An asteroid between 130 and 300 feet wide has a slight chance of hitting Earth in 2032 — but the precise odds have been a moving target since the space rock was first detected nearly two months ago.
NASA’s latest estimate for the asteroid, known as 2024 YR4, is that it has just a 0.28% probability of striking Earth — about a 1 in 360 chance. But the agency had put the probability at 1.5% on Wednesday, and the day before that, NASA estimated the chances of a collision to be a record high of 3.1%, or 1 in 32.
Those prior odds made the space rock the first object officially classified as a Level 3 out of 10 in a measurement known as the Torino scale, which astronomer Richard Binzel first proposed nearly three decades ago.
A Level 3 classification means the object merits attention by astronomers, and that it poses a threat of a “close encounter.”
Named after the Italian city where astronomers officially adopted it in 1999, the Torino Impact Hazard Scale is a way to communicate to the public the risks that near-Earth asteroids and other cosmic objects may pose to the planet.
The scale is color-coded, with categories from 0 (white for no hazard) to 10 (red for certain collisions). When 2024 YR 4 was thought to have a 3.1% chance of hitting Earth, that placed it in the scale’s intermediate yellow zone.
The flurry of news coverage about the asteroid has brought the Torino scale more attention than ever before. Binzel said this type of situation is precisely why he created the system in the first place.
“The idea was to be as transparent as possible about what astronomers know,” said Binzel, a professor of planetary sciences at the Massachusetts Institute of Technology. “But it’s a bit like being between a rock and a hard place because we want to report what we know to maintain public trust, but we also don’t want to raise any unnecessary anxiety.”
The challenges of walking that fine line have been apparent with asteroid 2024 YR4.
The reason the impact probability fell this week, NASA said, is that ground-based telescopes were able to get a better look at the space rock overnight on Feb. 19 and 20. Based on those observations, astronomers have been able to refine models of the asteroid’s orbit, allowing them to more accurately estimate the space rock’s trajectory. This in turn helps scientists predict with more certainty where 2024 YR4 will be on Dec. 22, 2032, the date when the asteroid will likely have its close encounter with Earth.
Although the shifting probabilities have been somewhat dizzying, it’s perfectly normal for a near-Earth asteroid’s chances of impact to dance around like this, according to Binzel.
“If you think of it like a baseball game, the amount of information we have to track this asteroid is like trying to figure out where a fly ball will land at the crack of the bat,” he said.
In other words, it will take more time to observe and measure the asteroid’s size and path before figuring out if it’s still a threat at all.
When Binzel debuted the Torino scale to the astronomical community decades ago, his goal was to foster greater public awareness of impact risks. The idea, he said, was to implement something similar to the Richter scale that measures an earthquake’s magnitude, or the Saffir-Simpson scale that assigns categories to a hurricane based on its wind speeds.
But when he first presented the system — at a United Nations conference in 1997 — it was not well received, he said. Some astronomers, according to Binzel, were skeptical that it would be useful for the public.
That same year, though, scientists discovered an asteroid dubbed 1997 XF11 that some thought could hit Earth in 2028. It didn’t take long for astronomers to rule out a potential collision, but the doomsday narrative proliferated in news reports, followed by blame for what was perceived to be a big mistake.
“It ended up being a bit of an embarrassment,” Binzel said. “It wasn’t that astronomers had made an error, but we didn’t have a good way of communicating uncertainty. So that became the motivation for proposing a simple system for categorizing any object for which we can’t immediately rule out that it will miss the Earth for centuries to come.”
Binzel presented the scale once again at a conference in Torino in 1999 attended by representatives from NASA and the European Space Agency. At that workshop, the system was finally adopted by the International Astronomical Union, a nongovernmental organization made up of professional astronomers that functions like a governing body for the astronomical community.
In addition to its use of colors and numbers corresponding to various risk levels, the scale also includes descriptions of the potential consequences, the likelihood that risk assessments may change, and what actions governments or members of the public should take.
A Level 6 threat in the orange zone, for instance, describes a likely close shave by “a large object posing a serious but still uncertain threat of a global catastrophe.”
Astronomers are advised to give this object “critical attention” in order to figure out conclusively if a collision will occur. “If the encounter is less than three decades away, governmental contingency planning may be warranted,” the scale says.
On the other hand, a Level 3 threat means: “Current calculations give a 1% or greater chance of collision capable of localized destruction,” according to the scale’s description. “Most likely, new telescopic observations will lead to re-assignment to Level 0. Attention by public and by public officials is merited if the encounter is less than a decade away.”
Unsurprisingly, then, Binzel correctly suspected when asteroid 2024 YR4 was considered a 3 on the scale that it would eventually be downgraded.
But during that brief time, 2024 YR4 did hit some uncomfortable milestones: It was the only known asteroid with a classification above Level 1, and it set records for reaching the highest impact probability and for spending the longest time with a probability greater than 1%, according to the European Space Agency.
The only time an asteroid of similar or larger size has been classified higher on the Torino scale was in 2004, when the asteroid Apophis was briefly ranked as a Level 4, with an estimated 2.7% chance of hitting Earth.
To witness the scale function in real time has been gratifying, Binzel said, though he is happy to be discreet about his connection to it.
“At the end of the day, if the Torino scale is helpful, there’s enormous satisfaction in that. It really makes my day,” he said. “But it is nice to hide my name behind the location where the scale was adopted. That helps keep my phone from ringing too much.”