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Science and Tech

A Boulder Astrophysicist Discusses His New Theory on the Dinosaur-Killing Asteroid

We spoke with David Nesvorny about the supercomputing model he used to figure out where the giant space rock came from, as well as when another one could be headed our way.

The major extinction event that wiped out all of the non-avian dinosaurs some 66 million years ago is commonly thought to be a result of the impact that created the Chicxulub crater in the Yucatan peninsula. But the question of where exactly in the solar system the asteroid that formed the giant pit came from, as well as how exactly it got here, has puzzled scientists for years.

Thanks to new research from David Nesvorny, an astrophysicist at Southwest Research Institute in Boulder, we are one step closer to answering those questions.

Most craters on the Earth were caused by space rocks on the inside of the asteroid belt (read: closer to the earth). But the Chixculub crater was caused by a type of asteroid known as a carbonaceous chondrite, which is found much farther out in the solar system. Much like how a basketball player is less likely to hit a full-court shot than a free throw, the far away carbonaceous chondrites should be less likely to hit earth.

That idea led scientists to come up with a series of fringe theories about how the asteroid got here. One, for example, suggested that the crater was caused by a comet that Jupiter’s gravity redirected and slingshotted toward earth. All of them, though, have been debunked or received significant criticism.

Nesvorny’s research aimed to improve the models that produced some of those hard-to-believe theories. (He helped make the old models, so he knew better than most what could be improved.) One major flaw he wanted to correct was that the model didn’t account for how many asteroids were in each section of the asteroid belt.

After adjusting the numbers, he used a supercomputer, which took a month to simulate a billion years of solar-system movement. It found that one of these asteroids from farther out in the belt was likely to be on a collision course with Earth roughly every 300 million years. That is five times more often than the previous model and lines up with the amount of craters of that size that currently exist on earth.

Ahead of those findings being released in the science journal Icarus next month, we chatted with Nesvorny about the model, when we might see another massive asteroid, and how we are going to stop it.

5280: How do we know the asteroid that left the Chicxulub crater killed the dinosaurs?
David Nesvorny: We think this is probably the case because we have dated the crater using radioisotopes [radioactive atoms that can be analyzed to figure out how old something is], and it lines up with the extinction.

We also know that it had a global influence because the impact caused tiny glass spheres to be flung into the atmosphere. And a layer of them can be found all around the world. There are only three craters of this size that have ever been found on Earth. It is difficult to prove these things for sure, but it would be such a big coincidence that this impactor hit right when this major extinction happened.

What else can we learn from this model?
The model isn’t just for Earth. It was developed for the whole inner solar system. So I know how many impacts should be happening on Mars, Venus, and the moon, for example. [Editor’s note: The number on Venus should be similar to Earth, while the ones on Mars, the moon, and Mercury are expected to be way lower.] Of course, these are different worlds so there are other factors at play like different atmospheres, etc. But this is helpful because we often use crater records to determine the age of the surface of planets and satellites.

What makes crater records look different on different planets?
Well, for one thing, the atmosphere makes a difference. It turns out that impacts are the most important geophysical process that happen on some other worlds. Here on the Earth, we have tectonics, which is important. But go to the moon, go to Mars, and you will see a clearer crater record.

On Earth, it is actually pretty hard to find craters. If the meteor that made Chicxulub had landed just 100 km away, it would have been in the sea, and it would have eroded much faster and might be impossible to find by now. Even if it is only on land and hasn’t been eroded, a crater can be hard to find. Chicxulub, for example, is so big you can’t really perceive it. You could be walking on the rim and you wouldn’t know unless someone told you. So big craters can require satellite imaging and detailed geophysical analysis.

When is the next giant asteroid expected to hit Earth?
In this paper, we found impactors of this size hit once approximately every 300 million years. The Chicxulub impactor hit 66 million years ago. Statistically, it’s likely that within the next 300 million years, there will be another global extinction. I can’t say exactly when, but the probability in the next 100 million years is something like 30 percent.

Smaller asteroids are more likely to hit sooner. One asteroid named Apophis will come very close to the earth in 2029. We know that it’s not going to hit. So there is no danger. We don’t have to do anything. But it will happen in the future, maybe 100 to 10,000 years from now.

Is anyone doing anything about it?
Yes, the people at NASA work very hard on this. Given the long time scale of these very large impactors, they aren’t concerned with those right now. But for the smaller asteroids, NASA is developing techniques. Should we destroy it by exploding it? Or if we find it earlier, maybe land big rockets on it and push it aside. Or maybe use radiation effects: Paint it black on one side, leave it white on one side, and let the photons push it away from the earth’s trajectory. People are suggesting lots of things.

At the moment, we don’t have any capability to destroy even the smallest one. But as the technology develops, it will be possible to deal with bigger and bigger impactors. And that kind of works out because the smallest ones are the most frequently impacting. For the big ones, we will have more time.

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