Earth is welcoming a temporary mini moon

Space is populated by countless objects. And when one of them gets trapped by Earth’s gravity, we have a new ­— lower-case — moon, although only temporarily. The 2024 PT5, a rock the size of a truck, is such a mini moon and will hang around for the next two months, according to calculations made by researchers.

When an object that passes close to the Earth, known as NEO, is drawn in by the Earth’s gravity, it moves in an elliptical orbit around us and is referred to by scientists as a mini moon. “The term mini moon was invented by my former supervisor Robert Jedicke of the Institute for Astronomy at the University of Hawaii,” explains astronomer Peter Veres of the Harvard-Smithsonian Center for Astrophysics and the Minor Planet Center.

Five temporary mini moons have been recognized so far. The first one, 2006 RH120, was the size of a small car, and circled the Earth four times between 2006 and 2007. It was followed by 2020 CD3, which stayed with us for about three years. “These objects remain in a chaotic orbit near Earth for days or weeks; their orbits are wildly affected by the Moon, and they are eventually ejected from the Earth-Moon system,” Veres says. Two other objects, 1991 VG and 2022 NX1, did not stay long enough to complete an orbit, so some scientists dispute their status as mini moons.

This is also the case for the new 2024 PT5, an asteroid of about 11 meters discovered in August 2024 by the ATLAS project through its Sutherland telescope in South Africa. ATLAS co-director John Tonry of the University of Hawaii explains that it is “the only telescope system that is spread out around the planet, so it is always looking at the whole sky.”

Once 2024 PT5 was discovered, the calculation of its trajectory was left in the hands of Carlos and Raúl de la Fuente Marcos, research brothers at Madrid’s Complutense University who are globally recognized experts in orbital dynamics. According to their study, which has just been published by the American Astronomical Society, the 2024 PT5 turns out to be a mini moon pulled into Earth’s gravity that will draw a loop around us from September 29 to November 25, before emerging from Earth’s gravitational pull and spinning back off into space.

Uncertain origin

As for the origin of 2024 PT5, Carlos de la Fuente Marcos explains that it belongs to the Arjunas, “a small secondary belt of asteroids that follows an orbital path similar to that of the Earth. The Arjunas can come from the main asteroid belt, or from lunar material ejected during the kind of impacts that create craters, or they could come from the region of the Solar System inside the Earth’s orbit.” According to De la Fuente, in the case of our new mini moon, a lunar origin would seem to be the most reasonable working hypothesis, according to the evidence.

So, does Earth have only one moon, or more than 170 million? According to the European Space Agency (ESA), 170 million is the approximate number of space junk pieces larger than one millimeter orbiting the Earth. As Ian Whittaker, an astrophysicist at Nottingham Trent University, wrote: “While the term planet has a clear definition, there is no strict definition of a moon.” According to NASA, only “naturally formed bodies” count as moons, which rules out space junk.

But when it comes to naturally formed objects, the Moon is not the only one that hovers near us. At the end of the last century, an asteroid called 3753 Cruithne, discovered in 1986, was presented by some media outlets as the Earth’s second moon. In reality, this rock, measuring about 3 miles, does not revolve around the Earth but around the Sun, yet it follows a path similar to ours — a horseshoe-shaped trajectory with respect to us. Several naturally formed objects are known to be co-orbital with the Earth; some are called quasi-satellites because they seem to revolve around the Earth when, in fact, theirs is a non-elliptical orbit as they are really going around the Sun in our vicinity.

But even if none of these objects stay with us for long, they often return. “They are temporary because the conditions that contribute to their stability — an absence of disturbers — are not present,” explains De la Fuente Marcos. The calculations of the orbits make it possible to know when the asteroids will come back; 2024 PT5 will do so in 2055. Monitoring NEO trajectories is vital to predicting possible impacts. Projects such as ATLAS, and Pan-STARRS in Hawaii, are dedicated to monitoring these risks: the acronym ATLAS stands for Asteroid Terrestrial-impact Last Alert System.

Veres, who participated in the discovery of thousands of minor planets while working on the Pan-STARRS project, also got to work on ATLAS, and adds that this latter system, while smaller than Pan-STARRS, has a field of vision “much larger and it can cover much more of the night sky every night.” Thus, ATLAS is today the best choice for detecting small objects that pass us almost in the blink of an eye. “If an asteroid is visible only a day or two before colliding with us, ATLAS has the best chance of seeing it,” Tonry says.

According to Tonry, the risk of threatening space objects is often misunderstood: “The probability of a serious impact is low, but the chance of a person dying from an asteroid impact is greater than from a shark attack or a plane crash,” he says. In other words, it is not a constant threat, but if there is a collision, its consequences can be disastrous. However, Tonry points out that the good thing about an asteroid collision, compared to other natural hazards, “is that it can be seen to be coming and is one hundred percent predictable.”

Tonry explains this system could warn of an event like the one in 2013 in Chelyabinsk, Russia one day in advance, and of a major one like the one in 1908 in Tunguska, Siberia one week in advance, while “a civilization-destroying event could be detected years in advance.” New observatories such as Vera C. Rubin or LSST, which will start operating in Chile in 2025, and NASA’s future NEOSM space telescope, will improve the chances of predicting potential space threats.

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