Of all the asteroids that have endangered the planet, 2024 YR4 is unparalleled. Soon after it was discovered in December 2024, worldwide telescopic observations quickly positioned it as the most dangerous space rock ever discovered—one that had a 3.1 percent (or 1-in-32) chance of crashing into Earth on December 22, 2032. If it were to hit one of the cities that have potentially been in its orbit, this force in the 60s would have unleashed a steroid atom that could compared with several. bombs, destroy the unfortunate metropolis.
An Earth impact was finally ruled out last February. But a late plot twist revealed that 2024 YR4 had a 4.3 percent (1-in-23) chance of slamming into our moon on the same date. Now, a concerted effort by astronomers indicates that the asteroid will also comfortably miss our alabaster companion – by 21,200 kilometers.
Remarkably, this revelation comes from the James Webb Space Telescope (JWST), an observatory that was designed to look at ancient black holes, distant galaxies, convulsing stars and distant planets – not help defend the planet from rogue asteroids. However, its incredibly sensitive infrared vision managed to track the asteroid in February when it was 450 million kilometers from Earth – a feat no other telescope could manage.
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“We think this is absolutely the faintest Solar System object ever observed,” says Andy Rivkin, an astronomer and planetary defense scientist 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 has been able to do for us with a real-life, short-term response to an asteroid threat,” said Kathryn Kumamoto, director of the Planetary Defense Program at Lawrence Livermore National Laboratory in California.
Some may lament that a seemingly harmless 2032 lunar impact—one explosive enough to be visible to the naked eye—is no longer in the cards. But there was a real risk that some of the impact debris ejected from the moon could have sliced open several Earth satellites. If JWST had determined that 2024 YR4 was headed for a violent encounter with the Moon, experts would have six extremely short years to try to deal with it. “It’s very good that we’re not forced to moderate this asteroid on that time scale,” Kumamoto says.
The NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope network first spotted 2024 YR4 shortly after Christmas Day in 2024. At first, it appeared to be nothing to worry about. But additional observations from other observatories give a 1 percent chance of an Earth impact in 2032. Those impact odds eventually rose to their unnerving high of 3.1 percent in mid-February 2025.
All the relevant researchers were interested in finding out whether these impact odds would continue to rise or fall. But refining 2024 YR4’s orbit was a tall order: it was moving quickly away from Earth, and by May 2025 it would have faded from view until it swung back 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.
That would give astronomers just four years to prepare if a catastrophic asteroid strike became likely. Even eight years, according to planetary defense experts, was insufficient to prepare a spaceflight mission capable of knocking the Earth-bound asteroid away.
Astronomers first needed to determine its true size. Visible light observations can reveal only rough estimates of a space rock’s dimensions. But when viewed in infrared, the thermal glow of an asteroid corresponds almost exactly to its size.
The same month 2024 YR4 was discovered, a study concluded that JWST could be used to hunt for small asteroids of interest. So when YR4 in 2024 ambushed everyone, Rivkin and his colleagues submitted a proposal to delineate it with the $10 billion telescope. It worked wonders: they found that the asteroid was 60 meters across, making it a comfortable urban wreck.
In May, once an Earth impact was ruled out, scientists placed the odds of a lunar collision at 4.3 percent. Aside from the fact that there would likely be both American and Chinese astronauts on the moon by 2032, who certainly wouldn’t appreciate being pancaked or blasted into space by 2024 YR4, modeling studies suggested that a shotgun spray of debris could knock several of Earth’s communications satellites out of the sky. “It would have had potential global consequences,” says Rivkin.
That prompted planetary defenders to outline a plan to prevent the lunar impact, which they described in a arXiv preprint. “If significant threats to space resources from an impact were detected, there is a reasonable chance that we would have tried to do something to stop the asteroid from hitting,” says Kumamoto. But “you couldn’t really divert it” in the remaining time. That left three options: ram it with a spacecraft to shatter the rock into tiny pieces, vaporize it with a spacecraft with nuclear devices, or let the impact happen.
“When we saw that it could hit the moon, we wanted to follow up,” says Rivkin. “JWST was the only facility that could do it 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; which would allow them to monitor the movements of the asteroid with great precision.
During JWST’s observations, “the asteroid was four billion times fainter than the human eye 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 2024 YR4’s orbit. The result? The moon was also safe from harm.
2024 YR4 may no longer be a danger. But NASA’s Near-Earth Object Surveyor space observatory (2027 launch) and the soon-to-be operational Vera C. Rubin Observatory in Chile are set to find hundreds of thousands of potentially hazardous asteroids over the next few years. That JWST can help protect not only the Earth, but also the Moon is welcome news.
“We are prepared to face future threats,” Cano says. “And they will come.”
