Looking up at a clear night sky, devoid of light pollution and clouds, one typically sees stars, an occasional satellite whizzing by, and Earth’s faithful companion, the moon. But every so often, Earth’s gravitational pull draws in another celestial guest, like a “mini-moon.” On Sunday, Sept. 29, Earth welcomed one such visitor, known as 2024 PT5.
Unlike the moon, 2024 PT5 will not be visible to the naked eye, measuring only a few meters across. This tiny asteroid will complete just a partial orbit around Earth before the sun’s gravitational pull sets it free again in about two months.
While 2024 PT5 isn’t the first mini-moon to pay Earth a visit, each encounter offers a unique opportunity to learn more about these fleeting guests. To understand more about this phenomenon, Penn Today caught up with two astronomy professors in the School of Arts & Sciences, Gary Bernstein and Bhuvnesh Jain.
What exactly is a mini-moon?
According to Bernstein, mini-moons like 2024 PT5 are essentially tiny asteroids or near-Earth objects (NEOs) that find themselves temporarily captured by Earth’s gravity. “This NEO is roughly the size of a school bus or small classroom, similar to the handful of other known temporary satellites which are just stopping by Earth for a brief visit,” Bernstein says, likening PT5’s journey to someone popping into a convenience store for a quick hoagie before heading back to work or home.
Bernstein explains that this mini-moon belongs to a group of asteroids known as the Arjuna asteroids, which orbit the sun in a path similar to that of Earth’s. These NEOs occasionally cross paths with Earth, leading to short-term captures like PT5’s current visit.
“If this were a tug of war for this mini-moon, we’d certainly lose to the sun,” he says, emphasizing that PT5’s gravitational bond with Earth is incredibly weak compared to the sun’s overwhelming pull. The mini-moon won’t even complete a full orbit around Earth, meaning it will only be what astronomers call “temporarily captured.”
Interestingly, Earth’s permanent moon plays a critical role in ejecting mini-moons like PT5, according to Bernstein. “Our moon acts as a kind of celestial bouncer,” he says. “It exerts a destabilizing force that makes it difficult for any other object to coexist with it in orbit around Earth. It’s like a sibling rivalry where the bigger sibling always wins.” This gravitational competition ensures that mini-moons don't linger for long.
How are mini-moons named?
Bernstein explains that the designation 2024 PT5 reveals when the object was first discovered. “The first four digits simply refer to the year of discovery—in this case, 2024—and the letters and numbers that follow indicate the sequence in which it was found within that year,” he says.
The rest is a little less intuitive, Bernstein says, as the alphabet is divided into half-month intervals: “A” for the first half of January, “B” for the second half, and so on, until “Y” (excluding “I”), which means the second half of December. Therefore, “P” indicates that 2024 PT5 was discovered in the first half of August 2024. The remaining “T5” code means this was the 144th new member of the solar system discovered in that half-month.
He adds that once an asteroid has been well-enough observed the discoverers can propose a name to the International Astronomical Union. While names can be inspired by a wide range of sources—mythology, history, or even pop culture—there are clear restrictions. For example, Bernstein says, “You can’t name them after living politicians, and if you want to name it after a politician, they’d have to be dead for at least 100 years to even be considered.” This rule helps to keep contemporary political issues out of the heavenly realms.
How experts spot mini-moons
More than 1 million objects have now been discovered and their orbits around the sun carefully characterized, but those that turn into mini moons are rare, with fewer than 10 being detected in the last decade. Jain says that it’s the job of some astronomers at places like the Sutherland site of the South African Astronomical Observatory, the center that first spotted PT5, to regularly survey the sky for large cosmic bodies for a number of scientific reasons but also, “quite importantly, to warn people early on if there’s a chance an asteroid may impact Earth.
“Fortunately, this visitor means no harm,” Jain says, “but, unfortunately, this visitor is so harmless, so discreet, that we won’t be able to see it with our naked eyes. Amateur astronomers can follow it but only if they have a high-powered telescope and software to track the mini-moon.”
Most mini-moons are detected using advanced telescopes and specialized software capable of identifying fast-moving objects against the backdrop of stars, according to Jain. These tools are critical for tracking NEOs, which can provide valuable data on the composition and behavior of asteroids.
The next big survey: The Vera C. Rubin Observatory
Jain and Bernstein are both involved in a global effort that’s gearing to be one of the most ambitious projects contributing to the detection of mini-moons and other NEOs, the Vera C. Rubin Observatory in Chile, which is set to become operational in late 2025. “This is going to be a big game-changer for astronomy,” Bernstein says. The observatory will conduct the Legacy Survey of Space and Time during the next decade, capturing a complete image of the southern sky every few nights, taking images every 15 seconds to track asteroids and other celestial objects.
“The Rubin Observatory will have a 2-billion-pixel camera that will scan the sky, and it will be recording a time-lapse of the universe,” Bernstein says. “It’s going to generate a massive amount of data—over 20 terabytes per night—for a decade.” This cutting-edge technology will make it easier to spot not just mini-moons but also larger, potentially hazardous objects that might pose a risk to Earth and produce intricate maps of more than 1 billion stars in our Milky Way galaxy and of more than 1 billion galaxies through the universe.
