A YouTuber who has, ahem, come to my attention before has produced a video on the concept of the “Nazi Sun Gun.” In a nutshell, it’s the idea that the Nazis had plans to orbit a gigantic mirror in space; the mirror would focus sunlight to a point on the Earth and burn cities to ash. As a yarn it’s entertaining enough; as history it’s a bit dubious; as physics it’s laughable magical thinking up there with car engines that burn water.
There are two major problems with the “Sun Gun” story:
1: It is very poorly documented. There were a few news and magazine articles on the topic immediately after the war; both the New York Times and Life covered it. But none of these stories provide any documentary evidence for the claims. It *appears* that someone who didn’t know any better stumbled across Herman Oberth’s ideas for an orbiting mirror from the early 1920’s. And while his ideas were reasonable enough given the time, his ideas were to provide some illumination at night, not make cities burst into flames. In all probability, some reporter, or perhaps a military officer looking for some press, heard something they didn’t quite understand and, using the journalistic integrity that CNN has demonstrated so well, blew it far out of proportion for the 1940’s equivalent of internet clout.
2: The physics does not work *AT* *ALL.*
The difference between providing useful levels of illumination and light so intense that wood catches fire is many, many orders of magnitude. For example: on Pluto, the sunlight is about 1/1500 less intense than it is on Earth… and that’s still more than adequate to read by. The full moon, which is strong enough to do useful things in, is only 1/400,000 as intense as full sunlight. In contrast, starting a fire with light requires light *far* more intense than plain everyday daylight. Whether using a parabolic mirror or a glass lens, you have to focus a lot of sunlight into a small area to get fires going… and typically you have to hold it for a while to do that.
OK, so why is this a problem for a space mirror? Because the sun isn’t a point source of light. It is a distinct circular area, about one half of a degree in apparent diameter. This means a parabolic mirror or a lens can *not* focus the light to a point, but to a circle. This limits how intense the spot can be. To first approximation, the best you can do, given really, really good workmanship, reflectivity and aiming accuracy, is to make your mirror look as bright (from the viewpoint of the target) as the sun. If you do it right, and your mirror is as big in the sky as the sun, your target will receive the equivalent of full daylight. So if you aim this fantastic mirror at a city that’s currently in night-time – and it would be difficult to do so with a daylit city – you will provide the city with the equivalent of normal daylight. Blue sky, chirping birds, all that. But that is far, FAR from causing fires.
And even that would require a truly VAST mirror. If your mirror is orbiting at 200 miles, about ISS altitude, it would have to be 1.75 miles across to look as big as the sun. And think of the geometry: you’re trying to reflect sunlight down onto a city. But if you’re only 200 miles up, that means most of the time when you’d be in position to fry a city, there’d be a *planet* in the way. Your mirror would be in darkness. So, move it out to 5,000 miles, as the “Sun Gun” articles suggested the Nazis were planning. In order to be as big in the sky as the Sun now, since you are 25 times further away your mirror would need to be 43.75 miles in diameter. We’re getting on to about the size of the Death Star… and all you can do is turn night into a pleasant, brief day for some city or other. If you want to start fires, you need to be *hundreds* of times more powerful… which means you need to have tens the diameter. A 400+ mile diameter mirror is something that is beyond stupid.
This is not physics only discovered post-war; this has been known for centuries, ever since children discovered the psychopathic delights of frying ants with magnifying glasses. Imagine being that ant and looking up to see a magnifying glass being moved into position in order to burn you. In the moments before your compound eyes fail and your brain melts… just how much of the sky does that magnifying glass take up? A very large percentage of it. An orbiting mirror meant to burn cities would have to be equivalently huge.
This is not mysterious; this is basic. So whenever I see a discussion of the “Sun Gun” with no mention that the idea is simply unworkable fantasy that defies logic and optics, I get a little miffed.
