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Crash Course Geology explains what asteroids, meteoroids, meteors, and meteorites are, and how rarely large asteroids strike Earth. It recounts the Chicxulub impact that helped end the dinosaurs about 66 million years ago, and describes how NASA now tracks asteroids and tested deflection with the DART mission. The conclusion is that dangerous impacts are extremely rare and detection and defense are improving.
- About 48.5 tons of space rock hits Earth daily, mostly tiny debris that burns up in the atmosphere.
- Asteroids are rocks orbiting the sun over 1 m across, meteoroids are smaller, meteors are their atmospheric flashes, and meteorites are fragments that reach the ground.
- An asteroid about 25 m wide or less usually burns up, but the 2013 Chelyabinsk, Russia event released about 30 times the energy of the Hiroshima bomb and injured 1,500 people.
- An asteroid about 140 m wide hits roughly every 20,000 years, while globally devastating impacts happen at most every 100 million years.
- The Chicxulub asteroid, over 12 km wide, struck off the coast of present-day Mexico 66 million years ago, triggering tsunamis, fires, and sulfur-driven climate change that killed about 75% of life on Earth.
- Luis and Walter Alvarez discovered iridium-rich clay at the Cretaceous-Paleogene boundary in 1977, a key clue to the impact.
- NASA tracks asteroids with space telescopes and runs planetary defense exercises every two years.
- NASA's DART spacecraft successfully crashed into a 160 m asteroid, slightly changing its path, demonstrating a deflection method.
- 0:00Daily Space Rock Bombardment
- 0:37What Asteroids Actually Are
- 1:44Asteroid, Meteoroid, Meteor, Meteorite
- 2:58Chelyabinsk and Impact Frequency
- 3:41The Chicxulub Impact Sequence
- 5:57Discovering the Iridium Clue
- 7:09Finding Ancient Impact Evidence
- 8:21Tracking and Preparing for Asteroids
- 8:57NASA's DART Deflection Test
- 9:31Final Perspective on Asteroid Risk
Daily Space Rock Bombardment
0:00The sky is falling. Sort of. Our planet gets pummeled by about 48.5 tons of space rocks every day. That's the weight of 7 and 1/2 elephants falling from the heavens 24/7. Most of that cosmic debris is harmless. It's so tiny it burns up in the atmosphere before it rains down on Earth. But what about the really big rocks?
0:17The asteroids that could wipe out life as we know it. Truth is, they're out there and they've collided with Earth before. Today, let's talk straight about rocks from space. They're at the top of my things to worry about list. Hi, I'm Sage and this is Crash Course Geology.
What Asteroids Actually Are
0:37You've seen the movies. An asteroid is headed straight for Earth. Everybody panic. Run for your lives. Oh, and thanks Dwayne. I really should go on more movie auditions. But despite all that drama and disaster, space rocks are sort of run-of-the-mill here on Earth. Hitting its surface daily like a light drizzle.
0:52Most of the time. So what exactly are they? Most space rocks are leftovers from the moment our solar system formed some 4.6 billion years ago. We differentiate them in a few ways. An asteroid is a rock that orbits the sun and is more than 1 m across. Basically, anything with a diameter longer than a baseball bat but not big enough to be a planet with its own gravitational pull.
1:12That's why asteroids are sometimes called a minor planet. Mm, burn. Most asteroids hail from the asteroid belt, an area between Mars and Jupiter that's chock-full of space rocks. Some of them hundreds of miles across. A trip to the asteroid belt would be like uh what am I thinking of? Oh, that video game where you dodge barrels that a gorilla throws at you.
1:29Uh Dwayne has just informed me that there's a video game literally called asteroids and apparently it's very famous. Mm, my bad. Now when a space rock's diameter is less than a meter across, it's a meteoroid. Usually a meteoroid is a chunk of an asteroid that got blasted off during a collision with another asteroid.
1:42You'd think that wouldn't happen with all that, you know, space but it does. So, big space rock, asteroid. Small space rock, meteoroid. And if a meteoroid leaves space and enters a planet's atmosphere, it becomes a meteor. These are the flashes of light we spot in the sky and call shooting stars. Meteors are little space rocks that burn up and vaporize into dust as they hurtle through the atmosphere.
Asteroid, Meteoroid, Meteor, Meteorite
2:04If a piece of meteor doesn't completely obliterate before reaching the ground, it becomes something else, a meteorite. As for asteroids that enter the Earth's atmosphere or hit the surface, they're still called asteroids. But, here's some good news. While our planet gets pelted by tons of space rocks every day, there's only one recorded instance of a meteorite actually striking a person.
2:27In Alabama in 1954, a meteorite broke through Ann Hodges' roof, interrupted her nap, and left her a mean shiner on her thigh. And most of the space rocks that hit Earth aren't big enough to completely wreck it. If an asteroid less than 25 m wide enters Earth's atmosphere, it's likely to burn up and disintegrate.
2:46In fact, an asteroid the size of a car enters the atmosphere and explodes before reaching the surface almost every year. That's what happened above Chelyabinsk, Russia in 2013 with an asteroid about two and a half times as long as a London bus. It entered Earth's atmosphere and burned up 30 km above the city, releasing 30 times more energy than the atomic bomb dropped on Hiroshima.
Chelyabinsk and Impact Frequency
3:08And the sound wave that followed was so powerful, it shattered windows across six cities, injuring 1,500 people. So, even asteroids that burn up can have an impact, especially if they're big. An asteroid 140 m wide hits the Earth roughly every 20,000 years. That's big enough to wipe out a metro area or even whole US states.
3:26An asteroid with the potential to cause global devastation and kill most living things is extremely rare, reaching us every 100 million years at most. But, it does happen. And we know that because it's happened before. Let's dig a little deeper. 66 million years ago, an asteroid wider than Mount Everest is tall hurtled through Earth's atmosphere, and it didn't burst in mid-air.
The Chicxulub Impact Sequence
3:51It directly collided with our planet off the coast of what's now Mexico. And the world changed. In the first minute of impact, what's now known as the Chicxulub asteroid carved a crater over 20 km deep into the ocean floor, vaporizing some of the rocks and water around it. Within 3 minutes, its sheer force caused the ground in the center of the crater to liquefy and rebound up into a giant peak.
4:17Then it collapsed again, sending out shock waves in every direction. That collapse dumped chunks of rock at the edges of the crater, forming a ring around the molten gap at the center. And that's just the first 30 minutes. Within an hour, the ocean surged over the molten crater where the water began churning and spitting up steam.
4:38For a whole day, the ocean sloshed back and forth as rocks from the crater's rim toppled and stirred up water, waves spitting them back onto the crater's rim. Also, everything within 1,400 km of the impact was on fire. 25 trillion tons of debris had been hurled up into the sky, some of which landed far from the crater, setting fires ablaze all over the planet.
5:01And there was a massive tsunami with waves 1.5 km high. The shock wave then triggered more tsunamis, earthquakes, and volcanic eruptions across the globe. If that wasn't enough, the rocks vaporized by the asteroid's impact unleashed up to 325 gigatons of sulfur into the air, creating toxic sulfuric acid.
5:23This might have blocked the sun and chilled the climate for years or tainted the rivers and oceans with showers of acid rain. All told, these effects were eventually enough to kill roughly 75% of all plants and animals on Earth, including the dinosaurs, except the very few that evolved into today's birds.
5:48Those guys are resilient. Can we just appreciate for a moment that human brain power can reconstruct a thing that happened millions of years ago, but I can't remember where I left my keys? Has anyone seen my keys? To be fair, it took decades of shared brain power to get to this level of detail. In 1977, an American father and son team, Luis and Walter Alvarez, made a discovery.
Discovering the Iridium Clue
6:13In Italy, they found a very thin layer of clay sandwiched between rocks with tons of marine fossils and rocks with barely any. It turns out you can find this thin clay layer in rocks from the same time period all over the world. The Alvarezes and their collaborators discovered that this layer is full of soot and iridium, an extremely rare metal on Earth, most often found in asteroids.
6:34This was the first clue. All that iridium must have been ejected from a space rock, a really big space rock. Today, that thin layer is called the Cretaceous-Paleogene boundary, or the KPG boundary for short. It marks the moment about 66 million years ago when a 12-km asteroid rammed into the planet and left a geological stamp.
6:58The other big piece of evidence was the Chicxulub impact crater, which marks the spot where the asteroid hit just off the coast of what's now Mexico. It's a whopping 180 km wide and 900 m deep. But it's also buried a kilometer below the surface, partially under the ocean. So, it wasn't even discovered until the 1990s.
Finding Ancient Impact Evidence
7:15Chicxulub, can't see me. It's pretty much impossible to find evidence of asteroid impacts from that long ago. Erosion and plate tectonics are constantly exfoliating Earth in slow mo, erasing the evidence. Like the Vredefort impact structure in what's now South Africa marks the spot where an asteroid up to 25 km wide slammed into the Earth a little over 2 billion years ago.
7:37But the main clue left behind isn't even a crater anymore. It's a dome, an area where the ground lifted after an asteroid obliterated the ground surrounding it. Hm? Oh, yeah. Dwayne doesn't like that I use the word obliterated. If you're wondering how worried you should be about an asteroid doing the same thing to us, you're not alone.
7:54But, I have more good news. We've got one thing the dinosaurs didn't have. And that What's that, Dwayne? Chapel Run? Well, yes, definitely. Oh, thumbs? Uh I think you could argue iguanodons did have thumbs, but we're getting off track here. We've got one thing the dinosaurs didn't have, and that's human brainpower.
8:13Remember how I said that before? That's foreshadowing, baby. Over the years, we've developed different ways of tracking asteroids like they're pizzas we never want delivered. Today, NASA uses powerful space telescopes to take pictures of asteroids' movements and calculate their orbits. And none of the asteroids big enough to be damaging, let me repeat, none of them are on track to hit the Earth anytime in the foreseeable future.
Tracking and Preparing for Asteroids
8:34If an asteroid does someday pose a threat, we'll know. And there's an organized effort to prepare for these risks if they ever arise. Every 2 years, NASA's planetary defense team leads a hypothetical tabletop exercise where key leaders and experts run through the what-ifs of a potential asteroid impact.
8:51Someone should tell them to play that asteroids game I just learned about. There's also ongoing research into redirecting an asteroid's path by crashing a spacecraft into it. And get this, we've done it. Like, not in a movie, in real life. NASA sent a spacecraft on a one-way trip to smack into an asteroid, and it worked.
NASA's DART Deflection Test
9:08The craft, called the Double Asteroid Redirection Test or DART, had to travel for more than 10 months to reach the asteroid, but it successfully headbutted the 160-m rock onto a slightly different path. That's one small smack on an asteroid, one giant leap for humanity. So, the sky is falling, but it's far more likely to be a shower of cosmic brownie sprinkles than a massive dino killer.
Final Perspective on Asteroid Risk
9:30At the time we're filming this, there are almost 1.5 million asteroids in our solar system that we know of. But altogether, their total mass is less than that of our moon. And there's even more good news. The last time a large asteroid had a major impact, some life managed to survive, adapt, and evolve into new life forms, including a weird, complicated ape that's invented marvels like novelty socks, pizza with cheese in the crust, and ways of tracking asteroids before they become a threat.
10:01All right, can we strike asteroid impact from my things to worry about list? Presently, it is directly above the server being mad at me because I corrected my order. And I'm mad at me, aren't you, Katie? Next time, we'll learn how geology fuels our world. See you then. Thanks for watching this episode of Crash Course Geology, which was filmed in our studio in Indianapolis, Indiana.
10:20It was made with the help of these stellar people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.
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