Good news! Over the next few years, we’re going to see a flotilla of new missions headed to Jupiter and Saturn. Why aren’t we seeing more missions to the outer planets, like Uranus and Neptune? It turns out, those places are far away. Today let’s talk about the challenge of exploring the outer Solar System.
Mars Perseverance Rover (NASA)
MarCO (NASA JPL)
New Horizons (JHUAPL)
Hohmann Transfer Orbits (NASA)
681: Gravity Wells (Explain xkcd)
Parker Solar Probe (JHUAPL)
The Expanse (IMdB)
Voyager (NASA JPL)
Ion Propulsion (NASA)
International Space Station (NASA)
Kim Stanley Robinson (Macmillan)
Power Systems (NASA)
Mars Curiosity Rover (NASA)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast, episode 620, “Why Getting to the Outer Worlds is Difficult.” Welcome to Astronomy Cast where weekly facts-based journey through the cosmos, where we 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 is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute, and the director of Cosmo Quest. Hey Pamela, how are you doing?
Dr. Pamela: I am doing well Fraser, it is one of those picture-perfect fall days where the wind is howling, the leaves are blowing, and you can just tell that snow is coming at some point.
Fraser: This is the first sunny day that we’ve had in all of November, and probably for the last two weeks into October, as well, it has been a wet, wet fall. So, yeah, it’s kind of nice to be able to stand outside and feel the burning orb of the sun on your face.
Dr. Pamela: You do live in a tropical rainforest.
Dr. Pamela: Temperate rainforest, you live in a temperate rainforest.
Fraser: Yeah. Apparently, that comes with a lot of rain.
Dr. Pamela: It does!
Fraser: Yeah, it’s right there on the packaging, true that. No snow, yet, but it’s close. We see the snow line inching down day after day, and so, it’s just a matter of time before the snow gets to us.
Dr. Pamela: Yeah, it’s five hours drive north of us right now, but I live where it’s flat, so, it’s not like you can gain anything by going up a hill.
Fraser: Right. All right, good news, over the next few years, we’re going to see a flotilla of new missions headed to Jupiter and Saturn. Why aren’t we seeing more missions to the outer planets like, Uranus and Neptune, even Pluto? It turns out, those places are far away. Today, let’s talk about the challenges of exploring the outer, outer solar system. I think one of the things that we’ve talked about for a longs time, is how sad it is that there aren’t any missions planned for Uranus, Neptune, Pluto, the Oort Cloud –it sucks. Why?
Dr. Pamela: Yeah. So, there’s two different problems, and which one you worry about more depends on the science you’re trying to do. The first problem is, just getting out there fast enough so that you can accomplish your science within a human lifetime, or – At least with the next generation, we do build missions where we know that it’s going to be the next generation of scientists who do the science. So, getting there fast enough is problem one, and problem two is, if you want to do more than a very brief flyby, you have to figure out how to drop all that velocity that you’ve gained, all that momentum that you’ve gained, and somehow get yourself in orbit around one of these worlds, so that you can do lasting data collection.
Fraser: All right, and I think that’s going to be a bit of a surprise to people, so, let’s tackle that first. So, you’ve got this idea, I guess – You say you’re going to launch a mission to Mars, and you have two options for how you do this mission to Mars, one option is that you can to a flyby, and so, you just take some pictures, you don’t get caught in Mars’ gravitational field, and you just fly on through. The other option is that you attempt to go into orbit around Mars. So, how does that change between those two mission parameters? How does that change what kind of a flight, what kind of a mission timeline are you looking at?
Dr. Pamela: In the getting there part, it doesn’t necessarily have to change anything. We have this fabulous example where with Perseverance, we have this excellent example of these two little CANSATs that flew along with the mission that went to mars, and then, they did that flyby, and kept on going to explore the outer solar system, not that they’re really returning signal anymore. But the difference between these two different missions was, the CANSATs really didn’t have the fuel, the maneuvering ability, or anything that would allow them to dump velocity to go from orbiting the sun, to instead, orbiting Mars.
Perseverance instead, was able to break using a whole bunch of different techniques from very carefully maneuvering so it could just graze the atmosphere and aerobrake down through – Well, of course, it had parachutes and retro rockets that all together, allowed it to land safely.
Fraser: And so, you have this situation where you’re blasting off at a high speed to, you’re following this perfect trajectory out to Mars, that uses the minimum amount of propellant to arrive there. But as you say, in the case of Perseverance, it had a parachute, back shell, landing crane, aerodynamics, to be able to actually set down softly on the surface of Mars, while those CANSATs, they’re just little bricks that flew right past, they had no way to go into orbit, and so, they had no way to stop. And so, if you wanted to have those things go into orbit around Mars and do a longer mission, they would’ve had to have booster rockets all on their own.
Dr. Pamela: And they did not. And with Mars, at least you have the ability to utilize not just it’s gravity as something that can grab onto you, because that’s always going to be there to help, but that alone is not enough, it was that atmosphere that really made the ultimate difference and allows them to slow down these missions without having to use a whole lot of energy.
Fraser: And so, the implication than is, you can either have retro rocket or science payload.
Dr. Pamela: Yeah.
Fraser: And so, in the case of the Mars CubeSats, they chose payload and no rocket or landing system, but the landing system is heavy, and you have to make that tradeoff every time.
Dr. Pamela: And when you chose both – So, it’s really that classic pick two of three, you either have to make it affordable, you have to make it so that you optimize for science, or you have to make it that you optimize for getting yourself onto the surface. If you get rid of the optimizing for cost, that leaves you room for both science and those engines that slow you down and put you in the correct orbit, or in this case, even allow you to just plain land. Pick two.
Fraser: So, let’s look at something in the outer solar system then, let’s look at say, the New Horizon’s mission to Pluto, which we know took about 10 years. And what kind of a flight plan did that mission take?
Dr. Pamela: Go fast, fast, fast, fast, fast. Fast, go fast.
Fraser: Right, faster, faster.
Dr. Pamela: It was amazing. They took this little, teeny tiny space craft, as far as spacecraft can be considered, they took this itty bitty, little, tiny, space craft, put it on top of the biggest rocket they could reasonably get, and just launched straight escape from the planet Earth, no once around, they didn’t bother with any of that, they just went from ground toward Pluto. Now, they did have some encounters along the way that’s allowed them to test their instruments, they did get some gravitational boosts, but that wasn’t the focus of their orbital route.
The focus of their route was, “We’re not going to do a Helmholtz orbit that has the earth and Pluto in this nice, easy, low energy transfer between the two. No, we are simply going to do an escape velocity from the sun, heading straight out.” It was kind of awesome.
Fraser: Right. And at times, New Horizons was the fastest, or I think it had some of the fastest velocity going through the solar system. It’s since slowed down now, because it’s essentially climbing out of the sun’s gravity well, and the voyagers are faster, but for a while there, it was a very fast space craft. I think now, like the Parker Solar Probe, has taken the record for the fastest space craft that humanity has ever launched. So, then again, back to this idea, you’ve got this mission flying like a bullet, picking up more velocity, thanks to Jupiter, there’s just no way to slow it down, it’s going to hurdle past Pluto at high speed.
So, what if mission planners said, “We want New Horizons to be able to go into orbit around Pluto.”? How would that have changed the dynamics of the mission?
Dr. Pamela: It wouldn’t have changed just the dynamics; it would’ve also changed how rugged the mission had to be. Imagine being on a set of roller skates with a safety harness on, that is attached with a solid steel cable to a pole, and you fire those water jets that people now use out over the ocean, to go as fast as you can in a straight line, and then, you hit the end of your cable and you suddenly get jotted sideways, that sudden change is not going to be comfortable.
And what you’re looking at is, you either do the “Go, go, go, go, go! Stop!” as hard as you can and hang a sharp corner as you put yourself into orbit, or you have to plan for a substantially longer period of time to get there, where you don’t get to as high of speed, or you start decelerating much earlier, so that it’s a gradual process that allows you to more casually enter orbit around Pluto.
Fraser: Now, I worked on an article a few years ago about this idea of homing transfers, where –and this is what you were talking about earlier on, that you follow this perfect trajectory, and I’m sure we’ve all seen this animation many times. You’ve got this trajectory where your spacecraft leaves the earth, it’s on this larger ellipse, and it just happens to end up exactly where Mars is crossing, and boom, you’ve got the least amount of energy and the easiest deceleration on the other side. So, if you didn’t do that straight shot to Pluto –
If you want to do a homing transfer to Pluto, you’re looking at about 46 years of flight time, to be able to do that, plus, you need a deceleration system. So, no one has time for that.
Dr. Pamela: Yeah. And you and I did not start learning orbital mechanics as toddlers, so, that mission would have had to have been started by a prior generation for us to have any hope of working with the data.
Fraser: Right. And so, you’ve to this enormous, just immense flight time, this 45 years – That’s way too long, obviously, and so, the way you cut that time down, is you put a rocket on your space craft that can decelerate and knock years off of that time. It speeds up, and then, it turns around and slows down, kind of like, in The Expanse or other TV shows that we’ve seen.
Dr. Pamela: It’s the classic, your spacecraft flips over, points ascensions in the other direction, it was on Foundation, I think last week, or two weeks ago, it works, it works. This is why science fiction authors use it.
Fraser: But the challenge then is, you have to give up science instruments for this material, for your propellant, for your engines, etc. So, let’s talk about some missions then, that were on the books that have since been canceled, pushed back. I think the most famous example, fairly recently, was the Trident mission.
Dr. Pamela: Yeah. That was actually a flyby mission – Of course.
Fraser: Of course, because – No, no other good time for an orbiter, yeah.
Dr. Pamela: I keep hoping, I keep hoping. So, Triton was a mission to the moon, Triton, that is a captured Kuiper belt object bigger than Pluto, and we know it’s captured, because it’s going the wrong way around its world, which is Neptune. And the way its surface is modeled is consistent with the release of energy that would occur while going from a highly elliptical capture orbit, down to the circular-ish orbit it’s in today. And it would’ve been awesome.
Fraser: Right. And, unfortunately, it was not selected while two missions to Venus were. I think we’ve gone on quite a bit about this.
Dr. Pamela: But it can be re-submitted. It’s not dead, it’s just not happening right now.
Fraser: We’ve got the Titan Dragonfly, which is leaving for Saturn later on this decade, but it’s going to probably take I think, eight years. It’s going to take a long time to get to Saturn, and Saturn is incredibly close compared to Uranus, Neptune, and Pluto.
Dr. Pamela: And so, they’re taking care not to put too much velocity onto the mission getting out there, which is a cost savings and energy savings, going slow is easy. And then, the other thing that Saturn and Jupiter both have going for them is they’re much more massive. And that additional mass allows them to just put that extra pull onto the space craft to keep it.
Fraser: And of course, with the Dragonfly mission, once again, you’ve got an atmosphere that you can use, the Titan atmosphere is way thicker than Mars. It’s a piece of cake, relatively speaking to aerobrake into the Titan atmosphere. And so, if you didn’t have that atmosphere of Titan, to be able to absorb that energy as your spacecraft was flying at it – Cassini had a much trickier job to be able to go into orbit around – That was a full 10 years, I think, to actually get out to Saturn, while – When you go back to say the Voyager missions, the Voyagers launched in ’77, they hit Jupiter in ’80, Saturn in ’81 –
Dr. Pamela: And they just went full tilt boogie the entire time.
Fraser: Full tilt, yeah. Full flyby, yeah, exactly. And so, you’re going to see these places that actually have an atmosphere, are going to be the first places that are going to get landers, possibly orbiters. There’s this cool idea of aerobraking that’s been done.
Dr. Pamela: And there’s also the potential, but they have complicated magnetic fields that you have to watch out for – There’s still the potential to break your spacecraft, not apart, but slow it down brake it, in the atmospheres of Saturn, Jupiter, Neptune, Uranus. And so, these gas giants give us an opportunity we don’t have with rocky things like, Pluto – Or at least icy things like, Pluto, Triton, the other Kuiper belt objects.
Fraser: Yeah, you would be lithobraking into those objects.
Dr. Pamela: Which is both forms of braking.
Fraser: Yeah, exactly. All right, so, let’s talk solutions. We’ve been talking about problems, we’ve been talking about how our expectations for a fully explored outer solar system have been delayed and we’re sad, so, what are some ideas? We pitched a couple already, this idea of using atmospheres to I guess, remove the need for bringing a rocket. What else can be done to try to lower those flight times to the outer solar system?
Dr. Pamela: Well, ultimately, you want to have the planets in perfect positions to allow you to get just the right set of gravitational assists to speed you up and slow you down. We’re not going to get the planets in the right places to do that for a long time, so, that’s not useful for any of the next several generations of astronomers, but it’s a possibility, it is a possibility.
Fraser: That’s an interesting idea. So, you could launch from Earth, do a gravitational slingshot around Jupiter to speed you up, and then, do a gravitational sling shot around Saturn, but go the other way to slow you back down, and then, you would have an easier time of being able to slow yourself down when you actually got to Uranus or Neptune.
Dr. Pamela: I’m not sure it would work that simply, but it allows you to do something like, take off from the earth, heading inwards, get your acceleration off of Venus, and then, on the outside, slow yourself down. And it gets tricky orbital mechanics, it’s not a straightforward problem. But these are the types of creative solutions you can start to look for. And beyond that, it’s just a matter of figuring out what is the correct way to combine technologies?
Fraser: Right. So, let’s talk about some advanced propulsion systems that could fill that gap.
Dr. Pamela: Well, it – So, while we generally think of ion drives as being the slow solution to things, there’s that potential that you zip yourself out with a rocket, turn on the ion drive when you need to, and very slowly, slow yourself down, and this gives you a low mass solution that will do what you need, and it’s a compromise, it’s not the perfect solution, but it’s a compromise.
Fraser: Yeah, we’ve seen ion engines capable of boosting spacecrafts to vastly higher speeds than any other –than a chemicalrocket could ever do, just because they are so efficient with their propellant.
Dr. Pamela: They just do it over years.
Fraser: Right, and if your flight time – If your cruise time is going to be years anyway, maybe you could accelerate up fast, turn around, slow yourself down.
Dr. Pamela: Well, you don’t know how to – If you build it right, you don’t even have to turn it around, you have – Your jet propulsion sets you up on the accelerating, and then, your ion drive sets you up on the slowing down.
Fraser: Right, yeah. And then, I think the other opportunity that’s coming up is a new class of launchers, like say, Starship.
Dr. Pamela: And this is where it’s a matter of, “And now we don’t really care about how much mass we use, we just take all of it. Take all of it.”
Fraser: Yeah, exactly. Yeah, it’s hard to wrap your head around how much fuel Starship could theoretically carry onboard if you filled it. If you launched Starship, re-fueled it to capacity, and then, set off for the outer solar system, you could just full burn, turn around, full burn, and you could knock years and years off the flight times with a rocket that big.
Dr. Pamela: The thing that got me about Starship, was someone threw out the other day, it could have taken up the entire International Space Station in three launches. You just defined my entire teenage through adulthood –transition to being the adultier adult, was defined by the construction of the International Space Station, and you’ve reduced it to three missions, and it’s just –
Fraser: Yeah, and it could be the same rocket, and it could do it in one day.
Dr. Pamela: I don’t think they’re going to get three launches per day out of the same Starship, but –
Fraser: A Starship, in theory, will be able to fly three times in a day, and a Super Heavy will be able to launch eight times in a day. So, in theory, this one Starship could launch the entire International Space Station in one day. I agree, that seems unlikely, but on paper, that’s the plan, but of course, we’ll see.
Dr. Pamela: Yeah, I just look at it – Sure, you could also fly a 747 back and forth from Boston to London three times a day, but they’re probably not going to use the same 747. Cleaning time people, cleaning time.
Fraser: And so, there are some other interesting propulsion systems that have been proposed to go vastly farther out into the solar system, places like, out into the –even out into the Oort Cloud, ideally. So, I don’t know if you’ve seen this as the idea on an electromagnetic sale.
Dr. Pamela: No.
Fraser: So, you – Yeah, so, you have this –it looks kind of like a wire net, and in a big circle, and you electrify the wire with solar power, and then, what it does is it then interacts, you make it positive, I think, and then, it interacts with the negative ions coming from the suns solar wind, and they give this thing a kick. NASA awarded a NIAC grant to one of these proposals a couple years ago, and the theory is thinking you could reach deep areas into the outer solar system, essentially to wherever the solar wind starts to really tail off, which is out at the Heliopause.
Dr. Pamela: Dang.
Fraser: And you could do it within again, just a matter of years, the acceleration is tremendous. But then, it’s really hard to turn it back off again, because you’ve got all that speed, so, you’re back to flybys, right?
Dr. Pamela: Right, right, right. But you can get there fast, you just said – So, one of the ideas that –it’s an imperfect solution, but I really like it, is – We’ve seen a variety of science fiction books, Kim Stanley Robinson has put this forward, the focus behind the expanse has put this forward, the idea that you have a variety of transport vessels, asteroids that are hollowed out, whatever you wish, that our systematically entering this transfer orbit, such as that they synch up, and you can jump from one means of getting outward to the other relatively simple.
So, you take a transfer orbit that gets you to a transfer orbit, that gets you to a transfer orbit, and you essentially have a set up as stations. I imagine the day where you can essentially spend your life working on your PHD while building the instrument on the way out.
Fraser: Right, yeah. So, you really come along for the ride.
Dr. Pamela: Yeah.
Fraser: The other idea that we’ve been reporting on is this idea of taking an RTG, a nuclear RTG, like what Perseverance or Curiosity has onboard, but just like the raw heat producing RTG, electricity producing RTG, bolting it to an ion engine, and then, having a very small payload that goes along with it. And so, now, you’ve got a spacecraft that is capable of ludicrous amounts of change in Delta V, so, like, 40km/second, change of Delta V, 100km/second of Delta V. One proposal for this is to go and chase down Oumuamua, grab a sample, and return back to Earth with it.
So, I think when you think about traditional chemical rockets, you got to be thinking in decades, but we’re now at the point where there are enough really fascinating new technologies that are coming online, either just massive rockets, like, Starship, or these advanced –
Dr. Pamela: Which is still chemical.
Fraser: Right, or these advanced technologies like, ion engines, RTGs, there’s also nuclear rockets, and even there’s a fusion rocket proposal that NASA is working on as well. So, I think we’re going to see this drought of outer solar system exploration, start to wrap up in the next couple of decades. That still sounds like a long time, but the drought is almost over.
Dr. Pamela: So, the science fiction story, I now want to find something to write, is that future where the graduate student and a 3D printer gets to work on flying their way out to the outer solar system through this series of Helmholtz Transfer Orbits, and they have to complete their instruments before they get there and start their return.
Fraser: Very cool. Let me know when you finish writing it.
Dr. Pamela: Oh, I would find someone else.
Fraser: Oh, okay, I don’t think that’s how that works. All right, thanks Pamela.
Dr. Pamela: Thank you Fraser.
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