Space News Deep Dive: Incoming Asteroid (or Not?)

You know, we in the space news community really thought the big asteroid news this week would be the latest results release from the sample from the asteroid Bennu that was returned to Earth by the OSIRIS-REx mission in 2023. That, obviously, has not turned out to be the case thanks to a cosmic interloper currently known only as 2024 YR4 (I’m betting this thing gets a more charismatic name on the quicker side. Or at least I hope so).

You’ve probably already heard the news: current data gives this rock a roughly 1-in-83 chance of hitting Earth on December 22, 2032. You’ve probably (hopefully) also heard the reasons why this isn’t as scary as it initially sounds. If not, or if you have and just want some more details, or if you’re just super into space rocks (which I very much get), I’m going to run you through it.

Let’s talk about 2024 YR4 and what might happen.

Vital Statistics

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Let’s start with the basics. 2024 YR4 was on December 27^th, by the Asteroid Terrestrial-impact Last Alert System (ATLAS). It had actually made its closest approach to Earth two days before, on December 25^th, coming within about 515,000 miles of us.

It’s still new to us, so there’s a lot we still don’t know about this rock. We know how bright it looks in our telescopes and roughly its distance, but we don’t know if it’s something smaller and more reflective or something bigger and less reflective. We can use what we see to narrow down the potential size range. YR4 is thought to be somewhere between 130-300 feet (40-100m) across. 

We also don’t know exactly what it’s made up of. Rock, yes, because it’s an asteroid, but a lot of asteroids have big amounts of other stuff mixed in, such as metals. A couple of telescopes have been able to take some spectra of it, and at this point our best guess is that YR4 is an S-type asteroid. The S stands for “stony” but these types of asteroids have healthy amounts of nickel and iron mixed in. S-types also tend to be brighter for their size, so if YR4 is an S-type, it actual size is more likely to be at the smaller end of the estimated range.

The current orbit model for YR4, based on the observations we have, suggest a very elliptical (oval-shaped) orbit that takes about four years to complete.

Okay, that’s the rock we’re dealing with, now what’s the fuss about?

Uncertainties

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When we find a new asteroid we observe it as much as we can and use these observations to build a rough orbit model. That early model is usually correct in the broad strokes but has a level of uncertainty that builds up if you use it to try and figure out where the asteroid will be a few years down the road. Our broad strokes orbit model for YR4 means that we have an area of space where we know it will be on December 22, 2032, but we don’t know exactly where in that area it will be.

The problem is that this area includes the spot where Earth will be on December 22, 2032. If it turns out that the exact spot in that area where YR4 will be on that date is the same spot as Earth, YR4 will hit us. The odds of Earth and YR4 being in the same spot at the same time on that date is currently calculated at 1 in 83, or about 1.2%. 

Now if you’re a betting type, you’re probably rolling your eyes right now. That is the definition of long shot odds if you’re talking about something like a team’s chances to win the World Series. But that’s not how threat assessment goes when it comes to space rocks. Odds like these are concerning and get a rock a bunch of attention. A 1.2% impact chance will, in fact, get you a score of 3 on the Torino Impact Hazard Scale, and that is…well, it’s higher than normal.

Grand Torino

If you’ve never heard of the Torino scale, that’s hardly surprising. It’s a threat assessment scale for asteroids that’s only been in use since 1999. At the moment every rock we know about (with the obvious exception of YR4) is at a 0 on the scale—no threat. That’s either because they’re not going to hit Earth or would do absolutely nothing but cause a pretty streak in the atmosphere if they did.

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The different levels are determined both by the certainty level of the impact and the amount of damage an impact would do. The scale goes up to 10, which is a certain collision that will wreck civilization as we know it. A level 3 is considered “merits attention”, indicating a still low-level threat from a rock that could potentially do a bit of damage (more on that later).

Only one asteroid has ever rated higher on the Torino scale than YR4, the asteroid Apophis. When we first found Apophis in 2004 it was determined to have a 2.7% chance of hitting Earth in April 2029. As it is substantially larger than YR4, this earned it a 4 on the Torino scale, the highest any asteroid has ever reached. Today, however, Apophis is rated at a 0. 

That’s because an asteroid’s Torino scale rating can and frequently does change (always downward thus far). As we observed Apophis more and more, we were able to refine our model of its orbit to the point where it became absolutely certain on April 13, 2029 Apophis will miss Earth. It won’t miss by much—it’s going to shave by us less than 20,000 miles (30,600 km) away—but since Apophis is neither a horseshoe nor a hand grenade, almost hitting us doesn’t matter.

And this is very likely going to happen to YR4 as well. As we learn more about it, that 1.2% chance of impact will mostly probably drop to zero, and so will its rating on the Torino scale.

Throwing DARTs

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But suppose it doesn’t. Just suppose we’re a little too unlucky and it turns out our big rock and this small rock will, in fact, be in the same place at the same time. We may have options.

In 2022 we smacked an asteroid with a spacecraft. The mission was called DART, Double Asteroid Redirection Test. It was essentially a mass with some engines and a camera on board and we whacked it hard into the asteroid Dimorphos to see if we were capable of changing Dimorphos’ trajectory. 

And we did! By even more than we thought we might, in fact. So the DART mission proved we’re able to alter the path of something at least the mass of Dimorphos, and the earlier we can hit it the more we can divert it. And Dimorphos is larger than YR4. We don’t know how much more massive it is (again, we don’t fully know what YR4 is made up of), so we can’t yet calculate how much a DART-like impact would push YR4, but it wouldn’t have to push it that much.

So if it turns out the odds are not in our favor, we might be able to change them and actually move YR4 out of the way of the Earth. Boy do the dinosaurs wish they’d had that option.

Worst Case

But let’s say, for the sake of argument, we’ve lost the odds game and for one reason or another no DART-like mission occurs. Let’s say YR4 hits us. How bad are we talking?

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Not that bad, at least when looked at from a global perspective. Anything within, say, 30 miles (50 km) of the impact site would be a very bad place to be. That’s the death zone. That doesn’t sound very far, but it encompasses an area of over 2,000 square miles (nearly 8,000 square km). Even that, though, could be not much of a deal depending on which 2,000 square miles you’re talking about.

In 1908 something roughly the size of YR4 entered the atmosphere over Tunguska, Siberia. There’s a lot we don’t know about what this object was made of, what angle it came it at, and just how fast it was going, but some combination of those variables mean it didn’t make it all the way to the ground but exploded in the atmosphere several miles up. 

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The explosion measured between 3-50 megatons. Hundreds of square miles of forest were flattened. Entire reindeer herds (and a few people? Maybe? The record is very uncertain. We are talking 1908 Siberia after all) were vaporized. This would have been an 8 on the Torino scale, if it had existed back then.

And…nobody really noticed, apart from those just outside the immediate area. Those monitoring seismic waves assumed there had been an earthquake somewhere. Some observatories noticed an increase in dust in the atmosphere. But because it happened at such a remote site, the effect on the functioning of the rest of the world was nil.

If YR4 were to hit Earth, and hit the atmosphere above a major population center, that is A Big Problem. If it hits the ocean, that’s A Possible Problem (speed and angle matter!). If it hits somewhere in the middle of nowhere (and again, for all we’re used to cities and towns, most of Earth is the middle of nowhere) then it’s…Not That Much Of A Problem. 

Playing the Odds

The tricky bit here is that it may take us a while to refine our orbit model for YR4. We found it after it had made its closest approach to Earth. It’s heading away from us, nearly in a straight line, which makes measuring fine details of its movement hard. It’s small enough that soon it’s going to become nearly impossible for telescopes to spot.

If we can’t refine our orbit models enough before it fades from view, we’ll have to wait until it swings back close to Earth again, which will be in 2028. At that point we’ll definitely be able to tell if YR4 is an actual threat or not and whether it’s time to scramble DART 2.0.

But again, by far, by far, so very much by far the most likely scenario is that this is going to be the second coming of Apophis (the asteroid, not the ancient Egyptian god of chaos for which it’s named): we’re going to find with additional observations that this rock will sail right past us. We can all wave at it as it goes by. I’ll bring the popcorn.