2024 YR4 LINKED TO REGION NOT KNOWN FOR COLLISION COURSE OBJECTS
Maunakea, Hawaiʻi – Astronomers using W. M. Keck Observatory on Maunakea, Hawaiʻi Island have determined the physical properties and potential origin of 2024 YR4, the Earth-crossing asteroid first discovered by scientists in December 2024. 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.
“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 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.
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.
“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 hit the Moon,” added Bolin. “If it does, it would give scientists a rare chance to study how the size of an asteroid relates to the size of the crater it creates—something we haven’t been able to measure directly before.”
Origins and Characteristics of YR4
Asteroids that are 100 meters or larger are often what astronomers call “rubble piles,” made up of fragments from a larger parent asteroid that broke apart in a collision. The pieces from that breakup clump together to form new, loosely held-together bodies. On the surfaces of these rubble-pile asteroids, we often see large boulders—some as large as ~60 meters. At 50-60 meters in size, YR4 falls into this size range, suggesting it could have been a boulder that once sat on the surface of a larger rubble-pile asteroid.
Scientists rely on the Yarkovsky Effect to explain and predict the orbits of asteroids. There is a subtle force that happens when an asteroid absorbs sunlight and then re-emits that energy as heat. That heat, in the form of infrared light, pushes back slightly on the asteroid, slowly changing its orbit. How strongly an asteroid responds to this effect depends on its thermal inertia – how quickly it heats up and cools down.
Small, 50-meter-sized asteroids, like YR4, may have low thermal inertia, which suggests they’re made of solid rock. This differs from larger, rubble-pile asteroids, which tend to have surfaces covered in loose material and higher thermal inertia. By studying how these small objects respond to sunlight, scientists obtain indirect evidence that they may be solid chunks of rock, possibly chipped off from bigger rubble-pile asteroids.
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 U.S. 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. “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.”
ABOUT MOSFIRE
The Multi-Object Spectrograph for Infrared Exploration (MOSFIRE), gathers thousands of spectra from objects spanning a variety of distances, environments and physical conditions. What makes this large, vacuum-cryogenic instrument unique is its ability to select up to 46 individual objects in the field of view and then record the infrared spectrum of all 46 objects simultaneously. When a new field is selected, a robotic mechanism inside the vacuum chamber reconfigures the distribution of tiny slits in the focal plane in under six minutes. Eight years in the making with First Light in 2012, MOSFIRE’s early performance results range from the discovery of ultra-cool, nearby substellar mass objects, to the detection of oxygen in young galaxies only two billion years after the Big Bang. MOSFIRE was made possible by funding provided by the National Science Foundation.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaiʻi feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. For more information, visit: www.keckobservatory.org