The asteroid 2024 YR4 is still in the news, but this time the news is better: the chance of an impact (as I write this) is now less than 1%. It’ll very likely drop even more when JWST observations are made in March.

Not to say I told you so, but I told you this would happen. It usually does.

In earlier articles I’ve written that due to uncertainty in the exact motion of the asteroid now, its possible location in the future is inside a cone with the vertex at the asteroid and spreading out ahead of it. If the Earth is inside the cone then there’s a chance of impact, though usually a small one (the Earth occupies a very small fraction of that cone). 

That’s true in a general sense, but in some cases we can do better. We can nail down some aspects of the orbit (say, its tilt relative to Earth’s) while other aspects are less well known. That can flatten the cone, for example.

As the orbit is better quantified, the possibility of impact can actually go up, because the Earth may still be occupying some “probability space” for the asteroid’s location at a future date. If that space shrinks but the Earth is still inside it, the impact chance goes up. But at some point that space may shrink so much that Earth is outside it, and the chance goes down.

That’s where we are now. This graphic from the European Space Agency can help show you how this works.

The graphic shows Earth as a blue dot with the Moon’s orbit around it (which is about 770,000 km wide). The red dots show possible locations of the asteroid 2024 YR4 when it gets closest to Earth in December 2032. They are calculated using the “Monte Carlo method”: taking the known shape of the asteroid orbit and running it forward in time to get its position, then changing an aspect of it (like its tilt or eccentricity) a little bit (informed by the uncertainties in the measurements we have now) and running the simulation again. This done over and over to get a suite of possible positions at some later date.

The yellow dot is the most likely location of the asteroid’s closest approach given the observations that had been made up until the date shown at the top. As time goes on and we nail down the orbit better, the future locations of the asteroid shrinks, until eventually the Earth is no longer in the path. But you can see that for a time the range of the dots shrinks but Earth is still inside them, so the chance of impact goes up!

This is how it usually works. The chance of an impact goes up for a time, then suddenly drops. The current estimate of impact as I write this is 0.36%, or 1 in 280. Put another way, that’s a 99.64% chance it’ll miss. Those are good odds.

Interestingly, it will pass close to the Moon as well, though how close is not perfectly known either. The nominal (most likely) miss distance is about 30,000 km, but the currently calculated minimum possible distance is only 1,500 km! That’s close. Though unlikely, I wouldn’t rule out a lunar impact just yet.

If it hits the Moon that would be interesting. We’re not sure how big 2024 YR4 is; it might be anywhere from 40 to 90 meters across — the JWST observations coming soon will help narrow that range. But either way it’ll be an energetic event.

It would create a very large flash of light that would last a few seconds as the energy of motion is converted to light, but after that fades the impact spot might glow for a while, certainly in infrared since it’ll be hot. It would make a crater a kilometer or two wide, which on Earth would be devastating but on the Moon would quickly get lost among the millions of other craters that size. There could be some Moon rocks ejected from the event, but very little of that would make it to Earth. It would also likely be far smaller than the asteroid itself and moving much more slowly, so they would be much less dangerous than from the asteroid itself hitting Earth.

What if we do find out with further observations that it will hit us? That’s a good question. We know it’s possible to hit asteroids with space probes; we’ve done it more than once (the DART mission recently, but also the Deep Impact mission of 2005). Like hitting a quarterback and knocking them out of bounds, hitting the asteroid could push it into a non-impacting trajectory, either by moving it to the side or slowing it down (or speeding it up) a bit so that it passes after (or before) the Earth occupies that same volume of space. It’s like crossing a street with a car coming; you can run faster to get across before the car passes, or slow down and wait for the car to pass first.

It’s not clear we can do this with 2024 YR4 though. Eight years isn’t much time to put a mission together. Mind you, it’ll pass us again in 2028, and we’ll have many more observations then (plus a much longer time baseline) so that’ll nail down the orbit for sure. If it’s still looking like it’ll hit, well. The damage it can do is large — the energy released on impact could be equivalent to dozens of megatons — but localized, much like the Chelyabinsk impact in 2013. Given the rock’s smallish size it would likely be an airburst, like the Tunguska impact of 1908. The good news is that a lot of its path will be over the ocean, but that path is also over South America, Africa, and India, with a lot of heavily populated cities. An impact there would be bad.

At the moment I know some asteroid defense folks (like my friends at the B612 Foundation) are taking the chance of impact seriously, even though it’s very small. If it were just money at stake I’d be willing to bet a fair amount on a miss, but when lives are at stake, better safe than sorry.

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