A Fleeting Moment in the Solar System

For a brief time, Earth had two moons.

The first, of course, was the one we’ve always had. The other was a fairly recent addition, about the size of a compact car— too tiny to tug on any oceans and invisible to the naked eye. In a chance encounter, gravitational forces had pulled the space rock away from an orbit around the sun and tossed it into one around us.

The discovery of the miniature moon, known as 2020 CD3, was announced in February, to the delight of people around the world, including those well beyond the usual crowd of stargazers. According to astronomers’ calculations, the little moon had been circling Earth for at least a year, maybe longer. What a wonderful surprise, to be ambling along our well-worn path in the cosmos, flanked by the usual scenery, only to find a new companion on our journey around the sun.

But this fellowship was fleeting: The mini-moon has left us, for a bigger, brighter attraction.

“There’s no question it was still in orbit around the Earth in early February, and there’s no question now that it’s in orbit around the sun,” Bill Gray, an astronomy-software developer, told me.

Read: Earth has had a secret second moon for months now

Gray places the shift around March 7. Since then, 2020 CD3 has been bound by the sun’s gravity, which means it’s not our mini-moon anymore.

This was always going to happen. The mini-moon was what astronomers call a “temporarily captured object.” It was never meant to stick around.

“I’m sad to lose this one,” Gray said. But Earth’s gravitational pull is strong enough to catch another passing hunk of space stuff. “There will always be another rock out there.”

The story of Earth’s short-lived moon begins, as stories of astronomy discoveries sometimes do, at a mountaintop observatory. The telescopes of the Catalina Sky Survey, a NASA-funded project in Arizona designed to track asteroids and comets near Earth, caught the object during nightly observations in February. “It wasn’t any different from the other near-Earth asteroids that we find,” says Kacper Wierzchos, one of the astronomers who was working the night shift then. Just a few pixels of light zipping across a dark background.

Read: The night sky will never be the same

On the other side of the country, in his attic office in Maine, Gray downloaded the observations. Gray makes and runs software that professional and amateur astronomers, including those at Catalina, use to track asteroids, comets, and other celestial objects. There was something unusual about that bright cluster of pixels, he thought, and when he calculated its orbit, the numbers suggested that, unlike the other objects in the data, this one was circling the Earth instead of the sun.

Weird, certainly, but not shocking. It might be space junk, Gray thought. “Once every year or two, we spot some bit of debris that’s been orbiting the sun for decades and comes back to visit us briefly,” he explained. In 2003, for example, a discarded rocket booster from the Apollo 12 mission entered Earth’s vicinity, looped around several times, and then escaped again. (Since the booster was human-made, it was considered space junk, not a new moon.)

But the new mystery object was intriguing, so Gray flagged it to fellow asteroid observers and soon professional and amateur astronomers around the world started tracking it. When they confirmed that it was indeed orbiting Earth, they were thrilled: Astronomers had detected a temporarily captured object only once before, in 2006.

Read: Fall in love with the sun again

Because of jostling gravitational forces from both our planet and its main moon, 2020 CD3 traced a rambling path around Earth, rather than a stable, neat loop. Based on orbital data, astronomers could see immediately that it was headed away from Earth—all it needed was a little push to escape our planet’s orbit. The mini-moon stopped being ours when it traveled beyond Earth’s Hill sphere, the region around a planetary body where its gravity dominates, attracting rocks and artificial debris toward its orbit.

While astronomers feel certain that our former moon isn’t space junk, they’re not sure yet what kind of rock it is. It could be one of the many asteroids that float near Earth, slowly nudged here from the asteroid belt between the orbits of Mars and Jupiter. Or it could be a piece of our own moon, cleaved from the surface during an impact from yet another space rock. (Seriously, there are a lot of rocks out there.) Astronomers are working on identifying it; some might already be analyzing data that could determine the color of 2020 CD3 and reveal whether it resembles the hue of an asteroid or a lunar rock.

In the meantime, 2020 CD3 continues on its path away from Earth, growing fainter by the second. Now that the rock is coasting in interplanetary space instead of pinballing around the Earth-moon system, its journey is more predictable. Gray predicts that 2020 CD3 will make another close pass to Earth in March 2044, but not close enough to be captured again. Someday, a more intimate relationship between Earth and its former mini-moon could start back up, but “someday” in cosmic time could mean thousands of years from now. And 2020 CD3 might not be so lucky next time; instead of escaping Earth’s grasp, the rock could plunge into its atmosphere and put on a sparkly meteor show for anyone watching below.

Don’t lose hope: Earth may have another mini-moon, or several, hidden out of sight from telescopes. Some astronomers estimate that the planet has at least one tiny moon around at any given time. According to one study, the typical mini-moon would orbit Earth for about nine months, but some could stay for decades.

“We’re continually in a transient ballet with small objects that are changing their orbits,” says Michele Bannister, a planetary astronomer at the University of Canterbury in New Zealand. “Sometimes, one of them will be on an orbit that is sufficiently close, exactly the right parameters where we can pick it up and dance with it for a while. And these are short dances.”