On August 7, 2024, NASA’s ATLAS Potentially Hazardous Asteroid Early Warning System detected object 2024 PT5. It appeared to be just another object on the list of near-Earth objects, or NEOs. near-Earth objects).
Although humanity has only been exploring the world of near-Earth objects for a few decades, more than thirty thousand of them are already known, and the discovery of each of them no longer causes the same sensation as in the past. But it quickly turned out that 2024 PT5 had something special: it was destined to become, in just a few months, Earth’s second natural satellite.
Indeed, a study by the Complutense University of Madrid revealed that between September 29 and November 25 this small asteroid, whose size is estimated at only around ten meters in diameter, would be trapped by the gravitational embrace of the Earth. In fact, it moves through our environment as if it were a mini-moon.
Captured by negative energy
For a planet, Earth or any other, catching an asteroid in its path and making it its satellite is not as simple as it seems. An object that approaches Earth from a distance and is not gravitationally bound to it has positive total energy. The law of conservation of energy would require that this energy be conserved, so the object would have to maintain the same positive value forever, which would eventually separate it from our planet.
However, the Earth and the asteroid are not alone in the universe, and the disturbances of the Sun and the Moon alter this simple framework and open the door to energy exchanges capable of modifying the panorama.
567,000 km from the center of the Earth
The ATLAS system captured the asteroid hours before its closest approach, which occurred on August 8, when the space rock was just 567,000 km from the center of the Earth, or 48% further than the distance of the Moon (which rises to 384,000 km). Despite this, the celestial body’s small size makes it invisible in the sky except for large telescopes.
Since then, 2024 PT5 and Earth have performed a cosmic dance in which distance and relative speed have changed.
We are losing the mini moon
The asteroid has moved away from our world but, at the same time, it has reduced the speed at which it moves from our point of view, until it was moving at only 680 km/h on October 29, a ridiculous speed in terms of astronomy.
During this complex multiband interaction, the asteroid’s energy measured from Earth has become slightly negative since the end of September and will remain so until the last days of November. This means that, in this interval, planet Earth will have had two natural satellites!
At the same time, the asteroid has traced a complex loop that will eventually move it away from Earth after recovering the energy necessary for its geocentric balance to become positive again. Although it will approach the planet again in January 2025, we will not have 2024 PT5 as Earth’s “mini-moon” again until 2055.
The busy patio of my house
Before the turn of the century, interplanetary space was considered much emptier than we think today. There was speculation about possible unknown natural satellites of Earth and other planets, and for a time it was believed that a second moon had been discovered, which was eventually named Lilith, although all indicates that this was an observational error.
However, systematic campaigns to locate potentially dangerous objects have revealed that Earth inhabits a very lively cosmic neighborhood. Among the more than thirty thousand bodies close to the Earth (most of them tiny), some are found in exotic orbits subject to capricious and varied gravitational effects.
The abundance of quasi-satellites
There are objects that can be captured by the Earth “in passing”, in a very transient manner, while others end up accompanying our planet for months or years, with which they come to make several complete revolutions around us , which makes them authentic “mini-moons”. This can happen several times a decade.
The so-called “quasi-satellites” are not captured (they still maintain positive energy), but they appear to orbit the planet, in apparent highly inclined orbits. Earth has nine quasi-satellites and Venus has at least one.
Asteroids with “horseshoe” orbits approach the Earth “from behind” then change trajectory without passing us, and they move backwards and backwards so much that they eventually reappear in front after a complete revolution. When they get there, they accelerate again, run away and repeat this cosmic swing a thousand times. Gravitational disturbances can cause objects in horseshoe orbits to become transient mini-moons.
Trojans are made of stone
Finally, there are Earth’s Trojan satellites, objects that follow our planet on its path but sixty degrees ahead or behind it in the same orbit.
The patio of our earthly home is special, and meteors rain there, but blocks of slightly larger size also circulate. None have yet been discovered that should really frighten us, so in the meantime, let’s enjoy the cosmic curiosities that gravitational games offer us, which introduce us to mini-moons, horseshoes and the acrobatics of the Tyrians and Trojans.
This article was originally published in The conversation. You can read it here.
