NASA’s James Webb Space Telescope recently turned its watchful eye toward asteroid 2024 YR4, which we now know poses no significant threat to Earth in 2032 and beyond.We spoke with Andy Rivkin of the Johns Hopkins University Applied Physics Laboratory, the principal investigator of the Webb Director’s Discretionary Time program used to study the asteroid’s characteristics from the observations.What is important for people to know about these Webb observations?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. 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.Why did you request to use both NIRCam (Near-Infrared Camera) and MIRI (Mid-Infrared Instrument) in this investigation? What did each tell you?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.The chance of Asteroid 2024 YR hitting Earth in 2032 has greatly decreased, according to NASA’s Planetary Defense Coordination Office. Why are these observations still important?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.What have you learned? Was there anything surprising about the data?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.How do the Webb observations fit within the larger picture of the study of this asteroid (and other near-Earth asteroids)?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!
Since near-Earth asteroid 2024 YR4 was first discovered in December 2024, NASA and the worldwide planetary defense community have continued to observe the asteroid, which was ruled out as a significant impact risk to Earth. New infrared observations from NASA’s James Webb Space Telescope have decreased the uncertainty of the asteroid’s size and 2024 YR4 is now estimated to be 174-220 feet (53-67 meters), about the size of a 10-story building. The previous size estimate of 131-295 feet (40-90 meters) was derived from visible light measurements from ground-based telescopes. Experts at NASA’s Center for Near Earth Object Studies at the agency’s Jet Propulsion Laboratory have updated 2024 YR4’s chance of impacting the Moon on Dec. 22, 2032 from 1.7% as of late February to 3.8% based on the Webb data and observations from ground-based telescopes. There is still a 96.2% chance that the asteroid will miss the Moon. In the small chance that the asteroid were to impact, it would not alter the Moon’s orbit. After mid-April, asteroid 2024 YR4 will be too far and faint to be observed by ground-based telescopes but Webb will observe the asteroid again in May.
What sort of tether do you think could influence the velocity of a trillion tonnes of rock.
While asteroid 2024 YR4 is currently too distant to detect with telescopes from Earth, NASA’s James Webb Space Telescope collected one more observation of the asteroid before it escaped from view in its orbit around the Sun. With the additional data, experts from NASA’s Center for Near Earth Object Studies at the agency’s Jet Propulsion Laboratory in Southern California further refined the asteroid’s orbit. The Webb data improved our knowledge of where the asteroid will be on Dec. 22, 2032, by nearly 20%. As result, the asteroid’s probability of impacting the Moon has slightly increased from 3.8% to 4.3%. In the small chance that the asteroid were to impact, it would not alter the Moon’s orbit.[...]NASA expects to make further observations when the asteroid’s orbit around the Sun brings it back into the vicinity of Earth in 2028.
On 2032 December 22 the 60m diameter asteroid 2024 YR4 has a 4% chance of impacting the Moon.Such an impact would release 6.5 MT TNT equivalent energy and produce a ∼ 1 km diameter crater.We estimate that up to 108 kg of lunar material could be liberated in such an impact by exceeding lunar escape speed. Depending on the actual impact location on the Moon as much as 10% of this material may accrete to the Earth on timescales of a few days. The lunar ejecta-associated particle fluence at 0.1 - 10 mm sizes could produce upwards of years to of order a decade of equivalent background meteoroid impact exposure to satellites in near-Earth space late in 2032. Our results demonstrate that planetary defense considerations should be more broadly extended to cis-lunar space and not confined solely to near-Earth space.
ConclusionIn this work, we have provided an overview of deflection and robust disruption mechanics and techniques, with an emphasis on impulsive techniques utilizing Kinetic Impactors (KIs) or standoff detonation of Nuclear Explosive Devices (NEDs). We have described the notional requirements for deflection or robust disruption of 2024 YR4, based on current knowledge and accounting for current uncertainties in asteroid physical properties (principally mass), lunar impact location (should the asteroid indeed be on a lunar-impacting trajectory), and deflection/disruption system performance (e.g., the momentum enhancement factor, β, for KIs). We have analyzed and discussed spacecraft mission trajectory options for flyby reconnaissance, rendezvous reconnaissance, deflection, and robust disruption. We have also generated exemplar mission campaign options based on the trajectory analysis results, noting key dates and discussing when associated decisions would need to be made in order for missions to be deployed. The best reconnaissance mission options launch in late 2028, leaving only approximately three years for development at the time of this writing in August 2025. Deflection missions were assessed and appear impractical. However, kinetic robust disruption missions are available with launches between April 2030 and April 2032. Nuclear robust disruption missions are also available with launches between late 2029 and late 2031.
Future JWST observations scheduled for early 2026 will further refine the orbit and may reduce the impact probability. Regardless of the outcome, this case illustrates how next-generation survey telescopes and infrared assets, such as JWST, transform planetary defense from an Earth-only effort to a whole Earth–Moon system strategy.
