Imagine a weapon of mass destruction with no warheads, no chemicals, and no nuclear fallout, one that weaponizes sunlight itself. This episode explores the Sonnengewehr, or Sun Gun, a theoretical orbital weapon developed by German scientists during World War II that would use a giant concave mirror to focus the sun's rays and incinerate cities below.

We trace the concept from the myths of Archimedes' heat ray and the accidental melting power of London's Walkie Talkie building, through Hermann Oberth's optimistic 1929 space station, to the apocalyptic blueprints drawn at the Hillersleben proving grounds. We then follow the idea into the modern era, including Russia's Znamya space mirror, proposals to warm Mars, and the unsettling feasibility of satellite mega-constellations.

• Why the proposed mirror was made of metallic sodium and  ...