On August 7, 2024, NASA’s ATLAS potentially hazardous asteroid early warning system detected object 2024 PT5. It seemed like just another on the list of near-Earth objects, or NEOs. near-Earth objects).
Although humanity has only been exploring the world of NEOs for a couple of decades, at this point more than thirty thousand are already known, and the discovery of each one of them no longer causes the same sensation as in the past. However, it was soon proven that 2024 PT5 had something special: it was destined to become, for just a couple of months, the 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 about ten meters in diameter, was going to be trapped by the gravitational embrace of the Land. Indeed, it has been moving around our environment as if it were a mini-moon.
Captured by negative energy
For a planet, Earth or any other, to catch an asteroid in its path and turn it into its satellite is not as simple as it seems. An object that approaches the Earth from a distance and is not gravitationally bound to it has positive total energy. The law of conservation of energy would require that that energy be conserved, so the object would have to maintain the same positive value forever, which would end up separating 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 that 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 with the space rock just 567,000 km from the center of the Earth, 48% further than the distance to the Moon (which amounts to 384,000 km). Even so, the small size of the celestial body has kept 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 lose the minimoon
The asteroid has moved away from our world but, at the same time, it has reduced the speed with which it moves from our point of view, until it moved at only 680 km/h on October 29, a ridiculous speed speaking of astronomy.
In the course of this complex multi-band 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. That means that, in that 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 “minimoon” again until the year 2055.
The busy patio of my house
Before the turn of the century, interplanetary space was considered to be much emptier than it is now thought. There was speculation about possible unknown natural satellites of the Earth and other planets, and for a time it was believed that a second moon had been found, which was eventually named Lilith, although everything indicates that this was an observation error.
However, systematic campaigns to locate potentially dangerous objects have revealed that Earth inhabits a very busy cosmic neighborhood. Among the more than thirty thousand bodies near the Earth (most of them tiny) there are some in exotic orbits subjected to capricious and varied gravitational games.
The abundance of quasi-satellites
There are objects that can be captured by the Earth “in passing”, in a very transitory way, 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 “minimoons”. This can happen several times each decade.
The so-called “quasisatellites” are not captured (they always maintain positive energy), but they give the impression of rotating around the planet, in very inclined apparent orbits. Earth has nine quasi-satellites, and Venus at least one.
Asteroids with “horseshoe” orbits approach the Earth “from behind” and then change paths without overtaking us, and they move backwards and backwards so much that they end up appearing again ahead after one full revolution. When they get there they accelerate again, flee forward and repeat that cosmic swing a thousand times. Gravitational perturbations can cause objects in horseshoe orbits to become transient minimoons.
Trojans are made of stone
Finally, there are the Earth’s Trojan satellites, objects that follow our planet on its path but sixty degrees ahead or behind it in its same orbit.
The patio of our terrestrial home is particular, and meteors rain down there, but blocks of somewhat more considerable size also circulate. None have yet been discovered that should really scare us, so, in the meantime, let’s enjoy the cosmic curiosities that gravitational games give us and that present us with mini-moons, horseshoes and the acrobatics of Tyrians and Trojans.
This article was originally published in The Conversation. You can read it here.
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