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Fears that a 100-metre-wide asteroid could be on course to collide with the moon appear to have been misplaced, according to new observations.
Discovered in December 2024, asteroid 2024 YR4 was briefly considered the “most dangerous asteroid” in decades after scientists initially estimated it had a 3.1% chance of colliding with the Earth in 2032. Closer observations quickly ruled out a “city killer” scenario, but instead astronomers calculated there was a 4.3% chance that the moon lay in the path of impact.
Although there would not be any danger to the Earth, lunar debris kicked up during a collision could interfere with satellites used for navigation and communications. But new observations appear to have eliminated this risk, showing that the asteroid will safely pass the moon at a distance of more than 20,000km (12,427 miles).
“It would’ve been a very interesting science experiment but probably, given the small risk of debris, it wouldn’t be one we’d want to try out,” said Colin Snodgrass, a professor of planetary astronomy at the University of Edinburgh. “Some of us in the scientific community might be a little disappointed.”
As the asteroid hurtled away from the Earth, fading from view, astronomers had not expected to be able to nail down the asteroids trajectory until 2028, when it came back into the Earth’s line of sight. However, an international team of scientists identified two five-hour windows in February, when they believed the James Webb Space Telescope could detect and track the asteroid, which is four billion times fainter than the faintest star visible to the unaided eye.
“[Asteroid] 2024 YR4 is exceedingly faint right now, reflecting about as much light as an almond at the distance of the moon,” said Dr Andy Rivkin, of Johns Hopkins University, and Prof Julien de Wit, of Massachusetts Institute of Technology, who co-led the observations. “Webb is the only observatory that could hope to make these measurements, as it is the only one with the required sensitivity and stability combined with precise moving-target tracking needed to follow and study objects like this.”
The observations allowed 2024 YR4’s orbit to be more closely refined, showing that it would miss the moon in 2032.
The ability of increasingly powerful telescopes, including the James Webb Space Telescope and the Vera Rubin Observatory, to track asteroids means that more objects of interest will be detected, according to Snodgrass. “We might get these brief scares slightly more often,” he said. “But we also have the technology to track these things much better and will normally be very quick to rule them out.”
“The moon is safe, 2024 YR4 poses no danger, but the work continues,” the European Space Agency noted in response to the findings. “The Planetary Defence team in ESA’s Space Safety programme continues to detect and track near-Earth objects to ensure that if a genuine danger ever emerges, we will not be caught unaware.”
