An asteroid’s chances of hitting Earth keep shifting. Here’s why that is happening

In this artistic rendering, two asteroids fly past Earth. (ESA via AP) (Pierre Carril/AP)

A recently discovered asteroid, named 2024 YR4, is now the riskiest asteroid ever detected. On Tuesday, NASA calculated that the space rock had a 3.1 per cent chance of hitting Earth in 2032, while the European Space Agency’s risk assessment sits at 2.8 per cent.

The narrow difference is due to the two agencies’ use of different tools for determining the asteroid’s orbit and modeling its potential impact. But both percentages rise above the 2.7 per cent chance of collision once associated with an asteroid discovered in 2004 called Apophis, making 2024 YR4 the most significant space rock to be spotted within the past two decades.

However, another update shared by NASA on Wednesday showed that 2024 YR4 has a 1.5 per cent chance of colliding with Earth in December 2032, based on new observations now that the full moon has passed. Astronomers have anticipated that such fluctuations are possible as they gather more observational data.

Measuring 1,148 feet (350 metres) in diameter, Apophis was previously considered one of the most hazardous asteroids, with a chance of hitting our planet and reaching a 4 out of 10 on the Torino Impact Hazard Scale, a tool for categorizing potential collisions of space objects with Earth. The rating meant that the asteroid merited astronomers’ attention and tracking. But scientists revised that assessment after a precise analysis of the asteroid’s orbit in 2021.

And scientists are expecting similar forecasting trends with 2024 YR4, which currently sits at a 3 on the Torino Scale and is estimated to be 131 to 295 feet (40 to 90 metres) wide, comparable to a large building.

“For asteroids larger than 30 metres (98.4 feet) in size, 2024 YR4 now holds the record for the highest impact probability reached, and the longest time spent with an impact probability greater than one per cent,” according to the ESA.

“It is important to note that this rising impact probability is an expected result as we continue to improve our knowledge of the asteroid’s orbit. … As more observations of the asteroid are made, the uncertainty region will continue to shrink and the impact probability may continue to rise. If we reach a point at which Earth is no longer inside this region, the impact probability will quickly drop to 0.”

Astronomers are using a multitude of telescopes to observe the space rock to understand its size and orbit, which will likely reveal lower chances of a 2032 collision than what the current data shows.

Ruling out risk

Apophis’ ranking of 4 was the highest level ever reached on the Torino Scale, and there are currently no other known objects above zero apart from 2024 YR4, said Richard Binzel, inventor of the scale. Binzel is a professor of planetary science, joint professor of aerospace engineering and MacVicar Faculty Fellow at the Massachusetts Institute of Technology.

Apophis reached a 4 because it was a larger object capable of regional damage, while 2024 YR4 is a 3 because it would cause local damage, Binzel said.

Binzel presented his concept for the scale at a 1995 UN conference. A working group of the International Astronomical Union adopted it at a meeting in Torino, Italy — hence the name of the scale — and published it in 1999. Binzel said he believed a scale was needed to calculate the risks of near-Earth objects, which were becoming increasingly detectable thanks to new asteroid surveys.

“For many of these objects, uncertainties in their preliminary orbits will result in calculations yielding nonzero probabilities of collision over the next century,” Binzel shared in his 1999 paper. “An index system is proposed which places the hazard posed by any given close approach into an easily understandable context that allows simple and efficient communication between astronomers and the public.”

According to the Torino Scale, 2024 YR4’s ranking of three means “current calculations give a one per cent or greater chance of collision capable of localized destruction. 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.”

But it’s perfectly natural that the impact chances will “bounce around a bit” before dropping to zero, Binzel said.

Predicting the future

Part of the uncertainty lies within understanding 2024 YR4’s orbit and how close it will come to Earth in the future, he said. Astronomers are at the beginning of measuring the space rock’s four-year orbit around the sun, making it difficult to pinpoint where it will be in years or decades.

While it’s likely that the asteroid will harmlessly whiz by Earth, as Apophis is expected to do in 2029 (when multiple spacecraft are expected to study it), it’s crucial for astronomers to keep actively watching and tracking the asteroid to gather more data, and Binzel confirmed they are “on the job.”

Using a pasta analogy as a nod to the Italian scale he created, Binzel likened the uncertainty around the asteroid’s future location to a long fettuccine noodle that spans across the orbit of the moon around Earth.

“Earth just happens to lay underneath that noodle, and the fraction that the Earth occupies is the probability of impact,” Binzel said in an email. “More data on the asteroid shrinks the noodle. As the noodle shrinks, but still happens to include the Earth, it can make the calculated probability go up. Eventually we will pin down the asteroid’s position to that of a single grain. Most likely that grain will not be on top of the Earth. It could be even farther than the Moon.”

The asteroid was discovered after passing by Earth in December, and after April, it won’t be visible again until June 2028, when it is expected to fly harmlessly by our planet again.

“At its farthest, YR4 will be near to Jupiter’s orbit,” Binzel said. “YR4 presents a challenge because it is small and headed away. Telescopes on the ground can track it for a few more months. Then we’ll call (the James Webb Space Telescope) into service to track it even further, if needed.”

Webb is expected to begin observing the asteroid in March to help astronomers pin down the space rock’s orbit and size.

“While certainty for 2024 YR4 missing the Earth is the outcome we expect, it’s not up to us. It’s for nature to decide,” Binzel said. “In fact, nature already has settled the question. We just don’t know that answer yet. That’s why our tracking efforts continue.”