Virginia geologist explains what we're learning from the Bennu asteroid
The Bennu asteroid is about one-third of a mile wide, and NASA says it’s possibly on a trajectory to Earth within the next 200 years.
Scientists recently revealed the contents of a sample of the 4.5-billion-year-old rock that was brought back to Earth last month.
Chuck Bailey is chairperson of the William & Mary Department of Geology. He’s teaching a course this year on planetary geology and, of course, talking with students about Bennu.
Morning Edition Host Phil Liles recently spoke with Bailey.
Below is a lightly edited excerpt from their conversation.
Phil Liles: So, what goes through a geologist's mind when there's talk of asteroids?
Chuck Bailey: That is a great question. Asteroids are these mysterious bits of rock, some larger than others, that inhabit parts of our solar system. And I think for most geologists, there are two things that get us excited about it.
One is, if you're interested in the history of the solar system, asteroids, in many instances, probably have clues about the very, very earliest parts of our solar system’s history. So, that's one of the reasons we're interested. Secondly, especially for those of us who live on planet Earth, we're interested in asteroids because they occasionally collide with Earth and create all sorts of mayhem — mass extinctions, large impact craters. The concern is, looking forward, are there other asteroids out there that are potentially on a collision course with Earth?
Scientists recently brought back fragments of Bennu. Can you explain why they did that and what we're trying to learn from that?
I'll give it a shot. So, it was, in some ways, a completely fabulous mission. We launched the spacecraft in 2016. It got to the asteroid. It orbited it. It effectively mapped out the surface. And it really looks like a sort of a dirty snowball with little bits of fuzz floating off it. And all that fuzz is material that formed early in the solar system’s history. Then they deployed, effectively, a probe that very delicately, if you will, landed on the surface, punched a tiny hole in it and captured the regolith or the debris that's on the surface of that asteroid. Then it went back out into space and returned to Earth with its little payload of asteroid dust.
So, why are we interested? This is a very dark asteroid. It's not very reflective, it's kind of grimy looking. That probably tells us that this is a carbonaceous material. There's a fair bit of carbon in it. And carbon is one of the building blocks of life. One of the big mysteries out there is how do we put carbon and amino acids together to ultimately build a beautiful world that we live on? Has that happened in other places? Those are some of the aspects of why we wanted to sample this.
Thirty-five million years ago, another asteroid slammed into Virginia, creating the Chesapeake Bay. Is there anything we've learned from that event that can help us understand Bennu?
Well, I think that is a fair question. That was one bad day in Virginia 35 million years ago.
The disruption from what we call the Chesapeake Bay impactor was pretty significant. There are thoughts that there were tsunamis that would have rushed outward from this wave at hundreds of miles an hour. There were lots and lots of organisms living in a very shallow sea. In fact, much of eastern Virginia would have been underwater prior to the meteorite arriving. Then it altered the geography of eastern North America ever since then.
That is a very rare event. But they've happened in Earth’s history. We've had a number of mass extinction events that are likely related to big asteroids or comets, which have impacted planet Earth. These mass extinctions in some ways allow life to reset. Some organisms don't fare very well, and they go extinct. And then others find sort of new opportunity and sort of the post-apocalyptic world. In some ways, that's how mammals got going. At one point, they found new niches, and ultimately, that led to us.
If we think about deep time and Earth time, big changes in our history have happened because of impacts on planet Earth.
How different would the impact of Bennu be from that of the asteroid that hit Virginia so long ago?
Well, a few things are probably different. The size is going to be different by probably an order of magnitude, maybe by a factor of 10. Bennu is about a quarter mile across from one side to the other. If an impactor of that size landed on Earth, it would be pretty catastrophic. We think that the impactor that arrived in the Chesapeake Bay would have been on the order of a few kilometers across— maybe five to 10 kilometers across — and that was epically catastrophic.
The other thing I'll say about Bennu is it's not very well put together. It's kind of a mixture of fairly light things that are not very cohesive. One of the things that hopefully would happen if Bennu arrived in our atmosphere, is a lot of it would break up upon entry. Rather than one very large projectile doing a lot of damage, we would probably end up with literally thousands and thousands of tiny space rocks. Hopefully, most of them would vaporize in the atmosphere, but it would be calamitous, nonetheless to wherever it fell.
Scientists say we still see the effects of that asteroid's impact today. How so?
In this instance, our geography was certainly impacted by the fact that the Chesapeake Bay impact structure created a pretty big hole in the ground. And what effectively happened is that crater was then partially filled by rivers and streams that were flowing towards that low spot. So, a lot of the mid-Atlantic’s geography is impacted by — that was a pun — by modifying the landscape. And so, the river is adjusted.
Then our modern geography of where the rivers are, in some ways, was set by that one event that happened back then. To me, the thing that's cool about the Chesapeake Bay impact structure is, it really is not evident. You look on a map or a satellite image of the Chesapeake Bay, the crater is sort of buried beneath the mouth of the bay. And it was buried by later deposits — the comings and goings of sea level. It took a lot of scientists working pretty hard over the course of about a decade to actually figure out it was there. One of the things about geology is that they're exciting stories, but then the geologic record is oftentimes pretty good at covering its tracks.
Are you afraid of this asteroid? Or is it more of a fascination at this point?
I would say at this point, it's the fascination. This asteroid certainly is one that is on a potential collision course with Earth.
People who worry about orbital dynamics say that we have a chance of that hitting us a few thousand years out. But the other thing we can learn from taking space probes to places like this is, we can get a sense of whether we actually can affect the trajectory of these. Could we nudge an asteroid out of the way? And if we want to do that, to get it to miss the Earth at some point, how much energy are we gonna have to put into putting the spacecraft on or around an asteroid? Are there consequences we haven't thought about for doing that?
In some ways, this was one of the test cases to see, can we actually get a spacecraft on an asteroid? And how could we mess with its orbital dynamics?