First proposed as early as 1596 and based on Greek mythology, the Scottish mathematician John Napier put forward the idea of a giant mirror being used to set fire to the sails of enemy ships by reflecting the Sun directly at them. The idea was finally suggested as a possible space-based weapon by the German physicist Hermann Oberth in 1929. His plans included a 100 metre concave mirror in earth orbit which could concentrate intense sunlight upon any target on the earth.
Amazingly, while the Second World War was under way, the research and development for the artillery weapons of the German army brought the plans well into the development stage with the Sun Gun. It was determined that such a mirror of 9 square kilometres orbiting above the earth could produce a massive heat ray that would boil a city into liquid within minutes.
The time frame for putting the weapon into production was estimated at before the year 2000. Considering this was at a time when the Reich was supposed to last a thousand years, this was a realistically ambitious roll-out date for the German research team at the time.