A space rock is about to become the new "mini moon" of the Earth

() – Earth is about to get a new “mini-moon,” but it won’t be around for long.

The newly discovered asteroid, named 2024 PT5, will be temporarily captured by Earth’s gravity and orbit our planet from Sept. 29 to Nov. 25, according to astronomers. Afterward, the space rock will return to a heliocentric orbit, which is an orbit around the sun.

Details about the ephemeral minimoon and the horseshoe-shaped path it follows were published this month in the journal Research Notes of the American Astronomical Society.

Astronomers first spotted the asteroid on Aug. 7 using the South Africa-based NASA-funded Asteroid Terrestrial-impact Last Alert System observatory, or ATLAS.

The asteroid is likely about 11 meters (36 feet) in diameter, but more observations and data are needed to confirm its size, said study lead author Carlos de la Fuente Marcos, a researcher at the Faculty of Mathematical Sciences at the Complutense University of Madrid.

The space rock could be between 5 and 42 meters in diameter, potentially larger than the asteroid that entered Earth’s atmosphere over Chelyabinsk, Russia, in 2013. That asteroid, between 5 and 42 meters in diameter, could be larger than the asteroid that entered Earth’s atmosphere over Chelyabinsk, Russia, in 2013. 17 and 20 meters in sizeexploded in the air, releasing between 20 and 30 times more energy than the atomic bomb dropped on Hiroshima, Japan, generating a brightness greater than that of the Sun. Debris from the space rock damaged more than 7,000 buildings and injured more than 1,000 people.

But as a minimoon, asteroid 2024 PT5 poses no danger of colliding with Earth now or in the coming decades, de la Fuente Marcos said. The space rock will orbit about 4.2 million kilometers away, or about 10 times the distance between Earth and the moon.

An animation shows what the mini-moon might look like when it passes around Earth. Credit: /Adobe Stock

Minimoons can be of two types, according to de la Fuente Marcos.

In long episodes, asteroids called temporary capture orbiters are involved, which complete one or more revolutions around our planet that can last one or several years. During short episodes, however, the asteroid does not even complete a full orbit around the Earth.

These short-duration episodes, also known as temporary capture flybys, are, like the 2024 PT5, mini-moons lasting a few days, weeks or a few months, he said.

Earth has previously captured other temporary minimoons, such as asteroid 2020 CD3. Although that asteroid was first spotted orbiting Earth in February 2020 and moved away a couple of months later, research showed that it had orbited our planet for a few years before being detected.

Asteroid 2020 CD3 is considered a long-capture minimoon, while the newly detected asteroid 2024 PT5 is a short-capture moon.

Short minimoons can occur several times per decade, but long minimoons are rare and only occur every 10 to 20 years, de la Fuente Marcos said.

It is not easy for asteroids to become mini-moons because they have to travel at the right speed and direction to be captured by Earth’s gravity.

“To become a minimoon, an incoming body has to slowly approach Earth at short distance,” de la Fuente Marcos said. Asteroids that become minimoons come within 4.5 million kilometers of Earth at speeds below 3,600 kilometers per hour, he added.

“Whether an asteroid is captured by Earth is independent of its size or mass, it depends only on its speed and trajectory as it approaches the Earth-Moon system,” Robert Jedicke, an emeritus specialist in solar system bodies at the University of Hawaii’s Institute for Astronomy, said in an email. “Almost all asteroids that approach Earth do so too quickly and at the wrong angle to be captured, but sometimes the combined tugs of all the objects in the solar system manage to allow a particular (slow) object at the right angle to be briefly captured.”

Jedicke was not involved in the new study.

Asteroid 2024 PT5 comes from the Arjuna asteroid belt, made up of small asteroids that have orbits around the Sun similar to Earth’s orbit.

“We think there is about one dishwasher-sized minimoon in the Earth-Moon system at any given time, but they are so difficult to detect that most of them go undetected for the time they remain bound to Earth,” Jedicke said. “2024 PT5 could be about 10 meters (33 feet) in diameter, making it the largest captured object discovered to date.”

Minimoons could also be asteroids originating from the main asteroid belt between Mars and Jupiter, or they could be fragments of the lunar surface thrown off by asteroid impacts millions of years ago, Jedicke said.

“Determining where they come from could help us understand the process of crater formation and how material is ejected from the Moon’s surface,” he said.

De la Fuente Marcos and his colleagues plan to observe 2024 PT5 to gather more data and details using the Gran Telescopio Canarias and the Twin Two Meter Telescope, both in Spain’s Canary Islands. But the asteroid will be too small and faint to be observed by amateur telescopes or binoculars, he said. It will not create any observable effects on Earth.

After 56.6 days, the Sun’s gravitational pull will return asteroid 2024 PT5 to its normal orbit.

But the space rock is expected to fly by Earth from 1.7 million kilometers away on January 9, 2025, before “leaving Earth’s vicinity shortly thereafter until its next return in 2055,” according to the study.

And when asteroid 2024 PT5 comes back again, astronomers expect it to become Earth’s mini-moon for a few days in November 2055 and again for a few weeks in early 2084.