Staggering in price, formidable in technical sophistication, unprecedented in level of national commitment—Zheng's voyages remain the closest functional equivalent to the cost, effort, and risk required to travel into deep space. Trying to picture what settling other planets might entail? One place to look is 15th-century China.

Zheng was an unlikely candidate for a life of far-flung adventure. At the time of his birth, China was torn by war between the Yuan dynasty and surging Ming rebels. Zheng was born into a Muslim family in the remote Yunnan province, then a battleground between Yuan and Ming. When he was about 10, invading Ming forces captured him and slaughtered most of his family. The boy was castrated. Forced to serve the Ming crown prince, Zheng eventually became his confidant and trusted adviser. After the last Yuan emperor fled in 1368, Zheng became part of an elite group of eunuch adventurers and troubleshooters at the Ming court in Beijing.