Later during World War II, a group of German scientists at a research centre in Hillerslebenbegan to expand on Oberth’s idea of creating a superweapon that could utilize the sun’s energy. This so-called “sun gun” would be part of a space station 8,200 kilometres (5,100 mi) above Earth. The scientists calculated that a huge reflector, made of metallic sodium and with an area of 9 square kilometres (3.5 sq mi), could produce enough focused heat to make an ocean boil or burn a city.[1] After being questioned by Allied officers, the Germans claimed that the sun gun could be completed within 5 or 10 years.[1][2]