I have some bad news: Someday, another  asteroid is going to hit Earth.

Which isn’t necessarily a reason to panic. Space rocks pummel us all the  time, like, every single day.

Have you ever seen a shooting  star going across the sky? That’s a space rock streaking  through the atmosphere. And if you can see me talking to  you right now, we’re still alive!

For now, at least. The only time we need to worry is  when the asteroid is really large. Because a space rock of a  certain size could wipe out a neighbourhood, a city, or even humanity itself.

So let’s talk about how big an  asteroid would need to be to do that … and which ones scientists  are keeping a close eye on. [intro music] When it comes to asteroid tracking, there are two things that NASA  scientists are concerned with: size, and proximity to Earth. And that makes sense, because  for an asteroid to hit Earth, it has to be, you know, near Earth. Researchers call these  asteroids near-Earth objects, for reasons I hope you can figure out.

There are more than 30,000 of them out there, but the vast majority are smaller than a shoe box, and they usually just burn up  on entry into our atmosphere. About once a year, we get a  bigger one … say, car-sized. But that’s still not big enough  to make it to the surface.

So even those don’t really pose much of a threat. And just a note: When they’re in space orbiting  the sun, they’re called asteroids. Once they crash-land on Earth,  they get bumped up to meteorites.

If they survive to hit the ground, that is. The odds of any meteorite hitting  any one spot are super slim: about 1 in 250,000, according to one calculation. To put it in perspective, in the U.

S., you’re 16 times more likely  to get struck by lightning than you are to be hit by a meteorite. So the small fry asteroids are no issue. But what about the bigger ones?

A space rock the size of a football field tends  to hit our planet about every 2,000 years. And when something like that crashes into Earth, it’s less great. Still, the damage is pretty localized.

So if an asteroid landed in Boston, it’s not that likely that New  Yorkers would be affected. But once every few million years, we get pummeled by a super-large meteorite. A space rock that is large  enough to threaten civilization.

More on that in a minute. Of the asteroids that have a chance of  hitting Earth with disastrous consequences, the most likely one is called Bennu. It’s a medium-sized asteroid that’s  going to pass near our planet in 2182.

But astronomers seem to have a pretty  different definition of what “medium” means. Let’s use the Burj Khalifa for scale. It’s the tallest building in the world at  828 meters tall, or roughly half a mile.

You know the super tall building in Dubai that Tom Cruise scaled in Mission: Impossible 4? That one. Yeah, Bennu is about 500 meters  wide at its biggest point, making it over half as wide as the Burj is tall.

And that’s a medium. Medium-sized asteroids like Bennu tend  to hit Earth every 100,000 years or so. If Bennu faceplanted into our planet  right now, that would be a very bad thing.

Like Game of Thrones, winter is coming, level bad. But the scale and type of badness  would depend on where it struck. If Bennu hit land, the disaster would  start with a massive earthquake.

If it struck the ocean instead, we’d get a tsunami like no one has ever seen. Either way, boy oh boy, would there be dust. As much as 400 million tons of dust  would be thrown into our atmosphere.

It could linger there for as long as four years, dimming the sunlight and devastating life  on Earth with slow-motion starvation. If you know your dinosaur history, you might remember that  when the asteroid hit Earth at the end of the Cretaceous Period, it wasn’t the immediate impact  that killed most of the dinos. They died as plants stopped photosynthesizing and our planet fell into a global darkness.

And while it’s not as big as the  asteroid that killed the dinosaurs, Bennu could lead to a similar, years-long winter. So that’s a medium asteroid.  Let’s talk about large ones. NASA has its eye on all the really  big asteroids in our solar system, under a program called Sentry.

Their biggest and most threatening entry right now has a pretty bland name: 29075. But it’s much scarier than it sounds, because it’s almost a mile in diameter. If 29075 hit us, it’s estimated that several million people  would die in the zone around the impact site.

For example, if it hit the Atlantic Ocean, it would impact with so  much force that the asteroid would carve a hole in the sea floor. That’s three miles below the ocean’s  surface, and one heck of a cannonball. Water would first flow back into  the pit it left in the seafloor, and then it would spread out again,  in a series of circular waves.

The first few hits on the  coast would seem manageable. But as more waves arrived, they  would get bigger. Way bigger.

The tsunami waves would be  several hundred feet high, and could crash right through the  largest cities on the coastlines. The highest wave would be about  the same size as the cavity that the space rock carved out in the ocean, which one estimate suggests  could be over 17 kilometers high. And even when the waters would recede, we wouldn’t be out of the woods yet.

Not only would the flood cleanup be really bad, but we’d still have the whole dust  in the atmosphere issue to deal with. Whole ecosystems would be absolutely annihilated, forever. People and animals would die in droves.

But the key thing is, that still wouldn't  be enough to wipe out every living thing. An asteroid powerful enough to essentially  sterilize our planet would need to be much bigger. So, how big?

Well, it depends. Anything from where the  asteroid hits to how dense it is could alter the likelihood  of a full-on life-ender. But one estimate suggests any comet  or asteroid that’s 95 kilometers wide would be enough to turn our  planet into a wasteland.

If something that size were to hit our planet, scientists say the effects would be  “catastrophic” for the entire ecosystem. For starters, the asteroid  would generate a huge fireball that could incinerate everything within  4,800 kilometers away from the impact zone, which is … let me do some math … a lot of places. The whole planet’s circumference  is about 40,000 kilometers, so that’s over a tenth of the  entire planet’s surface up in smoke.

As the asteroid slammed into our atmosphere, it would compress the air and  create the worst air-blast ever, wiping out everything from cities to forests. Even the crust of our planet wouldn’t survive! It would be removed or melted, depending on how close it was to the space rock..

One paper suggested that an asteroid of this size is not only enough to sterilize the planet, but to boil the oceans. Yes, boil. The upper 200 meters or so of the  ocean is known as the photic zone, or the area of water that’s  most hospitable to life.

A reasonably dense asteroid  that is 95,000 kilometers wide, flying in at over 100,000 kilometers per hour, would be enough to turn all this water deadly-hot, sterilizing it entirely. Could we hide from this kind of devastation? Unless you want to go off-planet, probably not.

The authors of this paper  say that kind of space rock would be enough for the extinction of humanity, and probably, everything else with it. And there are 17 objects in our solar system that are the right size to get the job done. None of them are currently  hurtling in our direction, but it’s not a comforting thought.

Luckily, humans have something the  dinosaurs didn’t have: spacecraft. No, it’s not practical for all  of us to get on these machines, like everybody did to escape the  aliens in Battlestar: Galactica. But we may be able to use these spacecraft to steer bad space rocks out of the way.

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Let’s say we’ve identified a killer asteroid. We’re sure that it’s going to hit Earth. And we’re sure that much of humanity is at risk.

What are we doing to do about it? Our best bet is diverting the asteroid  away from our planet towards … something else. We’re really not picky, as long as  it’s in a not-this-way direction.

And we’ve tested some of  this tech in space already! In 2022, NASA launched a mission called DART, or Double Asteroid Redirection Test. The scientists involved wanted to see if they could change the direction  of an asteroid called Dimorphos.

It had been happily orbiting another  asteroid called Didymos for eons. But when NASA’s spacecraft sent  an impactor toward Dimorphos, its path changed. Not much.

It’s still circling Didymos, but every orbit takes a  little less time than before. And that’s better than it sounds! If we can even slightly divert the  path of a harmless asteroid like that, it’s possible we could do the same  for one that is bearing down on Earth.

We might even be able to use  lasers, or a nuclear warhead. How we do so will take a lot  of discussion, and testing. But since we haven’t identified a world-ending  asteroid that’s heading our way yet, it’s all theoretical at this point.

Although it may sound scary, an asteroid impact is far from the most  likely culprit in ending life as we know it. Actually, we’re doing a pretty  good job of that ourselves. But it’s nice to know that scientists  are constantly scanning the solar system for rocks that might be a problem.

Because I, for one, do not want to get hit with something  the size of a football field. or even the size of a football [ outro ]