Earth is getting a tiny mini-moon. It won't be the last (and isn't the first)
The mini-moon, 'borrowed' from the听Arjuna asteroids, won't be visible to the naked eye.
Earth is going to have its very own mini-moon from听. The regular Moon鈥檚 new, temporary friend is 2024 PT鈧�, an asteroid captured from the Arjuna asteroid group (called the 鈥淎rjunas鈥�).
Our new mini-moon is approximately 10 metres in diameter and will be captured by Earth鈥檚 gravity for 57 days. It鈥檚 small and faint, so it won鈥檛 be visible by the eye or with small telescopes, but will be visible to larger telescopes.
What is a mini-moon?
听defines 鈥渕oons鈥� as 鈥渘aturally formed bodies that orbit planets鈥�. Intuitively, we think of moons as big rocks that stick around for a while, like our very own Moon.
听have defined mini-moons as asteroids or comets that are gravitationally captured by a planet for a temporary amount of time. Small mini-moons are very common, but ones big enough to spot are more rare.
2024 PT鈧� is only the听听mini-moon captured by Earth.
Orbiting or just flying by?
A mini-moon is a 鈥渢emporarily captured orbiter鈥� if it completes at least one full orbit of the Earth before returning to its usual orbit around the Sun.
If a mini-moon is captured by Earth鈥檚 gravity but doesn鈥檛 make it around for a full orbit, it鈥檚 a 鈥渢emporarily captured flyby鈥� instead.
Two of Earth鈥檚 mini-moons were 鈥渢emporarily captured orbiters鈥�, while the other three (including 2024 PT鈧�) fall into the 鈥渢emporarily captured flybys鈥� category.
Where do mini-moons come from?
Mini-moons are asteroids from the large population of听听that are temporarily grabbed from their orbit around the Sun.
Near-Earth objects are defined as natural physical objects floating in space, such as asteroids (space rocks) or comets (dirty space snowballs made of rock and ice), that are orbiting the Sun and approach to within 1.3 times Earth鈥檚 distance from the Sun at some point in their orbit.
Because these objects are around the same distance from the Sun as the Earth, they can sometimes be captured by Earth鈥檚 gravity.
2024 PT鈧� and and a previous mini-moon called 2022 NX鈧� were both captured from a group of asteroids orbiting the Sun at a similar distance from Earth, called the Arjunas. Arjuna is one of the main characters of the Hindu epic, Mah膩bh膩rata.
The first mini-moon 鈥� and a fake one
The first known mini-moon was called听. It arrived in late 1991 and left in early 1992, and like 2024 PT鈧� it was around听.
The mini-moon 1991 VG (marked with green lines). This image was made by combining seven images from the European Southern Observatory (ESO) Very Large Telescope (VLT). All images were taken on May 30 2017. The images were taken by tracking the asteroid position, so background stars appear as streaks.听,
In 2002, amateur astronomer Bill Yeung found what he thought was a second mini-moon:听.
However, on closer inspection the object displayed a spectrum of light suggesting it was coated in听. Then, a study of how the object鈥檚 brightness changed over time found its shape resembled something like the听.
Astronomers now believe J002E3 is the third stage of the Apollo Saturn V rocket () instead of a natural mini-moon.
Images of the false mini-moon J002E3 taken by amateur Bill Yeung on 3 September 2002. Bill Yeung / Bob Denny via Wikipedia, CC BY 听
More discoveries with a note of caution
The plot twist of J002E3 made astronomers a bit more cautious. When another mini-moon (2006 RH鈧佲倐鈧�) was spotted on September 14 2006, it was first classified as artificial. However, after more observations it turned out to be听. 2006 RH鈧佲倐鈧� stuck around from July 2006 until July 2007.
听
The next mini-moon,听, was gravitationally captured by Earth for听, making it the longest-captured mini-moon observed to date. It escaped Earth鈥檚 orbit in May of 2020. This mini-moon was around听.
Amateur astronomers Grzegorz鈥� Duszanowicz and Jordi 鈥塁amarasa discovered听听using the听. Similar to 2006 RH鈧佲倐鈧�, it was initially thought to be an artificial object from a past space mission.
It was later determined to originate from the Arjunas, just like 2024 PT鈧�. While it was discovered in 2022, it was temporarily captured by Earth鈥檚 gravity in January 1981 and June 2022. It鈥檒l be captured again in December 2051.
2022 NX鈧� was later found to be a听听5鈥�15 metres in diameter.
Will we find more mini-moons?
听听that, at any given time, Earth has at least one captured mini-moon less than 1 metre in diameter.
Even though astronomers think we always have a mini-moon, these bodies are challenging to detect. This is because they鈥檙e small and faint. They are usually found by projects specifically looking for asteroids near Earth.
2024 PT鈧� was discovered using the Asteroid Terrestrial-impact Last Alert System (), which is an ongoing project specifically designed to search for asteroids. 2006 RH鈧佲倐鈧� and 2020 CD鈧� were discovered using the ongoing Catalina Sky Survey () for Near Earth Asteroids. 1991 VG was discovered by the听听using the Spacewatch Telescope.
These projects will continue to search for asteroids, including mini-moons. We can also look forward to new discoveries and investigations when the听听Legacy Survey of Space and Time () starts observing in the next couple of years.
This enormous project will take images of the entire sky every few days for a period of ten years. First light of the Vera C. Rubin observatory听.
This article was first published in .
