Lux. People read indoors at ~150 Lux. Moderate local streetlighting is ~5 Lux. A full moon is ~0.1 Lux.
If you do 1/100,000th of the brightness of the sun, that's more light than a full moon provides. This makes the mirror size more like ~30 meters, which can be provided by a big e.g. mylar structure.
The thing I doubt the most is the claim that a device capable of lighting a city from 35,786km (22,236mi) away would have no effect on nocturnal animals.
"Our two-party system is like a bowl of shit looking at itself in a mirror." - Lewis Black
I think you misunderstood me. It is optically impossible to image an extended light source onto a point, no matter how many fancy lens and mirror elements you combine.
"[Etendue] is the product of the area of the source and the solid angle that the system's entrance pupil subtends as seen from the source. (...) Etendue is important because it never decreases in any optical system where optical power is conserved. A perfect optical system produces an image with the same etendue as the source." - https://en.m.wikipedia.org/wik...
Extra mirror elements would allow you to image sun spots and coronal discharge onto the earth's surface with low aberrations, but that's not what you'd use this system for; it would actually be undesirable if a car driver suddenly finds themselves in the dark as they enter a sun spot.
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Seriously, we should have several in GEO, but then be able to rotate them with the earth rotation. In particular, use these for disaster areas. This will allow for rescue crews to not have to bring in massive amounts of lights, generators, and fuel. This would save a LOT of lives.
I prefer the "u" in honour as it seems to be missing these days.