In the second half of the 23rd century, space cities existed as military outposts and overpriced spas. However by 2300, molecular chemists developed techniques for growing pure crystals and organic chemicals in Zero-G. Freed from gravity's pull, molecules proved easy to separate using magnetic centrifugal spin cycles. Then, in perfect proportion and freed of impurity, elements combined into crystalline forms with perfect "dense packing" between each molecule. As a result, in the 24th century, pharmaceutical refining and Nano-Assembly plants moved off-world.