September 25, 2024
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A new mini-moon appears on Earth
The tiny asteroid, 2024 PT5, will operate as a mini-satellite close to Earth for the next two months before blasting off into deep space again.
Artist’s impression of a small asteroid in deep space. If such an object were to come within a few million kilometers of Earth and move slowly enough relative to it, it could be captured as a moon.
In just a few days, Earth will be hosting a mysterious visitor from deep space, but he won’t be staying for long.
First spotted by a South African telescope on August 7 as it swooped to within about 500,000 kilometers of Earth, the 10-meter-wide object “2024 PT5” looks like a relatively ordinary asteroid — a member of the sparsely populated “Arjuna” family that follows Earth around the Sun. But what it does next is pretty special. Just after 4 p.m. EDT on September 29, the tiny outward-bound space rock will reach a distance of about 1.5 million kilometers, far beyond the reach of Earth’s gravity. Slowed down by the gravitational pull from that great distance, the object will not escape Earth’s strict bondage, but rather enter a trapped path that winds around the planet. At that point, 2024 PT5 will become another satellite of Earth, a so-called mini-moon.
But its stay with Earth will be so brief that the interplanetary visitor won’t even have time to complete a full revolution: by November 25, the gravitational force of the Sun will surpass that of the Earth, and the tiny moon, along with the rest of its Arjuna family, will return to heliocentric orbit.
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You won’t need to break out your backyard telescope to catch a glimpse of this temporary guest; unfortunately, the tiny moon is too small and too dim to be seen with such rudimentary equipment. Carlos de la Fuente Marcos, a co-discoverer of 2024 PT5 and an astronomer who studies small moons at the Complutense University of Madrid in Spain, told Space.com that you’ll need a telescope at least 30 inches (76.2 centimeters) in diameter and a digital detector to photograph the object.
If you’re feeling a little down after missing out on a telescopic snapshot of the faint, fuzzy dot, you can find solace in the future. Based on 122 measurements over a 21-day orbit, 2024 PT5 is predicted to be briefly captured by Earth again in 2055, giving you 30 years to plan ideal observations. (Tracing 2024 PT5’s orbit over the past few years suggests that the object also briefly became a mini-moon in 1960, but that no one noticed at the time.) But the impatient among you, don’t fret: Mini-moons are common, and some last longer than others.
In the paper announcing the discovery of 2024 PT5, de la Fuente Marcos and his brother Raul de la Fuente Marcos, co-author also of the Complutense University of Madrid, point out that four other mini-satellites have been confirmed and reported in the scientific literature: 1991 VG, 2022 NX1, 2006 RH120, and 2020 CD3. Dynamically speaking, the first two objects are very similar to 2024 PT5 in that they were observed to remain in Earth’s hands for some time, but were blown away before completing one orbit around the Earth. Such “flyby” mini-satellite events, although largely undetected by astronomers so far, are thought to occur several times per decade. The latter two reported mini-satellites are of the less frequent but longer duration type, remaining as Earth’s satellites for at least a year and completing at least one orbit around the Earth. These “orbiting” objects are so rare that the number of encounters with them over the last century could be counted on the fingers of both hands.
The similarity of the mini-satellites’ orbits to Earth’s has led to speculation that some of these objects may be man-made. In fact, sometimes they are. One candidate discovered four years ago, 2020 SO, was eventually removed from the mini-satellite list after it turned out to be a discarded upper stage of the rocket that launched NASA’s interplanetary probe Surveyor 2 in 1966. 1991 VG was also suspected at one time to be a space-age artifact or even an alien probe monitoring Earth. However, subsequent observations and a peer-reviewed analysis by the de la Fuente Marcos brothers suggest that this particular mini-satellite is entirely of natural origin.
These natural mini-satellites remain attractive targets for future exploration because they occasionally fly close to Earth and their low relative velocity makes them easily reachable from Earth. Although they are barely visible in a typical consumer telescope and appear as a tiny collection of pixels in professional observatories, perhaps one day 2024 PT5 or one of its fellow mini-satellites will be available for everyone to see up close through images transmitted from visiting spacecraft.
