Slashdot Mirror
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."
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 ?
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.
Chengdu weather: Of the 8 days shown, 5 are cloudy. No illumination from space then.
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!)
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.