Back in 1945, Life magazine revealed to its readers how “U.S. Army technical experts came up with the astonishing fact that German scientists had seriously planned to build a ‘sun gun’.”
This, the magazine explained, would have been a gigantic orbital mirror that would “focus the sun’s rays to a scorching point on the earth’s surface.” The German army, readers were told, “hoped to use such a mirror to burn an enemy city or to boil part of an ocean.”
The idea had been originally proposed by the seminal rocket scientist, Hermann Oberth, in 1923. As late as 1957, he was still convinced that his space mirror would become a reality.
“My space mirror,” he wrote, “is like the hand mirrors that schoolboys use to flash circles of sunlight on the ceiling of their classroom. A sudden beam flashed on the teacher’s face may bring unpleasant reactions...I was a schoolteacher long enough to have collected certain data on the subject.”
Life magazine could only speculate on how the German space mirror could be built. Its editors and illustrator imagined it assembled from prefabricated sections. These would eventually form “a slightly concave disk one mile in diameter.” Life thought such a satellite should be built in a geosynchronous orbit. “The Germans,” the editors sniffed, “complicating the problem, planned to build their mirror at 5,100 miles.”
The Life version of the mirror would be a manned space stations, with 30-foot holes in which supply rockets could dock, hydroponic gardens to provide oxygen and solar powered generators for electric power. The entire surface of the mirror—-front, back and edges—-would be mirrored, “otherwise [the sun’s rays] would burn occupants of disk to death instantly.” Which might be a clue to how much Life's editors knew about outer space.
In reality the building of the mirror would begin with the launch of a single unmanned rocket. Once in orbit, it would unreel six long cables, each only 1/2 to 1 1/2 inches thick. Spinning the rocket on its axis would extend the cables radially.
Illustration from 1929 showing the first stages in constructing the space mirror and an illustration from Oberth's original 1923 description, showing the mirror-supporting web and how the mirrors could be used to illuminate and warm the polar regions.
These would span 90 miles. Astronauts—-working like spiders creating a web—-would use these cables as the foundation for fashioning a network of hexagonal cells, each “several kilometers across.” The rotation of the structure would keep the entire thing taut. Each of these cells would contain a movable circular mirror made of thin sodium-metal foil.
Oberth's space mirror, ca 1957
Oberth thought it might take ten to fifteen years to assemble a complete mirror at a cost of $3 billion. The pressure of sunlight on the vast surface would be used to maneuver it in orbit, with steering accomplished by adjusting the angles of the individual mirrors.
Life's editors pooh-poohed the entire notion of an orbiting solar death ray. The scheme, they said, "is proved physically impossible by a simple axiom of optics. This is that light cannot be brought to a sharp, pointed focus with lenses or mirrors unless it comes from a sharp, pointed light source. Since the sun appears in the sky as a disk and not as a point, the best any optical system can produce is an image of this disk....At the distance the Germans proposed to set up their mirror (3,100 miles) the image of the sun cast on the earth would be about 40 miles in diameter and not hot enough to do any damage."
The nearly 5000-square-mile mirror, he said, would create a bright, heated “spot” on the earth’s surface about 2000 square miles in area. This heat and light, he admitted, would be “no stronger than that normal at the equator.” But, he went on to say, if “the mirror were double the size mentioned...the irradiation would be four times as strong...The temperature on the surface...would be 392°F.” Maybe not enough to melt cities and blow up battleships, but certainly enough to give one pause for thought.