Earth Has Had a Secret Second Moon for Months Now

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.

“It’s just a chance occurrence,” Kat Volk, a planetary scientist at the University of Arizona’s Lunar and Planetary Laboratory, told me. “They just have to come in at the right speed and the right angle. The vast majority of things that are whizzing by the Earth do not get even temporarily captured into orbit; they just keep whizzing by, with their trajectory just a little bit tweaked by the Earth’s gravity.”

Astronomers have named the mini-moon, for now, 2020 CD3. As excited as they were to find it, they weren’t completely shocked. The Catalina Sky Survey found one before, in 2006. Although they’ve now seen only two of them, astronomers suspect more are out there. Some estimate that, considering how many bits of asteroids reside near Earth, at least one tiny moon is lassoed around the planet at any given time. Gravity, after all, has shown itself to be a skilled thief; some of the outermost stars in our Milky Way were torn from another galaxy as it passed by. A rock the size of a car is an easy steal for Earth’s gravitational forces.

These forces, along with the moon’s own gravity, have put 2020 CD3 on a pretty quirky orbit, unlike the other neat loops of the solar system. Below, the white band represents the orbit of the moon, with the Earth inside. The tiny moon’s orbit is in red, looping around like yarn:

(2/3) The object has just been announced by the MPC and its orbit shows that it entered Earth’s orbit some three years ago. Here is a diagram of the orbit created with the orbit simulator written by Tony Dunn: pic.twitter.com/2wsJGtexiO

— Kacper Wierzchos (@WierzchosKacper) February 26, 2020

Like other near-Earth objects, 2020 CD3 probably originated in the asteroid belt between the orbits of Mars and Jupiter. With the help of computer simulations, astronomers can try to trace its path back in time. “If you get enough data, you can conclusively trace these looping spaghetti paths through the Earth-moon system and find out where it entered the system,” says Eric Christensen, a University of Arizona astronomer who works on the Catalina Sky Survey, and who discovered the mini-moon in 2006.