China's Artificial Moon Satellites

According to many news sources, China is planning to launch ‘artificial moon’ satellites in the next couple of years, which they claim will light up major cities and eliminate the need for street lights, saving them hundreds of millions per year in electricity costs.

Here’s the story from Time magazine: China Plans Artificial Moons.

Here’s the problem: I don’t dee how this plan passes a basic smell test. For example, they say the four satellites will be in a 500km orbit. That means they will orbit the earth about once every 2 hours, and their time over a city will be measured in minutes - about like an Iridium satellite. How in heck is that supposed to provide constant light over a city at night?

There are a number of other issues with this idea, but let’s start here. Would anyone like to critique my critique? I keep thinking I’m missing something since this is written up in articles all over the world and I have yet to see anyone make this point. On the other hand, the state of science reporting is execrable, and people still keep writing breathless articles about Hyperloop, so maybe no one has actually bothered to think this through?

It makes me wonder if this story is cover for a military or spy satellite, and gullible reporters are accepting it without question.

An orbit that low also means that at night (when you need the light), the satellite is going to spend most of its above-the-horizon time in the Earth’s shadow.

You could of course put them in higher orbits, but then the already-considerable required size is going to be even larger.

This is a much better article.

The basic geometry noted by Chronos (the earth blocking the sun, i.e. high altitude sunset/sunrise) means that at 500km you can only provide light for a short period after sea-level sunset and a short period before sea-level dawn.

However, you can certainly set up a dawn/dusk sun-synchronous orbit (at similar altitude to the 500km cited) that “rides the terminator”. Such a satellite could be available to sequentially light up all locations under its orbit for a short period after sunset or before dawn.

Beyond that, I got nuthin’.

ETA: it won’t work the way I describe away from equatorial latitudes.

So, I guess the part of the geometry that’s key is that the sunlight must be reflected at an angle toward the illumination target that makes it quite low in the sky. But it still seems like you’d need much more than 500km altitude to make that work for a significant part of the night.

Darren’s article suggests that the 500km cited is probably just for a test system, and that realistic systems have been proposed at ten times that altitude. And multiple satellites.

Although it seems like a total whoosh, this recent news item has been picked up by numerous Asian outlets as well as at least one major American magazine, Esquire.

[China Is Launching A Man-Made Moon Into The Sky](http://This recent news item would appear to be a gigantic whoosh, yet it has been carried by numerous outlets, mostly in Asia but also at least one major magazine in the States, Esquire. [URL="

As the article explains, the idea is to have a huge reflective satellite to bounce the sun’s light onto Chinese cities in order to supplement or replace terrestrial street lighting. The concept is clear enough; it would “work” just the same as the real moon. But according to all that I’ve read, these satellites would orbit at an altitude of 500km, which implies an orbital period of about 94 minutes. Obviously, the engineers can’t just “hang” this moon over any particular point, although some of the articles use that very word. Instead, a satellite at that altitude will appear to slide rather quickly across the sky, at about the same apparent speed as a high-flying jet. Moreover, for almost half of its orbit, the satellite would be traversing the earth’s dark side, completely cut off from the sunlight.

Just as obviously, these satellites could not possibly remain positioned over China, but would periodically pass over every terrestrial point within the band described by its orbital inclination. So the presence of these “moons” has the potential to affect millions of people outside of China. Astronomers and stargazers find this proposal infuriating, not to mention incredibly arrogant. But how much impact could this really have? And, as noted, how can it possibly work?

I merged two threads on the same subject. The post above was the OP of the second.

General Questions Moderator

I’m starting to think it’s a national publicity stunt. In the end it will be much like it was in the early 1960s, when people would see the Echo satellite in the early evening. At any given spot it was only very occasionally overhead at the right time of day.

Well, that might be true, but these things have been proposed by serious scientists. The speed with which any one satellite crosses the sky is not an obstacle per se, because any system would involve multiple satellites and aimed mirrors. The principal odd thing in these articles was the limitation imposed by simple geometry from the proposed 500km altitude, but I think that’s explained by this being a test system as Darren’s article suggests. From my experience of journalists reporting issues where I understand the science better, the other slightly odd stuff in the article may just be attributable to a journalist or PR person not fully understanding what a scientist was telling him, and the error propagating through all the news stories.

The basic concept is not complete fantasy. I think the more worrying thing is that it’s not just a publicity stunt, with the light pollution this would entail.


This is the modern equivalent of the moonlight towers of the late 1800s A major issue with the towers, other than their expense and generally disagreeable appearance, was the casting of long shadows and a very eerie and unpleasant quality of lighting. If you could get a satellite or multiple satellites to stay directly overhead in geosynchronous orbit (which I assume would put them way too far away to be effective without being insanely huge), while also not being blocked by the sun, and thus shining straight down, it would mitigate the shadow effect somewhat. However, the light needs to be bright enough to overcome shading from trees and roofs, and even then I suspect it would still be rather repulsive. Street lighting isn’t entirely utilitarian, there’s aesthetic and design criteria that play into it as well. Bad lighting can make a good neighborhood look like a post-apocalyptic war zone, it can kill night life, and it can sometimes make safety problems worse. Even if such a system was ever launched, I suspect the unintended consequences would be immense.

Hermann Oberth proposed something like this in the 1920s, although his objective was a space weapon, with a concave mirror 100 meters in diameter that could fry the enemy like ants under a magnifying glass.

From the article linked to in the OP’s link:

Wow, I have never heard of such a thing. I bet the shadows would be a bit creepy. Something similar, but way more effective seems to be at use at highway intersections (six or eight big ass lights on very tall poles). That seems to work quite well, thought I wouldn’t want to live near one.

Am I wrong in thinking this proposal to be unbelievably arrogant? If carried to its logical conclusion, i.e. one of the “realistic” proposals, could it spell the doom of stargazing and most earth-based astronomy? Might this be the end of dark skies for the entire planet?
Or am I overreacting?

I can’t imagine the frustration of waiting two weeks for the moon to disappear, a situation well known to anyone who ever used a telescope or a camera on the night sky, only to find the sky full of fake moons launched by someone on the other side of the world.

ETA: Also, very high lights are not that great for illuminating city streets. Los Angeles tried this with arc lamps in the 1880s. The lamps were mounted atop tall poles, much taller than anything else at the time, when the small cow town was dominated by single story adobes and almost nothing else higher than two stories. One side of a street would be brightly lighted as a result, but the other side would be in deep shadow.

Nope, seems a reasonable objection, especially if multiple ones are planned- the effects on ecosystems and even crops could also be huge. If it worked as claimed, it could have unpredictable global effects.

Coral spawning is triggered by moon phase for example, this could mess that up. Most of the rest of the planet is trying to reduce ‘light spill’ from artificial lighting, because it has so many negative effects on wildlife, launching fake moons to deliberately increase it and spread it over huge areas seems bizarre and the sort of thing that will trigger international outrage once anyone actually stops and thinks about it.

If the mirrors are orientable (which they must be to target a specific place), then the light could be diverted towards the sky, for instance, when the “moons” aren’t lighting some Chinese city. Besides, I assume they also could be covered/closed/whatever, which would eliminate the issue completely.

No, because these lights would be aimed at cities. It doesn’t make any economic sense to launch additional mirrors to light up every square mile of the earth.

The geometry of the requirement that the satellite be in view of the sun when the target is in darkness means that in order to be effective for longer than an hour or two around dawn/dusk there must be multiple satellites, much higher than 500km, and they must aim the light at a shallow angle, i.e. from low in the sky relative to the target, nearer parallel to the earth’s surface than overhead. It seems to me that this implies a lot of light spillage away from the target. We’re talking about aimed mirrors, not laser beams.