For more than half a century, the moon had been mocking the best minds in science, and for Erik Asphaug enough was enough.

The taunting began three years before Asphaug was born. On Oct. 7, 1959, the Soviet Luna 3 spacecraft looped behind the moon, snapping off a series of grainy but distinct photos and then radioing them home. Because the moon’s rotation is perfectly synchronized with its revolution, one hemisphere always points toward Earth while the other always points away, unseen. Luna 3’s first-ever images of the lunar far side revealed an expanse of rugged, blandly gray highlands—a vista utterly unlike the near side’s charismatic, Man-in-the-Moon markings. It didn’t take a planetary scientist to recognize the weirdness of that split personality. “I remember as a boy seeing one of the news programs showing the far side of the moon, and thinking it was incredible that a planet could be so different on each side,” Asphaug says.

Now it was 2010 and here Asphaug was, a professor of earth and planetary sciences at the University of California, Santa Cruz, attending a colloquium, still waiting for an explanation for the moon’s aggressive asymmetry. He listened, increasingly skeptical, as his colleague Ian Garrick-Bethell sketched out his proposed answer. In this latest theory, Earth’s gravity raised powerful tides on the moon billions of years ago, while it was young and molten. The bulges then froze in place, giving rise to the thicker crust and distinctive geology of the far side. The concept made no sense to Asphaug. “You’d get a bulge on both the far side and the near side, just like when you have high tide on Earth,” he says. But the whole point of the theory was to put a bulge on the far side only. “So the answer has to be that some miracle happens to erase the other half. It makes the problem even worse than before.”

Asphaug was not only dubious; he was inspired. For years he had been working to develop models moons, which only later merged into the one we know?