For 57 days this fall, Earth will have a second moon
An asteroid is expected to make a “horseshoe path” around Earth over the next two months.
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, which dominates the orbits of all other bodies in the solar systems. But as the asteroid got closer, Earth’s mass deflected it enough that it could hang around. The orbit is not stable, though, and will eventually return to the sun’s orbit.
“To become a stable moon orbiting us, the object would need to lose a lot more speed,” Richardson said. That could happen only through another massive body acting on it or another kind of frictional force. As a result, “the dance will only last a little while.”
Objects flying around Earth are important to track for potential danger. This month, a three-foot asteroid harmlessly burned up as it entered our atmosphere. NASA’s Double Asteroid Redirection Test (DART) investigation team, including Richardson, successfully deflected a 530-foot asteroid slightly to prove that we can deter incoming hazards.
This asteroid will not pose a threat to us, but Earth is expected to see a close encounter with a dangerously large object in our lifetimes.
On April 13, 2029, asteroid Apophis will pass less than 20,000 miles from our surface – closer than some of our Earth-orbiting satellites. People in the eastern hemisphere will be able to see it without a telescope or binoculars. It is not expected to hit Earth, despite its close approach, but scientists will be sending multiple spacecraft to study the asteroid.
To comment, please create a screen name in your profile
To comment, please verify your email address
Conversation
This discussion has ended. Please join elsewhere on Boston.com