This article was updated on Oct. 24, 2024.
Maybe you’ve heard: Earth recently gained a second moon. On Sept. 29, a small asteroid with the prosaic name 2024 PT5 wandered a little too close and was captured by Earth’s gravity. Right now, it’s moving with us as Earth orbits the Sun, the same way our familiar Moon does.
But orbital dynamics are complex and ever-changing. On Nov. 25, our little house-size companion will break free of Earth’s gravity and resume its own orbit, which will put it on track for another short visit in 2055, then possibly again in 2084.
The asteroid was discovered on Aug. 7 by a NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in South Africa, one of four that make up an early warning system for near-Earth objects known as NEOs. ATLAS telescopes scan the sky for objects whose orbits might bring them dangerously close to us.
Our solar system contains millions of asteroids: rocky, icy, and metallic bodies ranging in size from tens of feet across to more than 100 miles in diameter. Most are leftover fragments from the early solar system — hunks of material that didn’t combine to form planets and now orbit in the asteroid belt that lies between Mars and Jupiter. Others share our orbit or (more alarmingly) cross it.
The interlopers typically make a close approach, then continue on with no harm done. Asteroid impacts are rare. But such impacts were frequent four billion years ago during Earth’s infancy. Look up at the next full moon — visible craters and maria are the scars of that period, called the Late Heavy Bombardment. All kinds of space junk careened around the solar system back then, striking each other and the nascent planets. Eventually things quieted, with the surviving objects settling into the relatively stable orbits that we observe today.
The Late Heavy Bombardment damaged Earth just as much as it did the Moon. But impact craters on Earth have disappeared over time because of erosion, tectonic movement, and volcanic activity. The Moon has none of these processes; its impact craters last essentially forever, preserving the history of that era. Only the slowly eroding ghosts of a few large impact craters survive on Earth, among them Lake Manicouagan in Canada; Karakul, a lake in Tajikistan; and Chicxulub crater, which lies beneath the Yucatan Peninsula and Gulf of Mexico. Chicxulub formed 65 million years ago when an asteroid six miles wide struck Earth. The cataclysm brought about the extinction of the dinosaurs along with 75 percent of all other species alive at the time.
No need to worry: 2024 PT5 won’t hit us. Right now, it’s more than two million miles away — contrast that with the Moon at 220,000 miles. It’s also small — far too small, in fact, at about 37 feet across, to see with the eye or any amateur telescope.
This is not the first such “short capture” astronomers have observed. They extrapolate that there are probably several per decade. We’re learning of more now because of the increasing capabilities of sky surveys like ATLAS.
2024 PT5 is likely a fragment of lunar ejecta — rock blasted into space after something large struck the Moon long ago. Since then, it’s been tumbling through space in an orbit similar to Earth’s.
An impact also created the Moon: 4.5 billion years ago, a proto-planet the size of Mars crashed into Earth, nearly destroying this planet and blasting a huge amount of molten rock into orbit. That material formed a ring of debris, and over mere hundreds of years, gravity merged that debris into the Moon.
An impact like that is nearly impossible today. There just isn’t anything that big on the loose in the solar system. So-called “extinction level events,” like the six-mile-wide asteroid that killed the dinosaurs, are very rare, occurring once every tens of millions of years or so. Smaller strikes with significant effects occur once every several decades, though. Most famous in modern history is the Tunguska event of 1908, when an asteroid about 200 feet wide exploded five miles above Siberia. The resulting blast leveled trees over 830 square miles of uninhabited forest.
In 2013, an asteroid again exploded at high altitude because of the sudden extreme heat generated when it slammed into Earth’s atmosphere at more than 12 miles per second. This one occurred over Chelyabinsk, Russia. The asteroid was estimated to be 60 feet in diameter and 10,000 tons in mass; its shock wave shattered windows and caused injuries.
ATLAS and other sky surveys watch for asteroids that cross our orbit and have a chance of hitting us. If we detect an object on a collision course early enough, we might be able to prevent disaster. But Hollywood movies have got it wrong: it’s a bad idea to try to destroy an inbound object with nuclear weapons. That would just create many smaller impactors, whose cumulative damage would be similar to that done by the larger parent body.
The most promising tactic is to push the object. Even a small push, delivered early enough, might nudge an asteroid just enough to miss us. NASA’s recent Double Asteroid Redirection Test (DART) demonstrated this, though on a small scale and with a harmless object. In 2022, a robot spacecraft launched a kinetic impactor (a heavy lump of metal) at the small asteroid Dimorphos, which itself orbits the larger asteroid Didymos. The impact shifted Dimorphos’s orbit measurably, yielding valuable data on how a similar mission might work against a larger (and actually dangerous) object.
Gravity is the weakest of the fundamental forces of nature, but, like electromagnetism, it can act over long distances, and it’s persistent. Given enough time, gravity can draw galaxies together into mega-clusters spanning hundreds of millions of light-years or collapse interstellar gas clouds into stars. It can also take hold of a little asteroid, send it our way for a while, then banish it again to wander alone. Clear skies!
Editor’s note: An earlier version of this article, published in print on Oct. 17, incorrectly implied that electromagnetism cannot act over long distances. Thanks to reader David Arnow for pointing out that error.
