Our planet has only one natural satellite – the Moon. However, from time to time, our planet can also receive temporary companions that can stay in its vicinity for quite a long time. These can be mini-moons, quasi-satellites, and Trojan asteroids. Here’s what they are and how they differ from each other.
Mini-moons
Mini-moons are near-Earth objects that move in horseshoe-shaped trajectories and approach the Earth at a low relative speed. As a result, their geocentric energy becomes negative for some time. Most often, they do not have time to make a single revolution around the Earth, after which they leave its vicinity.
An example of such a mini-moons is the small object 2022 NX1. In 1981 and 2022, it twice became a temporary satellite of our planet. Some astronomers suggested that 2022 NX1 could be a spent rocket stage. But observations have not confirmed this theory. We can also mention the tiny asteroid 2006 RH120, whose diameter is only a few metres. In 2006-2007, it was a temporary satellite of the Earth and managed to make four orbits around it, after which it moved into a heliocentric orbit.
The last known mini-moons to date was the asteroid 2024 PT5. It held this status from 29 September to 25 November 2024. The next time 2024 PT5 will approach our planet is in 2055.
Quasi-satellites
Quasi-satellites are objects that are in a 1:1 orbital resonance with a planet, which allows them to remain close to it for many orbital periods. Unlike a real satellite, the orbit of quasi-satellites lies outside the area in which the planet’s gravity is able to hold the celestial body, and therefore is not stable. Therefore, over time, they move into interplanetary space.
To date, astronomers have found seven quasi-satellites. In the near future, one of them will receive a full name. It will be chosen by online voting in a competition organised with the support of the International Astronomical Union.
And another quasi-satellite called Kamoʻoalewa is waiting to meet with an Earth envoy. In 2025, China plans to launch the Tianwen-2 mission, which will deliver a sample of its soil to Earth. Scientists have long suspected that Kamoʻoalewa could be a fragment of the Moon, knocked into space by an asteroid impact. Analysing the sample of its soil will allow us to confirm or refute this assumption.
Trojan asteroids
Trojan asteroids are objects located in the vicinity of the Lagrange points L4 and L5, and, accordingly, in a 1:1 orbital resonance with the planet. Jupiter has the largest population of such bodies. Astronomers have already discovered more than six thousand of its Trojan asteroids. Their study is the main goal of the Lucy mission.
Earth has a much smaller population of Trojan asteroids. Currently, astronomers know of only two such objects located at the L4 point. They are distinguished by their chaotic motion, which makes it difficult to create a long-term forecast of their orbit. Most likely, in the future, they will leave their positions and move to other orbits.
Gravitationally trapped bodies
All of the above objects can be called pseudo-satellites. They may be in the vicinity of the Earth, but they do not orbit it in stable orbits, as the Moon does. This raises a logical question: can the gravity of our planet trap an object and put it into a permanent orbit around itself?
Giant planets are capable of this. Their gravity is strong enough to hold asteroids, which then become their permanent companions. In theory, the Earth is also capable of this. But this requires a very precise coincidence of a number of factors: the size, speed and angle at which celestial bodies approach the Earth. For example, an asteroid the size of a bus that ‘hits’ the upper atmosphere may, in some circumstances, be trapped by Earth’s gravity.
Most likely, this happened – especially in the distant past, when the Solar System was much more ‘lively’. However, the fact that there are no such objects near the Earth today suggests that even if the capture scenario is realised, its orbit will still be unstable. Therefore, the satellite will either fall to Earth or be ejected into interplanetary space.
It is quite possible that our planet had such a satellite at the beginning of the 20th century. On 9 February 1913, residents of North America saw a procession of slow-moving meteors that were observed for five minutes. Probably, they observed the process of destruction in the atmosphere of a captured asteroid that had been in a circumpolar orbit near the Earth for some time.
