The destructive power of an impacting asteroid is primarily estimated by knowledge of its size. The near-Earth asteroid 2024 YR4 reached a peak 2032 impact probability with Earth of 3%, motivating a desire to determine its size. Owing to its infrared capabilities, the NASA/ESA/CSA James Webb Space Telescope is uniquely suited for such assessment. Johns Hopkins University astronomer Andrew Rivkin and his colleagues used two Webb instruments to measure a diameter of 2024 YR4.
These Webb images show the near-Earth asteroid 2024 YR4. Image credit: NASA / ESA / CSA / STScI / A. Rivkin, JHU/APL.
2024 YR4 was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) station at Río Hurtado on December 27, 2024.
The asteroid made a close approach to Earth at a distance of 828,800 km (515,000 miles) two days before its discovery.
Initial observations of 2024 YR4 gave a diameter of between 40 and 90 m (131-295 feet).
“The initial diameter estimate of the asteroid was limited to visible light measurements, which depend on both the size and reflectivity of the surface,” said team member Dr. Eric MacLennan, an astronomer at the University of Helsinki.
“However, thermal emission at infrared wavelengths is a direct indicator of the size.”
Dr. Rivkin, Dr. MacLennan and their colleagues observed 2024 YR4 using Webb’s Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI).
The data from NIRCam show reflected light, while the MIRI observations show thermal light.
“The observations were taken to study the thermal properties of 2024 YR4, including how quickly it heats up and cools down and how hot it is at its current distance from the Sun,” the astronomers said.
“These measurements indicate that this asteroid does not share properties observed in larger asteroids.”
“This is likely a combination of its fast spin and lack of fine-grained sand on its surface.”
“Further research is needed, however this is considered consistent with a surface dominated by rocks that are roughly fist-sized or larger.”
The new Webb observations indicate that the asteroid measures roughly 60 m (197 feet).
“2024 YR4 is the smallest object targeted by Webb to date, and one of the smallest objects to have its size directly measured,” the researchers said.
“The new observations from Webb not only provide unique information about 2024 YR4’s size, but can also complement ground-based observations of the object’s position to help improve our understanding of the object’s orbit and future trajectory.”
The team’s findings were published in the Research Notes of the AAS.
_____
A.S. Rivkin et al. 2025. JWST Observations of Potentially Hazardous Asteroid 2024 YR4. Res. Notes AAS 9, 70; doi: 10.3847/2515-5172/adc6f0
