Understanding Juno’s Orbit: An Interview with NASA’s Scott Bolton

The intense radiation around Jupiter has shaped every aspect of the Juno mission, especially Juno’s orbit. Data shows that there is a gap between the radiation belts that encircle Jupiter, and Jupiter’s cloud tops. Juno will have to ‘thread the needle’ and travel through this gap, in order to minimize its exposure to radiation, and to fulfill its science objectives. Adding to the complexity of the Juno mission, is the fact that the design of the spacecraft, the scientific objectives, and the orbital requirements all shaped each other.

I wasn’t sure what question to start this interview with: How did the conditions around Jupiter, most notably its extreme radiation, shape Juno’s orbit? Or, how did the orbit necessary for Juno to survive Jupiter’s extreme radiation shape Juno’s science objectives? Or, finally, how did the science objectives shape Juno’s orbit?

Scott Bolton, NASA Principal Investigator for the Juno mission to Jupiter. Image Credit: NASA

As you can see, the Juno mission seems like a bit of a Gordian knot. All three questions, I’m sure, had to be asked and answered several times, with the answers shaping the other questions. To help untangle this knot, I spoke to Scott Bolton, NASA’s Principal Investigator for the Juno mission. As the person responsible for the entire Juno mission, Scott has a complete understanding of Juno’s science objectives, Juno’s design, and the orbital path Juno will follow around Jupiter.

EG: Hi Scott. Thanks for taking the time to talk to me today. Jupiter’s radiation is a big hazard that Juno has to contend with, and Juno’s titanium vault is designed to protect Juno’s electronics. But Juno’s orbit is partly shaped by the radiation around Jupiter. How has the radiation around Jupiter shaped Juno’s orbit?

“…we knew that the region around Jupiter is really bad, hazardous, and harsh with radiation…”

SB: Well, it limited our choices, let’s say. Juno’s orbit was chosen through a combination of the opportunities for scientific measurements, which needed a certain kind of geometry or location of the spacecraft to perform, and the fact that we had to avoid as best we could the most hazardous region, basically, in the solar system. This required us to be very close to Jupiter, and polar in orientation. We go over the poles of Jupiter. And we knew that the region around Jupiter is really bad, hazardous, and harsh with radiation, but we also had never gone in there with a spacecraft. So we’re not quite sure how harsh it is, or exactly how it’s shaped. We just have some ideas.

But through analogies with the Earth, and through modelling, we were able to figure out a way to accomplish the scientific objectives we wanted and still stay out of the worst regions. Juno comes in over the poles, and will dip down very close to Jupiter in a way that we believe will be between the radiation belts and Jupiter’s atmosphere itself.

At Earth there’s a tiny window between our own radiation belts—which are not nearly as dangerous as Jupiter’s but are shaped in a similar way—and Earth’s atmosphere. There’s a gap there, and we have evidence that there’s a gap at Jupiter as well, and we’re threading that needle.

The Van Allen Belts around Earth. The inner red belt is mostly protons, and the outer blue belt is mostly electrons. Image Credit: NASA
The Van Allen Belts around Earth. The inner red belt is mostly protons, and the outer blue belt is mostly electrons. Image Credit: NASA

EG: Where did the evidence for that gap come from, other than just looking at Earth’s Van Allen belts? Were there any observations from any of NASA’s observatories that showed there would be a similar gap around Jupiter?

SB: We used radio telescopes like the VLS (Very Large Array) and other radio telescopes around the world that can look at Jupiter, and at certain frequencies they’re seeing what’s called synchrotron radiation. Synchrotron radiation is very high energy electrons that are moving at near the speed of light, and they give off radio emissions. They give it off in a very specific geometry based on relativistic physics. We can see that, and it tells us something about how the radiation is shaped, and how the population of high-energy electrons is distributed. That is used in models, and we’re able to indicate that there should be a little bit of a gap, partly because when we look at that radiation, it looks like it tails off as it gets very close to Jupiter. But we have a limited resolution, so while there’s an indication that a gap exists between Jupiter and its radiation belts, there’s no positive proof.

EG: So Juno itself will be the positive proof that there is a gap between Jupiter and its radiation belts?

SB: Yes. And then we have one other measurement that helps us understand this. The Galileo spacecraft that orbited Jupiter back in the mid ’90s contained a probe that went into the atmosphere of Jupiter to find out what it was made of. That probe took some measurements with some very crude instruments, almost like Geiger counters, and the data from those measurements indicated a peak in radiation and then a gap close to Jupiter. So that gave us further evidence that a gap exists. Though it’s a very limited data set, it’s consistent with the models from the radio telescopes.

EG: You must have had certain science objectives in mind for the Juno mission, so how did this understanding of Jupiter’s radiation belts, and the orbit required to avoid them, shape the science objectives of the Juno mission? Did it force any objectives to be abandoned altogether?

“In fact, it was the science objectives that basically drove the orbit.”

SB: No, not at all. In fact, it was the science objectives that basically drove the orbit. That’s what drove us to want to get really close. The question was how close can we get that is safe, and how many times can we orbit? So I would say that what the radiation does, is it didn’t change our orbit so much as limit the number of times we can orbit. So we had a limited life time, and because of that limited life time, we went into an orbit that allowed us to map the planet as quickly as possible. We want to fly by it very closely, at many different longitudes that are evenly spaced.

The science objectives and the constraints of the radiation belts told us that Juno is only going to last so long, so you’ve got to get the map done in a limited amount of time. So there’s a little bit of a trade-off. Maybe there was a way to protect Juno longer with more titanium, more shielding, to last a little bit longer, but it gets so bad at the end, that I’m not sure if we protected it more that it would last any longer.

“Had I been able to put enough fuel on board, I could’ve changed the orbit in the middle of the mission…”

EG: Diminishing returns, I guess?

SB: Right. So the limitations of the engineering and the practicalities of what we can launch on a rocket are really what limited us. Had I been able to put enough fuel on board, I could’ve changed the orbit in the middle of the mission to allow us to last longer. That would require an enormous amount of fuel though. What happens is, when you’re close to Jupiter, it’s not perfectly symmetrical, so it starts to change the shape of Juno’s orbit.

EG: So you’d need to make corrections then, to maintain the orbit?

SB: Yeah, but we can’t. We don’t have enough fuel to do something like that, so you have to live with what Jupiter does to the orbit. So it starts to twist the orbit around, and each time we come by Jupiter it starts to twist the orbit a little bit more. We use that scientifically a little bit, but the reality is it’s just something we have to live with. For the first half of the mission, if the modes are correct, we won’t have to deal with the maximum amount of radiation, but towards the latter half of the mission it starts to get worse. We can’t avoid the radiation belts as much as we could at the beginning. That basically is what limits the life-time of the Juno mission.

EG: So Jupiter is constantly affecting Juno’s orbit, and you have a limited capacity to deal with that?

SB: That’s correct. It’s because Jupiter is not a perfect sphere.

EG: And one of the objectives is to map Jupiter’s gravity?

SB: Yes, to find out how exactly imperfect of a sphere it is [laughter.] And then to learn from that about what it’s interior structure is like, and hence how it formed.

EG: This seems like a good time to ask what the shape of Juno’s orbit is? How close to Jupiter will it get, and how far away will it get during its orbit?

“…we’re out near the outer moons, near Callisto or so.”

SB: It’s an ellipse, like most orbits, and its closest approach point is about 5,000 km (3100 miles) above the cloud tops or so, and that’s called perijove. On the other side, we’re out near the outer moons, near Callisto or so.

EG: Quite a distance away, then.

SB: Yes, it’s quite a distance away. It’ll take Juno 14 days or so to complete an orbit. And then the other orientation is just right over the poles. Right over the north and south poles. But we’re not getting into that orbit immediately. We first have to fire our rockets and we get into a much bigger orbit that takes about 53 days to go around, and the distance we go away from Jupiter is so much further. The over the course of the first few months, we have enough fuel to modify the orbit to get what we eventually want, and that takes a few months to do.

EG: So Juno is also solar-powered, other than its fuel to change its orbit. You have to stay exposed to the sun, so that must have been an additional  when designing your orbit?

“…in general, we avoid shadows or occultations by Jupiter.”

SB: Yeah, that was an additional constraint in the sense that I want to avoid going into the shadow of Jupiter. I want the solar panels to always see the sun. We can go short periods of time without that, but in general we avoid shadows or occultations by Jupiter.

EG: Is that one of the reasons that the orbit takes you so far from Jupiter? To avoid going into Jupiter’s shadow?

SB: Yeah, that’s right. Although you could avoid it even if you were so close, if you were orbiting sideways. I don’t have to go behind Jupiter, even if the orbit was small. But you have to calculate all that out and make sure.

EG: Will all of Juno’s instruments be active throughout all of it’s orbits? Or are some of the orbits dedicated to certain sensors and instruments?

SB: In general, all of the instruments are active. But we have orbits that are focussed on certain things based on pointing requirements. For instance, the gravity measurement. When we want to measure the gravity field, we have to make sure the antenna is pointed at the Earth as much as possible. That’s how you measure the gravity field, is you look at the signal that Juno sends back to Earth, and you measure the Doppler shift of the radio signal and that tells you how the gravity field has pushed and pulled on Juno.

When we’re not measuring the gravity field, we have other instruments that would prefer to be pointing directly at Jupiter. They can still take the data while we’re measuring the gravity field, but it’s better if they’re pointing directly at Jupiter. We can tolerate that because the solar arrays are still pointed at the sun, and we can still stay in communication with the spacecraft, we just can’t get the full gravity field measurement.

“…at the very end of the mission the solar cells are not expected to perform as well as they do at the beginning.”

So we have some orbits that are dedicated to that geometry. Of course when we’re dedicated to that it used to be that we can just shut off the gravity system if we weren’t using it. But I think our estimates are now that our power is sufficient that we may be able to keep these both on at the same time. Whether or not we do that, it’s not required, but at the very end of the mission the solar cells are not expected to perform as well as they do at the beginning.

EG: That’s because of the radiation? For the same reason that the electronics are sensitive, the solar cells will degrade over time?

SB: That’s right. So we have them protected, but we don’t know how well that’ll work exactly. We don’t have it in our plans, but we can accommodate it with the idea that at the end of the mission, if we don’t have enough power to run everything, we can start to shut some of the instruments off that have done most of the science that we wanted them to do. We can sort of take turns for which instruments are on and which are not.

EG: So that gives you some mission flexibility if the radiation is more severe than modelling suggests? You’ll have some flexibility to prioritize near the end?

SB: That’s correct. Right now our models suggest that we won’t have to do that, but we’re able to turn that dial if we need to.

EG: I’m wondering about the detailed modelling that you’ve done for Jupiter’s radiation and the Juno mission, and looking at the information available on NASA’s websites and other sources. It’s suggested that all of Juno’s instruments aren’t expected to survive the 33 orbits, is that right? Is there sort of a best case scenario for instrument survival? I’ve read that JIRAM (Jupiter Infrared Auroral Mapper) and maybe Junocam might only last until the 8th orbit, and the Microwave Radiometer might only last until orbit 11. Is that sort of a best case scenario? Or more a middle of the road model that you’re following those orbit numbers?

SB: We hope that’s the worst case scenario. They’re designed to survive that with a factor of 2 margin in radiation. It’s probably a little bit greater than a factor of two. So they should be able to do that without a problem. It would be a surprise if they didn’t last that long. Our expectation is that they’ll probably go to the end of the mission. But I don’t count on that, and I don’t require that. It came from the fact that a couple of those instruments don’t have their electronics inside the <titanium> vault.

EG: Is that because they don’t require all 33 orbits to fulfill their mission? Are instruments prioritized to be inside the titanium vault based on how many orbits they require to complete their mission?

“In the vault with all the electronics can be quite a warm place, and some instruments are a little better off when it’s cold.”

SB: That’s right. So that’s how we made that choice. They obviously needed some protection from Jupiter’s radiation, so there’s little boxes around them, but not like the giant vault. There are also some other reasons they’re not in the vault. There are some benefits to moving them out. In the vault with all the electronics can be quite a warm place, and some instruments are a little better off when it’s cold. So there are different trades that have gone on. But you’ve characterized it well in the sense that we’re not required in order to satisfy the science objectives to have them last the whole mission. But my expectation is that there’s benefits if they last longer, so we have hope when we designed them that they would last longer.

Juno's payload. Image Credit: NASA
Juno’s payload. Image Credit: NASA

EG: Scott, what’s your formal title at NASA?

SB: Officially it’s called Principal Investigator. So I’m the Principal Investigator of the Juno mission. That’s an official title that only means something to NASA people pretty much.

EG: So you’ve been in on the mission design right from the beginning of Juno?

SB: Oh yeah. I kind of created the whole thing, or the whole process. What Principal Investigator means to the average person is I’m responsible for Juno. For anything and everything associated with Juno, I’m responsible for its success. Whether it be the design, the engineering, the science, getting it built on time, spending too much money, the schedule, all that kind of stuff. Another way to say that is that if anything goes wrong, I’m the one who gets blamed [laughter.]

EG: Well, I think a lot of it is going to go right [laughter.] So, like myself, you must be pretty eagerly anticipating Juno’s arrival at Jupiter. What’s the most interesting and exciting part of Juno’s mission, if you had to choose one thing? I’m sure that’s almost impossible to answer. And what might be a surprise to you? When we look at New Horizon’s arrival at Pluto and the surprising things we found there, or Cassini finding ice geysers, there always seems to be a surprise waiting for us. What do you think is most exciting about Juno, or what do you think might be a surprising finding?

“…the exciting part of Juno is that we’re going somewhere that nobody’s ever gone before.”

SB: Well, by the definition of surprise, I can’t guess. None of those things could’ve been anticipated, which is why they were surprises. But you know, the exciting part of Juno is that we’re going somewhere that nobody’s ever gone before. We’re going to make measurements that have never been made. We have instruments that simply have never been created before, let alone getting them into this unique orbital geometry where you can make special measurements. So I think the anticipation of learning something brand new that will surprise us is the exciting part.

What are we really going to learn that’s going to change our ideas of where we came form and how we got here? What is Jupiter really like? There’s so many puzzles about it, and it’s so important. Even today, the things we’ve learned about our own solar system, and the things we’ve learned about other solar systems as we’ve been able to start to see exo-planets, have only made Jupiter even more important to us. It really holds the key, and I think the exciting part is that we’re finally going to unlock one of the doors to those secrets. We’re helping make the path for future missions to learn even more.

The other thing I find exciting is even though I’m what’s called the Principal Investigator, and if you ask NASA what that means and they tell you I’m responsible for everything, the real truth is that it’s not one person. It’s an enormous team that made this happen. That helped design it, that created a way to do it, that understood the constraints, that understood how it could work, that figured out the technologies we needed to make it happen, and that basically had the vision to create it, and had the capability to implement it and bring that vision to reality. I’m excited that I’m part of this team of people that are accomplishing this, and that that team is actually just part of our society and humanity, which is all reaching out trying to figure out things. Things like how we fit into nature and how the universe works. I’m just generally excited to be part of something that’s trying to do something like that.

EG: It’s awesome and I agree completely with your words, and I think it’s exciting for myself and for readers of Universe Today. It’s a huge mission, and we can’t wait to start getting some results back. And some picture. It’s super-exciting.

SB: Me too. [laughter]

EG: Thanks for taking the time to speak to me today Scott. Hopefully we can talk again. I know that people are keenly interested in the Juno mission.

SB: You’re welcome. Have a good day.