Category: Podcasts

We live in a dangerous Universe. Our tiny home planet is at risk from many extraterrestrial threats: asteroid strikes, solar flares, rogue black holes, supernovae. Now add gamma ray bursts to the list – those most powerful explosions in the Universe. Even 10 seconds of radiation from one of these events would be a deadly setback to life on Earth. Before you start looking for another planet to live on, Dr. Andrew Levan from the University of Hertforshire is here to explain the probabilities of a nearby explosion. It looks like the odds are in our favour.
Continue reading “Podcast: We’re Safe From Gamma Ray Bursts”

Artist illustration of New Horizons with Pluto and Charon. Image credit: NASA/JPL. Click to enlarge.
Listen to the interview: There Goes New Horizons (4.5 MB)

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Fraser: Congratulations on the launch of New Horizons. That’s got to be a big relief.

Alan Stern: Yeah, it’s wonderful to have a spacecraft on the way.

Fraser: So you’ve got 9 years ahead until you reach Pluto. Can you describe the path the spacecraft’s going to take, and what you might be seeing along the way?

Stern: Sure I can. First, it’s easy to remember, we’ve got 9 years on the way to the 9th planet. Our trajectory takes us first to Jupiter for a gravity assist. And we will have closest approach to Jupiter on the 28th of February next year, 2007. Following that, we have a long coast out to Pluto. About 8 years worth. And then we begin the encounter in the early months of 2015.

Fraser: Now as I understand, you’re going to just be going right past Pluto, get some photos on the way, but you’re definitely not going to be able to stick around.

Stern: Well, we have a long encounter. It’s about 150 days of observations of the system on the way in. And then we’ll make some observations on the way out as well. We have 7 scientific instruments, so it’ll be a pretty intensive course of observations. I think it would be selling it short if I characterized it as taking a few pictures of Pluto.

Fraser: And you’re going to be able to do a close flyby of its moon as well?

Stern: Well, you know Pluto has 3 moons. We’re flying through the Pluto system with very specific targeting because there are specific events that we want to make happen. Like we want to make the Sun rise and set so we can study Pluto’s atmosphere; and make the Earth rise and set for similar reasons – for both Pluto and Charon. And so, as we go through the system, all of our closest approach distances are set by those constraints. But we have very good telescopic cameras, and they’ll be studying Pluto, its 3 known moons, and any other moons that we find between now and 2015.

Fraser: And I think that one of the exciting parts for a lot of people is just to see it in photographs up close, because right now, all you get to see are some blurry pixels from Hubble. But just getting some pretty pictures isn’t everything. What’s some of the science that you’ll be pulling from this mission?

Stern: Well, quite a bit. First, this is the first exploration of a fully new type of object – these so-called ice dwarf planets. And so our objectives are very broad. To map Pluto and all the Pluto objects in the system. To map the surface composition as well, so that for every pixel we have a spectrum to determine what things are made of. And to assay the structure and the composition of Pluto’s atmosphere. Those are our 3 main objectives. We’ve got about a dozen others. But unlike a mission like Cassini or Mars Reconnaissance Orbiter, where we’re going back to a target we have visited in the past several times, this time it’s a real first time exploration, so our objectives more have to do with the data sets that we want to collect, and the specific answers we’re answering. We expect to be surprised when going to a new type of object; it’s always been the history in this type of planetary exploration.

Fraser: Well, I guess that’s the thing. Each mission tends to come up with some surprises. Obviously you don’t know what things are going to surprise you, but do you have some hunches on some stuff that you might be finding out?

Stern: We’re very interested to know the structure of Pluto’s atmosphere; what its dominant constituent is. We think we know from the ground, but we’re not sure. We have a hypothesis that Pluto’s surface will be young because the atmosphere is rapidly escaping. It’s removed the ancient terrains by escaping into space. There may be some evidence that Pluto is internally active, so we’ll be looking for evidence of that. For example, in the form of geysers or volcanoes; recent tectonic features, or flows. Similarly on Pluto’s largest moon, Charon, we’re going to be looking for ancient terrains; we’re going to be looking to count craters that tell us the history of the ancient Kuiper belt. And we’re going to be looking to see if we find ammonium hydrates, which have been detected at unfortunately tantalizingly low signal to noise by ground-based observers. But it would say a lot about small worlds.

Fraser: I heard recently that Pluto’s colder than people were expecting. That Charon is actually warmer. Will you be able to do some followup on this?

Stern: I’ll say a word or two about that, because I saw that reported in the press. It’s an incorrect story, in fact, exactly that result was obtained in the 1990s by two groups, published both in Science and the Astronomical Journal. So, I think the press release was flawed. Those results had been obtained about 12 years before.

Fraser: Not new… okay.

Stern: It’s correct, Pluto’s colder than Charon. It’s not colder than expected, because we’ve expected since the early 1990s. Pluto’s exactly the temperature that was found.

Fraser: Right, well I guess the hypothesis though, is that Charon is the result of a large object slamming into Pluto and turning it into a moon, sort of like our own Moon was created.

Stern: That’s right, but it has nothing to do with the surface temperature.

Fraser: Once the spacecraft gets past Pluto, and heads out, where do you want to go next?

Stern: Well, our secondary objective of the mission, and to a lot of scientists, it’s the primary objective of the mission is to see Kuiper Belt objects; the building blocks out of which Pluto and Charon were made. And so, our plan is to go onto one or two, or possibly even more Kuiper belt objects in the years following the Pluto encounter as we move further outward in the trans neptunian region.

Fraser: And I guess that’ll tell you how Pluto might be similar or different to those objects.

Stern: Right, exactly, we want to look and understand the composition of these bodies, learn their histories, and see whether they have atmospheres, the nature of small satellites around them. Count craters on their surfaces to compare to the bombardment of Pluto, and understand the accretion of these bodies.

Fraser: And if you had more time for a longer mission, or more advanced technology that you could put into the spacecraft, more powerful propulsion, what were some stuff you wished you could have added onto the mission if you had more budget?

Stern: I don’t really have any thoughts about the propulsion, and other fantasy land things. We built the mission when we could, and of course in the future decades or centuries, you could always do it, but it was time to do a Pluto mission. You have to build it with the best technology available. If spaceflight’s typically about the very real world engineering problems, were you have constraints of budget, time, mass you can send, things like that. But if we could suspend all belief, and remove those, it would have been very much to our liking to have flown a longer wave infrared spectrometer, so we could look for things like oxides of sulphur on the surface of Pluto and other bodies that we fly by. Perhaps a magnetometer as well.

Artist illustration of a galactic exile. Image credit: CfA. Click to enlarge.
Listen to the interview: Galactic Exiles (6.2 MB)

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Fraser Cain: Can you tell me about the stars you observed and how they’ve come to be kicked out of our galaxy?

Dr. Warren Brown: What we discovered are two stars in the far out regions of the Milky Way that are traveling at speeds that no one has ever really seen stars in our galaxy, at least stars outside of the galactic centre. Except that these stars are hundreds of thousands of light years away from the galactic centre. And yet, the only plausible explanation for their velocity is that they were ejected by the supermassive black hole at the centre of the galaxy.

Fraser: So they strayed too close to the supermassive black hole and were kind of kicked out?

Brown: Yeah, so here’s the picture. This scenario requires three bodies, and astronomers say that the most likely way that it happened is if you have a pair of stars. As you may be aware, something like half the stars in the sky are actually systems containing a pair, or sometimes more stars. And so if you have a tightly bound pair of stars that, for some reason, travel too close to the supermassive black hole, at some point the black hole’s gravity will exceed the binding energy between the pair of stars and rip one of those stars away. It’ll capture the one star, but the other star then leaves the system with the orbital energy of the pair. And that’s how you get this extra boost of velocity. It’s that the supermassive black hole is basically able to unbind one star, capture it, and leave the other one with the entire amount of energy that the pair used to have. And that star then gets ejected right out of the galaxy.

Fraser: Then if a regular, single star came too close, it wouldn’t have the energy to be ejected. I think I’ve seen some simulations where the star gets too close to the black hole and kind of changes the direction of its orbit, but it’s still continuing to orbit around.

Brown: Sure, you could imagine it’s like a spacecraft that gets slingshot around Jupiter or something. You can imagine that you might be changing the trajectory, and gaining some speed. But there’s no mechanism in the galaxy to gain this much speed for something that’s the mass of a 3-4 solar mass star. That requires a three body interaction to create the velocity we see. And what we observe is their motion with respect to us. They’re moving away from us at a velocity of about 1-1.5 million miles an hour.

Fraser: How fast would the stars have been going when they came in to meet their breakup?

Brown: I don’t know for sure. Probably something 10 times that, right before that moment when they’re swinging past the black hole. Of course, as you leave that gravitational potential well of the black hole, they slow down pretty suddenly. Their final escape velocity is what we observe now; it’s on the order of a million miles an hour. And that’s well over twice the velocity that you need to escape our galaxy altogether. These stars really are exiles. They’re being outcast from the galaxy and they’ll never return.

Fraser: And one star is kicked out. What happens to the other star?

Brown: That’s an interesting question. In fact there’s a theory paper that some theorists have written that suggested that these stars in very long elliptical orbits around the central massive black hole might be the former companions to these so-called hypervelocity stars that we’ve discovered. And that’s the sort of orbit you’d expect. Unless the star is so unlucky as to fall straight into the black hole, if it misses just a little bit, it’s going to just swing around and then be on a very long elliptical orbit around the central massive black hole.

Fraser: And where did the pair originate? Is this a fate that might affect some nearby binary stars?

Brown: Well, that actually gets to the bigger picture. The galactic centre is an interesting place. It has lots of young stars. Three of the youngest massive star clusters discovered in the galaxy come from right near the galactic centre. And they contain some of the most massive stars in the galaxy. So there’s lots of young stars orbiting around down there. The question is, how do you get a star to tweak its orbit so that it shoots straight towards the supermassive black hole, instead of just orbiting around it, like the Earth orbiting the Sun. And that’s an open question. And one thing that these hypervelocity stars we’ve discovered are starting to give us hints about maybe how that mechanism works. Because, for example, one idea is that with these star clusters we’ve observed. Perhaps by dynamical friction, as they encounter other stars, they can sink slowly down towards the galactic centre where there’s the black hole. And it that were to happen, you could imagine that suddenly there were a whole bunch of stars right by that massive black hole. You could get a burst of these hypervelocity stars. There’s all sorts of stars to eject. And yet the stars that we observe all have different travel times from the galactic centre. This is only suggestive, but already we’re starting to be able to say something about the history of stars interacting with the supermassive black hole. And what appears so far, is that there’s no evidence for star clusters falling into the galactic centre.

Fraser: There could be some kind of conveyor belt that stars are born and then they slowly sink down and then they’re kicked out as they get too close.

Brown: Yeah, that’s sort of one idea. For that conveyor belt to work, you need some kind of massive place like a star cluster for that conveyor to work. To be able to sink something down towards the massive black hole. As a massive object encounters lots of massive objects, it turns out the less massive objects will tend to give off a little more energy. As the massive object, in this case a star cluster, loses energy, its orbit decays and it gets close to the galactic centre.

Fraser: With the few number of stars that you’ve found, and the large number of stars in the galaxy, it must have been a pretty difficult job to track these guys down. What was the method that you used?

Brown: Yeah, that’s actually one of the exciting results of this time. The first discovery, a year ago, after the first hypervelocity star, it was something of a serendipitous discovery. And this time we were actively looking for them. And the trick was that these things ought to be very rare. Theorists estimate that there’s perhaps a thousand of these stars in the entire galaxy. And the galaxy contains over a 100 billion stars. So we had to look in a way that gave us a pretty good chance of finding more of them. And our strategy was twofold. One is that the outskirts of the Milky Way contain mostly old, dwarf stars. Stars like the Sun, or less stars that are red. There’s no young, blue massive stars, and that’s the kind of star that we decided to look for; stars that are young, and luminous so that we can see them far away, but where there shouldn’t be these stars like that in the outskirts of the galaxy. And the other part of the strategy was to look for faint stars. The further out you go, the less background galaxy stars you have to contend with. And the more likely you’ll come across these hypervelocity stars, as opposed to another star that’s just orbiting the galaxy.

Fraser: And what’s the method you use to actually tell how fast that the star is moving?

Brown: For that we had to take a spectrum of the star. Using the 6.5 MMT telescope in Arizona, we pointed the star at one of our candidate stars and we take the light from that star and we put it into a rainbow spectrum and take a picture of that spectrum. And the elements in the stellar atmosphere serve as a fingerprint. You can see absorption lines due to hydrogen and helium and other elements. And it was using the motions, the Doppler shifts – in this case the red shifts – of those wavelengths told us how fast the stars were moving away from us. And most of the stars in our sample were normal galaxy stars; they were moving fairly slow velocities, and then two of these happened to be traveling quite fast, and that’s the two that we announced just now.

Fraser: And what do you think this tells us about the formation of stars, or the centre of the galaxy, or…

Brown: Well, that’s actually an interesting part of the story this time around. Now that we actually have a sample of these, these are really a new class of objects, these hypervelocity stars, we can start to say something about where they come from, which is the galactic centre. These stars are uniquely suited for telling us the story about what’s been happening at the galactic centre. Their travels times tell us something about the history, what’s been happening, but also the kinds of stars we’re seeing. In this case, these young, blue stars – these 3-4 solar mass stars – which astronomers call them B-type stars. The fact that we’ve seen two in our survey region, which we’ve carried out for about 5% of the sky, is consistent with the average distribution of stars you’d see in the galaxy. But inconsistent with what a lot of these stars clusters you see in the galactic centre. So just the fact of the type of stars you’re seeing is starting to tell us about the population of what’s been shot out of the galaxy. In this case it doesn’t look like it’s these supermassive clusters of stars, but rather your average star that’s wandering through the galaxy.

Fraser: And if you had some kind of super Hubble telescope at your disposal, what would you want to look for?

Brown: Oh, we’d want to look for the motion of these stars in the sky. So all we know if their minimum velocity. The only thing that we can measure is their velocity in the line of sight with respect to us. What we don’t know is there velocity in the plane of the sky, the so called proper motion. It’s possible to do that with Hubble, if you have 3-5 year baselines with which to see these stars move. It should be a very small motion. If you had a super Hubble, maybe you could see it in a year. So that would be very interesting to know. Not only would that tell you for sure that these really are coming from the galactic centre, and not from some place else, but also their trajectories. If you knew exactly how they’re moving out, any deviation off a straight line from the galactic centre tells you about how the gravity of the galaxy has been affecting their trajectory over time. And that’s also very interesting to know.

Fraser: Right, so that would help with plotting out the distribution of dark matter.

Brown: Exactly, exactly. So astronomers infer the presence of dark matter. We see stars orbiting the galaxy faster than they should be just because there appears to be mass that we can’t account for holding them in their orbits. And this dark matter, it’s hard to get a handle on how it’s distributed around the galaxy. But these stars are already at the outskirts of the galaxy, and as they pass through it, this perturbation, this gravitational pull of dark matter as these things travel through the galaxy slowly adds up as they go. So they’re actually measuring the distribution of this dark matter, just on their orbits. So if you could measure their motion, of a sample of stars, it actually starts giving you a handle on how the dark matter is distributed around the galaxy.

Asteroid 951 Gaspra taken by the Galileo spacecraft. Image credit: NASA/JPL. Click to enlarge.
Listen to the interview: Gravity Tractor Beam (4.8 MB)

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Fraser Cain: Dealing with asteroids that are going to hit the Earth, now as I understand it, you need to find a crew of top quality oil miners. And you need to put them on the Space Shuttle and send them with a bunch of nuclear bombs to the asteroid to blow it up. Now you’re telling me that maybe this isn’t the best way?

Dr. Stanley G. Love: Well, it depends on what your goal is. If your goal is to make a movie that’s going to make a ton of money, then go wild; that’s exactly the right way to do it. If your goal is to actually prevent an impact with the Earth, though, we’re hoping there might be a simpler method of dealing with this.

Fraser: All right, so what’s the simpler method that you’re suggesting?

Love: Well, the method that we’re suggesting is to send a relatively large and heavy spacecraft – not so large and heavy that we can’t imagine it – to the asteroid, and instead of trying to blow up the asteroid, or land on it and push the thing aside (both of those ideas have been suggested, but they have some difficulties), we’re suggesting you just park the spacecraft next to it and let it hover there. And if you let it hover there for something like a year, very very gradually, the tiny gravitational pull between the asteroid and the spacecraft is going to pull the asteroid over in the direction of the spacecraft. The spacecraft is hovering in a constant distance from the asteroid, and what this means is that it’s very gradually pulling the asteroid off course using gravity as sort of a tow line. And if you can get enough warning on your asteroid – if you know it’s coming 20 years or so before it’s going to hit – then you can get the spacecraft out there and have it pull for about a year, you can pull it enough so that instead of hitting the Earth, it will miss the Earth.

Fraser: Now all the media, and all of those disaster movies revolve around some astronomer spotting a dangerous asteroid three months before it’s going to hit. It sounds like your solution is more in the 20 year range. Do you think that’s the more realistic scenario now these days?

Love: It’s hard to know. We haven’t really discovered all of the asteroids that could potentially hit the Earth yet. We’ve got a lot of people very busily working on that problem; there are searches going on every night. I think a lot of them are automated, and not some lonely guy on a mountaintop with his eye to the lens of a telescope there. And it is possible that tomorrow we could realize that there’s something coming that could hit us that we didn’t know about and it could be three months away from impacting the Earth. That would be certainly unfortunate. But in the future we are likely to know all these things; know all their orbits, and we can predict a hit long before it’s going to hit us. And that’s the sort of scenario that our solution will be able to deal with.

Fraser: And so what size of asteroids would you be able to deal with?

Love: A couple hundred metres in size. So the size of a football stadium or convention center.

Fraser: And what would the spacecraft itself look like? What kind of components would it have on it?

Love: When we came up with the idea for our little paper, we pulled a spacecraft design essentially off the shelf. It’s NASA’s Prometheus project, where they were going to send a large nuclear powered spacecraft to orbit Jupiter’s moon Europa, and do a lot of interesting science there. It’s a 20-ton spacecraft with electric thrusters, that is it uses electric power to heat a gas to extremely high temperatures and squirt it out the back. You get marvelous fuel economy; a lot of ability to move a spacecraft with a small amount of fuel, but the thrust is really low. You can only get a newton, or so (a fifth of a pound) of force. So you have a large electric propulsion, nuclear powered spacecraft – this is probably going to be a long skinny thing, because you’ll need a lot of radiators to reject the waste heat from the nuclear reactor. It’s going to have a set of thrusters, a fuel tank, and some guidance and navigation components. Depending on how you set this spacecraft up, we decided that if you put the reactor, which is heavy, and the fuel tank, which is heavy, down close to the asteroid – hanging from the thrusters – then you get more mass close to the asteroid, and that increases your gravitational pull as gravitational pull decreases rapidly as you increase the distance between the two masses. And it also helps stabilize your spacecraft and just helps you all around if you put your heavy components hanging down by the asteroid with the thrusters up at the top.

Fraser: Oh, I see, it would almost be if you had a ball at the end of a rope, hanging down with the heavy part – the reactor and all the fuel – hanging as close to the asteroid as you can, while all the thrusters are further up the rope pulling it away.

Love: That’s exactly right. Of course you need to tip your thrusters out away so the plume of hot gas coming out of them doesn’t hit the asteroid. It does no good trying to pull an asteroid closer to you with gravity and at the same time that you are pushing at it with your thruster plumes. So you need those outward so the plumes miss the asteroid and that will help improve your towing force.

Fraser: Now do you have any targets that you think might be a good victim of this kind of movement strategy?

Love: We were sort of developing the idea as a generic idea, and fly to anything. However, there’s Asteroid 99942 Apophis which is supposed to make a close pass of the Earth I think in 2029. And if that asteroid happens to pass through exactly the right point in space as it goes past the Earth, it has a chance to come back in 7-8 years and hit us, which would be bad. And that asteroid is an excellent target for this kind of a mission. If we can get to it before that first Earth flyby, that would line it up for impact the second time around. And the reason for that is that these flybys warp the path of the asteroid so that a tiny tiny change in the flight direction before the flyby gives a huge change in the flight direction after the flyby. So it’s like a bank shot in pool. A little tiny mistake on the first part, after the bounce, the mistake gets multiplied. So you could use a gravitational tractor that wasn’t nuclear powered and didn’t weight 20 tons. You could use a 1-ton, chemical-propelled gravity tractor to pull this asteroid just slightly off course before that Earth flyby so the asteroid is going no where near us.

Fraser: I see, if you have an asteroid that coming towards us 20 years out, you could move your big ion engine-powered tractor. How long would you need to have it spend next to the asteroid?

Love: About a year.

Fraser: But if it’s just about to do the flyby, you could give it a very small change and it would still kick it out of the bad orbit and into a good orbit.

Love: Right, you’re going to use that flyby of the Earth to multiply the tiny effect you put on the asteroid with your spacecraft before the flyby. And then after the flyby, the effect is much greater.

Fraser: So what’s the stage of your proposal now? What’s the future for it right now?

Love: Well it hard to know. Right now we’ve made a proposal, we’ve gotten the idea out there, and people are talking about it. My co-author, Ed Lu and I have written many scientific papers for publication, and none of them have received even a tenth as much attention as this one. So the idea’s out there, and we’ll see what happens. I think the debate will become much more pointed if we actually do discover an asteroid that’s on a collision path with the Earth. Then we’ll really need to get together and decide what we’re going to do about it.

Fraser: Well that’s my concern with the whole process of protecting the Earth from asteroids. There’s a lot of uncertainty in predicting when and where an asteroid is going to hit. The better you can mark the orbit, the better you can know if it’s going to be a risk. In many cases, if you’ve got these ones that are 30 years out, decision makers and lawmakers might say: well, let’s wait until we know better. And yet, the more you know better, the less chance you have of changing its orbit.

Love: Yes, that’s always true, and human nature plays into this a lot. Nobody’s every suffered an asteroid strike, so it’s hard to compare it to things that we have suffered, like tsunamis and hurricanes to take a couple of recent examples. The things that we know about and experience in a person’s lifetime are always easier to visualize and understand. And to get people to pay attention to something that seems kind of esoteric and science fictiony; is this real, or are people just making it up? I don’t know a good solution to that, but the fact that people are talking about the idea and thinking about it – and not just in the elevated circles of academia – all over the world, I think is a good sign. At least we’re thinking about the problem and how to solve it.

When I mention to people that I record a regular podcast, I often get some kind of response like, “oh, I can’t listen to that, I don’t have an iPod.” Yikes! Podcasts, despite their name, don’t require an iPod, or even a portable MP3 player of any kind. If you like to listen to radio shows, you’ll love podcasting. I’ve written up an explainer to help you get up to speed, and listen to a whole Universe of free audio content – especially my own podcast. 🙂

Fraser Cain
Publisher
Universe Today