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.
“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.
Mini-moons like 2020 CD3 are, unfortunately, “temporarily captured objects.” The object discovered in 2006 escaped Earth’s orbit and went on its merry way, less than a year after it was found. 2020 CD3 will eventually leave us, too. “This isn’t an object that is stably orbiting the Earth like the moon is,” Christensen says. “This is a fairly tenuous connection to the Earth. It’s getting tugged on by the moon and tugged on by the Earth.”
The latest observations suggest that 2020 CD3 is already moving away from Earth for good. “Unfortunately, we are catching this one on its way back out,” says Bill Gray, who provided astronomical software that helped pinpoint the object. “It’s getting fainter. Already, it’s faint enough that if the Catalina Sky Survey looked at it now, it wouldn’t see it.” Gray predicts that the mini-moon will escape Earth’s orbit in a matter of weeks. It will most likely return to orbiting the sun, although there’s a chance it could someday head straight to Earth, where it would burn up in the atmosphere in a glittering meteor display.
The thought of losing a new moon so soon after uncovering its existence is a little depressing, so I asked Volk whether, someday, Earth’s gravity could ensnare an object to stay, perhaps even one that we could see in the night sky, shining alongside the original moon. “It would be possible, but it would be extremely unlikely,” Volk said. “You would need the [object] to come in and have a gravitational interaction with our existing moon in just the perfect configuration that would tweak its orbit and put it onto a stable orbit around the Earth. You can’t really come in from a heliocentric orbit and get captured into a stable orbit.”
Sigh. Back to marveling at our usual moon, then, that reliable glow in the night sky, as enduring as the stars around it. From our vantage point, the skies can seem predictable and immutable. The fleeting miniature moon provides a lovely reminder that our corner of the universe is, in fact, rather lively, sometimes more than we can know.