Webb observations confirm safe lunar pass for asteroid 2024 YR4

by Clarence Oxford
Los Angeles CA (SPX) Mar 30, 2026
NASA has used the James Webb Space Telescope to make new observations of asteroid 2024 YR4, confirming that it will safely pass both Earth and the Moon in 2032. The asteroid had already been ruled out as a significant threat to Earth, but the latest data eliminate a previously non-zero chance of a lunar impact during that year.

The Webb campaign targeted 2024 YR4 under a Director's Discretionary Time program designed to refine the asteroid's orbit using the observatory's extreme sensitivity and moving-target tracking capabilities. Without Webb, astronomers would have had to wait until 2028, when the asteroid returns to the inner solar system, to obtain comparable measurements and reduce remaining uncertainties in its predicted path.

At present, 2024 YR4 is extraordinarily faint, reflecting about as much light as an almond at the distance of the Moon and shining at roughly magnitude 30, about 4 billion times fainter than the dimmest stars visible to the unaided eye. Webb is currently the only observatory able to detect and track such a faint object with the required precision and stability, enabling several hour-long exposures in which the asteroid did not move even a single pixel.

A combined stack of these exposures from February 26, 2026, shows the asteroid clearly detected at a position offset by about 22 pixels, or roughly 0.5 arcseconds, from where it would have appeared if a 2032 lunar impact were still possible. This positional shift is enough to rule out the impact scenario and confirms that the asteroid's 2032 trajectory will miss the Moon. The new measurements extend the observational arc of 2024 YR4 from May 2025 to late February 2026, nearly doubling the time span over which its position has been tracked.

Accurate asteroid trajectory predictions depend on both precise positions and long observational arcs, and the Webb data significantly improve both for 2024 YR4. Orbital dynamicists can now compute the asteroid's future path with high confidence, removing a potential concern for satellite operators and astronauts who rely on the stability of the cislunar environment. The refined orbit shows that a lunar impact in 2032 is no longer a possibility under the updated solutions.

The observing campaign also provided an opportunity to test and validate new techniques for extracting reliable astrometry from images that include both extremely faint objects and much brighter field stars. The team had to overcome challenges associated with the enormous contrast between the asteroid and reference stars used to pin down its sky position. Webb's Near-Infrared Camera and optical design delivered data of sufficient quality for these techniques to succeed, demonstrating that similarly demanding measurements can be repeated for other targets in the future.

Beyond the immediate case of 2024 YR4, the results highlight Webb's role as a tool for planetary defense, complementing other assets by providing unique capabilities for early orbit refinement of hard-to-observe objects. Webb can measure both the physical properties and precise positions of asteroids at distances or brightness levels beyond the reach of ground-based telescopes and many space-based observatories. This allows NASA to characterize certain potentially hazardous objects long before they become accessible again to conventional survey telescopes.

NASA notes that Webb is not the only astrophysics mission with applications to planetary defense and near-Earth asteroid studies. The upcoming Nancy Grace Roman Space Telescope will survey wide areas of the sky and is expected to detect asteroids serendipitously, with the potential to better constrain their orbits. The planned Habitable Worlds Observatory mission concept is also expected to provide unprecedented sensitivity, offering another pathway to refine asteroid trajectories at earlier stages in their discovery and tracking.

In parallel, NASA is developing the Near-Earth Object Surveyor mission, which will apply the agency's heritage in space telescopes directly to the asteroid hazard. Together, these assets exemplify a broad, coordinated approach to planetary defense that spans multiple divisions within NASA Science. The Webb observations of 2024 YR4 serve as an early demonstration of how this evolving toolkit can address specific risks and validate techniques that will be applied to future potentially hazardous objects.

The experience gained in planning, executing, and analyzing the Webb observations of such a faint target is considered as important as the specific result for 2024 YR4. By successfully detecting and precisely locating the asteroid under these demanding conditions, the team has shown that similar campaigns are feasible for other objects that may pose questions about potential impacts. As new near-Earth asteroids are discovered and cataloged, NASA will be able to draw on this work to rapidly design follow-up observations and refine their orbits well before they approach Earth or the Moon.