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This is going to sound preposterous, but I promise it’s true: Earth has another moon.

It is not the kind that will illuminate the night sky. It’s invisible to the naked eye and too tiny to do any classic moon moves, like tugging on the planet’s oceans. But it’s there, orbiting the Earth, accompanying us on our journey around the sun.

A pair of astronomers discovered the miniature moon on the night of February 15. It showed up in the nightly observations of the Catalina Sky Survey, a NASA-funded project in Arizona. The survey is designed to study asteroids and comets near Earth, the kind that could potentially menace the planet if they got too close. To Kacper Wierzchos and Teddy Pruyne, the mystery object appeared as a few pixels of light moving quickly across a dusky, fixed background.

Researchers at other observatories and amateur astronomers around the world raced to monitor the newcomer in the sky, collecting as much data as they could. When they calculated its orbit, they were baffled. The object wasn’t a newcomer at all. So far, their work suggests that the object has been moving around us, gravitationally bound to the Earth for the past many months—at least a year, but potentially closer to three. We’ve had a tiny new moon all this time, and we didn’t know about it.

So what exactly is this thing?

Astronomers don’t know everything yet—it’s been less than two weeks!—but they’ve identified some traits. The object is about the size of a compact car and traces a rambling loop around Earth about every four months or so. As the object passed by Earth on its path through space, the planet’s gravity pulled it close. And in that moment, it became a moon.

At first, astronomers thought the new moon could be a piece of space junk, a rocket part discarded after a successful launch. To say conclusively, astronomers would need to use powerful telescopes to study the sunlight reflected off the object, which can reveal its composition from afar. There’s at least a small chance that it could be a chunk of our moon that broke off after an impact, one astronomer told me. But the latest observations suggest that the object is probably an asteroid, one of the many floating around near Earth.

RK_Striker_JK_5 wrote:Let's hope it at least stays in a stable orbit. I don't relish this thing coming down.