New data shows asteroid 2024 YR4 will narrowly miss the moon

By Ashley Strickland, CNN

(CNN) — The possibility that a huge space rock — once deemed the riskiest asteroid ever observed — could hit the moon now appears to be off the table.

Discovered at the end of December 2024, asteroid 2024 YR4 at first seemed a serious threat to Earth, with scientists estimating as much as a 3.1% chance of it impacting our planet on December 22, 2032. A series of observations from ground- and space-based telescopes quickly helped rule that out, but by June 2025, a new concern emerged: a 4.3% chance YR4 would slam into the moon instead.

While Earth wouldn’t face any significant physical danger if the building-size asteroid struck the moon, researchers suggested that any astronauts or infrastructure on the lunar surface at the time could be at risk — as could satellites that we depend on to keep vital aspects of life, including navigation and communications, running smoothly.

Astronomers didn’t expect to get a chance to better assess the risk of a YR4 lunar impact until the asteroid came back into view from Earth’s perspective in 2028. However, Dr. Andy Rivkin, planetary astronomer from the Johns Hopkins University’s Applied Physics Laboratory in Maryland, and Julien de Wit, associate professor of planetary science at the Massachusetts Institute of Technology, spied an opportunity for an earlier glimpse.

Rivkin and de Wit applied and received approval to use the James Webb Space Telescope, or JWST, the only observatory with a chance of spotting the asteroid before 2028.

Their observations, taken on February 18 and 26, improved the certainty of the asteroid’s future position. Rather than colliding with the moon, YR4 will pass it from a relatively close distance of 14,229 miles (22,900 kilometers) — narrowly ruling out a once-in-a-lifetime lunar impact that humanity would have witnessed.

Rivkin and de Wit’s Webb observations were among the faintest ever made of an asteroid, according to NASA and the European Space Agency — and the detections weren’t easy to come by given the narrow window of time to capture them.

As the most powerful space telescope, Webb is perhaps a natural choice to aid the search for a potentially dangerous asteroid that could impact Earth or the moon. But YR4 presented a challenge.

The researchers had to develop new techniques for using Webb’s instruments to detect the asteroid as a nearly invisible speck amid the vastness of space, and their innovations could help future efforts if another similar threat arises.

Searching for a nearly invisible object

Scientists have used Webb to observed a multitude of celestial wonders — many of them on a grand scale — since the telescope’s first images were released in the summer of 2022. Sprawling galaxies and cosmic structures that expand for light-years have often been the focus of the observatory’s infrared gaze, but so have distant, faint objects.

A team led by de Wit demonstrated in December 2024 that Webb was capable of spotting 138 new asteroids ranging from bus- to stadium-size in the main asteroid belt located between the orbits of Mars and Jupiter that were not observable with ground-based telescopes. The finding showed that Webb could study extremely faint objects in the solar system, de Wit said.

When it was time to focus on YR4, de Wit and Rivkin leaned into Webb’s capability as a planetary defense tool — except the challenge was greater.

YR4 is about 60 meters (about 200 feet) in diameter and in February was located millions of miles from Webb’s orbit, which for de Wit and Rivkin was like looking for a dust particle against a backdrop of blinking stars.

Previous Webb observations of YR4 had helped to determine the space rock’s size in the spring of 2025. However, the asteroid appeared even fainter through the telescope’s instruments last month, reflecting as much light as a single almond would at the distance of the moon, according to de Wit and Rivkin in a NASA release.

Webb’s sensitivity and stability, as well as its ability to accurately track moving targets, make it an excellent tool for making hours-long observations of YR4, they noted.

Capturing images of the faint asteroid against bright stars required a novel approach to using the telescope’s Near-Infrared Camera, which is normally used to study extremely distant galaxies or exoplanets that appear fixed, rather than moving. YR4, on the other hand, moves much more quickly when compared with distant stars.

Rivkin and de Wit’s team knew they only had a couple of five-hour windows to secure the observations in February due to the slim chances of YR4 appearing just bright enough to be detectable, as well as the restrictions around the direction in which Webb could look without interference from sunlight.

Dr. Artem Burdanov, team member and research scientist at the MIT department of Earth, Atmospheric and Planetary Sciences, identified the two brief observation windows with the best chances of seeing YR4 with Webb, Rivkin noted.

The techniques employed during the observations were a blend of ideas the team developed in advance, as well as those that couldn’t be tested until the data was streaming in during the observations, requiring the astronomers to adapt quickly in the moment, Rivkin said.

“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.

Carefully timed exposures enabled the team to detect the asteroid, which was 4 billion times fainter than what the naked eye can see and 20 to 30 times fainter than the smallest asteroids detectable with other observatories, de Wit said. The timing also meant that the precisely known position of the stars visible behind YR4 acted like a reference, allowing the astronomers to track the asteroid’s position with extreme accuracy.

Three independent analyses of the observations were made by different members of the team, and despite varying approaches they all agreed very well, Rivkin said.

“In effect, we adapted an instrument optimized for deep cosmological imaging into a precision tracker for a rapidly moving asteroid, which is quite different from its usual use,” de Wit explained.

A paper detailing the observations and techniques will soon be available, he added.

The new Webb results are exciting, said Dr. Paul Wiegert, a professor of astronomy and physics at the Western University in London, Ontario, and lead author of a paper analyzing the potential lunar impact. He was not involved in the observations.

“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,” Wiegert wrote in an email.

Reducing the unknowns

The team’s observations that the asteroid will pass about 14,229 miles (22,900 kilometers) from the moon, with a margin of error that is plus or minus 497 miles (800 kilometers), may not seem like a large distance, astronomically speaking. However, one of the key factors when assessing the future trajectory of an asteroid is reducing the unknowns of its orbit, de Wit said.

Observations improve precision in understanding an asteroid’s position and reduce uncertainties, according to NASA, and the researchers are confident that a lunar impact can be ruled out.

“Every time we observe an asteroid, we reduce the range of possible trajectories,” de Wit said. “In this case, the JWST observations both provided very precise positional measurements and significantly extended the time span over which the asteroid has been observed.”

The absolute distance YR4 will pass from the moon is small compared to the usual miss distances, but it is pretty big when compared with the size of the moon itself, Rivkin said.

“While calculations of the close approach distance may shift a little bit closer (or further!) when YR4 is next observed, we expect those shifts will be minimal, within the current margin of error, and will not include a lunar impact as a possibility,” Rivkin wrote in an email.

Multiple new space observatories, including the Near-Earth Object Surveyor and the Nancy Grace Roman Space Telescope, are in development at NASA, along with the concept of the Habitable World Observatory. Such observatories could be used to spot asteroids or refine their orbits. But defining YR4’s size and orbit have also demonstrated the role that Webb can play in protecting the planet from potentially wayward space rocks.

“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, and we have gained important experience in designing and analyzing those measurements,” Rivkin and de Wit noted.

The-CNN-Wire
™ & © 2026 Cable News Network, Inc., a Warner Bros. Discovery Company. All rights reserved.