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A passion for asteroids

Atmospheric nuclear tests can reveal a lot about exploding asteroids.

Mark Boslough spends his time learning about natural and nuclear atmospheric explosions and looking at climate change from a national security point of view. The Sandia National Laboratories physicist has also worked with the Federal Emergency Management Agency and other government agencies and become a public figure, appearing in many popular lay science programs. Earlier this year, for example, he appeared in Sacred Sites: Ireland, a film by the Smithsonian Channel that explores how celestial events may have influenced religious beliefs and practices in ancient Ireland. He has also been involved in three NOVA episodes, including the 27 March 2013 "Meteor Strike," about last year's airburst above Chelyabinsk, Siberia. (See also "Chelyabinsk: Portrait of an asteroid airburst," the article he wrote with David Kring, in Physics Today, September 2014, page 32.)

Mark Boslough collected data used to re-create the trajectory of the 2013 Chelyabinsk asteroid from videos obtained from car dashboards. This image is from the NOVA program

Mark Boslough collected data used to re-create the trajectory of the 2013 Chelyabinsk asteroid from videos obtained from car dashboards. This image is from the NOVA program "Meteor Strike." CREDIT: WGBH

In his childhood home in Colorado, says Boslough, "there was a left-brain right-brain thing going on, with fiction and nonfiction in the same household. And a lot of it centered around science." His mother would wake him up before dawn to watch NASA space launches. And on a camping trip with friends and family, Boslough saw a bright object fly across the sky. "It was a big inflatable shiny Mylar satellite. And my dad was the only parent who knew what it was and could explain it to everybody. I wanted to be like that," he says. Despite his early interest in science, he says, "I managed to be a slacker in high school." Then, at Colorado State University, "I had no idea what I wanted to study." He liked art and writing, but didn't think he was talented enough to make a living at those. He took a physics class, and within a year he was majoring in physics. "It sort of seemed like such a fundamental basic field that you could do anything with," he says. He earned a PhD in applied physics at Caltech and from there joined Sandia. Physics Today caught up with Boslough in Albuquerque, New Mexico, and by phone to hear about the trajectory of his career.

PT: How did you end up at Sandia?

BOSLOUGH: At Caltech, we had weekly colloquia from various professors from around campus that were looking for students. One guy, Tom Ahrens, came and talked about impact craters. That combined my interest in Earth and planetary science, and I immediately decided I wanted to do that. We got to do some really fun things. We had a two-stage light gas gun, made from a big old navy cannon. You burned powder in the breech and shot a piston into this pump tube that pumped hydrogen up to a very high pressure. It seems incredibly dangerous and not something you would want to have graduate students work on. That was my entry into scientific research. My first physics conference was a topical conference in the field. I met all these people from Sandia and other national labs. The Sandia people had written the codes I was using, and there was an immediate kinship. A couple of years later, when I got my degree, I came here [to Sandia] for my first job interview. I strongly felt this is where I wanted to come, but I also realized you don't get married after your first date. That was in 1983. I've been here ever since.

PT: Describe your work.

BOSLOUGH: I was an experimentalist the first 10 years I was here, and still used these two-stage light gas guns. One of my colleagues developed a way to effectively put a third stage on these two-stage guns, so we could launch materials at superhigh speeds and test debris shields, like the shield that protects the International Space Station from space debris. Sandia had this brand new computer; it was the world's most powerful computer. I wanted to get involved with it. In 1993 this comet was discovered: Shoemaker–Levy 9. I went to my boss and said, Wouldn't it be cool if we could simulate it and make some predictions? We ended up getting permission and we calculated what the effects might be when it collided with Jupiter. It was like a big splash—a hypervelocity splash. And that was about the time when funding for experimental work was on the downswing, and funding for computational work was on the upswing. So I kind of jumped off one and onto the other with really lucky timing.

PT: Is computational work still your main focus?

BOSLOUGH: Yes. In the late 1990s, I was still interested in planetary impacts and how impacts on the Earth were a threat to people, to agriculture, to everything. If the Earth got hit by a big comet like Jupiter did, what would happen? I was very interested in coupling the impact problem to the climate problem. And I think that's about the time I became sort of the self-appointed gadfly at Sandia. I made the case that global warming is very much a national security and a global security issue. Taking national security in a broad sense, the health of the nation depends on the health of Earth, and so I tried to make that case to management. Things have evolved, and now all the national labs do take it very seriously.

PT: Do you choose what you want to work on?

BOSLOUGH: It follows lab needs. One of the things that makes Sandia great is that you do have a lot of flexibility to choose what you want to work on. It's almost like a market, where there are people who run projects who need people to staff projects. In a lot of ways it can be informal.

PT: How much of your work is classified?

BOSLOUGH: That changes with time. There have been times where my work has been dominated by classified work, and other times like now, where I'm not really doing any at all.

PT: What is the connection between your work in weapons and your work with asteroids and impacts?

BOSLOUGH: My asteroid work is my passion. And in terms of physics, of course it overlaps strongly with other work at Sandia. The hydrocodes that were developed for modeling nuclear explosions are the same ones that I use to model asteroid airbursts. They are the same codes you would use to look at how you would deflect an asteroid—by hitting it with a kinetic impact or setting off a bomb next to it. It's not 100% overlap but a lot of the physics is the same—high pressure shock compression of condensed matter.

PT: In an airburst, there is an explosion in the atmosphere and the asteroid vaporizes to create a hot, glowing fireball. Is that right?

BOSLOUGH: The actual definition comes from nuclear weapons. I have adopted the same term for asteroids. But it's an exploding asteroid, not an exploding bomb. The physics of what leads to the explosion is very different, but the end result is very similar. There are some other fundamental differences between the two. Unlike the weapon residues, asteroid residues have this huge momentum. They're still moving fast, even though you've converted most of the kinetic energy to thermal energy, and it has a lot of mass. That's the big difference. So an asteroid is like a directed energy weapon. It's going to keep going toward the thing it was going to if it hadn't exploded. The bottom line is that an asteroid explosion actually does more damage on the ground than a nuclear explosion of the same energy, of the same yield. It's because of this directed effect.

PT: How did you happen to go to Chelyabinsk, and what did you do there?

BOSLOUGH: Interestingly, the way I first found out about the airburst was on Facebook. And somebody posted a video on YouTube, which I saw about an hour and forty minutes after it happened. The western media hadn't picked up on it yet. We have this international monitoring network that's tuned to look for atmospheric explosions. One aspect of that is infrasound—very low frequency sound waves that travel across the Earth at the speed of sound. So it would be many hours before those waves would get to this hemisphere, whereas social media travels at the speed of light.

PT: And what about your involvement?

BOSLOUGH: I got a call from WGBH TV, the station that produces NOVA, the next day. They asked if I'd be interested in being interviewed for a quick-turnaround program. A couple of days later they called back and asked if I would join a film crew in Chelyabinsk. I asked my management, and they immediately jumped on board and did everything they could to get me a visa. I was on a plane within a week. I spent two days and three nights there.

PT: What did you do in Chelyabinsk?

BOSLOUGH: In the daytime I was committed to the film crew, to being there and being interviewed. But at night I got to do research. What I did was set up a camera on a tripod at the exact location of a frame of a few of these dashboard cameras—there were lots and lots of them, over 700. I've been told the primary reason people have them is to prevent insurance fraud and police corruption. Anyway, using the frames, I could point my camera in the direction of the sky, do a time exposure, basically create a two-dimensional ruler, to calibrate, do astrometry, on the sky and determine the location of the meteor at each instant and work out a trajectory. Now I didn't work out the trajectory, my colleagues did that, but I provided the time-exposure photographs. So, I told you I was an experimentalist, and then I was a computer guy. But frankly I think my favorite thing to do is fieldwork.

PT: How did you get the idea?

BOSLOUGH: When NOVA asked me to go, I called up Peter Brown—we had analyzed an event together years earlier—and said what can I do that would actually be useful? He said, "Do a stellar calibration. We'll find the best videos, and by the time you are in Russia, we'll send you a list of locations that you should go to." I landed at the airport. It was midnight in Chelyabinsk, and the sky was clear. They had sent a Russian-speaking cab driver to take me to the hotel. I put in the coordinates in my smartphone, and a map came up and I pointed at it, and told him to go there, and he did. He stood around patiently waiting for this crazy guy to take pictures of the sky on a tripod in the middle of a busy road. The second night was cloudy. So on the third night I got in the car with this enthusiastic guy—Peter was getting a lot of emails from Russia from people who had witnessed it, and he got me a contact for a cab driver. He had his English-speaking girlfriend and another buddy with him, so there were four of us piled in driving all over the Russian countryside. When the Sun came up, they took me straight to the airport.

PT: What was learned?

BOSLOUGH: Our paper came out in Nature. One of our most newsworthy conclusions was that we probably underestimated the number of airbursts. There seem to be events happening more often than you would expect based on astronomically based population estimates of objects of this size.

PT: You mentioned that asteroids can do more damage on the ground than a nuclear weapon. Why make the comparison?

BOSLOUGH: We have a lot of data on atmospheric nuclear tests, and we don't have a lot of data on asteroids. A lot of data associated with Tunguska [the asteroid explosion over Siberia in 1908] is circumstantial. And our only example of a large airburst that is well documented is Chelyabinsk, but there are lots and lots of atmospheric nuclear tests. You do want to do a risk assessment. It's like any threat. There aren't any hurricanes bearing down on our coast at the moment, but we still want to understand them, and we still want to be able to make preparations if and when there is one. I think we need to treat the asteroid threat exactly the same way, but you need to discover them before impact. It would be great to fly instruments, set up spectrometers and lidar and geophones and whatnot, you could even have IR trackers to follow meteorites all the way to the ground. They're still glowing in infrared even after they are no longer emitting light, so you might be able to extrapolate and pinpoint where they land. What you need to carry out a campaign like that is more telescopes to find objects on a collision course far enough in advance.

PT: Why is it important to get the trajectories of asteroids?

BOSLOUGH: There are a bunch of reasons. One is that by getting the trajectory, you can learn about the population. What kind of orbit is it in? Are there related orbits? Are there families of orbits? What class of asteroid is it? But the part I am more interested in, is if you know its precise trajectory, and you can observe it as we did with Chelyabinsk, then you can do some serious modeling, and have a real case to validate your models. You can calculate dynamic pressure, so know what the pressure on it was when it broke up. You can extract a lot more about the asteroid and the process of airbursts.

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Scitation: A passion for asteroids
http://aip.metastore.ingenta.com/content/aip/magazine/physicstoday/news/10.1063/PT.5.9020
10.1063/PT.5.9020