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ATLANTA — Earth is about to gain a new "mini-moon," but it won't stay around for long.
The newly discovered asteroid, named 2024 PT5, will temporarily be captured by Earth's gravity and orbit our world from Sept. 29 to Nov. 25, according to astronomers. Then, the space rock will return to a heliocentric orbit, which is an orbit around the sun.
Details about the ephemeral mini-moon and the horseshoe-shaped path it travels were published this month in the Research Notes of the American Astronomical Society.
Astronomers first spotted the asteroid on Aug. 7 using the South Africa-based observatory of the NASA-funded Asteroid Terrestrial-impact Last Alert System, or ATLAS.
The asteroid is likely about 37 feet in diameter, but more observations and data are needed to confirm its size, said lead study author Carlos de la Fuente Marcos, a researcher on the faculty of mathematical sciences at the Complutense University of Madrid.
The space rock could be anywhere between 16 and 138 feet in diameter, potentially larger than the asteroid that entered Earth's atmosphere over Chelyabinsk, Russia, in 2013. About 55 to 65 feet in size, the Chelyabinsk asteroid exploded in the air, releasing 20 to 30 times more energy than that of the atomic bomb dropped on Hiroshima, Japan, and generating brightness greater than the sun. Debris from the space rock damaged more than 7,000 buildings and injured more than 1,000 people.
But as a mini-moon, Asteroid 2024 PT5 isn't in any danger of colliding with Earth now or over the next few decades, de la Fuente Marcos said. The space rock will orbit about 2.6 million miles away, or about 10 times the distance between Earth and the moon.
The making of a mini-moon
Mini-moon events come in two flavors, de la Fuente Marcos said.
Long episodes involve asteroids referred to as temporarily captured orbiters, which complete one or more revolutions around our planet that can last for one or more years. During short episodes, on the other hand, the asteroid doesn't even complete one full pass around the Earth.
These short-timers, also known as temporarily captured flybys, are — like 2024 PT5 — mini-moons for a matter of days, weeks or a few months, he said.
Earth has previously captured other temporary mini-moons, such as Asteroid 2020 CD3. Although that asteroid was first spotted whirling around Earth in February 2020 and departed a couple of months later, research showed it had orbited our planet for a few years before being detected.
Asteroid 2020 CD3 is considered a long-capture mini-moon, while the newly detected Asteroid 2024 PT5 is a short-capture one.
Short mini-moon events can occur several times per decade, but long mini-moon events are rare and only occur every 10 or 20 years, de la Fuente Marcos said.
It's not easy for asteroids to become mini-moons because they have to be traveling at just the right speed and direction to be captured by Earth's gravity.
"In order to become a mini-moon, an incoming body has to approach Earth slowly at close range," de la Fuente Marcos said.
Asteroids that become mini-moons come within 2.8 million miles of Earth at speeds under 2,237 miles per hour, he added.
"Whether an asteroid gets captured by Earth is independent of its size or mass, it only depends on its speed and trajectory as it approaches the Earth-Moon system," said Robert Jedicke, a specialist emeritus on solar system bodies at the University of Hawaii's Institute for Astronomy, in an email. "Almost all the asteroids that approach Earth do so too fast and at the wrong angle to be captured, but sometimes the combined tugs of all the objects in the solar system contrive 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 came from the Arjuna asteroid belt, which is made of small asteroids that have orbits around the sun similar to Earth's orbit.
"We think that there is about one dishwasher-size minimoon in the Earth-Moon system at any time, but they are so difficult to detect that most of them go undiscovered during the time that they remain bound to Earth," Jedicke said. "2024 PT5 might be about 10 meters in diameter, making it the largest captured object discovered to date."
Mini-moons can also be asteroids that come from the main asteroid belt, located between Mars and Jupiter, or they may be fragments of the lunar surface launched 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.
Future flybys
De la Fuente Marcos and his colleagues plan to observe 2024 PT5 to collect more data and details using the Gran Telescopio Canarias and the Two-Meter Twin Telescope, both on Spain's Canary Islands. But the asteroid will be too small and dim for amateur telescopes or binoculars to observe, he said. It won't create any observable effects on Earth.
After 56.6 days, the sun's gravitational pull will bring Asteroid 2024 PT5 back into its normal orbit.
But the space rock is expected to make a close flyby of Earth from 1.1 million miles away on Jan. 9, 2025, before "leaving the neighborhood of Earth shortly afterwards, until its next return in 2055," according to the study.
And when Asteroid 2024 PT5 comes back around 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.
