Astronomers identify rare Earth-crossing asteroid from unexpected source

Astronomers identify rare Earth-crossing asteroid from unexpected source
by Clarence Oxford
Los Angeles CA (SPX) Apr 14, 2025

Astronomers at W. M. Keck Observatory on Maunakea, Hawai'i, have traced the origin of 2024 YR4, an Earth-crossing asteroid discovered in December 2024, to a surprising location. Contrary to expectations, the solid, stony asteroid appears to have emerged from a central Main Belt asteroid family between Mars and Jupiter-a region not previously associated with Earth-crossing bodies.

"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, lead author and research scientist with Eureka Scientific. "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."

Initial projections raised concerns about a potential impact with Earth in 2032, but further analysis has ruled out that scenario. However, scientists estimate there is approximately a 2% chance that YR4 could collide with the Moon.

"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," Bolin added. "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."

Asteroids larger than 100 meters are often "rubble piles," aggregates formed from debris following a parent asteroid's collision. These bodies are loosely bound, often displaying large surface boulders up to 60 meters in size. YR4's size and characteristics suggest it may be a remnant boulder from such a larger body.

The team studied how the Yarkovsky Effect-a tiny force resulting from sunlight absorbed and re-emitted as heat-alters asteroid orbits. YR4 likely has low thermal inertia, supporting the theory that it's a monolithic rock rather than a loose agglomeration. This sets it apart from larger, rubble-pile asteroids that retain more surface material and exhibit different thermal behavior.

Infrared observations using Keck's Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) allowed Bolin's team to determine YR4's physical traits with a high degree of precision. The team supplemented Keck data with input from the Asteroid Terrestrial-impact Last Alert System (ATLAS) and the Gemini South telescope in Chile.

The narrow observational window, just 4 arcseconds wide, required extreme precision. "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," Bolin noted. "It was a serendipitous set of circumstances that allowed us to do these observations."

Originally scheduled to study trans-Neptunian objects, Bolin's team shifted plans due to technical delays, enabling them to capture critical data on YR4. That data is now being used to refine techniques for rapid asteroid characterization-a key capability in assessing and mitigating potential impact threats.

"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."

Research Report:The discovery and characterization of Earth-crossing asteroid 2024 YR