Notorious asteroid 2024 YR4 won’t crash into the moon after all

Of all the asteroids that have imperiled the planet, 2024 YR4 is unprecedented. Shortly after being spotted in December 2024, global telescopic observations quickly positioned it as the most dangerous space rock ever discovered, with a 3.1% (or 1 in 32) probability of crashing into Earth on December 22, 2032. Had it struck one of the cities potentially in its path, this 60 meter asteroid would have released a force comparable to several atomic bombs, devastating the unfortunate. metropolis.

A land impact was finally ruled out in February last year. But a late twist revealed that 2024 YR4 had a 4.3% (1 in 23) chance of hitting our moon on the same date. Now, a concerted effort by astronomers indicates that the asteroid will also miss our alabaster companion, by 21,200 kilometers.

Remarkably, this revelation comes from the James Webb Space Telescope (JWST), an observatory designed to observe ancient black holes, distant galaxies, convulsed stars and distant planets, without helping to defend the planet against rogue asteroids. Its incredibly insightful infrared vision, however, made it possible to track the asteroid in February when it was 450 million kilometers from Earth, a feat no other telescope could achieve.

On supporting science journalism

If you enjoy this article, please consider supporting our award-winning journalism by subscription. By purchasing a subscription, you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

“We think this is certainly the faintest solar system object ever observed,” says Andy Rivkin, an astronomer and planetary defense researcher at the Johns Hopkins Applied Physics Laboratory in Maryland, who led the JWST effort to track 2024 YR4.

“I’m really amazed at what JWST was able to do for us with a real, near-term response to an asteroid threat,” says Kathryn Kumamoto, planetary defense program manager at Lawrence Livermore National Laboratory in California.

Some might complain that a seemingly innocuous lunar impact in 2032 – explosive enough to be visible to the naked eye – is no longer on the cards. But there was a real risk that some of the impact debris dropped from the Moon could have cut up several Earth satellites. If JWST had determined that 2024 YR4 was on track for a violent rendezvous with the Moon, experts would have had six extremely short years to try to deal with it. “It’s really good that we don’t have to dim this asteroid in such a short time,” Kumamoto says.

The NASA-funded ATLAS (Asteroid Terrestrial-impact Last Alert System) telescopic array first discovered 2024 YR4 just after Christmas Day 2024. Initially, there seemed to be no reason for concern. But additional observations from other observatories indicate that there is a 1% chance that an impact on Earth will occur in 2032. These impact probabilities eventually reached their disconcerting peak of 3.1% in mid-February 2025.

All scientists involved were eager to know whether these impact probabilities would continue to increase or decrease. But refining the orbit of 2024 YR4 was a daunting challenge: It was rapidly moving away from Earth, and by May 2025 it would have disappeared from view until it returned years later. “We didn’t expect to observe the object again until spring 2028,” says Juan Luis Cano, an aerospace engineer at the European Space Agency’s Near-Earth Object Coordination Center.

This would have given astronomers only four years to prepare if a catastrophic asteroid strike became likely. Even eight years were, according to planetary defense experts, insufficient to prepare a spaceflight mission capable of repelling the Earth asteroid.

Astronomers first had to determine its true size. Visible light observations can only reveal rough estimates of a space rock’s dimensions. But seen in infrared, an asteroid’s thermal glow almost exactly matches its size.

The same month that 2024 YR4 was discovered, a study concluded that JWST could be used to track small asteroids of interest. So when 2024 YR4 ambushed everyone, Rivkin and his colleagues submitted a proposal to expand it using the $10 billion telescope. It worked wonders: they discovered that the asteroid was 60 meters in diameter, making it a comfortable city wrecker.

In May, once the hypothesis of a terrestrial impact was ruled out, scientists estimated the probability of a lunar collision at 4.3%. Besides the fact that there would likely be American and Chinese astronauts on the Moon by 2032, who certainly wouldn’t appreciate being panicked or thrown into space by 2024 YR4, modeling studies suggest that a jet of debris could knock several of Earth’s communications satellites out of the sky. “It would have had potentially global consequences,” Rivkin says.

This prompted planetary defenders to develop a plan to prevent the lunar impact, which they described in a statement. arXiv preprint. “If an impact demonstrated substantial threats to space resources, chances are we would have tried to do something to prevent the asteroid from hitting,” says Kumamoto. But “we couldn’t really deflect it” in the remaining time. This left three options: ram it with a spacecraft to break the rock into small pieces, vaporize it with a spacecraft armed with a nuclear device, or let the impact happen.

“When we saw that it could reach the moon, we wanted to do a follow-up,” Rivkin says. “JWST was the only facility capable of doing this before 2028.” They had a small window of opportunity for two observations in February, when 2024 YR4 would be close to several background stars whose positions astronomers knew with high confidence; this would allow them to monitor the asteroid’s movements with great precision.

During JWST’s observations, “the asteroid was four billion times fainter than human eyes can see,” says Julien de Wit, a planetary scientist at the Massachusetts Institute of Technology and a member of Rivkin’s team. And yet it worked. Then, NASA’s Center for Near-Earth Object Studies in Southern California and the European Space Agency’s Near-Earth Object Coordination Center in Italy used the observations to recalculate the orbit of 2024 YR4. The result? The moon too was safe from danger.

2024 YR4 may no longer be a danger. But NASA’s Near-Earth Object Surveyor space observatory (launching in 2027) and the Vera C. Rubin Observatory, soon to be operational in Chile, are expected to detect hundreds of thousands of potentially dangerous asteroids in the coming years. That JWST can help protect not only the Earth, but also the Moon, is good news.

“We are prepared to face any future threats,” says Cano. “And they will come.”