This is a hard one to catalogue.  Should it go in Future Space?  Future Living? Future War?  Death Rays?  Maybe even Future Farming?  It's hard to say, because it's one of those bits of Future Past that could go just about anywhere except maybe under Robot Dogs. 

Sunlight is a useful thing.  It's used as illumination, to heat the Earth, grow crops, generate power, and is generally regarded as handy to have about.  The trick is to get enough of it in the right spot at the right moment, such as in this example from a 1924 issue of Practical Electrics that predicts a future where huge mirrors and lenses will heat temperate farmlands so that tropical crops can be grown in Minnesota or Herefordshire.  How the cocoa and bananas are supposed to get along after dark is best left unasked because the inventor gets a bit sulky.

The idea of a landscape dotted with huge lenses may sound a bit on the ambitious side, but that's as nothing compared to a true bit of ambition thought up in Germany about the same time.  Hermann Oberth,  the father of modern rocketry, hit upon a remarkably simple idea; instead of building a load of mirrors close to the surface of the Earth to warm things up, why not build one dirty big mirror further away?  Say, in outer space?

In 1929, Oberth outlined his idea for his space mirror, as it came to be called, in Hermann Noordung's classic  The Problem of Space Travel. 

The space mirror was very simple in principle, but frighteningly ambitious in execution.  Oberth advocated constructing a giant concave mirror with a diameter of 100 kilometres (62 miles) in orbit at a distance of 400 to 700 miles.  The mirror itself are made in sections made out of metallic sodium foil.  That may sound like a dangerous choice, since pure sodium is extremely flammable and ignites at the merest hint of water in its vicinity, but in the dry vacuum of space sodium foil is perfectly safe and highly reflective.  The foil sections are supported on a latticework of wires and individually controlled by electric motors so they can be tilted like louvers.  This way, the mirror can be  focused on its target as required.  Tilting the mirror to point it in the right direction is achieved by means of a system of rocket motors similar to those used on modern spacecraft.   Solar generators mounted on the mirror provide the electrical power needed to operate it and it is controlled at a distance from a manned space station s short distance away.

Hermann Oberth

With an estimated cost of three million marks and taking 15 years to construct, the purpose of the space mirror was to provide the people of earth with sunshine on demand, anywhere on the globe.  With such a device, Oberth imagined that night would become a thing of the past.  Harbours, train stations, airports, and even whole cities could be illuminated from on high without expensive public lighting systems.  The great metropolises of the world would be islands of sunshine in seas of darkness.

Even winter would be abolished by the great mirrors.  The cold northern regions of the would could be warmed with redirected sunshine that would turn the arctic tundra into farmland and even in the lower latitudes the mirrors would expand the growing season so that where only two crops a year were brought in, three or even four might be harvested to feed the hungry people of the worlds.  By focusing the sun's rays on the Baltic Sea or along the coasts of the Arctic Ocean, once icebound harbours could be freed up year-round and sea lanes along the polar boundaries of the continents could be a reality.

The mirror could even influence the weather.  By selectively heating various parts of the Earth, wind and rain patterns could be altered, storms redirected, hurricanes dispersed, and fogs evaporated.

However, the space mirror had other applications that Oberth made no secret.  Orbiting hundreds of miles above the Earth, the space mirror has a perfect vantage point from which to observe the comings and goings below–military as well as civilian.  The men controlling the mirror would be able to observe every troop movement and every fleet deployment.  They could map every arms depot, every camp, every rail line, and every fortification. 

So far, that sounds like a modern spy satellite, but the space mirror could go one better and step out of the realm of spy and into pure Blofeld territory.  That's because Oberth realised what every schoolboy knows; concentrated sunlight burns things.  And with a 62 mile mirror, that's a lot of sunlight and a lot of heat.

Oberth declared that the space mirror was the ultimate weapon.  It could sink ships in an instant, detonate munitions dumps with a flash.  Whole armies could be wiped out and cities sent up in flames as easily as throwing a switch.  The nation with a space mirror would be invincible and invulnerable.

All this would have left the space mirror in the Popular Mechanics school of death ray design, except for an incident in 1945 when the victorious Allies started going through captured Nazi war plans.  As reported in Life Magazine's 23 July 1945 issue, the Allies learned that the Nazis had dusted off Oberth's proposals, updated them a bit and looked into the possibility of the Third Reich building a mirror weapon in geosynchronous orbit 22,236 miles above the Earth.  It was a case of "I'll see your A-bomb and raise you my death ray".

The idea of London or Washington going up in flames without warning sent the discoverers into a mild panic until someone who knew a bit more about optics than Herr Oberth pointed out one minor detail that the Nazis overlooked.

It wouldn't work.

 The Oberth mirror could not be used as a weapon because of the nature of how a concave mirror focuses light. To produce a pinpoint of intense heat on the target, the light source would have to be a point. However, the Sun covers several degrees of arc.  That means that the smallest focus that the mirror can produce is an image of the Sun.  Beyond that, the lines of light cross and the image goes out of focus and just gets bigger.

This is fine for short distances where what's called the "focal length" is very small.  That makes the image very small and the heat intense.  That's why a pocket magnifier can start a campfire or a solar power station can heat a boiler.  But a space mirror has a focal length of thousands of miles and the size of the image increases at about one inch for every ten feet of length.  At a distance of 22,000 miles, the image of the Sun on the target would be forty miles across. 

That's not going to destroy anything.  In fact, it comes more under the heading of "shiny".
 

Unfortunately, the Allies never found out about Plan B.

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