The space banquet has been troubled by the ghost of Leif Ericson all along, archeological confirmation of Norse voyages having been unearthed right at the start of the space age. Only some years later (at least for Westerners) did another ghost turn up at the feast: Zheng He, the Muslim eunuch who led the great Chinese 'treasure fleets' to the Indian Ocean nearly a century before the Portuguese got there from the other direction.
The Chinese may have gone farther than that. A 15th century Venetian monk and geographer, Fra Mauro, reported that a large 'zoncho' - junk - from the Indian Ocean sailed sailed 2000 miles into the Atlantic before turning back. The illustration on his map shows a ship of European type, but the artist was probably working from a second or third hand account, and knew only that the 'zoncho' was big.
(By the way, the Chinese probably did not 'discover America' in 1421 or any other year. File that one alongside the aliens who built the Pyramids.)
One puzzle about the treasure fleets turns out to have a simple (likely) explanation. The largest ships are reported to have been enormous, upwards of 150 meters long, even close to 200 meters. But Europeans later found that the maximum safe length for seagoing wooden ships was 60-70 meters, about 200 feet. Yes, the Chinese used very different constructional methods, but wood is wood, and early modern Europeans were not chumps when it came to wooden shipbuilding.
It is now thought that the mega junks were used only for river service on the Yangtze, while the largest ships in the treasure fleet were comparable in size to the largest European Indiamen a few hundred years later, with a load capacity of about 1500 tons. Which is just what you'd expect.
But the much bigger and sobering puzzle about the treasure fleets is the way their voyages suddenly ended. There was a power shift at the imperial court. The court eunuchs (among whom was Zheng He) had supported the voyages, and when they fell out of favor the budget ax soon fell. The treasure ships were laid up and left to rot away. More axes fell than that. Building large seagoing ships was forbidden, and China's entire maritime capacity rotted away. A hundred years later Europeans showed up, and the rest is history.
It makes a great cautionary story. Cut the NASA budget, and the next thing you know the red haired devils are at the door. Or something like that.
The real story is more complicated. For one thing, the treasure fleets were stupendously big and expensive. Really big, perhaps five or ten times the fleet tonnage of a Spanish treasure flota or a convoy of Indiamen. A vast and economically disruptive effort was called for to build the treasure fleets and send them out.
Imperial China was capable of it; whether it was a good idea was no doubt a matter of dispute. It was good if you were a merchant or timber supplier, or a member of the court with a taste for Indian or Middle Eastern luxuries, not so good if you were paying taxes for some fairly nebulous benefits.
So it is not surprising that there was a reaction. The whole enterprise had no deep foundations in China's economy or its political interests. It was a matter of imperial prestige, and only by one measure of prestige - fleets of junks impressed no one in Central Asia, far from any ocean. By comparison the British East India Company was deeply rooted in the folkways of England. (Crumpets and tea, anyone?)
The backlash, when it came, was as extravagant as the voyages themselves, but it did not cause the 'century of humiliation' 500 years later. Shipbuilding was banned, but the sky was big and the Emperor far away, and oceangoing Chinese shipbuilding did not cease. China lost the Opium Wars for other reasons, in another era.
If there is any warning here for space advocates it is about needless gigantism, and arguably the old hare and tortoise story. Perhaps the post-Apollo letdown was fortunate in that it happened early, before the inevitable backstroke could develop too much force. The post-Apollo program has been more modest, but in spite of setbacks it has proven fairly robust.
Related posts: Leif Ericson, a retrospective and prospective, a Solar System for this century, and thoughts on colonization.