NASA’s James Webb Space Telescope (JWST) has delivered critical new observations of asteroid 2024 YR4, offering reassurance that the near-Earth object poses no significant threat of impact with Earth in 2032 or anytime soon. The findings mark a major step forward in the use of powerful space-based instruments for planetary defense.
Infographic showing the effect of DART's impact on the orbit of Didymos B while deployment of italian LICIACube
Credit: NASA
The research was conducted under the Director’s Discretionary Time program of the Webb mission, led by Andy Rivkin, principal investigator from the Johns Hopkins University Applied Physics Laboratory. Rivkin and his team were granted access to Webb’s advanced instruments to carry out one of the most detailed studies yet of a small asteroid near Earth.
“Earlier this year, there was concern that asteroid 2024 YR4 could potentially strike Earth in 2032,” said Rivkin. “But by late February, further tracking allowed NASA to confidently reduce that risk to near-zero. While the threat has been dismissed, studying the asteroid remains crucial for understanding the risks posed by similar-sized objects.”
A First for Webb: Measuring the Smallest Target to Date
Asteroid 2024 YR4 is approximately 60 meters (200 feet) in diameter—about the height of a 15-story building—making it the smallest object the James Webb Space Telescope has directly measured. This feat underscores Webb’s growing role not just in deep space astronomy but also in near-Earth object monitoring.
To study YR4, the team employed both of Webb’s key instruments: the Near-Infrared Camera (NIRCam) and the Mid-Infrared Instrument (MIRI). While most ground-based telescopes rely on reflected sunlight to observe asteroids, MIRI allowed scientists to measure the heat emitted by the asteroid itself, yielding a far more accurate determination of its size.
“The combination of infrared emissions from MIRI and the reflected light data from NIRCam gave us both the size and surface reflectivity, or albedo, of the asteroid,” explained Rivkin. “This tells us not only how big it is but also provides clues to what it’s made of.”
Beyond size, the observations also revealed intriguing details about the asteroid’s thermal properties. Unlike larger asteroids, 2024 YR4 displays unusual heating and cooling behavior. Rivkin believes this is due to the asteroid’s fast rotation and a surface that appears to lack fine dust or sand.
“We suspect its surface is dominated by larger rocks—perhaps fist-sized or bigger,” he said. “That would influence how heat is absorbed and radiated, and it gives us new insight into how smaller asteroids behave differently from their larger counterparts.”
This new data could reshape how scientists model the surface characteristics and heat dynamics of small asteroids—information critical for both impact risk assessment and the design of future mitigation strategies.
While 2024 YR4 is now considered safe, the value of the observations goes far beyond this single object. With more sensitive asteroid search programs launching in the near future, the discovery rate of potential Earth impactors is expected to increase. Webb’s success in studying YR4 provides a working model for how to respond quickly and effectively when new threats emerge.
“This was an ideal opportunity to test how we can use the world’s most powerful telescope in a time-sensitive planetary defense context,” said Rivkin. “The lessons we’re learning now will help guide how we respond in the future, especially when we encounter objects with less favorable outlooks.”
A Global Collaboration for Planetary Safety
The observations from Webb were part of a broader global effort to study 2024 YR4. NASA’s Planetary Defense Coordination Office, along with members of the International Asteroid Warning Network (IAWN), contributed to the analysis with ground-based measurements of the asteroid’s orbit, spin rate, and spectral properties.
“These findings improved not only our understanding of 2024 YR4’s trajectory but also its physical makeup,” said Rivkin. “It’s a great example of how space-based and ground-based observations can come together to give us a clearer picture of objects that may one day pose a real threat.”
Ultimately, the research on 2024 YR4 has helped enhance preparedness across the planetary defense community and affirmed the role of space observatories like Webb in safeguarding Earth.
“As we prepare for the next potential threat, 2024 YR4 has provided a timely and valuable rehearsal,” Rivkin concluded. “Thanks to Webb, we’re better equipped than ever to understand and respond to what might be heading our way.”
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