Asteroid 2024 YR4 has grabbed headlines lately, after NASA calculated a 1-in-32 chance of an impact in 2032 – the highest chance ever for an asteroid of hazardous size. Thankfully, new observations have now halved that probability.
This worrying space rock was first discovered in late December, when it whizzed past Earth at about twice the Moon’s distance. Observations allowed its size to be estimated at 40 to 90 m (130 to 295 ft) wide.
Like all near-Earth objects (NEOs), the future trajectory of 2024 YR4 was then calculated. While it’s currently shooting away from us, it’s expected to come back for another close flyby in December 2028.
But it’s the next visit that raised alarm bells. NASA’s Center for Near-Earth Object Studies (CNEOS) found there was a non-zero chance of an impact with Earth in December 2032. In late January 2025, that risk surpassed 1%, meeting the threshold to report it to the International Asteroid Warning Network (IAWN).
Follow-up observations raised that probability higher. ESA’s Near-Earth Object Coordination Centre gave it a 2.8% chance of impact, while CNEOS calculations briefly hit a high-water mark of 3.1%. That’s the highest chance ever given to any asteroid of its size or larger.
Now, the Full Moon has waned and the skies have gotten darker again, allowing astronomers to see the asteroid with ground-based telescopes again. And after another night of collecting data with better visibility, CNEOS has reduced the impact probability to 1.5%.
That’s good news, especially keeping in mind that the chance was already fairly low. Still, astronomers will continue to observe the rock to narrow down the possible path it could take. This could drop the chance to zero, as usually happens – or it could potentially raise the percentage again.
How is risk calculated?
When an asteroid is discovered, astronomers observe its motion over time to determine its trajectory. It can start off like a funnel shape – we can reliably predict its position in the immediate future with high accuracy, but the further ahead we look the wider the range of possibilities becomes.
If Earth happens to lie anywhere within that funnel, it will have a non-zero chance of an impact. As further observations narrow down the range of paths the object could take, the impact chances will change.
Most of the time, objects that initially have a chance of hitting Earth will quickly drop to zero, as the possibility space shrinks to no longer include our planet. But other times, more observations can increase the odds of a collision – as we trim away paths that would be a miss, those that cross Earth take up a higher proportion of the total possibility space.
That’s what happened with asteroid 2024 YR4, which is why its odds increased after its discovery. Thankfully they’ve now dropped back down to 1.5% from the peak of 3.1%, but it could still go either way from here.
Percentage chance isn’t the only metric NASA and other organizations use to calculate an asteroid’s impact risk, however.
Torino scale
The Torino scale rates the hazard of near-Earth asteroids and comets based not just on the probability of an impact, but their size – and therefore destructive potential – as well.
Objects are ranked on a scale from 0 to 10. Those with very low probability start off at 0, as do objects under 20 m (66 ft) at any probability – even with 100% chance of impact, a space rock that small will burn up in the atmosphere, so isn’t worth worrying about.
A 10, on the other hand, means a collision is certain (over 99%) and the object is big enough to cause a planet-wide disaster – over 1 km (0.6 miles) wide.
A few times a year, astronomers discover objects that rank a 1 on the Torino scale. This is a normal part of the process, and most of the time they’re quickly reclassified as a 0 as more observations rule out impact.
Asteroid 2024 YR4 currently ranks a 3 on the Torino scale, which is the second highest rating ever given to an object. Even though it only has a slight chance of impact, it’s worth keeping an eye on because it has the potential to do some significant damage to anything unlucky enough to be in the firing line.
The rock has been nicknamed the “city-killer,” since that’s the rough area it could devastate. Its size is part of the concern – even at the small end of the estimated range it’s at least twice as big as the Chelyabinsk meteor, which exploded over Russia in 2013, injuring almost 1,500 people.
Its speed has even more destructive potential. The asteroid is estimated to be traveling at about 17 km (10.5 miles) per second, which would create a large airburst and shock wave in the atmosphere.
It’s important to note that 2024 YR4 is far from an Armageddon rock. It would take an object at least 1 km wide to cause global disruption to humanity, and the one that wiped out the dinosaurs is estimated to have been at least 10 km (6 miles) wide. Even if this one is at the high end of its predicted size range, 80 m is a pebble in comparison.
The only other object to have outranked 2024 YR4 is one you may have heard about: 99942 Apophis. Its impact probability peaked at 2.7%, but it scored a 4 on the Torino scale because it’s much larger – 450 by 170 m (1,480 by 560 ft). Thankfully, further observation ruled out an impact for at least the next century.
What happens next?
Astronomers will continue to monitor asteroid 2024 YR4 to get a better understanding of its trajectory. That includes the James Webb Space Telescope, which will study it in March. Its 2028 close pass could also provide a better opportunity to calculate its orbit for the next few years.
The most likely scenario is that this extra data will allow astronomers to reclassify it as a 0. But if not, there’s still time to prevent catastrophe.
In 2022, NASA successfully crashed a spacecraft into an asteroid as part of the DART mission. It wasn’t just to watch a cool space explosion – the goal was to see if such a collision could change the trajectory of an asteroid. And the mission was a roaring success, altering the orbit of a small asteroid around a larger one by 32 minutes.
DART was a test run for exactly the kind of situation we might now be in: if we find an object that’s on a collision course with Earth, we might be able to launch a planetary defense mission to nudge it away.
And with seven years to prepare, we might not even have to call Bruce Willis out of retirement.
