At this point in history, astronomers and engineers who grew up watching “Deep Impact” and “Armageddon,” two films about the destructive power of asteroid impacts, are likely to find themselves in relatively high positions at space agencies. Don’t Look Up also provided a more modern, if more pessimistic (or unfortunately more realistic?) look at what could potentially happen if a “killer” asteroid is found approaching Earth. So far, life has not imitated art when it comes to what could be one of the most catastrophic events in human history, but most space enthusiasts agree it’s worth preparing for when the time comes. A new paper by Maxime Devogèle of ESA’s Near Earth Object (NEO) Coordination Center and his colleagues, available as a preprint on arXiv, analyzes a test run that took place about a year ago with the discovery of asteroid 2024 YR4.
The article walks through the process of discovery, classification, escalation, scientific response and finally de-escalation that the asteroid went through in a matter of months. While the asteroid early warning system worked largely as intended, it’s worth going through this very first scenario for this particular asteroid type, as it certainly won’t be the last.
2024 YR4 was in the headlines from early January 2025, although it was discovered by the ATLAS survey on December 27th. Over the next month, additional observations actually increased the probability of an impact, unlike 99.9% of other newly discovered asteroids.
Fraser discusses how we can find killer asteroids.
One of the key aspects of the paper is the discussion of the Turin Scale. This scale was originally developed in 1995 by Dr. Richard Binzel of MIT and later updated in June 1999 at a conference in Turin, Italy. It is expressly designed to classify asteroid impact threats into one of eleven categories. Importantly, each category is based on a risk assessment of both the likelihood of impact and the potential harm it may cause. The categories are:
Scale 0 (White) – No Danger – The asteroid that will either miss us or burn up in the atmosphere. Scale 1 (Green) – Normal – Where most asteroids are classified when they are first discovered. Impacts are “extremely unlikely” and are usually downgraded to 0 within a few days. Scale 2 (Yellow) – Attention worthy – Typically involves a “close” but non-threatening pass, but a very unlikely impact. This happens relatively rarely. Scale 3 (Orange) – Concern – The chance of them hitting Earth and causing “local” destruction is over 1%. A good analogue would be the Tuskunga event in the early 20th century. This is the rating that the 2024 YR4 eventually developed into. 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. Never before has an asteroid been ranked so high. Scale 8-10 (Red) – Definite Collision – At this level, astronomers are certain the asteroid will hit us – each scale number only increases the potential destruction it will cause.
On January 27, 2025, YR4 was officially listed as 3 on the Torino Scale and finally reached an impact probability of 3.1% on February 18. This made it the first asteroid ever to reach this level of the scale. It’s important to note that this wasn’t technically the highest rank ever achieved on the scale – that distinction goes to the asteroid Apophis, which famously reached level 4 on the Turin Scale in 2004. In fact, Apophis could never have been classified as 3 on the Turin Scale, as its larger size would have caused regional rather than local devastation – but it is also important to note that the threat level was downgraded much more quickly than that of 2024 Year4.
The second part of planetary defense is stopping asteroids, as Fraser explains here.
Even the Scale 3 rating was enough to trigger the first official notification from the International Asteroid Warning Network – which did not exist at the time of Apophis’ discovery in 2004, but was only launched in 2014 after the Chelyabinsk asteroid explosion in 2013. Having reached this high up the scale, the discussion of 2024 YR4 shifted from purely scientific fact-finding to a broader consideration of a threat to potentially large swaths of humanity.
The system worked exactly as intended – it attracted attention. Certainly a lot from the public and even some policymakers. But perhaps more importantly, it caught the attention of more astronomers. The increased interest brought additional resources, most notably Directors’ Discretionary Time for some of the world’s most powerful telescopes.
When those telescopes, which included the Catalina Sky Survey, the Gran Telescopio Canarias and the Very Large Telescope, began to focus their optics on the potential threat, it looked a lot less imposing. By early March, these giants had characterized many of the asteroid’s crucial features, such as its rotation and taxonomy. With a rotation period of just 19.5 minutes, it spun much faster than typical “debris pile” asteroids. And it was classified as either an Sq-type or K-type asteroid, although there is still some debate on this as its albedo appeared to vary depending on the observation platform used.
This final clarification will be crucial because although the threat of an impact on Earth has decreased, the threat of an impact on the Moon has actually increased over the course of observations earlier this year. Currently, 2024 YR4 has about a 4% chance of hitting the Moon in 2032, which could cause widespread destruction to satellites orbiting Earth due to its debris cloud. Therefore, the planetary defense community’s job is not yet done – but so far it appears to be working exactly as intended. One day there will be a real need to galvanize the international response to a threatening asteroid – and hopefully the response will be better than the one portrayed in the movies.
Learn more:
Maxime Devogèle et al. – Rapid response characterization of near-Earth asteroid 2024 YR4 during a Turin Scale 3 alert
UT – Remember the asteroid that won’t hit Earth? We could send a mission to explore it!
UT – Webb scans asteroid 2024 YR4, which is 60 meters in diameter
UT – Yes, the chance of an asteroid hitting Earth has doubled. No, you don’t need to worry
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