While many citizens were peacefully enjoying the Christmas holidays, a telescope located in a remote area of Chile detected the most concerning asteroid for humanity in two decades.
Measuring between 40 and 90 meters in diameter, and despite its uncatchy name, 2024 YR4 quickly rose to fame and raised alarms due to a concerning percentage: according to NASA and the European Space Agency (ESA), there was more than a 1% chance of it hitting somewhere on Earth on December 22, 2032. After several updates, that percentage, which was 1.2% at the end of January, reached a record value of 3.1% this week, from which it quickly decreased, with NASA placing it at 0.36% and ESA at 0.16% on Friday.
Since monitoring dangerous asteroids, none have had as high a risk of impact as 2024 YR4. The previous record was held by Apophis in 2004, with a 2.7% risk, which fortunately was completely ruled out upon better orbit study.
Although a 1% probability like the one 2024 YR4 had is very low, it is much higher than the thousands of Earth-close asteroids discovered each year. Therefore, and because it measures over 50 meters, the United Nations activated the Planetary Defense Protocol for the first time, established about a decade ago, while astronomers focus on scrutinizing the rock to better understand what we are facing.
These are busy days at the ESA's Planetary Defense Office, led by veteran German physicist Richard Moissl. A department created in 2021 based in Frascati, Italy, dedicated to protecting humanity from cosmic threats.
"2024 YR4 keeps us busy, but the truth is that right now we are working more or less as usual, with a higher level of activity than usual, and we are more attentive," he explains reassuringly during a video conference interview from the ESA's European Space Operations Center in Darmstadt, Germany.
"Our priority is to accurately define the trajectory to confirm or completely rule out an impact, in addition to determining its size to estimate the potential damage and how to deal with it if necessary. We have only a couple of months to do so from the ground, as the asteroid, which had its closest approach during Christmas, is moving away from our planet and will not be visible again until June 2028, leaving only four years until that unlikely - but still possible - impact," the scientist points out.
The International Asteroid Warning Network (IAWN), led by NASA, is one of the two technical asteroid response committees linked to the UN. The other is the Space Mission Planning Advisory Group (SMPAG), chaired by ESA. While the former studies potentially hazardous asteroids and ensures that all observations are processed and coordinated, the latter focuses on the measures to be taken in case of a threat: determining the available time, offering options, making recommendations, and informing civil and defense authorities.
Currently, there are over 1.2 million known asteroids in the Solar System, with 36,000 close to Earth. Of these, 1,740 have been included in the so-called risk list, where all asteroids with a non-zero risk of impact are listed. "2024 YR4 quickly rose to the top of this risk list, and as observations were made, the probability of impact increased," Moissl summarizes.
In addition to leading this list, the new asteroid has reached level 3 on the Torino Scale, but on Friday, it dropped to level 1, where it had been placed shortly after its discovery. This scale, used to classify asteroids according to the danger they pose to Earth, consists of 10 levels, from 0 to 10. By default, all harmless asteroids are at level zero, while levels 8, 9, and 10 indicate a certain collision. "Every year, a couple of thousand asteroids are discovered, and only a few reach level 1 of the Torino Scale, on average one or two, so being already at level 1 means it is something to pay attention to and was observed with priority," he explains.
So far, Apophis has been the only other asteroid classified at a level higher than 1: it reached level 4 due to the risk of impact it posed for April 13, 2029, when it will approach Earth. "It was qualitatively a similar case to what we have now, but it is much larger, measuring over 350 meters in diameter. Fortunately, 2024 YR4 is much smaller. The data obtained through spectra suggest that we can rule out some of the larger sizes, so although our procedures indicate a range between 40 and 90 meters, we believe it is closer to 40 meters and measures around 50 or 50-something meters," he explains.
The James Webb Space Telescope will help them further restrict the size with greater confidence since it can observe in the infrared. Space agencies have requested to observe this new asteroid with the most powerful astronomical instrument available, which will target 2024 YR4 in early March and early May when it will no longer be visible to ground telescopes.
"The James Webb is in space, so it does not have to pass through any atmosphere and can stably point to a space area for a long time. Additionally, we can measure the infrared radiation coming from the object, while from Earth, in the visible spectrum, we can only make assumptions about the size based on brightness," he states, emphasizing that observations with the James Webb were not urgently requested but followed the usual procedures when something important is discovered that cannot wait, in this case, because it will soon no longer be visible.
The measurements made by asteroid monitoring organizations are independent, ensuring that "the information is entirely correct." "Due to the use of different tools, software, and methodologies, there are slight numerical differences, so the data will never be exactly the same, but we all agreed that the risk of impact was above 1%," he assures. Therefore, the percentages provided by NASA and ESA these days differ slightly by a few tenths.
He assures that the maximum values reached this week (3.1% according to NASA and 2.8% according to ESA) "were entirely within expectations," and as predicted, the percentage has dropped below 1%.
This Solar System specialist - who has been part of missions like Rosetta (to comet 67P/Churyumov-Gerasimenko), Bepi Colombo (to Mercury), and Hera (heading to the Dimorphos-Didymos asteroid system) - believes that the most plausible scenario is that the same as with Apophis will happen. That is, they will end up ruling out the collision risk. "Probably, at some point, the probability of impact will drop to zero. That's what we expect to happen," he states.
In the surveillance of potentially hazardous asteroids, another critical threshold has been established: if the collision probability of an asteroid reaches 10%, the level of attention would shift from notification to a potential impact warning. In such a case, the IAWN group would issue a communication to the United Nations, asking all potentially affected areas to prepare for possible civil protection actions.
In that phase, the Planetary Defense Office he leads from Italy, part of the Earth Observation Center (ESRIN) of ESA, would play a prominent role. Observing dangerous asteroids, providing data, and mitigating - that is, proposing measures to protect the Earth's population when necessary - are the three fundamental missions of this department, where physicists like Moissl work, as well as engineers and mathematicians.
As you may have noticed by now, the world of asteroids is a world of probabilities and uncertainties. "Every day, celestial objects collide with Earth, although the vast majority are very small and completely harmless," he points out.
On average, every 15 days, an asteroid one meter in diameter impacts somewhere on our planet.
Every 15 years, one of 10 meters.
Every 2,500 years, one of 50 meters.
And every 10,000 years, one of 100 meters.
The asteroid that ended the dinosaurs 66 million years ago far exceeded any of these sizes because, as Moissl recalls, it was about 10 kilometers in diameter: "It had a global impact on the climate and all life on Earth. It has nothing to do with 2024 YR4, which at most would threaten an area equivalent to a city.