You might’ve seen a second so-called “moon” has recently taken up residence in our orbit. The school bus-sized asteroid will be orbiting Earth until Nov. 25.
At least five times further from Earth than our true moon at its closest approach, the asteroid is much too tiny to be seen with the naked eye or without the help of research-grade telescopes. Even calling the object a “mini-moon” is a bit of a stretch, says Thomas Statler, the lead scientist for solar system small bodies at NASA Headquarters.
To become a second moon, the asteroid would have to be gravitationally bound to our existing Earth-Moon system and make multiple circuits around it. These conditions currently don’t apply to the asteroid, which was recently dubbed “2024 PT5.”
There are hundreds of moons in our solar system, most drawn to the bigger outer planets. A whopping 146 circle Saturn, 95 crowd Jupiter, while our neighbor Mars has two of its own. Even the demoted dwarf planet Pluto has five official moons.
Scientists monitor around a thousand objects known that come within 10 lunar distances of Earth every year, though most don’t stay very long and are quite small. Even though 2024 PT5 is notable in both respects, (being a bit larger and longer-lived in our orbit compared to other objects), its stay will be but a blip in the lifetime of our planetary system.
But its occupancy does leave us wondering: Is it possible for Earth to get another moon? How would it affect life here if we did?
What makes a moon?
Asteroids slip quietly by our planet all the time. Most of these small objects enter and leave our space without much fanfare; the large ones tend to make a splash only once in a few million years.
Become a subscriber and support our award-winning editorial features, videos, photography, and more—for as little as $2/mo.
GET STARTED
Like most other celestial bodies in the solar system, asteroids are propelled through space by the Sun’s gravity. Scientists make great efforts to detect and track their expected paths, both to assess potential threats via collision and to study their properties.
Though 2024 PT5 is tiny, that’s not why it won’t really be considered a moon. Regardless of an asteroid’s mass, composition, or shape, whether or not it can be declared a moon ultimately comes down to if it starts orbiting another body.
Our moon, for example, follows an elliptical orbit around the Earth that takes about a month to complete. During this circuit, the moon’s gravity tugs on Earth, shifting our planet’s mass and distorting its shape into what slightly resembles a football. This process is what influences the formation of tides, and has significant consequences for supporting life on the ground, such as supporting marine ecosystems and the reproductive activities of many sea animals. Humans also need to be able to predict tides for maritime travel and trade.
You May Also Like
“Just about every living being is affected in some subtle or strong way by the lunar cycles,” says Statler. “So we are really locked into the existence of our own moon in a major way.”
In contrast, 2024 PT5 won’t be around long enough to study that much.
“There have been mini-moons that are way more mini-moonier than this one is, and there will be in the future,” he says. Luckily, scientists are getting ever-better at detecting these minor objects: In 2016, researchers discovered that one quasi-moon, called Kamo‘oalewa, plans to hang around the Earth for about the next 300 years.
Named from the phrase ‘oscillating fragment’ in Hawaiian, Kamo‘oalewa is roughly the size of a Ferris wheel. Though it follows an elliptical orbit that seems to move in sync with Earth, it actually lies outside our planet’s gravitational influence, disqualifying this visitor as a true mini-moon.
In June, NASA also tracked the close approach of the asteroid 2011 UL21, discovering a binary system—a smaller asteroid, called a moonlet, orbited it from a distance of about two miles. Such precise moon-hunting is a huge leap from when 1991 VG, the first mini-moon to be detected, gained notoriety as people argued whether it was a natural object or if it had more extraterrestrial origins.
The real importance of 2024 PT5 and asteroid research
Despite the Earth-Moon system being no stranger to short-lived (and relatively quiet) space guests, experts can say with certainty that we won’t be gaining another satellite as big or as cosmically vital as our moon anytime soon. Fairly small objects have been orbiting in our vicinity for billions of years and we’ve never really noticed, Statler says, because nothing bad has happened because of it. “That's a good indication that this is not a major thing in terms of affecting life on Earth,” he says.
If in a stable configuration, having two moons of comparable size would also completely change the oceans’ tides as well as likely alter what scientists know about the history of astronomy, including about how those moons formed. But again, that’s almost certainly not happening soon, if the last several billion years are any indication.
So why is 2024 PT5 getting so much attention? Andrew Rivkin, a planetary astronomer at the Johns Hopkins University Applied Physics Laboratory, says it may be because this asteroid’s appearance marks one of the first times that scientists have been able to predict a mini-moon’s presence ahead of time. “It's a great demonstration of our capabilities to be able to find things, to be able to predict where they go.”
Rivkin was the investigation team lead for NASA’s recent Double Asteroid Redirection Test (DART) mission, which was a trial run for demonstrating asteroid deflection technology as part of humanity’s larger planetary defense ambitions. DART showed that for the first time, something man-made could alter a celestial object’s whereabouts in the universe.
As of today, one of the next largest near-Earth objects to come hurdling our way will be Apophis, a 1000-foot-wide asteroid currently expected to safely pass Earth by in April 2029. There’s little chance that the asteroid will be captured in our orbit since it’ll fly past Earth much too quickly, but at its extreme approach at ten times closer than the moon, it will be visible to the naked eye.
Still, while Apophis’ visit might feel a bit more harrowing than 2024 PT5’s, it is bound to be a fascinating encounter for a brand new generation of asteroid hunters. “[Asteroids] are not just things to be scared of,” says Rivkin. “The skies are full of great things, of wondrous things.”
