How Do Spacecraft Move Around the Solar System?

ERIC: From the Museum of Science in Boston, this is Pulsar, a podcast where we voyage for answers to the grandest questions we get from our visitors. I'm your host, Eric, and getting from place to place in space is complicated. The paths that spacecraft tread through the solar system, their trajectories, vary wildly. Sometimes when we talk about a particular spacecraft journey, our visitors ask how it got there. For the answer, I returned to our Charles Hayden Planetarium to ask Talia, the author of our Spacing Out newsletter about one of her favorite space subjects. Talia, thanks for inviting me to come back to the planetarium and talk a little bit more about some space stuff.

TALIA: Always happy to have you under the dome of the planetarium, Eric.

ERIC: So our question today is how spacecraft get around the solar system. And that has a lot of different answers. But I know that you are really passionate about stuff like the paths that spacecraft take, not necessarily the engines and the rockets but, like, how you figure out your trip from one planet to another, say.

TALIA: It is a really, really niche topic. And I am a huge spacecraft trajectory nerd. I think they're so cool.

ERIC: So when you leave the earth behind to go explore, say, another object in the solar system, the first step is to go into Earth orbit. And that's kind of like, to check out all your systems before you go too far away. You want to make sure antennae are deployed and stuff like that. Then what?

TALIA: Well, once you're ready to leave Earth orbit, you need to get out of Earth orbit, which requires generally a rocket burn. Because you're going to need to up your speed. You need to leave Earth's gravity well. And that requires speed. So pretty much everything involving spacecraft trajectories is based on mass and speed. Mass, because gravity is a huge player in this game. And speed, because that's how you get out of gravity wells and get where you're going. So the first step is you've got to, you know, put on a burst of brilliant speed and escape Earth's gravity.

ERIC: So usually, that's one big chemical rocket burn that lasts for a couple of minutes to go so fast that Earth's gravity can't keep you back.

TALIA: Generally, yeah, that involves a chemical rocket burn.

ERIC: And the further away you're going, the faster you gotta go.

TALIA: Well, you need to get onto the path that you want to get on. There are ways to speed yourself up along the way, of course. You don't have to have all of your velocity right at the beginning.

ERIC: So it's all about efficiency. Not necessarily, I'm going to build the biggest rocket so I can go faster and farther. But I'm going to get clever about what I'm going to do on the way to my target to get a little speed from the environment that I'm in.

TALIA: Exactly. building the biggest, fastest rocket that will get you there quickest, is also going to be stupidly, ridiculously, impossibly expensive. So you can save a lot of money if you're willing to add more time and be a little bit more clever, and as you said, efficient about how you get to where you're going.

ERIC: So the JUICE mission just launched, that's Europe's Jupiter Icy Moons Explorer. And that one is going to take like eight years to get all the way out to Jupiter. We've got there faster, but this is going to be an efficient one. So what is this mission going to do? What is this secret thing you can do on the way to actually get where you're going more efficiently?

TALIA: Yeah. So you have to keep in mind that you're not just dealing with, you know, the Earth's gravity well. You're also dealing with the Sun's. So if you want to head out farther from the Sun, you really do need to build up more speed. So if you want to get to Jupiter, you need to build up speed, and it's very inefficient to just do it with a giant rocket burn. So what we do with our spacecraft when we want them to go further out, is we'll often have them spiral around. Instead of going directly there, they'll spiral around and they will do what are called gravity assists, which is a little bit of a weird name. It's actually a momentum transfer. So what happens is the spacecraft flies by a planet, generally a planet, it could be a moon, theoretically, but it's almost always a planet, and steals momentum from the planet. The planet, of course, is huge and does not notice. But a little tiny transfer of momentum from the planet to the spacecraft speeds the spacecraft up significantly. So these spacecraft will take these big, long, looping paths where they will fly by multiple planets and steal momentum from each one to get enough speed to reach, in this case, Jupiter's orbit.

ERIC: So flying by the Earth a couple years after you left the Earth, if you fly by in just the right way, basically, Earth slows down in its orbit, the tiniest little fraction of a bit, and the spacecraft gets a boost that's the equivalent of if you had a bigger rocket or burned for a longer time.

TALIA: Exactly. And then of course this takes a long time, which is why it's going to take JUICE eight years to get out to a place where we could, you know, theoretically get a whole lot faster if we just went directly there. But it's going to be incredibly more fuel efficient, which means it saves a lot of money.

ERIC: I think that's the main thing that I had a reaction to with this question of how you get around, because the answer is not, well, you point your rocket at Jupiter, and you burn the engine until you get there. And then you're there. Because space doesn't work that way. Jupiter is moving, the Sun's gravity dominates everywhere in the Solar System except those little pockets, those gravity wells around the planets. So it's really like trying to get around a racetrack where all the cars are moving at the same time. And also, the racetrack is, like, tipped at 45 degrees. So if you stop moving, you fall down. Like it's so complicated.

TALIA: It really is. It's part of what is fun about it.

ERIC: So going out in the Solar System is something that we've done a lot of. Going out to Mars, Jupiter, further away. But it's actually pretty hard to get to the inner planets. Mercury and Venus are ones that are extra tricky to get to.

TALIA: Yes. So even though Venus is the closest planet to us, it actually can take us years to get a mission to Venus, because it has the opposite problem. You're falling in towards the Sun, the Sun's gravity is actually causing you to speed up. So if you pointed your spacecraft directly at Venus, by the time it got there, it would be going so fast that it would just zip right by Venus, and you can wave at it as you go by. So in this case, we need to slow the spacecraft down and take a long spiraling trip in where again, multiple flybys happen. But they happen on the opposite side of the planet. If you fly by the planet on one side, that you steal the planet's momentum. If you do it on the other side, the planet steals your momentum.

ERIC: It's kind of like if you leave for work. And your commute could be, like, 30 minutes if you drove straight from your home to work. But if you drove around your neighborhood and drove by your house a bunch of times and took seven hours to get to work, you'd actually use, like, 1/3 of the fuel.

TALIA: Yeah, so the record for most flybys of a spacecraft that's going to a planet is MESSENGER, which was trying to get to Mercury. It had to do flybys of Venus and Mercury to spill off speed in these momentum transfers. It finally approached Mercury going at a slow enough speed that it could enter orbit around it. Now that was going around the planet. I think, in fact, it might be the Parker Solar Probe that has the most, because it's going in orbit around the Sun.

ERIC: Orbiting the Sun super close, setting the record for the closest we've ever been to the sun with anything.

TALIA: So yeah, gravity plays tricks on your spacecraft paths and means you can't usually just take a direct line to wherever you're going.

ERIC: I like that the exception to this is when you want to leave the whole solar system behind.

TALIA: Oh, yeah

ERIC: You just blast the engines go as fast as you can, and then don't look back.

TALIA: And even then, the Voyager spacecraft did technically do flybys of the outer Solar System planets. Now, their mission was to explore the outer Solar System planets, so of course, they did go by them. But they also did these momentum transfers as they did, which means they picked up speed. There are spacecraft that launched before Voyager 1 that were heading out of the solar system but Voyager 1 caught up and passed them. So Voyager 1, because of the speed it picked up from these flybys of the giant outer planets, is the farthest object we've sent out in the space. And nothing's going to catch it at the rate it's going. New Horizons is going pretty fast as well. But I looked it up. It said New Horizons was never going to catch up to Voyager 1.

ERIC: No, once you're done with all those flybys, you have that speed, the momentum, the rockets aren't burning, but in space, nothing slows you down. So you have that speed. As long as it's higher than the Sun's escape velocity, as long as the Sun is not going to be able to gravitationally pull you back in, those five are the ones that have done that. Of all the spacecraft we've ever seen anywhere, those five aren't coming back, and none of them are going as fast as Voyager 1. So even if right now, we designed and built one that was going to end up going faster, it would probably take, like, probably hundreds of years to be able to actually catch up.

TALIA: Voyager 1 is going really fast.

ERIC: So we think about getting from place to place in the solar system. When you go into orbit around a place you don't really think day to day too much detail other than yes, okay, you're now at that location, and you're going around in a circle in space. But there's lots of different types of orbits and to see different parts of planets, you need to change whether you're going over the north pole of the planet or around the equator of the planet. And again, orbits don't work like that. You can't just burn your spacecraft and end up in a different orbit. You have to get clever with how you do it.

TALIA: Yeah, so my favorite example of this because, you know, frankly, it's my favorite spacecraft of all time. It's the Cassini mission to Saturn. So imagine you're orbiting Saturn, there's a lot of really cool things to look at. Let's say you want to look at the moons. The moons are going around the planet's equator. So if you want to study the moons, you need to be near the equator. But you also want to study the rings. From the equator, you can't see the rings: they're edge onto you. So you really want to be in a polar orbit if you want to study the rings. So Cassini had to switch orbits a lot, and that takes a lot of energy and the spacecraft didn't carry enough fuel to do that. So it did multiple flybys of Saturn's big moon, Titan, and did these momentum transfers again, and used the momentum that it stole from Titan to change its orbit multiple times so that it can look at different really cool things in the Saturn system. I love that spacecraft so much.

ERIC: I like the idea of one of the moons kind of flinging it off in a direction that seems random, but it's actually like, perfect for what you want to observe next.

TALIA: Yep. And it also had the benefit that Cassini got to see Titan up close multiple times.

ERIC: Well Talia, thanks for telling us all about how to get around the Solar System with our spacecraft.

TALIA: Thank you for asking me about my favorite niche nerdy space topic that nobody else ever asks me about.

ERIC: Next time you're at the Museum of Science catch a Science Snapshot presentation at the Current Science and Technology Stage - we just might be discussing the journey of one of the many spacecraft currently exploring the solar system. And while you're home, take a virtual tour around a scale model of the solar system spanning the entire Metro Boston area on our YouTube channel. Until next time, keep asking questions.

If you liked this episode, be sure to check out:

How Many Moons Are There In The Solar System?

Why Is it Important to Look for Life on Mars?

What's the Fastest Thing in the Universe? Part Two

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