Remember asteroid 2024 YR4? There was a lot of hand-wringing earlier this year when NASA and other space agencies calculated it had a chance of hitting Earth in 2032. Lucky for our planet, further data eventually ruled out an impact. The James Webb Space Telescope observed 2024 YR4 to find out just how big the asteroid is and what its surface might look like.
The asteroid might not be a danger to Earth, but scientists are still interested in understanding its properties. This won’t be the last worrisome asteroid to wander into our neighborhood.
“All together, we have a better sense of what this building-sized asteroid is like,” planetary astronomer Andy Rivkin said in a Space Telescope Science Institute statement on April 2, noting the data “gives us a window to understand what other objects the size of 2024 YR4 are like, including the next one that might be heading our way.”
City Killer Asteroid
The asteroid was widely referred to as a “city killer” that could potentially cause catastrophic damage to a city-sized portion of the planet. Early estimates pegged its size at between 130 and 300 feet wide. That’s a broad swing. Webb’s powerful instruments calculated the asteroid at about 200 feet, or roughly the height of a 15-story building.
The concern around the asteroid wasn’t just hype. At one point, it was estimated to have a 3.1% chance of impact. That placed it at the top of asteroid watch lists and temporarily earned it a 3 out of 10 on the Torino Impact Hazard Scale, used to categorize potential asteroid and comet impacts.
An object reaching a 10 on the Torino Scale would be a collision capable of causing a global catastrophe. With a 3 rating, 2024 YR4 merited attention from astronomers, the public and public officials. The asteroid is now logged as a harmless 0, meaning there’s no likelihood of a collision. Whew.
Webb Flexes Its Asteroid-Observing Muscles
The James Webb Space Telescope launched in late 2021. It’s a powerful, flagship telescope jointly run by NASA, the European Space Agency and the Canadian Space Agency. The Space Telescope Science Institute operates Webb.
Webb has peered into the early universe, scoped out distant galaxies and stars and even studied the planets in our solar system. An asteroid is another matter. “This is the smallest object targeted by the mission to date, and one of the smallest objects to have its size directly measured,” Rivkin said.
Scientists used Webb’s Mid-Infrared Instrument and Near-Infrared Camera to study the asteroid. MIRI measured the heat given off by the asteroid while NIRCam looked at light reflected by the asteroid. This gave researchers a solid estimate of its size and also clued them into the asteroid’s composition.
The asteroid has a very fast spin and seems to lack fine-grained sand on its service. Its heating and cooling cycles are different from what scientists have seen with larger asteroids. “We’ll need more data to say for sure, but it seems consistent with a surface dominated by rocks that are maybe fist-sized or larger,” said Rivkin.
Asteroid 2024 YR4 was first discovered in late 2024 and rocketed to space superstardom shortly after. It might feel like the asteroid snuck up on us, but it’s actually a testament to improvements in asteroid-spotting technology. We were able to see it, track it and understand its future trajectory. All of this took place within the span of a couple of months.
Webb’s observations add to our knowledge of this particular asteroid, but also asteroids in general. “We expect more possible impactors to be found in coming years as more sensitive asteroid search programs begin operation,” Rivkin said. “Observations using the most powerful telescope we have right now are invaluable.”
