HOUSTON — The asteroid known as 2024 YR4 is out of sight yet still very much on scientists’ minds.
The building-sized object, which initially appeared to be on a potential collision course with Earth, is currently zooming beyond the reach of telescopes on its orbit around the sun. But as scientists wait for it to reappear, its revised trajectory is now drawing attention to another possible target: the moon.
Discovered at the end of 2024, the space rock looked at first as if it might hit our planet by Dec. 22, 2032. The chance of that impact changed with every new observation, peaking at 3.1% in February — odds that made it the riskiest asteroid ever observed.
Ground- and space-based telescope observations were crucial in helping astronomers narrow in on 2024 YR4’s size and orbit. With more precise measurements, researchers were ultimately able to rule out an Earth impact.
The latest observations of the asteroid in early June, before YR4 disappeared from view, have improved astronomers’ knowledge of where it will be in seven years by almost 20%, according to NASA.
That data shows that even with Earth avoiding direct impact, YR4 could still pose a threat in late 2032 by slamming into the moon. The impact would be a once-in-a-lifetime event for humanity to witness — but it could also send fine-grained lunar material hurtling toward our planet.
While Earth wouldn’t face any significant physical danger should the asteroid strike the moon, there is a chance that any astronauts or infrastructure on the lunar surface at that time could be at risk — as could satellites orbiting our planet that we depend on to keep vital aspects of life, including navigation and communications, running smoothly.
Any missions in low-Earth orbit could also be in the pathway of the debris, though the International Space Station is scheduled to be deorbited before any potential impact.
Initially, YR4 was seen as a case study in why scientists do the crucial work of planetary defense, discovering and tracking asteroids to determine which ones have a chance of colliding with Earth. Now, astronomers say this one asteroid could redefine the range of risks the field addresses, expanding the purview of the work to include monitoring asteroids that might be headed for the moon as well.
“We’re starting to realize that maybe we need to extend that shield a little bit further,” said Dr. Paul Wiegert, a professor of astronomy and physics at the Western University in London, Ontario. “We now have things worth protecting that are a bit further away from Earth, so our vision is hopefully expanding a little bit to encompass that.”
In the meantime, researchers are assessing just how much chaos a potential YR4 lunar impact could create — and whether anything can be done to mitigate it.
‘City killer’ on the moon
The threatening hunk of rock appears as just a speck of light through even the strongest astronomical tools. In reality, YR4 is likely about 200 feet in diameter, according to observations in March by the James Webb Space Telescope, the most powerful space-based observatory in operation.
“Size equals energy,” said Julien de Wit, associate professor of planetary sciences at the Massachusetts Institute of Technology, who observed YR4 with Webb. “Knowing YR4’s size helped us understand how big of an explosion it could be.”
Astronomers believe they have found most of the near-Earth asteroids the field would classify as “planet killers” — space rocks that are 0.6 mile across or larger and could be civilization-ending, said Dr. Andy Rivkin, planetary astronomer from the Johns Hopkins University’s Applied Physics Laboratory in Maryland. The planet killer that slammed into Earth 66 million years ago and led to the extinction of dinosaurs was estimated to be roughly 6 miles in diameter.
Smaller asteroids such as YR4, which was colloquially dubbed a “city killer” after its discovery, could cause regional devastation if they collide with our planet. About 40% of near-Earth space rocks larger than 460 feet but smaller than a kilometer — capable of more widespread destruction — have been identified, according to NASA.
But astronomers have never really had a chance to watch a collision of that size occur on the moon in real time, Wiegert said. The latest glimpses of YR4 on June 3 before it passed out of view revealed a 4.3% chance of a YR4 lunar impact — small but decent enough odds for scientists to consider how such a scenario might play out.
A striking meteor shower — and a risk
Initial calculations suggest the impact has the largest chance of occurring on the near side of the moon — the side we can see from Earth.
“YR4 is so faint and small we were able to measure its position with JWST longer than we were able to do it from the ground,” said Rivkin, who has been leading the Webb study of YR4. “And that lets us calculate a much more precise orbit for it, so we now have a much better idea of where it will be and won’t be.”
The collision could create a bright flash that would be visible with the naked eye for several seconds, according to Wiegert, lead author of a recent paper submitted to the American Astronomical Society journals analyzing the potential lunar impact.
The collision could create an impact crater on the moon estimated at 0.6 miles wide, Wiegert said — about the size of Meteor Crater in Arizona, Rivkin added. It would be the largest impact on the moon in 5,000 years and could release up to 220 million pounds of lunar rocks and dust, according to the modeling in Wiegert’s study.
Even pieces of debris that are just tens of centimeters in size could present a hazard for any astronauts who may be present on the moon, or any structures they have built for research and habitation, Wiegert said. The moon has no atmosphere, so the debris from the event could be widespread on the lunar surface, he added.
On average, the moon is 238,855 miles away from Earth, according to NASA.
Particles the size of large sand grains, ranging from 0.1 to 10 millimeters in size, of lunar material could reach Earth between a few days and a few months after the asteroid strike because they’ll be traveling incredibly fast, creating an intense, eye-catching meteor shower, Wiegert said.
“There’s absolutely no danger to anyone on the surface,” Wiegert said. “We’re not expecting large boulders or anything larger than maybe a sugar cube, and our atmosphere will protect us very nicely from that. But they’re traveling faster than a speeding bullet, so if they were to hit a satellite, that could cause some damage.”
Not all lunar debris that reaches the Earth is so small, and it depends on the angle and type of impact to the moon, according to Washington University in St. Louis. Space rocks slamming into the lunar surface over millions of years have resulted in various sizes of lunar meteorites found on Earth.
The Key Takeaways for this article were generated with the assistance of large language models and reviewed by our editorial team. The article, itself, is solely human-written.
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