Topline
The media is full of reports of a “city killer” asteroid called 2024 YR4 that’s destined to impact Earth in 2032, with astronomers about to be left helpless as it moves out of view of telescopes until 2028. Here’s what experts think.
Key Facts
All asteroids orbit the sun, and some have orbital paths that cross Earth’s. That’s the case with 2024 YR4, one of about 20,000 Apollo-type asteroids that orbit the sun from about the same distance as Earth, so are a potential threat.
2024 YR4 orbits the sun every four Earth years, so it passes relatively close to Earth once every four years. That just happened. 2024 YR4 was found, for the first time, by the Asteroid Terrestrial-impact Last Alert System on Dec. 27, 2024, two days after its closest approach, when it got to within 515,000 miles (828,800 kilometers) of Earth, about twice as far as the moon.
Its four-year orbit has had astronomers scrambling to examine archive images from sky-survey telescopes from 2020, 2016, 2012 and so on. Its trajectory also means it’s next due to pass close to Earth in December 2028 and 2032, when its just-calculated orbit has it potentially impacting Earth on Dec. 22, 2032.
There’s currently a 2.3% chance of it hitting Earth. For that reason, it sits atop the Sentry list of objects whose orbital trajectories suggest a possible impact with Earth in the future. It also has a value of 3 on the Torino Impact Hazard Scale, the International Astronomical Union’s list of asteroids that come close to Earth. That’s higher than any other asteroid, though as more data on its orbit is obtained it’s expected to eventually be declared safe.
Should We Worry About Asteroid 2024 Yr4 Impacting Earth?
“People pick up somewhat arbitrary scare stories about killer asteroids, but I think this one is a little different,” said Danny Steeghs, head of the Astronomy and Astrophysics Group at the University of Warwick in the U.K, in an interview. “This is rare. Once every five years, you might have something like this, but mostly the threats disappear. Actual impact events are one every hundreds of years.”
An asteroid as threatening as 2024 YR4 is newsworthy because it doesn’t happen very often, yet it almost certainly won’t strike Earth. “It could pass by at 50 times the diameter of Earth,” said Luca Conversi, NEO Coordination Centre Manager and NEOMIR Study Scientist at the European Space Agency’s ESRIN site in Frascati, Italy, in an interview. Conversi has plotted the region where the asteroid could pass in 2032. “The uncertainty we have is much bigger than the distance between the Earth and the moon,” he said.
If it does strike Earth, it will do so on Dec. 22, 2032, with the “risk corridor” calculated to be across the eastern Pacific Ocean, northern South America, the Atlantic Ocean, Africa, the Arabian Sea, and South Asia, according to IAWN. Astronomers can be accurate about this because the plane of the asteroid’s orbit is known, so this risk corridor is unlikely to change much, though it would likely narrow if and when an impact is imminent.
How Big Is Asteroid 2024 Yr4?
Judging purely on how bright it appears in telescope images, 2024 YR4 is thought to be between about 130 and 300 ft. (40 and 90 meters) in diameter. That’s a huge range, which astronomers are keen to refine. After all, a 300ft./90m asteroid is about 11 times larger in volume than a 130ft./40m asteroid, making it far more destructive on impact. It’s the difference between, say, a commercial airplane and a large cruise ship. Nor do scientists know its density or what it’s made of, only that it’s not ice or metal.
When it was initially picked up in December by the Asteroid Terrestrial-impact Last Alert System — using a modest telescope — it was very bright. It was also detected by many other telescopes, including by Steeghs and his colleagues on Dec. 27, 2024, using the GOTO telescope in Australia, which scans the skies for transient objects so sees asteroids serendipitously. “It’s very quickly fading and now we need the largest telescopes available to us to potentially have further measurements of it in the next weeks and months to try to refine its orbit,” said Steeghs.
Key Background
According to Sky and Telescope, 350 observations of 2024 YR4 have been made so far by astronomers at over 50 observatories since the International Asteroid Warning Network issued a rare warning on Jan. 29, 2025, but it’s a race against time. Today, it’s half the sun-Earth distance from Earth in the Canis Major constellation, and receding fast. By April it will be invisible to ground-based optical telescopes. NASA’s James Webb Space Telescope will be used to make about four hours of infrared observations in March and May before it too loses sight of the object. Its data will focus on the heat of 2024 YR4, not its orbit, but may help refine where on Earth it might strike.
After these last few observations, it won’t be until late 2028, when 2024 YR4 approaches Earth next time that astronomers will get another view of it. Trouble is, that pass will be distant, with 2024 YR4 only getting to within 4.7 million miles (7.5 million kilometers). That will make radar observations — the only kind of observation that could pin down the asteroid’s size for sure — impossible. Radar observations won’t be viable until 2032, just months before it could strike Earth.
