Normally when I write about some asteroid that’s making headlines due to a potential impact with Earth, the odds are so low that I can comfortably say, no sweat. It’ll miss.

To be clear, this is still true with the newly found space rock called 2024 YR4. The thing is, the chance of an impact isn’t vanishingly small. While the odds are still pretty long, they’re not zero.

To be precise, there’s a 1.2% chance of an impact in December 2032. That’s a 1-in-77 possibility. If I were playing poker with those odds I’d fold — that’s essentially a 99% chance it’ll miss! But the stakes are a bit higher here, so astronomers are keeping an eye on it. If it helps, I’m not worried about it hitting us, but still, it would be nice to be certain.

2024 YR4 was discovered using the Asteroid Terrestrial-impact Last Alert System, or ATLAS, a series of 0.5-meter telescopes across the world; YR4 was found by the one in Chile. Its orbit is mildly elliptical, taking it a bit closer to the Sun than Earth, and then back out most of the way to Jupiter. It takes about four years to go around the Sun once.

We can’t directly measure the size of asteroids like this (yet), but given its distance, apparent brightness, and taking a guess at how reflective it is it’s estimated to be 40 – 90 meters in size. That’s bigger than I’m comfortable with for a potential impactor. The one that exploded over Chelyabinsk, Russia in 2013 was 19 meters in diameter, so this one is quite a bit bigger than that. The asteroid that slammed into Earth 50,000 years ago and created Meteor Crater in Arizona was probably 50 meters across. Now, that one was mostly metal, so the impact was more energetic than what we’d expect from one made of crumbly rock, which is likely what YR4 is like. But still, the energetic yield from such an event would be similar to a thermonuclear bomb.

YIKES! So hey, why am I not too concerned then?

It goes beyond just the long odds of impact. Those are the odds now. Almost certainly — and by this I mean I would happily bet a nice chunk of change on it — the chance of an impact will go down with time. Why?

Knowing whether or not an asteroid will hit us boils down to how well we know the orbit it’s on. We cannot know that perfectly, but there are ways of nailing it down pretty well.  The problem is that any small uncertainty in the calculations now propagates into the future, getting larger, and if the uncertainties are large enough we don’t know where the asteroid will be exactly at some later date.

As I’ve written a bazillion times before, it’s like a baseball game. You’re in the outfield, and the pitcher winds up and throws the ball. Now imagine that as soon as you see the batter hit the ball, you have to close your eyes and move to where the ball will come down so you can catch it. You can make a decent guess where it’ll fall, but nowhere near well enough to make it hit as small a target as your mitt. The lesson here is that the longer you can keep your eye on the ball, the better you can determine its trajectory, right down to when you catch it.

Same with asteroids. When a rock is first discovered there may be only a few observations of it, and the orbit determination is shaky at best. We need more observations with time to be able to calculate its path better (sometimes we get lucky, too, and we can trace the orbit backward in time and see if it shows up on any images taken years ago; that extends the baseline enormously and allows much better math). As I write this, the “observation arc” for YR4 is only about a month, so the orbit is still a little uncertain.

What that means is that the calculated path of the asteroid looks like a fuzzy cone extending away from its current position, with the mouth of the cone getting wider the farther ahead we try to look. In seven years it could be anywhere in a big volume of space, and it just so happens that Earth occupies a small part of that space. That’s why the chance of impact is 1.2%; that’s how much Earth occupies of this “probability space”.

As time goes on and we get more observations, that probability cone narrows. Earth is near the edge of it, so very likely any sharpening of that cone will leave Earth outside that volume of space, and the asteroid misses. This happens all the time with asteroids that have a small chance of hitting us. It’s almost certainly going to happen here, too.

But, well, to be honest it might not. It’s possible we’ll get hit, though unlikely. That wouldn’t be good. If the asteroid is a rubble pile — it’s made of crumbly rock, like the 2013 Chelyabinsk impactor — it’ll likely explode high off the ground as the immense pressure of its hypersonic passage through our air compresses and heats it. That creates an airburst, with a powerful shock wave that will pass through the air and slam into the ground. The pressure from this wave would be strong enough to knock trees down and, should it happen near a populated area, topple buildings. The heat would be enough to start fires, too.

This will sound familiar if you know about the Tunguska blast, which was probably from a similar 50-meter rubble pile asteroid that exploded over remote Siberia in 1908. It flattened trees for hundreds of square kilometers.

So yeah, that would be bad. But the good news here, again, is that it’ll almost certainly miss.

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