New data shows asteroid 2024 YR4 will narrowly miss the moon

The possibility that a huge space rock – once thought to be the most dangerous asteroid ever observed – could hit the Moon now appears to be ruled out.

Discovered in late December 2024, asteroid 2024 YR4 initially appeared to pose a serious threat to Earth, with scientists estimating a 3.1% chance of it hitting our planet on December 22, 2032. A series of observations by ground-based and space-based telescopes quickly ruled out this hypothesis, but in June 2025 a new concern emerged: a 4.3% chance for that YR4 hits the Moon.

Although Earth would not face significant physical danger if the building-sized asteroid hit the Moon, the researchers suggested that any astronauts or infrastructure on the lunar surface at that time could be in danger, as could the satellites we depend on to keep vital aspects of life functioning, including navigation and communications.

Astronomers did not expect to have the opportunity 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, a planetary astronomer at the Johns Hopkins University Applied Physics Laboratory in Maryland, and Julien de Wit, an associate professor of planetary sciences at the Massachusetts Institute of Technology, saw an opportunity for an earlier look.

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

Their observations, carried out on February 18 and 26, improved the certainty of the asteroid’s future position. Rather than colliding with the Moon, YR4 will pass it at a relatively close distance of 14,229 miles (22,900 kilometers), narrowly ruling out a single lunar impact witnessed by humanity.

Rivkin and Wit’s Webb observations were among the faintest ever made on an asteroid, according to NASA and the European Space Agency — and the detections were not easy to make given the narrow time window 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 the fourth year was a challenge.

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

In search of an almost invisible object

Scientists have used Webb to observe a multitude of celestial wonders, many of them large-scale, since the telescope’s first images were released in the summer of 2022. Sprawling galaxies and cosmic structures that span light years have often been the focus of the observatory’s infrared gaze, as have distant, faint objects.

A team led by de Wit demonstrated in December 2024 that Webb was able to spot 138 new asteroids ranging from the size of a bus to a stadium in the main asteroid belt between the orbits of Mars and Jupiter that were not observable with ground-based telescopes. This discovery 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 looked at 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 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.

Webb’s previous observations of YR4 helped determine the size of the space rock in the spring of 2025. However, the asteroid appeared even more faintly 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 statement.

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

Capturing images of the faint asteroid against bright stars required a new approach to using the telescope’s near-infrared camera, which is normally used to study extremely distant galaxies or exoplanets that appear stationary rather than moving. YR4, on the other hand, moves much faster than distant stars.

Rivkin and Wit’s team knew they only had a few five-hour windows to secure observations in February because of the low chance of YR4 appearing just bright enough to be detectable, as well as restrictions on the direction Webb could look without interference from sunlight.

Dr. Artem Burdanov, a team member and researcher in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, identified the two brief observation windows offering the best chance of seeing YR4 with Webb, Rivkin noted.

The techniques used during the observations were a mix of ideas developed by the team in advance, as well as those that could not be tested before the data came in during the observations, requiring astronomers to adapt quickly in the moment, Rivkin said.

“To observe the asteroid, we designed an observation strategy that allowed JWST to track a fast-moving target while preserving extremely precise astrometry, that is, measurements of the object’s position relative to background stars,” de Wit said.

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

Three independent analyzes of the observations were carried out by different members of the team and, despite different 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 fast-moving asteroid, which is quite different from its usual use,” explained de Wit.

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

Webb’s new results are exciting, said Dr. Paul Wiegert, a professor of astronomy and physics at Western University in London, Ont., and lead author of a paper analyzing the Moon’s potential impact. He did not participate in the observations.

“While a little disappointed that we weren’t able to study a large asteroid impact on the Moon, which would have been our first glimpse of this kind of dramatic event, it is amazing to see what science and technical know-how can do to help us navigate the future,” Wiegert wrote in an email.

Reduce 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 of plus or minus 497 miles (800 kilometers), may not seem like a great distance, astronomically speaking. However, one of the key factors when assessing an asteroid’s future trajectory is to reduce the unknowns of its orbit, de Wit said.

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

“Every time we observe an asteroid, we narrow the range of possible trajectories,” de Wit said. “In this case, the JWST observations provided both very precise position measurements and significantly extended the observation time of the asteroid.”

The absolute distance YR4 will travel from the moon is small compared to typical failure distances, but it is quite large compared to the size of the moon itself, Rivkin said.

“Although the close approach distance calculations may move a little closer (or further!) during the next observation of YR4, we expect these changes to be minimal, within the current margin of error, and do not include a lunar impact as a possibility,” Rivkin wrote in an email.

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

“If and when NASA’s planetary defense assets discover another potentially dangerous 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 these measurements,” Rivkin and de Wit noted.

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