For sailing in heavy seas, a single, strongly protected, scarf-jointed keel is the most that shipwrights judge acceptable for safety. According to today’s traditional ship- builders in Fujian, this would allow a boat in Chinese junk style to be at most 200–240 feet (61–73 metres) long. These vessels were highly seaworthy: between 1846 and 1848, a traditional Chinese junk, the Keying, sailed to the US and Britain, where it was visited by Queen Victoria.

Why Emperor Yongle commanded this enormous expenditure of resources is still the subject of much controversy. Today in China, these journeys are characterised as peaceable missions to display the flag, encourage trade and wield Chinese soft power. But the idea that Zheng He’s expeditions were done for no real political, economic or practical gain seems, on the face of it, unlikely.

These routes had long been travelled by Arab navigators, and the Chinese had first visited east Africa as early as the Tang dynasty (AD 618–907). In some places, such as the Strait of Malacca, there was a deliberate effort to develop commercial entrepots. For example, Palembang, in south Sumatra, became an important Chinese trading post and a base for journeys into the Indian Ocean. Zheng He’s voyages, then, were to advertise Chinese power, exact tribute, open up new trade routes and, in particular, to get the states of south-east Asia to acknowledge the power and majesty of the Ming.

When they had to, they engaged in warfare, destroying pirate fleets near Palembang and intervening in civil wars to establish friendly regimes in Sumatra and what’s now Sri Lanka. So though they were not trying to colonise or conquer, their goal was to establish a network of control between China and the Indian Ocean over peoples who, in Zheng He’s words, “resisted the transforming influence of Chinese culture”. To create trading relations mutually beneficial to China and its various tributary states and kingdoms, “the sea routes were cleaned up and made peaceful so the natives were able to pursue their vocations in peace”.

At that moment, with its technological lead over Europe, China seemed poised to dominate much of the world. In 1433, however, the voyages were abruptly stopped and the fleets broken up. In a memorable advert for the aerospace company Lockheed in the 1980s, that decision was compared to giving up the moon programme after Apollo 8. Though the greatest scientific inventors of the pre-modern world, the Chinese were dismissed as lacking the desire to push the bounds of knowledge.

But that is to misunderstand the concerns of traditional Chinese civilisation. There were two key reasons for stopping the voyages. First was the enormous expense, including inevitable losses at sea. But there was something else. The ruling Confucian ethic was the cultivation of the Middle Kingdom, not the conquest or domination of other peoples. As Matteo Ricci – a Jesuit in the Ming court – observed, it was the Europeans, not the Chinese, who were “consumed with the idea of supreme domination… always covetous of what others enjoy”.

So when I hear the SpaceX plans, I think of Zheng He. Curiosity and scientific interest no doubt will drive human beings to the uninhabitable wastes of Mars, if only to harvest its rare earth metals. But beyond? Is Musk right to think that humanity can become a multi-world species? The facts, as I understand them, are completely against it. The distances are too vast for humanity to travel to other habitable systems, even if they exist.

Our fate is on our planet – and here our efforts should be redoubled in this time of crisis. Confucius, I’m sure, would agree.

This column was first published in the April 2025 issue of BBC History Magazine