This episode was recorded on 3/8/2018 at 4:00 pm EST/ 1:00 PST/ 21:00 UTC!
It’s one thing to get from Earth to space, but sometimes you want to do the opposite. You want to get into orbit or touch down gently on the surface of a planet and explore it. How do spacecraft stop? And what does that even mean when everything is orbiting?
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Low Earth orbit
Geostationary transfer orbit
Hohmann transfer orbit
Why is Mars harder? Missions to Mars
Spirit and Opportunity Rovers landing
Curiosity Rover landing
“Seven Minutes of Terror” video
Lunar Landing game
Philae (spacecraft) from Rosetta landing on Comet 67P/Churyumov–Gerasimenko
Transcription services provided by: GMR Transcription
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Fraser: Astronomy Cast episode 483: Stopping in Space. Welcome to Astronomy Cast weekly fact based journey through the cosmos. We’ll help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today, with me as always Dr. Pamela Gay, the director of technology and citizen science at the Astronomical Society of the Pacific, and the director of CosmoQuest. Hey Pamela, how you doing?
Pamela: I’m doing well. And I just realized at you introduced us, I should remind everyone while you’re listening to this, if you’re not like, at the gym exercising or otherwise moving around doing useful productive things, you should go online to cosmoquest.org and do citizen science. We just upgraded all of our mapping interfaces. You can help us do new science with Mercury mapers, Mars mapers. There’s the Moon and Vesta waiting for your inputs. So – hey, if you’re looking for something to do while you listen or watch the show, go to cosmoquest.org and do science.
Fraser: I think it’s more than just looking for something to do. If you want to make a meaningful contribution to our scientific understanding of the universe, if you want to participate and be part of the science that’s getting done around this solar system go to CosmoQuest and help out.
Pamela: Thank you.
Fraser: It matters.
Pamela: And I will personally be very – very grateful.
Fraser: Perfect. You got that? Personally grateful. It’s one thing to get from Earth to space, but sometimes you want to do just the opposite. You want to get into orbit, or touch down gently on the surface of a planet and explore it. How does spacecrafts stop? And what’s that even mean when everything is orbiting? And we’re not just going to talk about stopping. We’re going to talk about transfer orbits and slingshots and all kinds of stuff. All right. Where shall we start? Well, let’s talk about this concept of stopping. So we’ve spent a whole bunch of episodes now talking about going and different methods of going, and why going is so hard, and now let’s talk about stopping, because it turns out stopping is also hard.
Pamela: Yeah. So you get yourself going. You’re whipping around the planet, 90 times a minute or so if you’re on the International Space Station, you might be going faster, you might be slower, or geosynchronous orbit, you’re going around every 24 hours, but you’re so high up you’re still booking it through space, and with these large velocities if you want to get back down to the surface you have to somehow reduce your velocity otherwise you’re just going to stay up forever.
Fraser: And, you know, we talked about how difficult it is. Like, when you go into orbit around the Earth you’re going from – say, into low Earth orbit you’re going from zero to 28,000 kilometers per hour around and around and around the Earth; that is orbit. That’s the trick. So you have to then, if you want to return to earth, say you’re sitting on the International Space Station and you want to return to Earth you need to go from 28,000 kilometers per hour to zero.
Pamela: Well, it’s not quite zero to be fair, because if you’re at the equator of the planet, you’re going around at a thousand miles per hour.
Fraser: Good point.
Pamela: But there’s still a whole lot of reduction between those two different speeds. And I’m using the word speed because your direction’s constantly changing, velocity – words, they’re hard, and in this case speed means how fast you’re moving through space without care of direction, and velocity has a direction that is constantly changing if you’re in an orbit, so don’t screw these words up your local physics teacher will want to punch you.
Fraser: You mentioned that you go 90 times a minute around the Earth. I think what you meant to say is you go once around the Earth every 90 minutes.
Pamela: I did – I did.
Pamela: Yes. Words – they’re hard.
Fraser: Yes, clearly, even for you.
Pamela: So with the Earth stage 1, is to throw yourself at the atmosphere. It’s true.
Fraser: I know – I know. And I love it – I love it. You just throw yourself at the atmosphere.
Pamela: And the way you throw yourself at the atmosphere is you have to reduce your orbital velocity, which if you’re in a low enough orbit you can just wait long enough. The Chinese space station is currently taking this particular approach to coming out of orbit. If you’re an astronaut you may lack that patience, you want to get home and experience gravity and tasty food. And in order to get back down you fire engines to slow your forward speed, to slow your orbital velocity, and when this happens you go from an orbit that will keep taking you around the planet to an orbit that intersects with the surface of the planet.
Fraser: One thing that really interesting – and I sort of discovered this doing a recent video, is that like say back with the space shuttle they would fire their retro thrusters and the amount of difference in orbital speed that they would do is actually very slight.
Fraser: Spatial orbit only need – I think it was a little faster than a fastball pitcher throws a fastball, so just a couple of hundred kilometers per hour and change in velocity was all the space shuttle needed to set it onto an orbit that would begin the process of throwing yourself atmosphere.
Pamela: And this is why they don’t need to have massive engines to get out of orbit. You can use those little tiny thrusters that they had on the shuttle. You can use the little tiny thrusters they have on the current Soyuz capsules. Get that little tiny delta ‘V’ and then let the atmosphere do most of the rest. Now, most is the key word here. Whether you’re the space shuttle or the Soyuz capsule you want to hit that atmosphere at just the right angle to make sure that you’re braking with the largest surface of your spacecraft that hopefully has a heat shield on it and it’s taking the brunt of the force.
The – in this case it’s a normal force being exerted by the atmosphere, and you’re going through the atmosphere, you’re getting frictionally heating as you push all of that atmosphere out of the way, the air around you is actually getting ionized, it looks like it’s on fire. You feel like you’re going to die is what I’ve heard. And this gets you significantly slowed down. This gets you down to the terminal velocity of whatever kind of thing you’re in. Now, the problem is if you hit the planet at your terminal velocity that probably isn’t healthy for humans or content inside the rocket.
Fraser: Just on a bit of a somber note, of course, there was the return of the space shuttle Columbia that was performing this maneuver, there was a crack in one of its wings that hot gases got into from the atmosphere as it was performing this deceleration maneuver and it was enough to tear the orbiter apart and all seven crew members were lost. It is a very dangerous time – you know, when you’re going around and around out in orbit you’re not going to run into anything unless there’s some space debris. But that hasn’t caused an issue so is far for astronauts. It’s that return flight is very – very dangerous and quite scary.
And you got to get that angle – as you mentioned, you got to have that angle of your spaceship exactly right when you’ve got a capsule, whether you’ve got a glider, you’ve got to bleed off that velocity right, and you’ve got to be able to make that transition from thing that tried to throw – throw itself at the atmosphere to thing that is able to gently return to Earth, and that’s a tough transition to make.
Pamela: And if you get the angle wrong you’re either going to get through the atmosphere okay, but be at far too high of a velocity, or you’re going to end up tumbling and lose control, both these things are bad and lead to death, which is not optimal. I’m trying to add humor to this because it’s such a black topic. And with the Columbia, the heat that got into the wing was sufficient to damage the aluminum structure. We’re talking the aluminum got melted in parts and this is extraordinarily high heat. So those heat shields they really – really matter, and you have to get through just right or you lose control or you come in too fast.
Fraser: And there’s no alternative. Like, if – and we’ll talk about this in a second here, but if there was no atmosphere then if you wanted to land, say, on an airless Earth, or let’s say the Moon, you just have to reverse the maneuver that you did that allowed you to take off in the first place. But you can’t do that with any place that has an atmosphere because you’ve got to deal with this atmosphere. It’s an advantage, you don’t need a lot of energy. You’ve got lots of energy. You need to get rid of that energy.
But the disadvantage is you have to deal with the tremendous forces, and there’s kind of no way to go around it. I think of it like you’re in a boat and you’re going down a set of rapids and you can’t – once you commit yourself the rapids you can’t get off and you can’t it stop and you have to just ride it down to the bottom. And if you watch like, Apollo 13, they have a great sequence where they’re coming back through the atmosphere and it’s quite suspenseful, and that is what it’s like for people trying to interact with the astronauts as they’re coming back through the atmosphere.
Pamela: And you lose communications with your capsule at this point because all of the ionizing radiation blocks out radio signals. So there’s this point where you don’t have any communications and it looks like the world outside your capsule is on fire due to all of the ionization that’s going on. And you just have to suck it up and trust.
Fraser: Yeah. And trust in the engineering, and trust in the technology.
Pamela: But we haven’t really stopped. We’ve just hit terminal velocity.
Fraser: Oh, right. You want to – you want to actually land now, okay, sure.
Pamela: So once you make it through the atmosphere you’re still going at the terminal velocity of your vehicle. This is the velocity that you fall at based strictly on the shape of what you are. A feather has a much lower terminal velocity than a brick, for instance. Now, with the space shuttle in order to slow down the rest of the way they actually did these amazing S-turns. So they would bank back and forth just like you and I have probably both done skiing to slow down. You lose all of your energy going through these turns and dropping it with the friction of the curve.
Fraser: That’s a great analogy. I really like that. That it’s just like how when you’re skiing, that really helps understand it.
Pamela: And it was actually – I learned about the space shuttle before I learned about skiing, so I learned that analogy in the opposite direction the first time. Have I mentioned I was a weird child? So with the capsule you don’t exactly have the capacity to do S-turns. So here they have to have parachutes. And they often have a first parachute that gets them down to one speed and then they let off another set of parachutes that are much bigger and would pretty much blow apart with the initial blast if you didn’t have a first parachute. So it’s a staged process. Use the atmosphere, then you use parachutes, even the space shuttle had a final parachute – in this case a drag chute that it deployed to break when it hit that runway.
Fraser: And it is still like the Russian Soyuz, they still use that parachute system. They land those capsules in the steps of Kazakhstan, it’s a rough landing. Now, SpaceX of course – it’s time for us to say nice things about SpaceX. Now, they’re not necessarily coming back from – they haven’t cracked doing a powered landing from orbit, but you can kind of see the direction that this is going with the way the booster rockets go.
Pamela: And here we start to get to the technology that also gets used in part on Mars, which is the idea of just land on those same engines that you took off on and fire them to slow yourself down. It’s what we all watched in the Jetsons growing up, and now it’s starting to become our modern reality.
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Fraser: So why is Mars so much worse than Earth? I mean Mars eats spacecraft for breakfast. What’s going on?
Pamela: Well, it eats spacecraft for breakfast for many different reasons. One of the problems that we have that lucky doesn’t seem to be the primary reason it eats spacecraft. The primary reason seems to be human issues. But Mars – it doesn’t have enough atmosphere to let atmospheric braking get very far. But it has so much gravity that using retrorockets just doesn’t quite work for the entire thing. So you have to have this combined approach of first drop your speed using what atmosphere you can, and then figure out how do I get the rest of the way there – and early on there were fabulous solutions in the modern Mars exploration era.
So the late 90s forward, where we tried things like Pathfinder was a bouncy ball, and that was fabulous. It was the – we shall simply make this thing very rigorous and we shall drop it with airbags, and the airbags will get rid of the energy because it’s not a fully elastic collision, so after a number of bounces and a bit of rolling you now have a no longer moving spacecraft that presumably like a weeble-wobble it always ends up standing upright and opens up and out deploys your spacecraft.
Fraser: But the size of those rovers was literally the maximum size you can use that technique, so they had to use something a little more complicated for the Curiosity, which is more like the size of a Mini – like it’s a much larger – larger vehicle. They couldn’t use that bouncy ball.
Pamela: And this is where we get the seven minutes of terror. This was another combined approach, so first it did aero-braking, then it did parachutes, then it did retrorockets, but they didn’t want to disturb the area around where they were going to land the Curiosity. So they dropped the Curiosity on a tether with retrorockets firing above it and off to the sides in this crazy sky crane approach. And then they landed the retrorocket part, or dropped it, as the case may be, a little bit off to the side.
Fraser: And you know, a lot of the spacecraft that have been sent to Mars, it’s this orbital entry landing approach that’s gotten them killed with – you know, again SpaceX, they’re talking about the – the BFR and the plans for the BFR to go to Mars, they’re shifting back to a powered landing because now the payloads are too heavy, you can’t use really much of a parachute. You can try to use as much of the atmosphere to brake as you can, but you can’t use the sky crane. You can’t put the BFR in a bouncy ball. You’ve got to land with rockets.
Pamela: And the past several years it’s been crazy watching NASA and other groups try and figure out how to engineer bigger and bigger parachutes, and they’re just got getting it to happen, so this is really a more feasible direction in a lot of ways. And what kind of amazes me is this is exactly what we saw in the Martian, where you have that rocket that landed and then is waiting primed, sitting there, ready to go in the same orientation that it landed in. This is new.
Fraser: Yeah, absolutely. And back to this idea of like, being able to build your fuel on site and be able to go from there. Let’s talk about some other places that we have landed on. Like, what about Venus? You know, the Soviets sent their trusty spacecraft onto Venus and they made it down to the surface.
Pamela: And then they melted.
Fraser: Yes. Well, you know. You can’t go to Venus without melting.
Pamela: Well, yeah, Venus isn’t really a problem to stop on. The problem is landing where you intend to because the atmosphere is so thick, it buffets you so much, it has really high winds, but it will slow you down and there are lots of really cool ideas for how to get to the surface safely. It’s just staying on the surface without melting that is really the issue.
Fraser: And then the other place that I think has been sort of a fascinating landing site is Titan. It’s like the perfect place to land.
Pamela: On every possible front because you do have to worry about landing in fluid. They do have lakes – great lakes on Titan, so be prepared to float. But the atmosphere is thick enough that if you attached icker style wings to you and I we could happily fly around in the atmosphere if we had a spacesuit to allow us to breathe.
Fraser: But you would need a pressure suit.
Pamela: No – no. You just need protection from radiation and protection from lack of oxygen and cold. But you could fly.
Fraser: Yeah. But – so the gravity’s not pulling you down as harshly. The atmosphere is much thicker than we have here on Earth. It is the perfect, safest place you could try to attempt an orbital reentry.
Pamela: And the only reason you want the bouncy ball airbags is if you land in a lake. So don’t land in a lake.
Fraser: Have a boat.
Pamela: Have a boat.
Fraser: And then what about the Moon? I mean we’ve all played our lunar landing game. How does that work?
Pamela: It’s another one of these low gravity environments, and here we nailed it on using rockets to do a powered landing. Just like we’ve seen the SpaceX Falcon 9s land, that’s the same technology. It – just a whole lot more advanced as what they had that they were using with joysticks and buttons to land on the lunar surface. We were able to do it in the 1960s and early 70s with the lunar landers with such low tech because there was such low gravity and that means the challenge is much less.
Fraser: And then you know, sort of as a connection to that, think about what it’s like to try and land on places like 67P, on the Rosetta Mission feel they had a really hard time and now we’re looking at the OSIRIS-REx mission, which is going to be doing something similar is the high Hyaboosta 2 mission which is going to be bringing back a sample from an asteroid as well. Why are asteroids so difficult to get to zero?
Pamela: Asteroids and comets have two different problems that you have to contend with. One is those suckers are rotating. And so you have to figure out how to match that speed that the surface is moving at, which may be a faster velocity than the one you want to stay in orbit, so you’re trying to like, not accelerate away to match the speed of the – it gets tricky. The reason this is so tricky also is it doesn’t have sufficient gravity to really pull you down. Felay felt the same way a piece of paper on the planet Earth feels when it comes to weight. So Felay had to harpoon it itself into the surface to hold on, and this was where it didn’t exactly succeed and it bounced around a bit before it finally settled.
Fraser: On its side.
Pamela: And because it did have essentially equivalent weight to a piece of paper on Earth, it did settle to the surface. There was gravity. It just wasn’t a lot. And if you’re trying to land on a rotating chunk of rock you have to match the orbit of velocity, you have to match the surface’s velocity, and you may only have a comparative weight of how a couple of pieces of paper feel.
Fraser: I mean, from what I understand Felay didn’t bounce as much as the researchers were expecting, but its retro thrusters failed, its harpoons failed, and so it just wasn’t able to use the two devices that it was supposed to have to be able to make this landing, and part of the problem is that the spacecraft was so far away that you can’t in real time adapt to the issues that are going on. And this is what we’re going to see time and time again as we explore the outer solar system, as we try to land things on Mars.
You talk about the seven minutes of terror. When Curiosity was making its descent through the Martian atmosphere it was set, you know, 15 minutes away from Earth or longer and so people on Earth had no way to stop it, change it, fix it, tell it to do something smarter. They had to just follow the programming and couldn’t adapt, and that’s one of the last reasons why it kind of makes sense to have humans do some of this stuff is that they can be there and they can adapt as the conditions change.
Pamela: And with OSIRIS-Rex we’re somewhat hopeful that the way the mission has been set up this will be a fairly straightforward approach. At grocery stores or carnivals you may have seen that grabby game where you have the claw that dropped down, grabs a toy, and drops it in a bin if you do it right, drops down and grabs nothing and drops nothing in a bin if you do it wrong. Well, this is a very similar approach to what OSIRIS-Rex is going to do. It’s going to come in and try and grab a rock and bounce away.
This is a fairly simple process. The only trick is going to be finding a rock and grabbing it quickly. And if you want to be one of the people who potentially helps with this, go to ComoQuest.org, prove yourself as a crater marker because we are going to be using rock markers later to find rocks for OSIRIS-Rex.
Fraser: Oh, that’s cool. Our zone in the chat as we’re doing this live show is noting – and I think it’s a great point, is that it’s now believed that the surface of Enceladus and Europa are actually too soft to even land on. That it could be that they’re actually quite squishy, and because of the low gravity and because of sort of what this material might be that it might be too slushy and that any lander might sink into it, so it’s not even about trying to get down to the surface, but it could very well be that there’s no hard surface for you to get on, so it’s like going back to like, maybe you need to take a boat.
Maybe you need to take big – great big floppy snow shoes on your spacecraft. And this is one of the concerns that people had about the Moon back in the day before they were able to actually land on the Moon. They weren’t sure that it was even going to be possible.
Pamela: They thought there were meters upon meters of lunar dust potentially, or inches upon inches which would still be enough that they couldn’t free their spacecraft later. But luckily it turned out they were wrong and so they were fine.
Fraser: And so let’s hope they’re wrong about Enceladus and Europa as well. But the bottom line is that really getting off Earth is really only half of your challenge. Landing and – you know, wherever it is that you want to go, arriving at your destination is the other half and as complicated as required. All right, well, Pamela I think we’ve filled up this episode. I’m not sure whether we’re done. We want to talk about some orbital maneuvers – maybe some rendezvous docking, transfers, and things like that, so maybe one more episode.
Pamela: One more.
Fraser: Yeah, we’ll see how it goes. Too much fun. I’m having too much fun with all the space flight stuff. All right, we’ll talk to you next week, Pamela.
Pamela: Sounds good, Fraser.
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Duration: 28 minutes