Planetary Defense

This week we talk about DART, extinction events, and asteroid 2024 YR4.

We also discuss Bruce Willis, Theia, and the Moon.

Recommended Book: Exadelic by Jon Evans

Transcript

In the 1998 action flick Armageddon, an asteroid the size of Texas is nudged into a collision course with earth by a comet, and NASA only notices it 18 days before impact.

The agency recruits a veteran oil driller, played by Bruce Willis, to fly out to the asteroid and drill a hole in it, and to detonate a nuke in that hole, which should destroy it before it hits earth, which undetonated, that rock not broken up ahead of time, would wipe out everything on the planet. It’s a fun late-90s flick loaded with some of the biggest names of the era, so I won’t ruin it for you if you haven’t seen it, but the crux of the plot is that there’s a lot going on in space, and at some point there’s a chance one of these big rocks hurling around in the void will line up just right with earth’s orbit, and that rock—because of how fast things move in space—would hit with enough force to wipe out a whole lot of living things; perhaps all living things.

This film’s concept was predicated on historical events. Not the oilmen placing a nuke on a rogue asteroid, but the idea of an asteroid hitting earth and killing off pretty much everything.

One theory as to how we got our Moon is that an object the size of Mars, called Theia, collided with Earth around 4.5 billion years ago. That collision, according to some versions of the so-called “giant impact hypothesis,” anyway, could have brought earth much of its water, as the constituent materials required for both water and carbon based life were seemingly most prevalent in the outer solar system back in those days, so this object would have slammed into early earth, created a disk of debris that combined that early earth’s materials with outer solar system materials, and that disk would have then reformed into a larger body, earth, and a smaller body, the moon.

In far more recent history, though still unthinkably ancient by the measure of a human lifespan, an asteroid thought to be somewhere between 6 and 9 miles, which is about 10 to 15 km in diameter hit off the coast of what is today Mexico, along the Yucatan Peninsula, killing about 70% of all species on earth.

This is called the Chicxulub Event, and it’s believed to be what killed the dinosaurs and all their peer species during that period, making way for, among other things, early mammals, and thus, eventually, humans.

So that was an asteroid that, on the low end, was about as wide as Los Angeles. You can see why those in charge back in the 90s tapped Bruce Willis to help them handle an asteroid the size of Texas.

Thankfully, most asteroid impacts aren’t as substantial, though they can still cause a lot of damage.

What’s important to remember is that because these things are moving so fast, even though part of their material will be burnt up in the atmosphere, and even though they might not all be Texas-sized, they generate an absolutely boggling amount of energy upon impact.

The exact amount of energy will vary based on all sorts of things, including the composition of the asteroid , the angle at which it hits, and where it hits; an oceanic impact will result in a whole lot of that energy just vaporizing water, for instance, while a land impact, which is less common because a little more than 70% of the planet is water, will result in more seismic consequences.

That said, an asteroid that’s about 100 meters in diameter, so about 328 feet, which is a lot smaller than the aforementioned 6 to 9 mile asteroid—a 100 meter, 328 foot object hitting earth can result in a force equivalent to tens of megatons of TNT, each megaton equaling a million tons, and for comparison, the atomic bombs dropped on Hiroshima and Nagasaki at the end of WWII ranged from 15,000 to 21,000 tons of TNT, mere kilotons. So a 100 meter, 328 foot asteroid hitting earth could generate somewhere between a few hundred thousand and a few million atomic bombs’ worth of energy.

None of which would be particularly devastating on a planetary scale, in the sense that the ground beneath out feet would barely register such an impact. But the thin layer of habitable surface where most or all of the world’s life exists, certainly does. And that’s the other issue here, is that on top of even a relatively small asteroid being a city-killer, wiping out everyone and everything in a large area around where it strikes, it can also cause longer-term devastation by hurling a bunch of water and soil and detritus and dust and ash into the atmosphere, acting as a cloak around the planet, messing with agriculture, messing with growth patterns and other cycles for plants and animals; the water and heat cycles completely thrown off. All of which can cause other knock-on effects, like more severe storms in unusual places, periods of famine, and even conflict over scarcer resources.

What I’d like to talk about today is a recently discovered asteroid that is being called a potential city-killer, and which is raising alarms in the planetary defense world because of its relatively high likelihood of hitting earth in 2032.

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Asteroid 2024 YR4 is thought to be around 130-300 feet, which is about 40-90m in diameter, and it has what’s called an Earth-crossing, or Apollo-type orbit. Asteroids with this type of orbit won’t necessarily ever intersect with earth, and some are incredibly unlikely to ever do so. But some relatively few of them, that we’re aware of, anyway, have orbits that periodically get really close to earth’s, to the point that even a small tweak to their orbit, caused by gravitational perturbances or maybe being nudged by something else in space, could put them on course to cause a lot of damage.

Global astronomical bodies keep tabs on these sorts of asteroids, and they keep an especially close eye on what are called PHAs, or potentially hazardous asteroids, because they are objects that are close-ish to Earth, are in orbits that could bring them even closer, perhaps even on an intersection path with earth at some point, and they have an absolute magnitude of 22 or brighter, which means they’re big enough to be fairly visible to our instruments, and that generally means they’ll be 500 feet or around 140m in diameter or larger, which puts them in the “will cause severe damage if it hits earth” category.

That latter component of the definition is important, as while the Chelyabinsk meteor that blew up in what’s called an air burst over southwestern Russia in 2013 caused a lot of damage—generating about 400-500 kilotonnes of TNT worth of energy, about 30-times the energy released by the atomic bomb that blew up Hiroshima, resulting in a shock wave that injured nearly 1,500 people sufficiently that they had to seek medical attention, alongside all the broken glass and thousands of damaged buildings caused by that shockwave (which in turn caused those injuries)—that meteor is considered to be pretty tame compared to what we would expect from a larger impact. It was only about 60 feet, around 18m in diameter.

That’s part of why asteroid 2024 YR4 is getting so much attention; it’s more than twice, maybe as much as five times that large, and current orbital models suggest that on December 22, 2032, it has a small chance of hitting earth.

Small is a relative term here, though, both in the sense that the exact likelihood figure keeps changing, and will continue to do so as we’re able to capture more data leading up to that near-future deadline, and in the sense that even very small possibilities that a city-killer asteroid will hit earth is something that we should arguably be worried about, out of proportion to the smallness of the statistical likelihood.

If you are told there’s a 1% chance you’ll die today, that means there’s a 99% chance you won’t, but that 1% chance is still really substantial in the context of living or not living.

Similarly, a 1% chance of a large asteroid impacting earth is considered to be substantial because that means a 1% chance that a city could be completely wiped out, along with all the maybe millions of people living in it, all the plants an animals in the region, too, and we could see all those aforementioned weather effects, atmospheric issues, and so on, for a long time into the future.

At the moment, as of the day I’m recording this, there’s a 2.2% chance this asteroid will hit earth on that day, December 22, 2032. Its likely impact zone, if it were to hit, stretches roughly along the equator, from just south of Mexico, across upper south america and the middle of africa, over to eastern India. If it’s on the larger side of current estimates, it’s possible that its blast could stretch for 31 miles in all directions from where it hits, because it’s a hard object the size of a large building traveling at around 38,000 miles per hour.

So just shy of 7 years, 11 months from now, which is around 2,870 days, that thing could plow into a span of earth that contains quite a few major cities—but it could also hit a stretch of ocean, causing a separate set of problems, ranging from tsunamis to borked weather patterns and loads of sun-concealing, globe-spanning cloud cover.

Again, though, the numbers here are weird because of the things they’re describing. Nearly 8 years is a long time in many ways, but if you’re staring down the barrel of a potentially city-killing asteroid, that begins to feel like not long at all; Bruce Willis only had 18 days, but he also lived in the world of Hollywood fantasy. In real life, spinning up that kind of mission takes a lot longer, and that’s after you settle on who’s going to pay for some kind of asteroid killing or deflecting program, how it’s going to work, and so on.

Fortunately for everyone involved, back in late-2022, NASA launched a project called the Double Asteroid Redirection Test, or DART, which entailed launching a spacecraft that rendezvoused with pair of asteroids with a known trajectory. That spacecraft shot an impacter, basically a little space bullet, at one of the asteroids, which allowed the craft, along with a supplementary satellite, to collect all sorts of data about what happened to the asteroid after it was hit.

The hope was that using this method, launching a craft that shoots space bullets at asteroids, we would be able to reduce the target asteroid’s orbit by 73 seconds, which is an orbital measurement. Instead, it shortened it by 32 minutes, which is way, way more, and generally considered to be a huge success beyond what the mission planners could have hoped for.

Not all of what was learned from the DART mission will be transferable to other possible missions, because asteroids have different compositions, have different spins and speeds, and some will be easier to hit than others, and to hit in a way that would move them beneficially: we want to move them away from a path that lines up with earth’s orbit, not in such a way that a strike becomes more likely.

But this success suggests that it may be possible to basically nudge asteroids away from a collision trajectory with our planet, rather than having to blow the things up with nukes, which would be a far more involved and dangerous undertaking.

We’ve also seen the costs associated with space launches drop dramatically over the past ten years, to the point where launching this sort of mission will cost a fraction of what it would have cost back in the 90s, which is fortunate, as historically governments have shown less enthusiasm for firing space bullets than for firing bullets planet-side, so if worse comes to worse, there’s a chance even a beneficent billionaire, maybe even a millionaire, could fund such a project in a pinch.

At the moment, it’s still overwhelmingly likely that asteroid 2024 YR4 will miss earth in 2032. A 2.2% chance of an impact is worrying, and we’ll hopefully start building the infrastructure we need to deflect such objects sooner rather than later, as even if we don’t end up using said craft this time around, it seems prudent to have those sorts of missions ready to go at a moment’s notice, should we someday find ourselves in an Armageddon situation, with only a few weeks before something really, really bad happens.

That said, even with today’s quite high likelihood, that still means there’s a 97.8% chance it won’t be anything to worry about. We should know a fair bit more by April of this year, after which point this asteroid will be really far away and thus trickier to see until 2028, when it loops back in our direction.

There would still be time to do something about it then, if warranted, but more time is typically better with this sort of thing—again, because we want to be sure any deflection attempt is successfully launched, but also that it deflects it away from us, not toward us. And our best bet to deflect would be during that 2028 close flyby, so it’s likely by April, or just after that, we’ll have some kind of decision by the folks in charge about whether to launch a deflection mission in 2028 or thereabouts.

All of which would be historic, but would also probably be a good idea and a worthwhile investment, wherever this specific asteroid’s path ends up taking it. As our space neighborhood is rich with these sorts of rocks and other astronomical bodies, and because, as our in-space sensory assets have become more numerous and sophisticated, we’ve been able to see just how lucky we are, that we haven’t had more horrible impacts, so far; there’s a lot of stuff flying around out there, and the moon probably helps by taking some of those bullets for us, but even with that extra layer of natural protection, we might want to play a more active role in managing our orbital neighborhood, soon, as it would be really embarrassing to have all this knowledge and these capacities, but to not be able to use them when we need them because we failed to plan ahead.

Update, February 24, 2025: Whew.

Show Notes

https://en.wikipedia.org/wiki/List_of_Earth-crossing_asteroids

https://cneos.jpl.nasa.gov/about/neo_groups.html

https://en.wikipedia.org/wiki/Chelyabinsk_meteor

https://x.com/Astro_Jonny/status/1886742128199336362

https://en.wikipedia.org/wiki/2024_YR4

https://www.nytimes.com/2025/02/08/science/asteroid-yr4-2024-impact-odds.html

https://www.usatoday.com/story/news/nation/2025/02/08/asteroid-hitting-earth-2032-nasa/78322607007/

https://en.wikipedia.org/wiki/99942_Apophis

https://science.nasa.gov/solar-system/asteroids/2024-yr4/

https://www.cbsnews.com/news/asteroid-2024-yr4-chance-hit-earth-what-to-know/

https://blogs.nasa.gov/planetarydefense/2025/02/07/nasa-continues-to-monitor-orbit-of-near-earth-asteroid-2024-yr4/

https://www.nasa.gov/news-release/nasa-confirms-dart-mission-impact-changed-asteroids-motion-in-space/

https://en.wikipedia.org/wiki/Double_Asteroid_Redirection_Test

https://science.nasa.gov/mission/dart/

https://www.space.com/nasa-dart-mission-dimorphos-didymos-asteroid-impact-reshaping

https://www.cnn.com/2024/02/27/world/nasa-dart-dimorphos-impact-scn/index.html

https://en.wikipedia.org/wiki/AIDA_(international_space_cooperation)

https://www.planetary.org/notable-asteroid-impacts-in-earths-history

https://en.wikipedia.org/wiki/Origin_of_water_on_Earth

https://en.wikipedia.org/wiki/Theia_(planet)

https://science.nasa.gov/earth/deep-impact-and-the-mass-extinction-of-species-65-million-years-ago/

https://en.wikipedia.org/wiki/Chicxulub_crater

https://www.lpi.usra.edu/publications/books/barringer_crater_guidebook/chapter_11.pdf

https://en.wikipedia.org/wiki/Armageddon_(1998_film)

https://www.history.com/news/7-major-asteroids-strikes-in-earths-history

https://en.wikipedia.org/wiki/Impact_event

https://world-nuclear.org/information-library/safety-and-security/non-proliferation/hiroshima-nagasaki-and-subsequent-weapons-testin

https://www.astronomy.com/science/earths-greatest-hits-a-history-of-asteroid-impacts/