The Webb Director's Discretionary Time program enabled principal investigator Andy Rivkin of the Johns Hopkins University Applied Physics Laboratory to gather a comprehensive dataset on the asteroid. Rivkin emphasized that despite the diminished impact risk, the asteroid remains a valuable subject for planetary defense research.

"While earlier in the year asteroid 2024 YR4 posed a potential future impact threat to Earth in 2032, by the end of February, NASA announced the risk of Earth impact had been significantly lowered to near-zero," said Rivkin. "Nevertheless, we are interested in using the observatory to measure its properties to understand what asteroids of this size are like to help inform the hazard they could pose to Earth. This is the smallest object targeted by the mission to date, and one of the smallest objects to have its size directly measured."

The team used both the Near-Infrared Camera (NIRCam) and the Mid-Infrared Instrument (MIRI) onboard Webb to study the asteroid. According to Rivkin, "Most telescopes observe asteroids by measuring sunlight reflected from their surfaces, and it's hard to precisely determine their sizes from this information. At mid-infrared wavelengths like those used by MIRI, however, the heat given off by asteroids themselves can be measured and used to directly give the asteroid's size. The NIRCam data covers the reflected light, and using it along with the MIRI data gives us not only the size but also a measure of how reflective its surface is, which is related to the asteroid's composition."

Images from Webb confirm that asteroid 2024 YR4 is approximately 60 meters across, making it roughly equivalent in height to a 15-story building. These observations represent a milestone for the Webb mission, as 2024 YR4 is the smallest body it has directly measured so far.

While the odds of impact have been nearly eliminated, Rivkin explained that the study offers key insights for future preparedness. "While we are confident that 2024 YR4 will not hit Earth in 2032, there is still great value in making these observations and analyzing the results. We expect more possible impactors to be found in coming years as more sensitive asteroid search programs begin operation. Observations using the most powerful telescope we have right now are invaluable. Understanding the best ways to use it and how to get the most out of its data is something we can do now with 2024 YR4. This will help us determine the best approach to use during a more urgent observing program should another asteroid pose a potential impact threat in the future."

Webb's observations also revealed unexpected thermal characteristics. "We found that the thermal properties of 2024 YR4, in other words how quickly it heats up and cools down, and how hot it is at its current distance from the Sun, are not like what we see in larger asteroids. We think this is likely a combination of its very fast spin and a lack of fine-grained sand on its surface. We'll need more data to say for sure, but it seems consistent with a surface dominated by rocks that are maybe fist-sized or larger. And of course, our main goal was measuring the size of 2024 YR4, which we estimate at about 60 meters (200 feet). That's just about the height of a 15-story building."

The broader planetary defense community, including the International Asteroid Warning Network, benefited from these findings. "2024 YR4 and other near-Earth asteroids are studied by NASA's planetary defense program and the international community of astronomers, orbit calculators, and impact physicists, among other scientists involved in the International Asteroid Warning Network. The new observations from this observatory not only provide unique information about 2024 YR4's size. They also added to ground-based observations of 2024 YR4's position to help improve our knowledge of its orbit and future trajectory. Some colleagues were also able to use other telescopes to make measurements of 2024 YR4's spin rate and spectral properties shortly after it was discovered. All together, we have a better sense of what this building-sized asteroid is like. This in turn gives us a window to understand what other objects the size of 2024 YR4 are like, including the next one that might be heading our way!"

Research Report:JWST Observations of Potentially Hazardous Asteroid 2024 YR4

Related Links
Webb Director's Discretionary Time program
Asteroid and Comet Mission News, Science and Technology