For a few days in mid-February, headlines around the world buzzed about the potential for an asteroid to hit the Earth in 2032 — specifically, asteroid 2024 YR4. The chance of this impact rose to a high of 3.1 per cent on Feb. 18.
The number has since dropped to near zero, but this news was a real-life Don’t Look Up moment, and a stark reminder of the threat that asteroid impacts pose to life on Earth.
As a planetary geologist, my research focuses on meteorite impact craters, the scars of large asteroid and cometary impacts in Earth’s past.
Impact Earth
There are countless numbers of asteroids and an unknown number of comets throughout our solar system. Most of these objects date back to the very beginnings of our solar system, around 4.5 billion years ago.
Research has identified approximately 200 locations where these asteroids or comets have struck the Earth in the past to form meteorite impact craters. It’s very rare that planetary geologists can tell whether it was an asteroid or comet that hit.
One of the most famous of these 200 or so impact craters is the 200 km diameter Chicxulub impact crater in the Yucatan Peninsula, Mexico. This impact wiped out 65 per cent of all species on Earth, including the dinosaurs, 66 million years ago.
One of the most recent and best-preserved craters on Earth is the 1.2 km in diameter Meteor Crater in Arizona, which formed 50,000 years ago.
Millions of craters
Two hundred craters over 4.5 billion years hardly seems like a big number or cause for concern however, this number is a tiny fraction of the actual record. Most impact craters formed on Earth have been erased due to plate tectonics, volcanic eruptions, and erosion by water, wind and ice.
To truly appreciate how common impact craters are, we need to look to Earth’s closest neighbour, the moon. Because of its proximity, objects that can hit the moon can also hit the Earth. In fact, because the Earth is bigger, which means our gravitational attraction is higher, more asteroids and comets would have hit the Earth over the past 4.5 billion years than the moon.
The best estimate is 1.3 million craters over one kilometre in diameter on the moon, with another 700,000 or so smaller ones.
Updated calculations
Asteroid 2024 YR4 was discovered on Dec. 27, 2024 by the Chilean station of the Asteroid Terrestrial-impact Last Alert System (ATLAS). It was immediately recognized to be a near-Earth object (NEO). Additional telescope observations enabled astronomers to better calculate its orbit.
In January, the probability of this asteroid hitting Earth surpassed one per cent, which triggered a series of international responses. The International Asteroid Warning Network coordinates telescopes around the world to make further observations and narrow down uncertainties in its orbit.
On Feb. 18, NASA and the European Space Agency announced that the probability of asteroid 2024 YR4 hitting Earth in 2023 was 3.1 per cent, the highest ever recorded for an object of this size. This represents one in 32 odds. For comparison, the chance of dying in a motor vehicle crash in the United States is one per cent, or one in 95; the chances of the asteroid hitting Earth were pretty significant.
Thankfully, the most recent estimates of the probability of impact have gone down to near zero, based on improved calculations of its orbit.
We’re off the hook… for now.
Potential impact
Bruce Betts, chief planetary scientist at the Planetary Society, was quoted as saying: “If you put it over Paris or London or New York, you basically wipe out the whole city and some of the environs,” leading to asteroid 2024 YR4 being dubbed “a city-killer.”
The average impact velocity for an asteroid on Earth is a whopping 17 km per second — this is 25 times faster than an F-35 Lightning strike fighter.
To calculate the mass of an asteroid, we need to know its size. Estimates for 2024 YR4 range from 40 to 90 metres. If we take the upper estimate of 90 m, we can calculate the energy released at approximately nine megatons, the equivalent of the explosive energy of nine million tons of TNT. For comparison, the atomic bomb dropped on Hiroshima in Japan in 1945 was only 0.015 megatons.
The crater formed by this 90 m asteroid would be approximately 2.7 km in diameter. This is just over twice the diameter of the Meteor Crater.
The destruction doesn’t stop there, however. Research on nuclear weapons suggests that each megaton can destroy roughly 50 square kilometres, so this impact could destroy up to 450 square km around the crater through a fireball, supersonic ejecta and seismic shaking.
Would this be a city killer as some reports suggested? Absolutely. With an urban area of 232 square kilometres, my hometown of London, Ont., with a population of around 420,000, would be totally destroyed.
Actual risks
The good news is that we estimate that the impact of a 90 m diameter asteroid will occur once in every 10,000 years. For a 40 m size asteroid, this drops to once every 1,000 years — but the destructive effects are drastically reduced. It’s worth pointing out that these numbers are very approximate, and they don’t really help us figure out when the next one might happen.
As the story around asteroid 2024 YR4 shows, there is more good news in that we are getting better at detecting asteroids. Thanks to the coordination of the United Nations Office for Outer Space Affairs, many space agencies around the world are collaborating, with the knowledge that this is a problem for our entire planet.
If the calculations had continued to show that the chance of asteroid 2024 YR4 hitting Earth in 2032 was high, with enough time, an attempt to deflect the asteroid could have been attempted. In September 2022, NASA’s DART spacecraft provided the first demonstration that deflecting an asteroid from its path is possible, something that had been imagined in Hollywood movies, but not proven to be possible until then.
- is a Professor in Earth and Planetary Science, Western University
- This article first appeared in The Conversation
