Astronomy Cast

Ep. 291: Shockwaves

Podcast: Play in new window | Download

Subscribe: RSS

As a meteor crashed into the atmosphere above Russia, the world discovered the importance of shock waves; how they’re caused and how they propagate through the atmosphere. Today we’ll discuss the topic in general and find many examples where shock waves can be created, here on Earth, and out in space.

Show Notes

 

Transcript: Shock Waves

 
Astronomy Cast episode 291 for Monday, January 28, 2013 – Shock Waves
 
Welcome to Astronomy Cast, our weekly facts based journey through the cosmos. Where we help you understand not only what we know, but how we know what we know.
 
My name is Fraser Cain, I’m the publisher of Universe Today. With me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville.
 
Fraser: Hi Pamela, how are you doing?
 
Pamela: I’m doing well, how are you doing Fraser?
 
Fraser: Doing great. So this is one of those episodes pulled out of time and space and it’s all wibbily-wobbily timey-whimey… in that, we’re recording this about three weeks after the date that we’re posting it. Today’s topic, which is shock waves, is kind of apropos because when we’re actually recording this is in February and the meteor just hit Russia. We already had shock waves on the list of topics but we thought it would be cool to cover the topic. When we’re talking about things that sounds like we’re predicting the future…
 
Pamela: We’re not.
 
Fraser: We’re actually not. We’re just taking a topic that was very relevant and throwing it into the recent stream of episodes. The other thing is that if people don’t catch this in time, we’re going to be at South by Southwest from March 7th– March 11th, but typically the 8th, 9th, and 10th. We’re going to be joining NASA, Microsoft and a bunch of other people to hopefully run some live star parties. There’s going to be amateur astronomers out with telescopes; we’re going to be haunting that area.
 
Pamela: (Laughs)
 
Fraser: There’s going to be a live model of the James Webb space telescope and we’re going to try and run some actual live events from South by Southwest so it’s going to be a really great time.
 
Pamela: We promise, no undead astronomers. Only living astronomers haunting the site.
 
Fraser: Well I just imagine that as we roam around looking at telescopes, showing people the night sky, doing live events, we’ve got lots of good stuff planned. It’s going to be a lot of fun. If you’re going to be at South by Southwest, by all means, look us up. Just look for the gigantic model of the James Webb space telescope and we’ll be nearby.
 
Pamela: We’ll be down near Zilker Park, so if you know Austin, that’s the area we’re going to be in, down along the river.
 
Fraser: I don’t know Austin very well.
 
Pamela: That’s fine.
 
Fraser: …but I’ll figure it out… Are there any more announcements?
 
Pamela: Not that I can think of.
 
Fraser: As a meteor crashed into the atmosphere above Russia, the world discovered the importance of shock waves, how they’re caused and how they propagate through the atmosphere. Today we’ll discuss the topic in general and find many examples where shock waves can be created, here on Earth, and out in space. As I mentioned, we’re recording this episode about four days after the meteor struck in Russia. It detonated in the atmosphere and created a massive shock wave that blew out the windows of the city and caused a lot of damage and a lot of injuries. I’d like to go back and relive that day and talk about what actually happened. We have a lot more details; we know the size and what happened so can you tell people what happened in that event?
 
Pamela: As briefly as possible, a 7000 ton meteorite decided to intersect Earth’s orbit and it did this by crashing through Earth’s atmosphere over eastern Siberia, streaking through the sky and then exploding and crumbling… chose your verb of choice. Right before it got to the Ural Mountains it did this at an altitude of about 32,000 feet. With this process you have very fast moving objects slamming through the atmosphere faster than the speed of sound. It created a shock wave, sonic boom if you will, propagating through the atmosphere. The sonic boom is estimated to have had a pressure that caused the air to move it approximately 500 mph, which was the gust that made very loud noises and blew out windows, window frames, and a lot of other stuff. The actual damage that is attributed to the meteor itself, which became a meteorite as it hit the planet, the shards appear to have gone into a lake, that’s confirmed. There is a factory that lost a chunk of wall for reasons that are still being sorted because if there are chunks of meteorite that will have to be found in the midst of a Zinc factory and it was made out of brick so it’s just a mess.
 
Fraser: I think, from what I hear, a third of the windows in the city we’re blown out?
 
Pamela: It’s like a billion rubles worth of damage.
 
Fraser: I was thinking about the scenario, imagine this: You’re standing in your house, eating your breakfast, having your coffee, it’s early in the morning and then you see this really bright flash that illuminates your entire room and house. You walk over to the window…
 
Pamela: This was 9:20 in the morning.
 
Fraser: Yeah, it was coffee time, breakfast time, whatever. You look up and you see the after effect of the impact and you see this contrail in the sky. You think, “That’s really weird”, and you take a picture and then two and a half minutes later everyone is sitting right in front of their windows looking up and bam, the shock wave hits and it shatters the glass on the windows. It was the perfect situation for people to stand right in front of their windows and two and a half minutes later get struck by shattered glass.
 
Pamela: 1200 people have been estimated to have sought medical attention in the three different cities that were affected by this. The time delay between when the object was seen streaking through the sky was traveling at the speed of light. When the sound wave of the sonic boom hit people and blew out their windows it ranged from right about a minute to two minutes twenty eight seconds… so it wasn’t good.
 
Fraser: No, no… lets go back and take another look at what the physics are in this situation. What is this shock wave and what really happened?
 
Pamela: Well in this case, it’s literally a matter of having an object traveling through the medium air at greater-than-the-speed-of-sound. The speed of sound is the rate at which a compression wave can move through an elastic medium; this would be air or dirt or any of the number of different things that are capable of compression and decompression. As that sound wave, the pressure wave, moves through the medium it has a certain speed. If something is trying to plow through that medium and the sound of it plowing through the medium can’t get away from it as fast as it’s going through the medium then this creates a discontinuity. On one side of the medium you have a very different pressure than on the other side of the medium that’s caused in this case by the object moving through medium… That’s a lot of uses of the word medium.
 
Fraser: Just so I understand, when I’m talking the pressure waves are coming out of my mouth at the speed of sound and it’s happy to absorb these pressure waves and move them through this medium. It’s just when you have something that is not playing by the rules.
 
Pamela: Yeah. This is the case where you imagine you are running along and you are shouting to your friend. The waves are exiting your mouth and traveling through the medium propagating in front of you and the waves are going faster than you are. What happens at that moment is that you’re, then, traveling at the speed of sound or faster than the speed of sound. People often ask this about the speed of light. There you’re lucky because you can’t actually travel faster than the speed of light because then time stops. With sound, you end up instead with sonic booms where these discontinuities in the medium where the shock wave is creating a pressure front where on one side of the medium it’s one pressure but on the other side it’s another pressure. That discontinuity moving out carries a lot of energy with it.
 
Fraser: What speed does this pressure wave move through the medium?
 
Pamela: That depends on what speed the object generating it is traveling at. All of these things get tied together. In the case of the Russian meteorite, some of the math that I’ve seen works out close to 500 mph.
 
Fraser: When the meteorite struck the atmosphere it was moving at 18,000 mph so definitely the sound wasn’t moving that fast; it had to slow down.
 
Pamela: No, it’s not only that but as the wave propagates, it loses energy. Part of that energy is the rate at which it’s moving at so it starts out moving much faster and then slows down as it moves out. The energy has to fill larger and larger volume and as it has a larger and larger surface area is what makes up the discontinuity.
 
Fraser: So do you get a situation where the energy is dissipated to the point that wave slows down to the compression speed of the medium?
 
Pamela: Yes. This is actually something that we all know has to be happening. If you think of supersonic airplanes and if the sonic boom from them didn’t stop we would have supersonic waves propagating around the planet. They do eventually taper off but it does take distance for the energy to get dissipated into the atmosphere.
 
Fraser: Right, so you brought up the other common form of shock waves that a lot of people are quite familiar with and this is a supersonic aircraft. In this situation it’s not like you’ve got this meteor streaking down at 17,000 mph and then hits the atmosphere and then slows down. You’ve got the opposite right? You start at the slower speed within the speed of… I guess… the compression within the medium and then it crosses the line. What’s going on there?
 
Pamela: Supersonic airplane is just an airplane that does what airplanes do. They speed up and speed up. With the supersonic ones, eventually they do cross the speed of sound and when this happens, suddenly, the pressure front created by it moving through the atmosphere stops making the normal happy airplane noise that we all hear on a normal basis… which I guess isn’t happy when you’re trying to enjoy the sound of birds. It goes from that to this pressure front that is the “disconnect” between the inside of it where things are moving faster and on the outside of it things are moving slower. When that discontinuity hits you or your windows it can often get mistaken for small earthquakes, you just have this “bam” of everything shaking. That’s the pressure front hitting you.
 
Fraser: What happens when you are inside the airplane? Do you experience this shock wave?
 
Pamela: No, it makes it harder and harder to move as this is occurring so you do actually end up with… it gets more difficult to travel the faster you go. Superseding the speed of sound, it becomes substantially more difficult to keep accelerating. That is one thing you have to take into account. Depending on the shape of your craft this can happen in a number of different ways. There are two different kinds of shock waves that we have to deal with: One is created when you have a rounded object like a meteorite moving through the object. This is your standard bow shock. As you have this big round object plowing through the medium it gets this volume of material that gets built up in front of it that creates a rounded shock wave where you have that discontinuity from the pressure front in front of the object that’s moving plus the normal speed of sound and the normal pressure outside of that shockwave. The other type of shock that we get is an oblique shock and this comes from having a sharp, sharp nosed aircraft from the wedge shape of airplane wings. In this case you have the shock wave connecting to the object that it’s moving through so you have that sharp edge that the shock wave propagates away from. In the case of the bow shock there is that disconnect between the object creating the shock wave where the shockwave occurs. Basically, one is pointy and one is round. They have slightly different physics but in either case they make a loud boom.
 
Fraser: Will you ever get multiple shock waves coming off of an object? …Or is it always just one for the leading edge of the object?
 
Pamela: You can get various surfaces interacting with the air in different ways that cause different shapes to the shock wave coming off but one object will generally create one sonic boom coming off of it. You don’t get a different sonic boom from the tail and from the nose of the aircraft.
 
Fraser: We’ve been talking about shock waves in air but that’s just a compressible medium. Where are a couple other places might we see shock waves and what kinds of events would it take to make them happen?
 
Pamela: In astronomy we see these all the time. They come from supernovae and they come from stellar winds interacting with the interstellar medium. The come anytime you have anything that is flaring or booming; even the ends of jets from extra-galactic objects, galaxies, or active galactic nuclei. These jets can create, within the intergalactic medium, shock waves as they compress that medium and there ends up that inside the compression wave there is a high pressure front and outside of it the normal pressures that the universe exists under.
 
Fraser: Give me a specific example. Let’s take a look at a super nova explosion. How is that going to be interacting with its environment?
 
Pamela: If you look at the Crab nebula, to give a very specific example, you see all of these scalloped edges and all of these tight knots within the medium. With all of those different places, what you have is the energy of the outer atmosphere of the former star that wasn’t the core of the supernova. As that light and as that material flies outward it presses on the medium and compresses it; you can think of this as if you scatter flour across your desk and gently blow on the flour, it will end up streaming outwards. Now if you go out and get some sort of a squeezy-thing that will allow you to give out jets of air you can use that on the flour to end up creating shock waves through the flour. The exact same principle applies with much more complicated math and much more beautiful structures when you look at supernovae.
 
Fraser: But here on Earth when we have the shock wave we hear it right?  I’m assuming we don’t hear a shock wave in a supernova explosion. What do we see where that shock wave is happening?
 
Pamela: The reason we don’t hear it is because the material is so diffused that even if we were to expose our ear drums to the horrors of interstellar space there just isn’t enough material to vibrate our eardrums. In this case what we see is those shocks are building up walls of material that are getting scooped and pushed. Within our own solar system we have noticed to a much smaller degree where our sun pushes out and clears out most of the material within our solar system through stellar winds. It pushes against the interstellar medium and there is actually a point between where the interstellar winds push out and then disconnect which is where the winds basically lose their ability to push things; you have pressure discontinuity. On the other side you have normal interstellar space.
 
Fraser: So basically whenever these particles get mashed together we get an increase in temperature and then a radiation that corresponds to that increase in temperature.
 
Pamela: By radiation, that’s just the light that is being given off. It all depends on what pressures are involved. With the supernovae examples you have gasses as they get compressed and get heated to temperatures that correspond to oxygen molecules giving off light to a variety of other molecules giving off light. We don’t have that point between where the solar wind and the interstellar medium interact. We don’t see that as a wall of light surrounding our solar system.
 
Fraser: That’s cool. So we’ve got the solar wind coming from the sun and we’ve got these supernova explosions. The other one that you mentioned that is really interesting are these jets that come out of these super massive black holes. Obviously it’s not coming out of the super massive black hole itself, it’s being generated by the accretion disks. Still you have these hundreds of thousands of light-year-long jets that are coming out. How are those interacting and creating a shock wave?
 
Pamela: One of the prettiest examples of this is M87. If you just Google M87 it will bring up beautiful images that are usually a combination of the Hubble space telescope and various radio and x-ray images. What you see is this long narrow jet of material that then at the end appears to fountain out. It has basically a fountain at the top and it will stream out as gravity pulls the water back. In this case the jet of material is not getting pulled back to earth by gravity; it’s getting pushed back by the pressure of intergalactic space. You have this jet which fires out hitting the intergalactic media and then it creates a shockwave which produces that discontinuity in pressures between inside and outside of the shockwave. The shock wave curves out as the energy propagates through space.
 
Fraser: One of the theories is that as these jets are creating this pile-up of gas, especially if they happen to interact with other galaxies, you might get these to be places of star formation right?
 
Pamela: We don’t really look for that as much but within our own galaxy the shock waves from supernovae we think it could be possible for compression star forming regions. It’s inside of galaxies where we typically get star formation. There are a few examples. They are finding when you look at light echoes from quasars hitting large pile-ups of gas in intergalactic space that those light echoes contain enough energy to compress the gas enough to trigger some amounts of star formation. The real intriguing usage is how is it that the energy from supernovae compresses? Well new star forming regions themselves inside of galaxies.
 
Fraser: I know as well that we have situations where you have events like on the surface of the sun where you can see these shock waves propagating around the atmosphere of the sun.
 
Pamela: Those are typically different forms of waves; we see convection cells moving through the sun but those are all traveling at the speed of sound or less, usually a whole lot less than the speed of sound. We see beautiful fountaining material that is a classic fountain. You can get shockwaves within stars though but those are pretty special events.
 
Fraser: If you smash a planet into one for example you might get something happening.
 
Pamela: It depends on the in-fall rate of the planet. That’s the cool part. You can trap a planet in nice and slow and gentle and it will cause effects just not supersonic effects.
 
Fraser: Earlier you mentioned something that was kind of neat. Shock waves can travel through any compressible medium, even dirt. How can you have a shock wave travel through dirt?
 
Pamela: Well, if you think about it, you can compact dirt and this is how certain types of earthquakes travel through our earth’s crust. There are different types of waves, S waves, P waves, and those are an entirely different shell. When the compression waves travel through the dirt it’s actually this energy front that essentially acts the way a wave can move through a Slinky. If you move the Slinky from left to right as it’s horizontally stretched between your hands you can see the compression move back and forth. Dirt will do the exact same thing. The speed of sound however is directly related to the density of the material and the higher the density, the slower the speed. It’s also related to the stiffness of the material. The stiffer the material, the slower the speed. This is why when you inhale helium gas your voice becomes much more highly pitched. It’s because the helium gas is low density so the waves can travel faster and you end up with that Mickey Mouse voice.
 
Fraser: Have you ever thought about the speed of sound in dirt? There must be a speed of sound in dirt.
 
Pamela: Every compressible medium has a speed of sound. Magma has a speed of sound. It’s one of those awesome things: If you can squish it, it has a speed of sound.
 
Fraser: There is a medium that we’re very familiar with and is not a compressible liquid and that’s water.
 
Pamela: Yeah. Water is mildly compressible which is why you still have sound moving through water. It’s not the most compressible of mediums though. It’s not as though you can take a container of it and squish it as readily as you can squish many other things out there. It’s not the most effective for propagating shock waves.
 
Fraser: This is why if you fall from a great height and hit water it’s the same as hitting concrete because the water is just not going to compress under you.
 
Pamela: You have to break that surface tension and push the water molecules away. In general a lot of the shocks that we’re familiar with end up creating things like tsunamis which are a continuous wave front. If you have a massive deep water event, the energy from that event will propagate through the water but you don’t have that same discontinuity where you have low pressure on one side and high pressure on the other. It’s that high pressure that drives things. Instead you end up with the energy distributed through continuous waves which, in their own way, can be much more devastating.
 
Fraser: Sometimes tsunami waves are traveling at hundreds of kilometers an hour and they can maintain their energy across the entire ocean sometimes.
 
Pamela: …and these are a very, very different type of wave with different physics involved. IT’s important to think of shock waves as a discontinuity between where the wave front is and the front and the backside of the wave front. Tsunamis don’t quite have that discontinuity the same way.
 
Fraser: Now that we’ve taught everyone about shock waves, let’s pretend we go back to Russia and there was another explosion happening. You don’t want to imagine that but I guess our advice would be to get away from the window. If you hear that explosion or see some event in the sky and you haven’t heard it yet…
 
Pamela: Treat it like a tornado. That’s the best way to think about this is to treat it like a tornado where you have that large differential pressure. That large differential pressure is driving high power winds that do all the damage.
 
Fraser: So if you see the flash and you see the explosion and you haven’t heard anything yet, there is a big noise coming and it could be very disruptive. Get away from the windows.
 
Pamela: One of the questions that I got on Twitter that I’m just going to answer here; I’ll follow up on Twitter in case the poor fellow isn’t watching. If a nuclear blast went off in the Earth’s atmosphere it would generate a different shaped shock wave simply because the meteorite went through the atmosphere. As it moved through the atmosphere it creates a cylindrical shock wave off of it. If you detonate a nuclear bomb, it’s a moving shock wave that is in a single place and it moves outwards from that single place. The experience from a single person at a single place is going to have similar experiences along this same distance away from where the shock wave was generated. It’s bad either way but at least the falling rock from space doesn’t have radiation with it. It just has a nasty sonic boom.
 
Fraser: Yeah but get away from the windows. That’s the lesson we’ve learned today.
 
Pamela: Yeah and don’t fly your planes faster than the speed of sound over cities. We don’t allow it because it does cause damage.
 
Fraser: Well thanks Pamela, I appreciate that and we’ll see you next week.
 
Pamela: Sounds good Fraser, talk to you later.
 
This transcript is not an exact match to the audio file. It has been edited for clarity.