The numbers 'just don't add up'

DW recently spoke with physicist and astronomist Scott Manley to talk about China's "artificial moon" plans.  

DW: What's wrong with the "China Daily's" story?

Scott Manley: The numbers that they give us just don't add up. They talk about the spacecraft being 500 kilometers away [roughly 300 miles], which is fine — that's a low Earth orbit. But then they talk about providing light for an extended period of time. And the problem with that is, at 500 kilometers up, satellites are moving very quickly. If you see a satellite, it moves across the sky in a few minutes. So they would only be able to direct light at a single point for a few minutes at a time. And that means if you really want to give light to a city, you need multiple satellites, and the satellites also need to be able to track the location on the ground.

Has this been done before?

The Russians obviously tried this back in the 1990s, and they didn't even adjust the angle of their illumination. They created a bright spot on the earth that would move along at seven kilometers per second, so you had to chase it if you wanted to get sustained brightness. Otherwise you would just see a flash of light for a second. Geostationary orbit [36,000 kilometers above the Earth] is conventionally the magic distance at which it could stay over a single area of the Earth.

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Can't China just put the artificial moon out there in geostationary orbit?

The problem is, as you go further and further out, you need to make your mirror larger and larger. Because the light being reflected disperses, there's a kind of fundamental physical limit: If you're in a 500-kilometer orbit, [a mirror of] 25 meters would give you something which is 10 times brighter than the moon. If you're going to do that out at 36,000 kilometers, it has to be hundreds of meters across. And if you've been following the James Webb Space Telescope — the next big space telescope from NASA — a big problem they're having is just unfolding their sun shield in zero g in space. It's actually quite complicated to make something that packs up very small and that can reliably unfold in zero g. especially since you can't really test it in zero g on the earth.

Other projects in the Earth orbit: Are astronauts stuck on the ISS?

But would the geostationary option work, at least in theory, to provide light to a medium-sized city?

You couldn't illuminate just a city. It would be a province that would be a fairly large area. If you've got a 600-meter mirror, sure ... There's this sci-fi trope of being able to do the "Archimedes light weapon," where you focus the sun onto a target. Thankfully, you couldn't do it from geostationary orbit to a target on the Earth, unless your mirror was really huge — we're talking hundreds of kilometers or more.

But in geostationary orbit, you also have the other problem that your spacecraft is going to be reflecting the sun. And we think of sun as just being light. But light has pressure, and that will act as a force. That means that the rays are essentially pushing your spacecraft. And that will change the orbit over time. So the spacecraft would not only have to reflect the sun. It would also have to make adjustments for the perturbations that the sun would cause to keep it in this orbit.

So did "China Daily" get it wrong? Would you say that?

I think they've just been very enthusiastic. I think realistically what they're going to do is test it in low Earth orbit. Then they have some claim that if they could continuously do this, yes, it could save hundreds of millions of dollars in street lighting. But that would then require multiple satellites and very carefully coordinated orbits, all coordinating themselves and making sure that they provide continuous illumination. And I don't think 500 kilometers is a good altitude for this. I think there's intermediate orbits as well that could be used. You could also put it in a polar sun synchronous orbit that passes over the city at the right times. 

And then of course, even if you did get into a situation where you could make it as bright as the moon every single night, that would actually mess with biological processes on the earth. There are animals that would see this thing, and that would mess with their various monthly rhythms, I guess. So you know, there's a lot of things to consider when you're doing these big grand schemes.

But this is, on some level, feasible.

It is, but not as described in their article.

Scott Manley, a trained physicist and astronomist, is best known online as an 'internet rocket scientist.'

The interview has been edited for length and clarity.