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An asteroid is expected to make a “horseshoe path” around Earth over the next two months.
The Harvest moon rises through clouds over the Atlantic Ocean off the coast of Camden, Maine, Tuesday, Sept. 17, 2024. Robert F. Bukaty / AP
By Kasha Patel, Washington Post
updated on September 20, 2024 | 12:50 PM
For the next two months, an unusual object about the size of a bus will be orbiting above our heads. Say hello to Earth’s temporary new mini-moon.
From Sept. 29 to Nov. 25, a passing asteroid will be pulled in by Earth’s gravity – sort of like a window shopper – before returning to its normal orbit around the sun. Objects that get captured by our gravitational force for a short time are known as mini-moons. This one, named asteroid 2024 PT5, came from the Arjuna asteroid belt near our sun, about 93 million miles away.
Of course, you won’t really be able to “say hello” without a professional telescope. At 33 feet long, the mini-moon is too mini for our eyes. Its closest approach is still five times farther than our permanent, non-mini moon. That challenge, though, makes it all the more exciting that scientists are able to detect the tiny rock in the first place.
“Captures like this must occur frequently. They are just hard to detect,” said Derek Richardson, an astronomy professor at the University of Maryland.
Asteroid 2024 PT5 was discovered in August by an early-warning system for incoming asteroids called the Asteroid Terrestrial-impact Last Alert System (ATLAS). The system automatically scans the sky several times each night looking for moving objects.
The asteroid is expected to make a “horseshoe path” around Earth over the next two months, researchers wrote in a recent study. After it completes its mini-moon episode, it will stick around Earth’s neighborhood until January. After that, it will not return until 2055.
Not many mini or temporary moons have been discovered around Earth, said Richardson, who was not part of the recent study. At least two other short events in 1991 and 2022 have been documented in scientific studies, but many more probably exist outside of scientific literature.
A mini-moon event requires a lot of physics to line up. To become a temporarily “captured moon,” an object must come very close to Earth at a relatively slow pace (about 2,200 mph). This is not very common, Richardson said, so it takes a lot of objects passing by closely before any are captured. Small objects are more likely to be captured, but they are harder to see with eyes or telescopes. More advanced surveys, such as ATLAS, are helping fill in those gaps.
The asteroid will not become a permanent second moon around Earth due to a tug-of-war with the sun, he said. It was originally orbiting our massive sun,