Astronomy Cast’s 2014/15 season begins! With Rosetta’s arrival at Comet 67/P, we’re about to see a comet up close and personal. What will it take to explore, exploit and enjoy the asteroids and comets hurtling around our Solar System. And how does science fiction have it all wrong?
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Announcer: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit Astronomy.swin.edu.au for more information.
Fraser Cain: Astronomy Cast episode 351, Asteroid Adventures. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos. 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 and with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville and the director of CosmoQuest. Hi Pamela, how you doing?
Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser Cain: Great. Welcome back to the 2014-2015 season of Astronomy Cast.
Pamela Gay: And isn’t this – so this is 2014, so this is our eighth season we’re starting.
Fraser Cain: Yeah, eight years of Astronomy Cast coming at you.
Pamela Gay: There are people that have started college and gotten their masters degrees while we’ve been doing this.
Fraser Cain: Totally. I would love to hear some stories. Oh, you should totally email us.
Pamela Gay: Yeah, actually that’s an awesome idea.
Fraser Cain: If Astronomy Cast has somehow influenced your educational career and you started early on and you’ve been proceeding through it, let us know. Email us at firstname.lastname@example.org. We’d love to hear it.
So if you’re hearing this hopefully you will have already heard our live DragonCon episode which will have gone over in the feed at some point in the last few days. So I hope you enjoy that. And that is a taste of things to come.
All right. Let’s get going. So Astronomy Cast 2014-2015 season begins with Rosetta’s arrival at Comet 67/P Chury-Gery. We’re about to see a comet up close and personal. What will it take to explore, exploit and enjoy the asteroids and comets hurtling around our Solar System? And how does science fiction have it all wrong?
Announcer: This episode of Astronomy Cast is brought to you by 8th Light, Inc. 8th Light is an agile software development company. They craft beautiful applications that are durable and reliable. 8th Light provides disciplined software leadership on demand and shares its expertise to make your project better. For more information visit them online at www.8thLight.com. Just remember that’s www dot the digit 8 T-H L-I-G-H-T dot com. Drop them a note. 8th Light. Software is their craft.
Fraser Cain: So, Pamela, we’re choosing this episode because holy-moly we’re about to land a spacecraft on a freaking comet.
Pamela Gay: Yes. And what I love is for the past year I’ve been hearing people from the Rosetta mission go, no, we’re not landing. We are harpooning, we are docking. We are not landing. There’s not enough gravity to land. And today through the entire Rosetta session at the European Planetary Sciences Conference, which is where I’m at, all day I’ve heard people say, we’re landing on an – because it’s impossible to pronounce – comet they said we’re landing on the comet. And all of their slides just said CG67P. It’s kind of awesome.
Fraser Cain: Yeah, so the Chury-Gery, this is from Emily Lakdawalla. So, you know, she works for the Planetary Society, one of the most knowledgeable people in planetary exploration. So that’s her name for it. I love it. That’s what we’re using from here on out, Chury-Gery.
Pamela Gay: The actual name, as near as I can pronounce it, is Churyumov-Gerasimenko.
Fraser Cain: Cool. Your Russian’s good. Okay. So let’s talk a bit about so the goal of this episode really is to talk about what does it take to reach these asteroids, to orbit these asteroids, to be able to actually set down on these asteroids and comets and —
Pamela Gay: And when he says asteroids, he means small bodies including comets.
Fraser Cain: Yes, asteroids and comets. All right. So let’s sorta like set the stage then about where these objects are in the Solar System and what’s involved to actually reach them.
Pamela Gay: So some of the things that we’re going after in general are going to be somewhere out beyond Mars. So they’re kind of a pain to get to. When I say beyond Mars, they could be a lot beyond Mars. The Rosetta spacecraft had to put itself into a rather severe elliptical orbit in order to match the orbit of the comet that it’s going to be harpooning and putting a Lander onto.
So you’re going out far enough that you can still use solar panels. But if you do you have to power yourself down a lot. You’re going out far enough that ice has become an issue. Staying warm is always an issue. And time is the real annoyance because you’re waiting ten years, eleven years longer to get your orbit correct to be able to do what you’re doing if orbiting and landing is your goal.
Fraser Cain: Right. And so what are the main missions that have been part of this? I mean, there was the mission to Eros with NEAR.
Pamela Gay: Right. So I have notes.
Fraser Cain: Oh, good. I don’t have to pull them out of my memory.
Pamela Gay: No, no, no, no. That’s just mean. So we’ve had Chang’e went within 5 kilometers of a little tiny cute asteroid. I’m just going over the ones that got really cute – not cute – really close – they’re cute as well but really close. We had Hayabusa –
Fraser Cain: Hold to the close missions, please.
Pamela Gay: Yes. We had Hayabusa actually returned – is in the process of returning a sample from Itokawa. We had Deep Space 1 got within 26 kilometers of Comet Borelly. Rosetta did a flyby of Comet Steins at 800 kilometers. NEAR Shoemaker landed on Eros and took all sorts of awesome images from 35 kilometers away.
But for asteroids in general, most of the images have come from 1,000 kilometers or further away. And when we start looking at comets, spacecraft are even less willing to get close in. So with the early missions that were trying to get in close to Halley, we were looking at Giotto got within about 600 kilometers. And that’s not that great compared to, well, Rosetta is going to be orbiting at about 30 kilometers up and then working itself down to a 10-kilometer orbit.
And then we’re looking at Stardust got within 240 kilometers of [inaudible] [00:07:14] 2. And Temple 1 was approached by Stardust at 181 kilometers. So comets are not something you usually get nearly as close to. And this is probably because a lot of these missions were trying to stay outside of the blast zone.
Comets are volatile. They periodically release all sorts of different things. My favorite comet of the day was looking at EPOXI results of Hartley 2. And Hartley 2 is a comet that they call it a bilobate nuclei. It means that it’s kind of barbell shaped and it has a waist or a neck, whatever you want to call it around its middle. And that middle skinny section gives off water vapor. And the far end of the tail part of this comet – I was waiting for one of the scientists to say derriere or something less appropriate – but the tail end of this comet was giving off C02.
So you have these really neat structures that are giving off jets of different volatile materials. And getting hit by a jet is gonna screw up your orbit. And you have to have enough fuel onboard to compensate for that. And a lot of these missions aren’t carrying enough fuel.
Fraser Cain: Right. And so, I mean, as you said, you’ve got these – the material that’s streaming off the comet that the spacecraft is gonna have to deal with. You’ve got the irregular shape and spin of the object that the spacecraft is gonna have to deal with. I mean, you can imagine the – one of these big asteroids tumbling and turning and trying to land, you’ve gotta match this strange rotation that the asteroid’s doing with the spacecraft.
Pamela Gay: You have a 12.4 hour rotation period of Chury-Gery is what we’re calling it – Chury-Gera?
Fraser Cain: 67P, yeah, Chury-Gery.
Pamela Gay: So you have a 12.4 hour rotation period of an object that is currently being described as a biolate duck. And over and over everyone was referring to it as a duck because if you rotate it, it does indeed look like a rubber ducky. It ‘s two nonaligned objects that are connected by a skinny neck of material. The entire thing is not that many kilometers across. You don’t really have gravity that’s all that much to speak of. It’s just barely enough to allow them to orbit. And so now you’re trying to maneuver in and to land on a rotating duck.
Fraser Cain: Yeah, yeah, not easy, which is spraying material at you.
Pamela Gay: Yes.
Fraser Cain: So it’s not an easy job.
Pamela Gay: It’s giving off a lot of the jets.
Fraser Cain: Yeah, yeah. So then – and I guess the other part of the problem is that the gravity is so low that any maneuvers that you make will push you – like it must be super hard to get captured by the very weak gravity. It’s almost nonexistent.
Pamela Gay: Yes. Well, and with missions like Dawn which [inaudible][00:10:24] it’s on its way to serious missions like Dawn which snuck up on [inaudible]. What you essentially do is you first match the object’s orbit as it goes around the sun. And as you’re matching the orbit you slowly just basically pop yourself into an orbit that is spiraling around that object as you co-orbit around the sun. It’s messy.
Fraser Cain: Yeah, you’re gonna have to put some Kerbal Space Program to really know how to do this maneuver. I highly recommend it.
Pamela Gay: They actually have full-on simulators where as it was described to one of the women on the Dawn mission. She goes into a very, very cold room where they have a secondary version of the spacecraft, plug all of the maneuvers into this cold storage version of the spacecraft, look to see what happens and then compare the output to what was expected.
Fraser Cain: Oh, that’s really cool.
Pamela Gay: Visually.\
Fraser Cain: Okay. So let’s specifically then talk about Rosetta because at the time that we’re – we don’t normally do this but at the time that we’re recording Rosetta is orbiting 67P. It’s about to send the Philae Lander down, the Philae harpooner down.
Pamela Gay: Right now they’re in the process of narrowing it down from five landing sites, two on the head of the duck and three on the body of the duck. They’re trying to narrow it down to one landing site and one backup landing site. That will be announced on September 15, so right after this episode goes live. They’re going to then say specifically which one of the primary and secondary sites it is in October. And then in November they’re gonna send that little Lander down.
And one of the big issues that they’re running into is when you’re trying to land on a moving object, you end up with error ellipse. We see the exact same thing when we land on Mars with the various rovers. There is this area on the surface that is defined by how well they think they can land based on wind in the atmosphere, rotation of the planet and all these other factors.
Well, Chury-Gery doesn’t have an atmosphere to worry about but it has these jets. And its gravity isn’t exactly knocked out perfectly and so you still end up with an error ellipse because it’s a rotating object. So it’s an ellipse not a circle. The ellipse is one kilometer in diameter. It’s called a [inaudible] [00:13:14] meter error ellipse or landing ellipse. And they haven’t found any flat areas that are that big. So landing is actually a much more difficult process than they thought it was going to be.
They are also struck by the fact that they can’t actually get in at the neck, for instance, because how do you maneuver in there? They just don’t have enough fuel on the Lander. There’s issues of, well, if you’re orbiting this way you can’t actually get to all parts of the spinning object, so there’s a ton of constraints in terms of just the dynamics of the system and what orbital mechanics and the amount of fuel. They refer to it as the amount of change in velocity that you have on your Lander and spacecraft.
So you take into account what you can do. You then start looking for sites that are big and smooth, get a little upset because you can’t find them, start looking for things that at least have the surface parallel to gravity. Because one of the other problems they’re having is, this is a silly looking object that is sloped in all sorts of crazy directions with respect that insane gravitational potential because it’s rubber duck shaped.
Fraser Cain: So let’s – so it’s hard?
Pamela Gay: Yes.
Fraser Cain: And they’re gonna figure it out and they’re gonna – and so in the case of Rosetta it’s gonna take this harpoon on the Philae Lander, it’s gonna jab it into the comet and then it’s gonna reel itself in and try and land.
Pamela Gay: Yeah, and this is another one of those things that they don’t take into account really. It’s like Armageddon. One of the things – one of the many things that had me kind of screaming at the television set during the few excerpts of Armageddon that people who love me still let me say. They land on this comet that is nominally kilometers across and they’re walking around.
Fraser Cain: It was 1,000 kilometers across. It was as big as Texas.
Pamela Gay: It’s still not gonna have that much surface gravity.
Fraser Cain: Yeah. Well, even in like Deep Impact it was only more like 10 kilometers, right? You wouldn’t – and that was a comet and you wouldn’t be able to stand on it, yeah.
Pamela Gay: Yeah, so you have this super low gravity surface. And if you try and just gravitationally land on it while it’s spinning past you, it gets tricky. So by harpooning it, it guarantees that you match that rotational velocity and you essentially reel yourself in. And suddenly it makes the dynamics a little bit less scary to deal with.
Fraser Cain: Yeah, I mean, we had an ad hoc landing with NEAR when it landed on Eros, right? It sort of –
Pamela Gay: Yeah, that was awesome.
Fraser Cain: Yeah, I know, I know. And it sorta slowly made its way closer and closer and closer and then just landed and survived –
Pamela Gay: Yeah.
Fraser Cain: — briefly and was able to provide data. But hopefully the Philae Landers can do a better job. So let’s talk about the science then. So what kind of science are astronomers looking at both in the landings they did with – on Eros and the Philae Lander and future missions. What unsolved questions are they trying to get to the bottom of?
Pamela Gay: Well, at the end of the day, asteroids and comets are leftover material from the formation of our Solar System. They haven’t been processed through all of the different things that happen on planets like earth that cause you to have silver mines in one place and uranium in others. All of these differentiation processes that we have and other large objects have, comets and small asteroids don’t have the same way.
So when you get up and explore these things it gives you a chance with the sample return missions, with the spectrometers, with all of the instrumentation to start to get a sense of what were the original ingredients that made up our Solar System before they got processed by all of the things that happen when you have chemistry happening on the body?
Here on earth we end up with all sorts of complex things that weren’t formed in the Solar System but were formed here because we have weather, because we have so much gravity and because we have tectonics. Now we find that objects like [inaudible][00:17:41] actually are differentiated. We didn’t know that and that’s kinda cool.
And the Dawn mission, it’s looking at bigger things. It’s looking at [inaudible] here it’s looking at two large almost planets that are on either side of the Solar System’s ice belt. So here you’re looking at things that formed from either edge of a very special mine in our Solar System’s formation.
But when we’re looking at all of these smaller things we’re basically going, okay, what were the raw ingredients before we baked the Solar System through planetary differentiation? What’s cool with Rosetta is they’re actually going in and they’re looking for organics that formed naturally. They’re looking to see what is the mix of chemicals? What are all of the awesome things that happen when you look at the debris that comes from the outskirts of the Solar System instead of the outskirts on the inner part of the Solar System?
Fraser Cain: Right. And 67P is a long – or is a short-period comet, right. So it’s one that’s bee orbiting within the relatively inner Solar System for billions of years. And so it isn’t fresh in the way that, for example, some of the new ones like the new siding springs one that’s gonna be going past Mars shortly is, things like that. And so it’s gonna be a different creature. I mean, that’s – man, that would be the dream, right, land a Lander on a comet where you don’t – that’s come from the outer – from the [inaudible][00:19:04] cloud, right?
Pamela Gay: But the problem with things coming in from the [inaudible] cloud is you don’t have enough years to match their orbit before they – between when they would get to [inaudible] out by Jupiter, if you’re lucky, and when they come in towards the inner Solar System.
` With this particular comet, it’s not even coming all that far into the inner Solar System. It’s gonna get in around 3AU and that’s awesome but that’s not a sun grazer and it’s actually a whole lot safer for the spacecraft because it’s not going to have huge amounts of activity like you would if you got too close to the sun.
But if you’re looking at one of those outer comets coming in, you’re not gonna get a super precise orbit. You’re gonna basically be throwing rocks at a moving target and you don’t have enough years to throw that rock. So, yeah, it’s awesome, it’s a dream but don’t know how you do it at this point. We just don’t have the ability to get things going fast enough.
Fraser Cain: Yeah, I believe that’s why I said, man, wouldn’t it be cool? That’s why I did my empty speculation because beyond that, right, being able to get up close and study one of those comets is important –
Pamela Gay: Yeah.
Fraser Cain: — because we need to understand what their constituency is. What are they made of? How dense are they? How do they behave? Because as we watch each impact, the goal will be to eventually be able to move, shift, adjust, harvest from these comets.
Pamela Gay: And, well –
Fraser Cain: And so to actually be –
Pamela Gay: — I’m not sure we’re gonna harvest a comet.
Fraser Cain: — harvest their precious helium three –
Pamela Gay: They’re like water and CO2. We’ve got those.
Fraser Cain: But part of the challenge with these, like if we can’t even get to one of these comets to be able to figure out how to move them or even study them, how are we gonna be able to move them and protect the earth from these if they’re gonna eventually impact us? So anyway that’s a whole other rabbit hole [inaudible][00:21:04] to go too deep down into that.
Pamela Gay: So a rabbit hole that you probably aren’t thinking about though that makes these things interesting in a different way is when the Solar System formed you ended up with a different mix of stuff at different distances from the sun. This is how we know that the earth and the moon came out of the same lump of stuff as we have the same isotopic ratios. Mars is made up of a different ratio of stuff. This is how we’re able to identify Mars’ meteorites when they land on earth. This is how we’re able to say these meteorites all came from the same parent asteroid.
As we look at things that came from different distances from the sun when they formed, we’re getting a different sampling of that original solar nebula. So objects that come from the Kyper Belt for their origins are gonna have a different composition than objects that came out from the [inaudible] cloud.
So beyond the whole protecting earth, which is underway to justify spacecraft with congress, the whole let’s get a teaspoonful of the outer Solar System, that’s basic science, basic research. You can at least usually get a small mission out of that sort of an idea.
Fraser Cain: Yeah, so one of the things as well when I look at the pictures of 67P is –
Pamela Gay: — the giant duck.
Fraser Cain: The flying duck, the – it looks like a little eagle, an eaglette to me – is how much it looks like an asteroid. Like it doesn’t look like a comet.
Pamela Gay: It doesn’t have jets that are visible with the contrast that allows you to see the surface. This is one of those things that is really frustrating is it does have jets, it does have a mission right now. It doesn’t have a giant awesome tail. It doesn’t have a big fuzzy [inaudible][00:22:44] but it is giving off stuff. But you can only see that stuff if you play with the contrast to the point that the surface of the comet is completely saturated. So when you turn down the contrast so that you’re making out the surface features, you lose all of the jets that make it look like your quick essential comet.
Now, at the same time I wouldn’t say it looks like an asteroid because the sucker isn’t all cratered. It doesn’t have craters. Also it’s much more jagged with many more sharp edges. And asteroids in general are a lot more rounded in their appearance and lack these sudden plateaus and chasms that you see with the ice fractures all over this object.
Fraser Cain: It looks like, and I don’t know if you get this there, but here in Canada when we go up to the mountain we have this great big – like it snows tons in the wintertime on the mountain because it rains here all winter long.
Pamela Gay: Yeah.
Fraser Cain: And so we get this almost a channel that you have to drive through on your way up the mountain where it can be ten years of snow that you’re driving up through.
Pamela Gay: Right.
Fraser Cain: And so you’ve got snow banks on both sides but then the actual ground is dirt and mud and muck and the cars are driving through it and they’re spraying up this dirt and it’s covering all this snow that looks like snow but it’s sort of half melted and ice and whatever and it’s all covered in dirt. And it is like gray, it looks like someone painted –
Pamela Gay: It totally looks just like that.
Fraser Cain: Yeah, someone just painted snow with mud. And that’s what it looks like to me.
Pamela Gay: One of the awesome things that happens is with a lot of organic compounds that form naturally, when you expose them to ultraviolet light, which the sun has, they blacken overtime. This is one of the awesome things that creates dirty snowballs out of really old comets.
And there’s actually a few objects that have been misidentified as asteroids because they’re very old comets that have used up the most volatile parts or have caked over the most volatile parts. So as they pass around the sun they’re just going, okay, I’m covered in organics. I might have a little jet over here. But in general they’re just coated in dark sludge. And this is a sludgy object still. It’ll shine up as it gets a little closer to the sun.
Fraser Cain: And that’ll be really interesting, right, which is – and I think that’s one of those scientific questions is what exactly happens as it warms up, as it gets to the point that these jets start to appear and start to erupt out of the surface of the comet? And wouldn’t it be amazing for the Lander to be close to one of these jets –
Pamela Gay: And that’s the plan.
Fraser Cain: — not under one of the jets. Not on the business side of one of these, although it probably won’t be too much of a problem for it. Like it’s not gonna be blasting jets. It’s gonna be –
Pamela Gay: Yeah, it will.
Fraser Cain: What’s it gonna be I guess?
Pamela Gay: I mean, so the thing to think about is if you’ve ever watched dry ice, it gives off nice pretty clouds of material. These get used to create fog when you don’t have a fog machine. But if you blast, and I don’t recommend this, if you blast that dry ice with a crembrule torch you will get a jet of material trying to escape as it rapidly sublimates. And that rapid sublimation is the same thing as a jet forming.
And what you end up with on comets is most of the surface is sludged over but as the surface melts you end up revealing a pocket of more pure material that just jets. And that’s kinda volatile. Now this probably won’t happen but there’s the miniscule chance that they’ll harpoon into something that just decides to jet back off. And that would be a bad day.
But what they’re hoping to do is actually land near activity but not on an active region.
Fraser Cain: But it’s hard to know when you’re looking down – when your map – I guess this is part of the landing process is they’re trying to guess and go, which parts will probably not have jets or –
Pamela Gay: And this is where they use spectroscopy to try and understand what are the different things in different parts of the surface. They’re trying to land near organic materials. They’re trying to land near an active region. They have a ton of constraints. I do not envy the group of people who are choosing that final landing site.
Fraser Cain: Yeah, yeah.
Pamela Gay: But they wanna do all the things you wanna do, so you’ll be happy.
Fraser Cain: Yeah – no, it’s just an amazing mission. So then before we wrap this up, I would love to know what would be sort of the next dream mission. If you could sit down and hammer out the requirements of another mission that would interact with a small body, a minor body in the celestial system, what would you want your mission to do?
Pamela Gay: So if I was looking at an ultra low-cost mission, I’d want to take one of the earth-crossing asteroids that gets fairly close but not too close and actually try out so what happens if we paint that sucker. That’s just a cool idea. And if you paint one and then you can send the spacecraft off to do more science, I don’t know how you would paint it at this point but I know there’s people that have worked to figure it out. And the idea of painting an asteroid just makes my heart giggle.
If it was a more expensive mission that allowed – if the sky was the limit on cost I – once you start –
Fraser Cain: — which it is. Allow me to write you a blank check.
Pamela Gay: I would love to be able to start doing – no matter if they’re only a few feet deep, I’d love to start doing core samples instead of just scraping off a small level. Just imagine being able to dig even 10′ into one of these objects and see how thick is the crust, see what is the diversity and start doing those core samples. It’s hard but –
Fraser Cain: Yeah, I mean, here on earth core samples tell us so much about everything. We use silt samples from the bottom of lakes to tell us about the plants in the region. We use core samples in Antarctica to look back at the levels of carbon dioxide and the constituents of the atmosphere. We use rock samples to look back at the layering and the – we final the mass distinction. So same thing, you dig in and produce a core sample on one of these comets or asteroids – a comet might be easier – you’re gonna get the history of the Solar System.
Pamela Gay: And where it starts to get problematic is, that would be a hugely energy-intensive mission. And it would need to be big and sturdy and able to withstand all sorts of shaking and it starts to becoming an engineering problem that we’re not quite there yet.
Fraser Cain: It should be covered by my blank check. Don’t worry about it. All right. Well, Pamela, thank you so much.
Pamela Gay: My pleasure. Thank you.
Fraser Cain: We’ll see you next week.
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