The possibility of a huge space rock — once considered the riskiest asteroid ever observed —
Discovered in late December 2024, asteroid 2024 YR4 initially appeared to be a serious threat to Earth, with scientists estimating up to a 3.1% probability of impact with our planet on December 22, 2032. A series of observations from ground- and space-based telescopes quickly helped rule out this possibility, but in June 2025, a new concern emerged: a 4.3% probability that YR4 collided with the Moon.
Although Earth would not face any significant physical danger if a building-sized asteroid struck the Moon, researchers have suggested that any astronauts or infrastructure on the lunar surface at that time could be at risk — as could the satellites we depend on to keep vital aspects of life, including navigation and communications, running smoothly.
Astronomers did not expect to have the opportunity to better assess the risk of a lunar impact from asteroid YR4 until it becomes visible again 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, associate professor of planetary science at the Massachusetts Institute of Technology (MIT), saw an opportunity for an early observation.
Rivkin and de Wit applied for and received approval to use the James Webb Space Telescope, or JWST, the only observatory with a chance of detecting the asteroid before 2028.
Observations made on February 18 and 26 increased certainty regarding the asteroid’s future position. Instead of colliding with the Moon, YR4 will pass by at a relatively short distance of 22,900 kilometers (14,229 miles) — virtually ruling out a unique lunar impact that humanity would have witnessed.
Webb’s observations of Rivkin and de Wit were among the faintest ever made of an asteroid, according to NASA and the European Space Agency — and the detections were not easy to obtain, given the small window of time to capture them.
As the most powerful space telescope, Webb is perhaps a natural choice to aid in the search for a potentially dangerous asteroid that could collide with the Earth or the Moon. But YR4 presented a challenge.
Researchers had to develop new techniques to use Webb’s instruments to detect the asteroid as a nearly invisible speck in the vastness of space, and their innovations could aid future efforts if another similar threat arises.
See astronomical discoveries from 2026
Since the telescope’s first images were released in the summer of 2022, scientists have used Webb to observe a multitude of celestial wonders — many of them on a grand scale. Vast galaxies and cosmic structures stretching light years have often been the focus of the observatory’s infrared gaze, but distant, faint objects have also received attention.
A team led by de Wit demonstrated in December 2024 that Webb was capable of detecting 138 new asteroids, ranging in size (from bus to stadium), in the main asteroid belt located between the orbits of Mars and Jupiter, which were not observable with ground-based telescopes. The discovery showed that Webb could study extremely faint objects in the solar system, de Wit said.
When it came time to focus on Year 4, de Wit and Rivkin explored Webb’s capabilities as a planetary defense tool — only the challenge was greater.
in February, it was spotted millions of kilometers from Webb’s orbit, which for Wit and Rivkin was like searching for a speck of dust in a starry sky.
Previous Webb observations of YR4 helped determine the space rock’s size in spring 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 statement.
Webb’s sensitivity and stability, as well as its ability to accurately track moving targets, make it an excellent tool for conducting long-duration observations of YR4, they noted.
Capturing images of the faint asteroid against bright stars required an innovative approach to using the telescope’s Near Infrared Camera, typically used to study extremely distant galaxies or exoplanets that appear fixed rather than moving. YR4, on the other hand, moves much faster compared to distant stars.
Rivkin and de Wit’s team knew they only had a few five-hour windows to secure observations in February, due to the slim chance of YR4 appearing bright enough to be detectable, as well as constraints regarding the direction in which Webb could look without interference from sunlight.
Dr. Artem Burdanov, a team member and research scientist in MIT’s Earth, Atmospheric, and Planetary Sciences department, identified the two brief observation windows with
The techniques employed during the observations were a mix of ideas the team developed in advance as well as those that could not be tested until data was being received during the observations, requiring astronomers to adapt quickly on the fly, Rivkin said.
“To observe the asteroid, we developed 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 visible to the naked eye 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 served as a reference, allowing astronomers to track the asteroid’s position with extreme precision.
Three independent analyzes of the observations were carried out by different team members and, despite the varied approaches,
“In practice, 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.
The new results from Webb are exciting, said Dr. Paul Wiegert, a professor of astronomy and physics at Western University in London, Ontario, and lead author of a paper analyzing the potential lunar impact. He did not participate in the observations.
“Although a little disappointed that we did not have the opportunity to study the impact of a large asteroid on the Moon, which would have been our first glimpse of this type of dramatic event, it is amazing what science and technical knowledge can do to help us navigate the future,” Wiegert wrote in an email.
The team’s observations that the asteroid will pass within about 22,900 kilometers (14,229 miles) of the Moon, with a margin of error of plus or minus 800 kilometers (497 miles), may not seem like a huge distance, astronomically speaking. However, one of the key factors in evaluating an asteroid’s future trajectory is reducing the unknowns of its orbit, de Wit said.
According to NASA, the observations improve accuracy in understanding an asteroid’s position and reduce uncertainties, and researchers are confident that a lunar impact can be ruled out.
“Every time we observe an asteroid, we reduce the range of possible trajectories,” said de Wit. “In this case, the JWST observations provided very precise positional measurements and significantly extended the time period during which the asteroid was observed.”
The absolute distance YR4 will pass from the Moon is small compared to usual approach distances, but it is quite large when compared to the size of the Moon itself, Rivkin said.
“Although maximum approach distance calculations may change slightly (closer or further away!) when YR4 is observed again, 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 in development at NASA, along with the Habitable World Observatory concept. Such observatories could be used to detect asteroids or refine their orbits. But defining YR4’s size and orbit also demonstrated the role Webb could play in protecting the planet from possible errant space rocks.
“If and when NASA’s planetary defense capabilities discover another potentially hazardous object of interest, we will know that we can make these measurements in practice, not just in theory, and we will have gained important experience in designing and analyzing these measurements,” Rivkin and de Wit noted.
