Ep. 84: Getting Around the Solar System

Fraser Cain: Hey Pamela.
Dr. Pamela Gay: Hey Fraser, how’s it going?

Fraser: Good. How’s Europe?
Pamela: I don’t know yet because we’re cheating and recording this before I leave.
Fraser: Oh, this joke never gets old. [Laughter] So once again, we’ve booked a bunch in advance. Where are you right now then?
Pamela: Right now I am probably over the Atlantic Ocean on my way home.
Fraser: Okay. Then you had a good time at the Pub [Laughter] with the mini astronomy cast meet up in the UK.
Pamela: Yes, exactly.
Fraser: Wonderful. Good. All right, let’s get on to this week’s show. Have you ever wondered what it takes to get a spacecraft off the Earth and into Space? And what are orbits? How does that work? What’s the difference between a polar and an equatorial orbit? And how do managers at NASA actually navigate spacecraft to another planet half a Solar System away? And who does these gravity assists? How do they get spacecraft into orbit around another planet? And how do they land? [Laughter]
All right, I’ve got so many questions. Okay, so this week the plan is we’re going to talk about every way that spacecraft get from the Earth out into Space and do their job. So let’s bring it right back home and let’s start with what are the mechanics of actually getting something off of the Earth?
Pamela: Well you need to actually fire more than just once. There’s this fascinating notion that if you give a rocket a big enough burst of energy that it will fly off into Space and happily orbit. But when you look at what NASA rockets actually do, they fire and fire and fire…and keep firing and the rocket makes it into Space and they keep firing and the rocket is in Space and then they shut off.
To our network television trained mind, it looks like the rocket takes off perpendicular to the Earth, flies straight into Space and the engines turn off and as soon as they turn off the rocket starts going around the planet. That’s not actually what happens.
You actually take off and you fling yourself in the direction of the Earth’s rotation. So you’re adding your velocity to the velocity that you already get from the planet’s motion and you take off such that you’re heading on an arc into Space.
You angle yourself such that once you get about where you want to be, you fire the rockets so that your orbit starts you going in roughly a circle around the planet.
Fraser: I think that’s a misnomer as you said. People imagine a rocket ship taking off and going straight up and out into Space. That never happens. The rocket takes off and almost immediately starts to angle itself downrange so that it’s almost flying parallel to the Earth’s surface and going faster and faster and faster, and getting higher and higher altitude. But in the end, it’s more that the rocket is going parallel to the Earth at 18,000 kilometers per hour, not straight up at 18,000 kilometers per hour.
This is why with the space tourism like the flights with Spaceship 1, which went up 200 kilometers and came back down, it went straight up, reached the limits of Space and then came back down. People said, “Oh, well then we’re really close to getting into orbit.” But in fact it’s a tremendously different amount of energy and velocity required to be going 18,000 kilometers sideways around the Earth.
Pamela: The fact is that if we didn’t have that extra firing, that extra maneuver, once you get up to the top of your arc that sets you going sideways, you just fall straight back down. Orbits are shaped like ellipses and they are perfectly happy to have one end of that ellipse hit the planet. To take an orbit and make it from one that goes up past the top of the atmosphere and comes back down and crashes onto the planet, you have to fire the engines up at the top of the arc, up at the top of the atmosphere beyond the atmosphere and set you moving sideways.
It’s that extra step, that firing at the top that allows you to keep going in a circle around the planet instead of returning back to the surface more violently than you might have wished for.
Fraser: The other thing you mentioned was the fact that the rocket is thrusting and thrusting and thrusting. Why don’t they just put one great big boom and get out into Space?
Pamela: Well, that might be a bit devastating to the asteroids. It comes down to how much acceleration can any one thing handle? If I wanted to, I could (and I had the technology which I don’t think we have right now), I could accelerate a rocket over the course of perhaps 30,000 feet such that it had all the velocity it needed to get to orbit, it would still have to do an extra firing once it got up to the correct altitude to get it going sideways.
I can get it going fast enough that we’d get to that highest point but I’d have to accelerate so fast that it would shake the tar out of the spacecraft and probably kill the astronauts by accelerating too fast.
Fraser: That’s what happens when a spacecraft is coming back through the atmosphere. It’s hitting the solid atmosphere at huge velocity and it’s heating up. So, if you were actually to try and accelerate to that speed right of way, I’m sure you would just vaporize it in the atmosphere.
Pamela: It would be bad. So, instead we do constant acceleration. By firing constantly, you don’t have to get going to a velocity that will carry you through the atmosphere you can keep pushing yourself through the atmosphere.
One way to think of this is if you’ve ever ridden a bike. You can get your bike going down the hill so fast that you don’t have to pedal up the next hill. But, you might have to get yourself going so fast that you’re a bit scared of what might happen if you hit a rock. If instead you go down that first hill a little bit more slowly, you can pedal up the next hill and you never have to get going as fast and you’ll still get to the top of the next hill.
If we gave the rocket enough energy close to the planet Earth to carry it all the way up through the atmosphere it would have to be going frighteningly fast. It would slow down and slow down as it goes through the atmosphere and encounters drag until it eventually pops out in orbit. By instead accelerating all the way through the atmosphere, it’s just a little bit friendlier to everyone.
Fraser: With that rocket firing constantly, the bulk of the energy that is coming out of the back of the rocket is just counter-acting its pull back from the force of gravity. If you take the amount of energy required to hold the rocket steady in mid-air that would be most of the energy that is coming out of the back of the rocket. Then you’re left with just a little bit of extra force and that’s what pushes it up.
As the rocket is going faster and faster, more and more of that energy that is coming from the rocket is used to accelerate it. That’s why when you see a space shuttle, especially like a Saturn 5, sitting on the pad when they first turn it on, it barely seems to move and then slowly inches up. Later on, it’s going faster and faster, much faster. The acceleration is increasing. [Laughter]
Pamela: One of the other things happening is you start off and the poor innocent engines not only have to pick up the spacecraft but they have to pick up all of the fuel. The higher up you go, the less fuel you’re trying to move. You’re firing engines to move smaller and smaller masses you go and that helps as well.
All these things build together to allow us to get things into orbit. Once we’re up there, we fire again so that we can stay in orbit.
Fraser: Okay, so let’s talk about some orbits then. What are the different kinds of orbits that a spacecraft might want to get into?
Pamela: The most common orbit for human beings is your standard run of the mill, low Earth, and kind of sort of equatorial orbit. These are orbits that pass over Cape Canaveral, that pass over the Soviet launch facilities.
They’re inclined to the equator, but most of the time the international space station, the space shuttle, stay over….
Fraser: Hubble.
Pamela: And Hubble too. They’re staying over the equator and they’re criss-crossing. They’re passing over Florida, they’re passing over Mid-America, and they’re passing over the plains of Russia. These are just nice happy little orbits, about 300 miles up that take about 90 to100 minutes to go round and round and round. What’s neat about these inclined orbits is every orbit they pass over a slightly different part of the planet.
You have an orbit and you can imagine a hula-hoop going around the planet where it crosses the equator in two places and then it has a high point and low point. Well, as the Earth rotates inside that hula-hoop, and in fact as the hula-hoop itself slowly rotates that high point is over a different point on the planet every moment.
That allows different parts of the planet to have the International Space Station straight overhead at different times, which is cool for amateur observers who like to go out and look at these things.
Fraser: Now you mentioned that a lot of stuff launches near the equator. They get a boost from the rotation of the planet there?
Pamela: That’s exactly correct. Depending on where you are on the planet, you’re going around the center of the planet at a slightly different speed. The entire surface of the planet rotates once every 24 hours relative to the sun. If you’re at the equator you have to travel a much larger distance than you have to travel if you’re up near the pole.
That extra velocity that you have by being near the equator helps throw you into orbit and gives you an extra boost as you’re taking off. So, Florida is a great place (if you want to stay in the continental United States) to stick a space facility because it’s pretty much as close to the equator as you can get in our country.
Fraser: Well, I think Sea Launch has the best one. [Laughter] They take a boat and an oilrig down to the equator in the ocean and they launch rockets right in front of the equator. Now let’s compare that with the polar orbits. How is that different?
Pamela: With a polar orbit you launch yourself into space and not only when you get into orbit do you give yourself a boost sideways to get yourself staying in orbit around the planet, but you also do an extra transfer so that you’re going over the North Pole and over the South Pole. There are a lot of different reasons to do this different type of orbit.
First of all, with a polar orbit you’re constantly passing over different parts of the planet. This allows you to slowly map the entire surface of the planet as you go around and as the planet rotates beneath it. You can still have a low earth orbit.
In general, we don’t have orbits that go exactly over the North Pole and exactly over the South Pole, just because that starts to get a little complicated. Our Earth isn’t actually a perfect sphere, we wish it was but it’s not.
Its rotation causes it to bulge out a little bit at the Equator and this affects orbits and it’s just a little bit simpler to have something that’s tilted slightly relative to North and South. It tends to get tilted naturally by this extra torque from going over the equatorial bulge.
Fraser: But you don’t get the boost from the Earth’s equator, right? In this case, you just have to get into space all on your own.
Pamela: Well, once you’re in space you can get the extra boost to get into space from launching near the equator, but then you have to tilt your orbit once you’re up there. You have to change the direction that you’re moving toward.
Think of it as you get your car moving and then you break the front left wheel. Now there’s no way in a healthy car to break only the front left wheel. You can imagine in an unhealthy car that if you break only the front left wheel it’s going to cause the entire car to keep moving, but change direction.
You get that extra velocity and you get that motion by launching at the equator. Then once you’re in orbit you either brake or you accelerate to tilt the direction that you are headed in until you get going in the polar orbit that you want.
Fraser: All right and then I guess the last orbit that really matters is a geosynchronist orbit.
Pamela: With a geosynchronist orbit you’re up really high. You’re actually up 35,970 kilometers. At this altitude you are orbiting instead of every 90 minutes, you’re orbiting every 24 hours. What’s cool with this type of an orbit is you stay over the exact same part of the planet all the time.
This is useful if you are a television station because you can stay over the people that you are trying to communicate with. It’s useful if you’re a weather satellite that’s tied to a specific part of the country because you stay over the part of the country you’re trying to take images of, that you’re trying to keep track of if there’s going to be a tornado.
Geosynchronist orbits are one of the most prized orbits for communication satellites and weather satellites. We are actually running out of space to put things into geosynchronist orbits because it’s one of the first places to fill up.
Now one of the problems with geosynchronist, is you are so high up that you can’t take detailed surface images. If we really want to do Google map style images, that is what’s called a sun synchronized orbit, but we’ll get to that one in a minute. The other problem with geosynchronist orbits is you have to stay above the equator if you want to stay over the same part of the planet.
If you go up to 35,970 kilometers and you get there such that you’re over Boston, you will be over Boston on one part of your orbit, and then you’re going to cross down over the equator and you’re going to end up over the southern hemisphere somewhere. You’re still orbiting over 24 hours, but because your orbit is inclined, where you are on a north/south line, that’s going to change constantly.
It’s like you’re tracing this line up and down the same part of the planet on the globe, but that’s not always useful. In fact, it’s rarely useful. If you are a poor individual living in northern Canada, Siberia, Norway or Finland and you need a communications satellite, you can’t see one over the equator real well.
So, another type of orbit that we use is a molniya, and I may be pronouncing that wrong. It’s a Russian word because the Soviets were the ones who had to come up with this orbit. Most of their country is far enough north that geosynchronist communication satellites aren’t useful to them.
Modification on the geosynchronist orbit is this orbit that for half the orbit when it’s at its furthest point from the planet is going along at the same rate as a geosynchronist satellite. It’s basically staying over the same part of the planet, staying over the same part of the planet, and then it accelerates and then it zips really close into the planet over a different point, zips back out, and lands over where it started originally.
With this modification of a geosynchronist, one that is inclined 63.4 degrees, so it’s getting up to the high northern or high southern latitudes. You have a 12-hour orbital period but for most of those 12 hours it stays over the exact same point in the globe.
What’s cool is if you have three satellites in this orbit, one of them (if you space them out correctly) will always be over the part of the planet you are interested in communicating with, so you can provide cell phone signals, you can provide television, you can provide weather imaging for people who live out near the poles.
Fraser: As you said, we’re talking about getting around the solar system, so let’s say we have launched some kind of robotic explore (that’s going to go to the moon for example). No, you know what let’s go with the most complicated. [Laughter] Let’s go with Messenger.
Pamela: You really don’t like me. [Laughter]
Fraser: Yeah, the Mercury mission. It launched from Earth. I assume it went into an orbit around the Earth, and then what?
Pamela: Well, it didn’t do what we had originally planned. One of the problems with really complicated orbits is that you have to figure out if this planet is here and this planet is here and this planet is here we can use these gravities. It’s like playing a really complicated game of pool. As long as everything is in the right place everything ricochets correctly and you end up getting all the balls in the holes. If one thing isn’t where you expect it to be you have to choose an entirely different shot.
With Messenger they missed the intended launch window. They ended up launching and they went into this orbit that carried this satellite that was destined for Mercury out past the Earth’s orbit. It took off and it headed on an elliptical orbit that carried it beyond where the Earth would ever go and then it got out there and it went too far.
Then it came back in and it came inside Earth’s orbit and ended up encountering the Earth again, spiraling, slowing down, took some images of the Earth and then spiraled in towards the sun after doing some braking maneuvers. It went past Venus, out past Venus’ orbit, came back in and then spiraled in and it’s this weird circles getting smaller and smaller but never really being circles.
They are ellipses and the entire system rotates and shrinks, rotates and shrinks, as it spirals in getting braked by different planets, taking images as it goes. In this complex spiraling set of shrinking ellipses, they ended up adding two years to how long it took to get to Mercury, all because the planets weren’t properly aligned.
Fraser: Okay, I think that was too complicated. So, let’s pick a simpler example. Let’s say that we are going to go from Earth to Mars.
Pamela: Yes.
Fraser: How does that work?
Pamela: That’s actually one of the coolest orbits. You time it just right so that when you look at a picture of the system, where Earth is when the mission takes off is exactly across the sun from where Mars is when the mission gets there.
So, what you do is take a satellite and put it in an elliptical orbit so the point where the object is closest to the sun is where the Earth is located and the point where the object is furthest from the sun is where Mars is located. It’s a nice friendly low energy type of transfer called a home-in transfer low energy maneuver.
The way orbits work, when you’re closest to whatever the primary mass in the system is, in this case the Sun, you are moving the fastest. When you are furthest from that object, you are going the slowest. When you are on a circular orbit you stay the same velocity all the time. When you are on an elliptical orbit velocities are constantly changing.
You can have orbits that cross the Earth and the Mars orbit, you can have orbits that cross Earth and Jupiter’s’ orbit. The easiest way to get yourself from one orbit to another is to put yourself first in a nice friendly circular orbit (around say Earth) and then when you are pointed in the right direction, you fire your engines so that you are going the velocity that’s necessary to be on the elliptical orbit that intersects the orbit of whatever you are trying to get to.
Now the trick is timing this just right so that the object you’re trying to get to is at that point in its orbit when you get to its orbit.
Fraser: I’ve seen animations of this. You see the spacecraft leave the Earth orbit and move into that elliptical orbit and it doesn’t look like it’s going to get to Mars at all. But then, the orbit of Earth with the spacecraft catches up to the orbit of Mars and it’s a bulls-eye.
Pamela: This is where launch windows are so important. If you miss a launch window you really can’t catch back up to the planet. It ends up requiring far too much energy. We have this narrow window of time when the Earth is exactly where it needs to be to get to Mars when it’s where it needs to be in its orbit.
As with all orbital transfers, it’s all about saying, okay, let me change my velocity, perpendicular to what I am going around now to get to what I what to be going around later.
Fraser: Now, you mentioned that this is a low energy orbit. This is the way to get from Earth to Mars with the least amount of energy. I am guessing if you just wanted to fire your thrusters non stop [Laughter] or you had some kind of really powerful nuclear rocket, you could get there in more of a straight line. You could just put Mars in your cross hairs and keep firing and you would get there.
Pamela: That would require a lot of energy.
Fraser: Right, and a shorter time. I guess the pay off or the balance is that with the low energy transfer it’s the lowest amount of energy required to get from Earth to Mars. There are higher energy orbits you could use, but those might be not even possible or too expensive.
Pamela: One of the neat things about doing orbits like this is that they work in the opposite direction as well. If you want to get from Mars back to Earth, you put yourself in orbit around Mars and then you slow yourself down. That slowing you down sends you on a path back to Earth.
Fraser: Now we’ve got some of the landers that have gone from Earth to Mars. How do they even get into orbit around Mars in the first place?
Pamela: Once they get to Mars they are actually not going to right speed to stay there unless they do some maneuvering to change their velocity. We change velocities in a number of different ways.
One of the really cool ways is through aerobraking. This is where you use an objects atmosphere to slow yourself down. As you go through the atmosphere all the particles hitting your spacecraft slow your spacecraft down. You can get whatever velocity you need as long as you started with a higher velocity. You can also use gravitational assists to speed yourself up if you’re going not quite the right speed by going too slowly.
Fraser: Why don’t we talk about the gravitational assisting in one more second? I see with the aerobraking, you’re dipping into the atmosphere, baying into a little bit of the atmosphere, but then you’re getting back out again. So, it’s not like your crashing, right? You’re skimming and slowly scraping with the atmosphere time after time until you are in an orbit.
Pamela: Its like watching a water plane do touch and go’s. They come in glide across the surface and then skip back off into the air. In this case, it’s a satellite skipping into the atmosphere and then skipping back off after slowing down.
Fraser: What if a planet doesn’t have an atmosphere?
Pamela: Then it gets a little bit more complicated. You would probably end up having to fire an engine or insert yourself. If you insert yourself in the opposite direction instead of gravitational assists you have gravitational braking. It’s all about what direction you are going as well.
Fraser: Now we talked about gravitational assists in a question show, but why don’t we bring that up again. That’s the process where spacecraft use planets to increase their speed to get to more distant goals faster.
New Horizons, which is on its way to Pluto, did a gravitational assist with Jupiter to increase its velocity. So what’s the process here? As we had it in a question show; how is it possible? It makes sense for a spacecraft to get pulled into the gravity of a planet like Jupiter to speed up. But then, as it goes back out of the gravity, it should slow down the exact same amount just like going down a hill and then coming back up again.
Pamela: Let’s return to our bicycle analogy. Imagine that you are going down a road that has a big dip in it. You are plugging along, you’re going a constant 30 miles an hour. That’s a little bit insane on a bike, but let’s imagine because it is a nice round number.
If you have a perfectly symmetric dip where you go down and you come back up and all friction is exactly the same, you are going to accelerate as you go down into that hill. You are going to slow down as you come back out of the hill. If you go into it at 30 miles an hour you should come out of it at 30 miles an hour.
Now imagine that somehow that dip is moving forward, and it’s moving forward not quite as fast as you are, but it is still moving forward. As it moves forward, what ends up happening is you end up spending more time going into the dip because the bottom of the dip keeps going forward and it essentially stretches out the downhill part. You are accelerating down hill for a much longer period of time.
Then as you come back out of it you’re spending less time going up the hill so all together you spent more time getting accelerated down hill than you spent getting slowed down going up hill so you end up going faster than that original 30 miles per hour.
Fraser: Right and I think the way we tackled it was that it’s true, you know as you are going to Jupiter that part does cancel out you get pulled into Jupiter, and then as you move away from Jupiter you slow back down again because Jupiter is pulling on you. The point is that you match Jupiter’s speed in its orbit. That’s where the bonus comes from.
Pamela: So it’s all about both of you are moving and if it’s moving forward and you’re moving forward its velocity forward gets helped to add push to you. Now if you’re going in the opposite direction you end up spending a lot more time getting slowed down than you end getting sped up so you can use the gravity assist to slow yourself down as well. It’s all about directions.
Fraser: That’s one of the techniques that they can use to go into orbit around a planet. They’ll come in and use successive gravity, reverse gravity assists, and gravity brakes to get them into orbit.
Pamela: It’s all about matching the angles and slowing yourself down.
Fraser: Okay so the last part of this puzzle then is [laughter]…how do you land?
Pamela: Landing is one of the more complicated things we’ve had to figure out how to do.
Fraser: Landing safely.
Pamela: Yes. It’s easy to crash things. We’ve been doing that for a long time. Well, landing you have to somehow shed velocity. When you have an atmosphere it’s fairly easy. You go through the atmosphere and the atmosphere wants to slow you down. It wants to burn you up actually. But, if you avoid the burning up part, it slows you down.
Mars, the atmosphere is a little bit thin so it’s usually this crazy combination of we’re going to fire rockets to slow us down. Then we’re going to launch parachutes to slow us down some more and let’s add some airbags and maybe bounce a few times. If you have no atmosphere, like when we land on the moon there is a lot of rocket firing going on. So, just as you can use a rocket to lift you up and take you off of a planet, you can use a smaller rocket to slow you down as you come in for a landing.
If you have ever seen one of those jet packs they have at Disney World or something, the guy fires the rocket a lot to take up over the magic castle. The rockets are firing less as he comes in for a landing. They use rockets first to slow themselves down and basically put themselves on a ballistic crashing into the surface of the planet, the moon, the object, orbit. Just as they are getting ready to crash violently, they fire extra rockets to slow them down even more. It’s very fuel intensive.
Fraser: Right and it’s that whole process in reverse. You start out in an orbit moving very quickly around the object and then you fire in the opposite direction that you are orbiting and that drops you out of orbit down towards the surface because you are slowing down.
You are no longer matching your outbound speed with your inbound speed, so you are starting to slow down. But, if you even look at the video of the Apollo landings, you can see the moon’s surface moving under them very quickly because they are still on an orbit.
It’s not until the very last few thousand meters that they are able to straighten out so that they are coming straight down. So they don’t scrape along the surface of the moon.
Pamela: Your motion is always an ellipse. If you slow yourself down at one point then you are just moving the other 180 degrees around the orbit point closer to the surface. If you move it close enough to the surface it’s inside the planet. And your orbit takes you to the surface of the planet.
Fraser: To put this altogether, spacecraft controllers will use all of these techniques. They will figure out which kind of orbit to launch a spacecraft. They will be able to use a combination of gravity assists and orbital transfers to move a spacecraft around the Solar System from planet to planet, or to get speed boosts, or to go past the objects they want to image.
If they want to go into orbit, they use this process in reverse either through gravity braking or through firing rockets or using the atmosphere. And every one of these missions are very complicated, they have to do a lot of work to figure out all of the mathematics involved.
I think the amazing part is when I see these missions, is how close they always get them. It’s amazing how when you think about the enormous distances involved, they’re able to get these spacecraft within their tolerances that they were expecting so they get right on target. It’s great.
Pamela: It’s really amazing and it all comes down to always carrying a little bit extra fuel just in case you need it.
Fraser: Right. In case you need to make one last correction. Great. So now you know and understand how spacecraft get off the Earth into orbit, around the solar system and land on other worlds. Well, thanks Pamela.
This transcript is not an exact match to the audio file. It has been edited for clarity. Transcription and editing by Cindy Leonard.