Asteroid 2024 YR4, once the most threatening near-Earth object ever tracked, will not hit the moon in 2032, thanks to pioneering James Webb Space Telescope observations that have rewritten its trajectory and demonstrated a powerful new tool for planetary defense.
In a landmark achievement for space situational awareness, astronomers have used the James Webb Space Telescope to precisely track the faint, fast-moving asteroid 2024 YR4, definitively ruling out a collision with the moon in 2032. The observations, conducted in February 2026, have shrunk the asteroid’s orbital uncertainty so dramatically that scientists can now state with confidence that the 200-foot-wide space rock will pass the moon at a safe distance of approximately 14,229 miles.
The Discovery and Escalating Risk
When asteroid 2024 YR4 was discovered in late December 2024, initial calculations revealed a startling 3.1% probability of Earth impact on December 22, 2032—the highest risk ever assigned to a newly found near-Earth asteroid CNN. Within weeks, improved observations eliminated the threat to our planet, but by June 2025, analysis unearthed a new concern: a 4.3% chance that the asteroid might strike the moon instead NASA. A lunar impact, while not dangerous to Earth, could have endangered future astronaut missions and critical satellite infrastructure that underpins global navigation and communications.
The moon impact scenario persisted because Earth-based telescopes could not refine YR4’s orbit further until the asteroid reappeared in 2028. This created a three-year window of significant uncertainty, with potentially catastrophic consequences for lunar ambitions if the impact probability had held.
James Webb’s Unprecedented Challenge
Dr. Andy Rivkin of the Johns Hopkins University Applied Physics Laboratory and Dr. Julien de Wit of MIT recognized an opportunity to reduce that uncertainty years ahead of schedule. They secured approval to use the James Webb Space Telescope, the only observatory with the sensitivity to spot the asteroid before 2028.
However, observing YR4 presented extreme difficulties. In February, the asteroid was millions of miles away and reflected as much light as a single almond would at the distance of the moon—20 to 30 times fainter than the smallest asteroids detectable by other telescopes. Previous Webb observations in spring 2025 had already determined YR4’s size, but the asteroid appeared even fainter during the critical February window.
“To observe the asteroid we designed an observing strategy that allowed JWST to track a fast-moving target while still preserving extremely precise astrometry, meaning measurements of the object’s position relative to background stars,” de Wit said. The team had only two five-hour windows to capture the asteroid, constrained by its faintness and the need to avoid sunlight interference with Webb’s instruments.
Innovative Tracking Techniques
The team developed novel strategies to use Webb’s Near-Infrared Camera (NIRCam) for this moving-target challenge. Unlike the telescope’s typical deep-space subjects, which appear fixed, YR4 traversed the star field quickly. Dr. Artem Burdanov of MIT identified the optimal observation windows.
Carefully timed exposures enabled detection of the asteroid, which was 4 billion times fainter than what the naked eye can see. The precisely known positions of background stars acted as a reference grid, allowing astronomers to track YR4’s motion with extreme accuracy. Three independent analyses of the data converged on identical results, confirming the approach’s success.
Definitive Results and Confirmation
The February 18 and 26 observations yielded detections among the faintest ever made of an asteroid NASA. The data revealed that YR4 will pass the moon at a distance of 14,229 miles (22,900 kilometers), with a margin of error of ±497 miles (800 kilometers) ESA. This miss distance, while small in astronomical terms, is vastly larger than the moon’s 2,159-mile diameter, comfortably ruling out any impact.
“Every time we observe an asteroid, we reduce the range of possible trajectories,” de Wit explained. “The JWST observations both provided very precise positional measurements and significantly extended the time span over which the asteroid has been observed.” Rivkin added that even if future observations shift the calculated distance slightly, it will remain well outside the moon’s diameter and within the current margin of error.
Implications for Planetary Defense
The mission proves that Webb, originally designed for cosmological studies, can be repurposed as a precision planetary defense instrument. The techniques pioneered here could be deployed if another potentially hazardous object emerges with limited observation windows.
“If and when NASA’s planetary defense assets discover another potentially hazardous object of interest, we will know that we could make these measurements in practice, not just in theory,” Rivkin and de Wit noted. Future space observatories like the Nancy Grace Roman Space Telescope will enhance our capabilities, but Webb has now demonstrated a critical stopgap role for faint, fast-moving threats.
A Missed Opportunity, But a Scientific Victory
While the chance to witness a human-era lunar impact has passed, the scientific dividends are immense. “Though a little disappointed not to get to study a large asteroid impact on the Moon, which would have been our first look at this kind of dramatic event, it’s amazing what science and technical know-how can do to help us navigate the future,” said Dr. Paul Wiegert of Western University, lead author of an independent analysis of the potential impact arXiv.
The successful observation campaign transforms Webb from a cosmic explorer into a guardian of the inner solar system, providing humanity with a powerful new method to neutralize the unknown when cosmic threats appear.
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