EXPERT REACTION: Global defence plan triggered as asteroid has small chance of hitting Earth in 2032

It is fantastic that as our telescopes improve we are able to get early detections of many more Earth-threatening asteroids.  They have always been out there, but we lacked an early-warning system.  Now with the amazing power of modern telescopes doing dedicated surveys of the sky we will be able to see the potential threats that are out there, and perhaps take action to avoid dangerous impacts. 
 
This asteroid is too small to cause a major threat, and there’s a 99% chance it will miss us, but it is worth monitoring because it could cause some local damage if it hits.  Once astronomers watch it for several months they will have detailed information about its trajectory and be able to give a more confident estimate of how safe we are.

2024 YR4 is a Near-Earth Object (NEO), which means that its orbit brings it to within 1.3 AU at perihelion, the orbit’s closest point to the Sun (one AU is one Astronomical Unit, the average Earth-Sun distance). This means that a NEO might, at some time, approach the Earth to within 0.3 AU.

2024YR was discovered on December 27th 2024, by ATLAS, a NASA-funded telescope network – developed by the University of Hawaii - that scans the sky, looking for asteroids. 

Whilst there was a very close approach of 2024 YR4 in December 2024 (two days before discovery), with a current estimated orbital period of 4.05 years, the Earth and this asteroid do not always come near every year, or even every orbit of 2024 YR4.

As more positional observations for the asteroid become available over time, its orbital characteristics will be refined, allowing us more accurately to predict its position in the future.

At present, it’s estimated that there is a ~1.3% chance of a collision in December 2032 (the same as a 98.7% chance of a miss) – but this is based on our current knowledge of its orbit, which will change with time. There is a good chance that the impact probability will be reduced, but we will have to wait to find out.

A discovery of this nature highlights the essential work being done by planetary defence programs such as ATLAS.

More observations of the asteroid will eventually refine the orbit enough to give us some clarity it will completely miss us in 2032 (most likely scenario), or potentially hit us. The problem is that small asteroids like this are bright enough only when they come close to the Earth and this one is moving away from us, fast. In two months time, even the biggest telescopes available to us will struggle to see it. So we need more observations, but crucially we need more observations spaced in time to precisely predict the asteroid's course. The asteroid came close to us in 2016, when it remained undetected, but there is a chance that it could still be in telescope archival images from that time. If we do not find archival images, then it is possible that we cannot sufficiently refine its orbit until 2028, the next time it comes close to the Earth. In the unlikely event that it is on a collision course to Earth in 2032, the asteroid will have an explosive energy comparable to the Tunguska impact in 1908. The atmospheric explosion caused by the asteroid in Tunguska caused local devastation, flattening a large forest, and also possibly killed people.

Even if unlikely, this asteroid could crash somewhere on Earth with potentially devasting consequences, even if only local. If it is headed for Earth, we’ll need a plan to prevent a collision, such as pushing it off course or even destroying it. But this plan will need to rely heavily on knowing what the asteroid is made of and its structure: you don’t deflect asteroids that are made of a pile of rubble loosely bound together the same way you do with the ones made of a solid chunk or rocks. The same applies if part of the asteroid is made of metal. So detailed observations are key, with potentially sending a spacecraft to take accurate measurements. Hopefully, with only 1 per cent chance of impact, it won’t come to this.

Asteroid 2024 YR4 is significant. Not because it is likely to actually hit Earth - with current predictions suggesting it will most likely pass us by without incident – but because it highlights the need for a global strategy for other asteroidal and cometary impact hazards that might. Planetary defense technologies have been developed that could redirect an asteroid on an impact trajectory, but the ethico-legal governance issues for if and how we should deploy such systems have not been settled. NASA’s Double Asteroid Redirection Test (DART) mission demonstrated proof-of-concept for kinetic impactor technologies in 2022, colliding with the Dimorphos asteroid and successfully altering its path. But whose responsibility is planetary defense on a global scale? Can we decide in advance what we should do in the face of a future threat? Asteroid 2024 YR4 reminds us that questions of international governance for planetary defense systems have not been answered, and that addressing them will require global cooperation and collaboration efforts in ethics and policy for guiding human interactions with the space environment.

The first thing to stress here is - don't panic! Whilst this sounds like a scary story, it's actually a really good example of how the search for potentially dangerous asteroids works. The asteroid in question - 2024 YR4 - was first discovered about a month ago, and as astronomers have tracked it since, we've gained a better understanding of its orbit around the Sun.

With each new observation that comes in, that orbit gets refined - which is why the estimate of how likely it is to hit Earth in 2024 keeps changing! By far the most likely outcome is that, as more observations are made, it eventually becomes clear the asteroid will miss the Earth in 2032 - but even so, it will still come pretty close! From what we've learned so far, the asteroid is probably comparable in size to that which hit Earth back in June 1908, over Tunguska. That impact leveled a huge area (around 2200 square kilometers) of forest - but that was the extent of the damage it caused.

The vast majority of Earth is empty space - and the bulk of the area at risk from the impact is open ocean - so the most likely outcome of an impact would that it produces a spectacular light show but no-one gets hurt. We'll know more in 2028, when the asteroid flies by the Earth again - and observations then will allow us to learn more about the potential impact risk, and to begin planning should it turn out that the impact is going to happen.

While the chances of the asteroid colliding with Earth are currently just under 1%, active monitoring of this asteroid should increase our knowledge of its trajectory. This means that the chances of the collision are going to be more precise.

Normally, the chances would go down as the asteroid gets closer because we have better ability to monitor and calculate its trajectory with a better precision. In an unlikely event of a collision, there is a potential for civil damage at potentially city-wide scale if it happened in an urban area. For comparison, the Chelyabinsk meteor that exploded in Russia about 10 years ago was approximately a fifth of this asteroid size.