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.