'Potentially hazardous' asteroid 2024 YR4 was Earth's first real-life planetary defense test

An illustration of an asteroid in outer space — (Image credit: NASA, ESA, CSA, N. Bartmann (ESA/Webb), Martin Kornmesser (ESA), Serge Brunier (ESO), Nick Risinger (Photopic Sky Survey))

At this point in history, astronomers and engineers who grew up watching Deep Impact and Armageddon, two movies about the destructive power of asteroid impacts, are likely in relatively high ranking positions at space agencies. Don't Look Up also provided a more modern, though more pessimistic (or, unfortunately, realistic?), look at what might potentially happen if a "killer" asteroid is found on approach to Earth. So far, life hasn't imitated art when it comes to potentially one of the most catastrophic events in human history, but most space enthusiasts agree that it's worth preparing for when it will. A new paper, available in pre-print on arXiv, from Maxime Devogèle of ESA's Near Earth Object (NEO) Coordination Centre and his colleagues analyzes a dry run that happened around a year ago with the discovery of asteroid 2024 YR4.

The paper walks through the process of discovery, classification, escalation, scientific response, and eventually de-escalation that the asteroid went through in a matter of months. While the asteroid early-warning system largely worked as intended, reviewing this first-ever scenario for this particular type of asteroid is worthwhile, as it surely won't be the last one.

One of the key aspects of the paper is its discussion of the Torino scale. Originally developed in 1995 by Dr. Richard Binzel of MIT, and later updated in June 1999 at a conference in Torino (Turin) Italy, this scale is explicitly designed to classify asteroid impact threats into one of eleven categories. Importantly, each category is based on a risk assessment of both it's probability of impact and also the potential damage it can cause. The categories are:

Scale 0 (White) - No Hazard - The asteroid with either miss us or burn up in the atmosphere
Scale 1 (Green) - Normal - Where most asteroids are classified when they are first discovered. Impact is "extremely unlikely", and they are typically downgraded to Scale 0 within a few days.
Scale 2 (Yellow) - Meriting Attention - Typically involves a "close" but not threatening pass, but very unlikely impact. This is relatively rare
Scale 3 (Orange) - Concerning - These have a greater than 1% chance of hitting Earth and causing "localized" destruction. A good analog would be the Tuskunga event in the early 20th century. This is the rating 2024 YR4 eventually grew to.
Scale 4 (Orange) - Threatening - Still a greater than 1% chance, but with "regional" devastation rather than localized.
Scale 5-7 (Red) - Threatening - A "credible threat" of regional to global devastation. No asteroid has ever been categorized this high.
Scale 8-10 (Red) - Certain Collision - At this level, astronomers are sure the asteroid will hit us - each scale number simply increases the potential destruction caused when it does.

On January 27th of 2025, 2024 YR4 was officially listed as a 3 on the Torino scale, eventually winding up with a 3.1% percent impact chance on February 18th, and making it the first ever asteroid to reach that level of the scale. It's important to note that, technically this wasn't the highest ever rank on the scale - that distinction goes to asteroid Apophis, which famously reached a Level 4 on the Torino scale in 2004. In fact, Apophis could never have been classified as a 3 on the Torino scale, because its larger size would have caused regional rather than localized devastation - but, it's also important to note that the threat level was downgraded much more quickly than that of 2024 YR4.

Even the Scale 3 rating was enough to cause the first ever official notification of the International Asteroid Warning Network - which didn't exist when Apophis was discovered in 2004, having come into existence in 2014 after the Chelyabinsk asteroid explosion in 2013. After hitting that high on the scale, the discussion about 2024 YR4 moved from pure scientific fact-finding to wider consideration of a threat to potentially large parts of humanity.

As these telescopes, which included the Catalina Sky Survey, the Gran Telescopio Canarias, and the Very Large Telescope began to turn their optics towards the potential threat, it began to look far less imposing. By early March, these behemoths had characterized many of the asteroid's critical features, such as its rotation and taxonomy. It spun much faster than typical "rubble pile" asteroids, with a rotational period of only 19.5 minutes. And it was classified as either a Sq-type or K-type asteroid, though there is still some residual debate on this point, as its albedo seemed different depending on what observational platform was used.

That final clarification will be a critical feature, as, though the threat of an impact to Earth decreased, the threat of an impact to the Moon has actually increased over the course of the observations earlier this year. As of now, 2024 YR4 has a ~4% chance of hitting the Moon in 2032, which could cause widespread destruction amongst satellites orbiting Earth from its debris cloud. Therefore, the planetary defense community's job isn't yet done - but it seems to be operating exactly as intended so far. Someday it will truly be needed to kick the international response to a threatening asteroid into gear - and hopefully the response turns out to be better than that depicted in the movies.

Andy has been interested in space exploration ever since reading Pale Blue Dot in middle school. An engineer by training, he likes to focus on the practical challenges of space exploration, whether that's getting rid of perchlorates on Mars or making ultra-smooth mirrors to capture ever clearer data. When not writing or engineering things he can be found entertaining his wife, four children, six cats, and two dogs, or running in circles to stay in shape.