Earth Has a Second Moon—For Another 300 Years, At Least

It’s easy to be brand loyal to the moon. We’ve only got the one, after all, unlike Jupiter and Saturn, where you’d have dozens to choose from. Here, it’s luna or nada. Or not. The fact is, there’s another sorta, kinda moon in a sorta, kinda orbit around Earth that was discovered only in 2016. And according to a new study in Nature, we may at last know how it was formed.

The quasi-moon—named Kamo’oalewa, after a Hawaiian word that refers to a moving celestial object—is not much to speak of, measuring less than 50 m (164 ft) across. It circles the Earth in a repeating corkscrew-like trajectory that brings it no closer than 40 to 100 times the 384,000 km (239,000 mi.) distance of our more familiar moon. Its odd flight path is caused by the competing gravitational pulls of the Earth and the sun, which continually bend and torque the moonlet’s motions, preventing it from achieving a more conventional orbit.

“It’s primarily influenced just by the sun’s gravity, but this pattern shows up because it’s also—but not quite—on an Earth-like orbit. So it’s this sort of odd dance,” says graduate student Ben Sharkey of the Lunar and Planetary Laboratory at the University of Arizona, the lead author of the paper.

None of this means that Kamo’oalewa has to have especially exotic origins. The solar system is littered with asteroids, some of which are captured by the gravity of other planets and become more conventional—if fragmentary—moons. Others don’t orbit other planets in the common way but fall into line in front of them or behind them and pace them in their orbits around the sun, like the flocks of so-called Trojan asteroids that precede and trail Jupiter.

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Either way, Kamo’oalewa was bound to get attention because its composition posed a stubborn mystery. Asteroids tend to reflect brightly in certain infrared frequencies, but Kamo’oalewa just doesn’t. It’s dimmer somehow—clearly made of different stuff, which suggests a different origin.

To investigate the mystery, Sharkey, under the guidance of his PhD adviser, planetary scientist Vishnu Reddy, first turned to a NASA-run telescope in Hawaii routinely used for studying Earth-vicinity asteroids. But even through the usually reliable instrument, the infrared signature seemed too faint. Instead they switched to a University of Arizona-run monocular telescope that, as Sharkey says, could “squeeze every last ounce of photons out of that object.”

That produced better, clearer results, but still they were incomplete. The rock was made of common silicates like other asteroids, but they were common only in their general composition, not in their infrared signature, which remained stubbornly off.

At last, the answer suggested itself. If Kamo’oalewa was behaving like a sort of quasi-moon, perhaps it was an artifact of the actual moon. Earlier in Sharkey’s PhD program, one of his advisers published a paper on lunar samples brought back by the Apollo 14 mission in 1971. When Sharkey compared the data he was getting in his telescope with what the earlier geologists came up with in the rock lab, the results matched perfectly. The kind of space-weathering lunar silicates undergo when they’re still on the surface of the moon precisely accounted for the differences in the infrared reflectivity between common asteroids and Kamo’oalewa.

“Visually, what you’re seeing is weathered silicate,” says Sharkey. “The eons of exposure to space environment and the micrometeorite impacts, it’s almost like a fingerprint and it’s hard to miss.”

How Kamo’oalewa shook free of our lunar companion is no mystery. The moon’s been getting bombarded by space rocks for billions of years, resulting in all manner of lunar debris getting ejected into space (nearly 500 bits of which have made it to the surface of the Earth as meteorites). Kamo’oalewa is one such piece of lunar rubble that spiraled away from the moon. But rather than landing on Earth or simply tumbling off into the void, it found itself a quasi-satellite in its own right.

“We see thousands of craters on the moon, so some of this lunar ejecta has to be sticking around in space,” says Sharkey.

Kamo’oalewa won’t stick around all that long, as its current trajectory is not entirely stable. According to estimates from Sharkey and others, the object will remain an earthly companion for only about 300 more years—nothing at all on the cosmic clock—after which it will break free of its current gravitational chains and twirl off into the void. Originally a part of the moon, then a companion of Earth, it will spend the rest of its long life traveling on its own.