Astronomers: YR4 asteroid will not hit Earth in 2032, but has ~2% chance of moon strike

Astronomers using the W. M. Keck Observatory at Maunakea on Hawaiʻi Island have determined the physical properties and potential origin of 2024 YR4, the Earth-crossing asteroid first discovered by scientists in December 2024.

Despite being a scenario widely pondered by astronomers and Hollywood writers for decades, further observations have since determined YR4 will not impact the Earth in 2032 as originally predicted. However, there is a ~2% chance it could hit the Moon instead, according to the study.

At about 50-60 meters in diameter (similar to the width of a football field), it’s one of the largest objects in recent history that could strike the moon.  

“YR4 spins once every 20 minutes, rotates in a retrograde direction, has a flattened,
irregular shape, and is the density of solid rock,” said Bryce Bolin, research scientist
with Eureka Scientific and lead author of the study. “The shape of the asteroid provides
us with clues as to how it formed, and what its structural integrity is. Knowing these
properties is crucial for determining how much effort or what kind of technique needs to
be used to deflect the asteroid if it is deemed a threat.”

The results of the study will be published in The Astrophysical Journal Letters. A reprint of the paper, “The discovery and characterization of Earth-crossing asteroid 2024 YR4,” is available online https://arxiv.org/abs/2503.05694.

The study reveals YR4 is a solid, stony type that likely originated from an asteroid family in the central Main Belt between Mars and Jupiter, a region not previously known to produce Earth-crossing asteroids. 

The size of YR4 suggests it could have been a boulder that once sat on the surface of a larger rubble-pile asteroid. Small, 50-meter-sized asteroids, like YR4, may have low thermal inertia, which suggests they’re made of solid rock, according to the study.

Using data from Keck Observatory’s Multi-Object Spectrograph for Infrared Exploration (MOSFIRE), Bolin and team were able to observe YR4 in the infrared, seeing properties of the asteroid that would otherwise be impossible to observe. The study uses additional data from the Asteroid Terrestrial-impact Last Alert System or ATLAS, developed by the University of Hawaiʻi and funded by NASA, as well as the Gemini South telescope in Chile, one half of the International Gemini Observatory, partly funded by the US National Science Foundation and operated by NSF NOIRLab.

The instrumental window Bolin and his team had for observing this object was only 4 arcseconds wide, projecting to a very small patch of the sky, requiring precision measurement only Keck Observatory could provide.

“This object’s orbit was so well determined we knew its position to within less than an arcsecond. It was moving less than 10 arcseconds per minute, if we were off target the background static stars would have been trailed, but we got it on our first try,” said Bolin. “It was a serendipitous set of circumstances that allowed us to do these observations.”

Serendipitous because Bolin’s original science case was imaging for trans-Neptunian objects, but due to technical difficulties, his team was able to pivot at the last minute to image the object, obtaining data that may one day play a crucial role in saving our planet from impact, researchers said.

“The data from our study will be used to assess the physical properties and shapes of potentially impacting asteroids, providing a great test case on the kind of rapid response observations that are necessary to characterize a potential threat like this object. The physical information about an asteroid’s physical property (rubble pile vs solid rock) is crucial for planning mitigation efforts if necessary.”