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A “city-killing” asteroid just zipped by Earth. Why didn’t we see it coming?

NASA tracks big asteroids. Small ones — which can still do damage — are harder to spot.

An asteroid, with Earth in the background
An asteroid that zipped by Earth yesterday could have done substantial damage if it’d entered the atmosphere.
Getty Images/Science Photo Library-Andrzej Wojcicki
Kelsey Piper is a senior writer at Future Perfect, Vox’s effective altruism-inspired section on the world’s biggest challenges. She explores wide-ranging topics like climate change, artificial intelligence, vaccine development, and factory farms, and also writes the Future Perfect newsletter.

On Thursday, an asteroid called 2019 OK, traveling at almost 15 miles a second, came unusually close to impacting Earth. The asteroid passed by about 43,500 miles away — closer to Earth than our moon is. It was one of the closest known approaches of an asteroid to Earth since we started closely tracking the movements of objects in space.

If you had binoculars and knew exactly where to look, you could have briefly seen 2019 OK in the sky.

NASA tracks large asteroids in order to identify any that might be on a threatening trajectory toward Earth. But 2019 OK was first seen a few days ago, and was only definitively identified as an asteroid yesterday — hours before it passed right by us.

How’d they miss it? Well, while 2019 OK could have done a lot of damage if we’d gotten very unlucky — as Swinburne University astronomer Alan Duffy told the Sydney Morning-Herald, the asteroid would have struck Earth with “over 30 times the energy of the atomic blast at Hiroshima” — it’s not actually all that big. The asteroid is estimated to be “between 187 feet and 427 feet in diameter.” The largest passenger aircraft in service today (the Airbus A380-800) is about 240 feet long, so spotting this asteroid would have been a bit like spotting a single big commercial jet in the vast expanse of space — traveling at 15 miles a second and coming toward us directly from the sun, which makes spotting it more difficult.

Even a small asteroid like 2019 OK could potentially do a lot of damage if it’d hit Earth, rather than missing by 43,500 miles, but for it to be, as lots of outlets called it, a “city killer” asteroid, we would have needed several more unlikely things to go wrong. A 45,000-mile near-miss is very close compared to how vast space is, but it’s still a fair bit — Earth itself is about 8,000 miles across. About 0.5 percent of asteroids that come this close or closer will actually hit us.

But what if it had?

My colleague Brian Resnick has written about how an asteroid strike would kill you. For an asteroid this size, damage would come from the air compressed as it flew through — and burned up in — the atmosphere. Here’s a casualties estimate for asteroid strikes from an analysis in the journal Geophysical Research Letters:

Data from Geophysical Research Letters

As you can see, the expected casualties from an asteroid like this one, between 50 meters and 100 meters, are greater than those from most natural disasters — though much smaller than the casualties if a larger asteroid hit.

Of course, “expected casualties” is a slightly misleading way to think about an asteroid at this size, which could have touched down in the ocean, killing no one, or in New York City, killing lots more than 10,000 people. While it might not seem that way to us, most of our planet is actually uninhabited ocean, desert, or remote wilderness. The asteroid could have been a “city killer” if it had touched down in one of our major cities, but the chances of that are fairly slim — and it easily could have touched down in the ocean, without killing anyone.

How common are big, deadly asteroids?

Impacts from asteroids the size of 2019 OK are very uncommon but not unheard of. An asteroid about that size hit Earth a bit over a century ago in remote Siberia, ripping apart 800 square miles of forest. Don Yeomans, manager of the Near-Earth Object Office at NASA’s Jet Propulsion Laboratory, has estimated that an asteroid like that will enter Earth’s atmosphere once every 300 years or so. (Bigger asteroids are much rarer; the asteroid that wiped out the dinosaurs was nearly 10 miles across.) And since such asteroids are hard for NASA to map and track, it’s hard to be confident that we would see one coming, or have the chance to react in time.

Bigger asteroids are both much rarer and much easier for NASA to track. That means that while an impact from an asteroid like 2019 OK is hard to rule out, we can be pretty sure we’d know about an impact from an asteroid like the one that killed the dinosaurs.

NASA is responsible for mapping near-Earth objects (NEOs) that are larger than 1 km across. It has mapped more than 90 percent of them, which means we know their trajectories and can be confident that they aren’t on their way to impact Earth. The handful of NEOs that are larger than 10 km across — that is, big enough to kill us all if they struck Earth — are all in safe, stable orbits.

There’s another category of objects to worry about, though: long-periodic comets that swing through our solar system so infrequently that astronomers haven’t mapped them yet. To guess how likely that is, we have only rough estimates from looking at the comets we’ve observed; some research has found that we can expect to encounter “on the order of one 2+ km impactor per 5 to 10 million years.”

So incredibly unlikely, though not impossible.

What could we do if we saw a big one coming? Scientists have, yes, thought about it, including attempting computer modeling of some options that are so wild, they sound like they’re out of the movies. As my colleague Brian Resnick writes, “even if there is an asteroid hurtling toward Earth, it wouldn’t make a lot of sense to launch the world’s entire nuclear arsenal at it in the hopes of blowing it up.” That’s because you’d need an astounding amount of force, and you’d need to apply it just right. Knocking the asteroid off course looks a little more promising.

But our best defense here is the sheer unlikeliness that an asteroid of that size will hit us in the first place. As I concluded in my look at ways that natural world could kill us, humanity is far more likely to go extinct not through a rock from space, but through our own mistakes. And as individuals, we’re in much greater danger from car crashes and air pollution than from rocks from the sky.

Moments like this are a great reminder that space is vast, unpredictable, and fascinating, and that we desperately need to learn more about it. And moderately sized disasters are more probable than you might think. But if we’re afraid for the future of our species, the likeliest culprits are all closer to home.

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