Gravity Assist Podcast: Jupiter, with Jared Espley
The Gravity Assist Podcast is hosted by NASA’s Director of Planetary Science, Jim Green, who each week talks to some of the greatest planetary scientists on the planet, giving a guided tour through the Solar System and beyond in the process. This week, he is joined by Dr Jared Espley, who is a planetary scientist at NASA’s Goddard Space Flight Center, and also the program scientist on Juno, one of planetary science’s fantastic missions, currently orbiting Jupiter.
Jim Green: It was a real nail-biter when Juno really arrived at Jupiter on 4th July 4th 2016. How tricky was it for engineers to get that spacecraft into orbit?
Jared Espley: Well, of course, there’s millions and millions of things that you have to do right to make a spacecraft work, and that it has all worked perfectly is a testament to our engineering colleagues. The actual event itself was the fire of the rocket engine, and they did it at precisely the right time and for precisely the right amount of time. It was superficially easy, but there’s millions of things that have to happen right to get you to that point.
Jim Green: Juno’s in a 53-day orbit that gives us fabulous close encounters over a two-hour period, and then we spend a lot of time planning instruments and doing other things. What’s happening during the rest of its orbit?
Jared Espley: Like you said, Juno gets super-close to Jupiter every 53 days, and makes a lot of these interior measurements we’ve been talking about. But, one of the other major science goals is to try and understand the near-space environment at Jupiter, what we call the magnetosphere. And in the magnetosphere, are all these particles and waves like we talked about before, which create those really awesome sounds from the Waves instrument. We want to try and understand how the magnetosphere works, so we’re taking continual data all throughout the orbit, especially with the magnetosphere-oriented instruments when we’re far away from the planet.
Jim Green: Compositionally, Jupiter and Saturn are pretty similar. We call them the gas giants, dominated by hydrogen and helium mostly. But based on their cloud shapes, they’re very different, particularly in the northern hemisphere. What’s the difference?
Jared Espley: Using JunoCam we’ve started to see polar vortices swirling around the north and south poles of Jupiter. At Saturn, there’s this hexagon, which is a really interesting storm that really catches your eye because you don’t often see angular features in nature like that. The storms are produced by the cloud features the wind patterns of their respective planets, and I think that’s something scientists are really interested in looking at in more detail.
Jim Green: Jupiter is, as we said, the big guy on the block, the planet that has got the most gas. It’s also pretty huge in size, and attracts small bodies that pass by, such as asteroids and comets. What’s been happening over time as we’ve been observing Jupiter and these small bodies?
Jared Espley: As we’ve observed Jupiter, we’ve actually even seen some of those small bodies be pulled in by Jupiter’s gravity. In particular, back in the 1990s, there was a comet [Shoemaker–Levy 9] that got pulled in very close and ripped apart . There were some fantastic images that were taken, back in 1994, of how [the fragments of] this comet impacted into Jupiter and created what looked like bruises in the planet’s clouds. Over the lifetime of the Solar System this has happened millions of times over billions of years as small bodies get pulled in. Many people think that, by having a large planet out there, it in some ways shields the inner planets from heavy bombardment by comets and asteroids.
But, there’s actually a little controversy about that. People still haven’t really worked out exactly all the mechanics of how that works. But certainly, it’s interesting to see how large planets can affect the bombardment history of the other planets.
Jim Green: Jupiter is further out than the Asteroid Belt and its massive gravity is also pushing asteroids around. It either takes those and pulls them out of the Solar System or throws them inward. So, you’re right – the controversy is, is Jupiter really helping us that much when, in reality, it’s scattering asteroids out of the Asteroid Belt and having them come inward where they can potentially threaten Earth.
Jared Espley: Exactly. It would be interesting to compare and contrast and try to get our modeling to try and understand a little bit better what those effects are. Again, that goes to the big picture of what roles do planets have in solar systems, not just our own but in many of those solar systems that we see elsewhere and what the histories of those solar systems would be and how they would be protected from impacts.
Jim Green: Jupiter also has certain gravitational nulls that orbit with it around the Sun. We call them Lagrange points. Why are they important?
Jared Espley: Those are places where the gravity between Jupiter and the Sun approximately cancels out just because they’re both competing and it averages out. And so objects that naturally enter into those locations would stay more or less stable and there are now a lot of small bodies, a lot of asteroids, that have collected in those places. We call them the Trojan asteroids. We actually have some upcoming missions that’ll be trying to visit some of those asteroids.
Jim Green: Yes indeed, the Lucy Mission is really all about going to L4, Lagrange point number 4, and L5, Lagrange point number 5, on either side of Jupiter in its orbit and visiting many of those Trojans. It was thought that those Trojans were perhaps asteroids, but they could be Kuiper Belt objects, comets. So, by going out there and really taking a good look at them, we’ll try to understand how they have been trapped and maybe that’ll give us an indication of the evolution of our planetary system.
Jared Espley: Yeah, exactly, because we have a lot of small bodies of different types, like you mentioned. They all tell a different story about the history of the Solar System.
Jim Green: In 1610 Galileo took a look at Jupiter with his telescope, and he was really surprised. He saw objects that were very close [to Jupiter] and studied those, and it turns out they were huge objects. Today, we call them the Galilean moons. What do we know about them?
Jared Espley: Those four major moons that Jupiter has are really significant worlds in their own right. I mean, they’re the moons of Jupiter, which would make them interesting, but each one of them – in order, Io, Europa, Ganymede and Callisto – are the moons that are potentially some of the most interesting in the Solar System. They are particularly interesting because they have all these really interesting features like volcanoes, subsurface oceans, ice on the surface. There’s all these really interesting features that we really like to try and explore in these worlds.
Jim Green: Ganymede, which is the largest moon in the Solar System, has its own magnetic field. How cool is that?
Jared Espley: It is awesome, and I say that as a person who studies magnetic fields for a living.
Jim Green: Out of those four Galilean moons, which are all special, there’s one that’s really intriguing, and that’s Europa. What can you tell us about that?
Jared Espley: Europa is a really interesting place. We know that it has a surface of ice, with cracks in that ice, with red streaks on it, and we know indirectly that there is a sub-surface ocean of liquid water underneath that icy shell. How thick that ocean is, how deep it goes, we don’t know. What’s in that ocean we don’t know, but it’s a really fascinating place, because on Earth every place where we find liquid water on Earth, there’s life. So, the natural question is, is there life at Europa? Nobody knows, but it’d be awesome to find out.
Jim Green: We’re working on a mission right now called Europa Clipper that’s planned to be launched in the early 2020s, and we’re determined to find out.
We’re getting some beautiful images of Jupiter from Juno, and they’re all rendered by the public. How is that happening?
Jared Espley: That’s one of the really neat things about the Juno Mission, which is that it has this camera, JunoCam, and its images are specifically designed to be processed by the public. We requested that the public help us out with that, and they’re doing a fantastic job. You can download the raw images and then rework them into an interesting technical result or even an interesting artistic result, and there have been lots of people doing that. It’s all freely available on the web. If people just do a search on Juno or JunoCam, they can find the archive, and they can help be part of history, making new images from Jupiter.
Jim Green: Actually, the team is really encouraging a lot of the amateur astronomers to look at Jupiter during certain times.
Jared Espley: That’s right. People observing here on Earth can use their own telescopes to look up into space and to see and then you can compare and contrast that with what our space robot is collecting there in orbit.
Jim Green: I ask every one of my guests a very important question, and that what was that “gravity assist” that propelled them into their field. Jared, what happened with you?
Jared Espley: Like many of us, I’ve been fascinated by space since I was a little kid. My dad was a big science fiction fan, so I always enjoyed going to science fiction conventions and reading science fiction with him. Then I realized that, in order to study space, you had to learn about physics and about math, and so I did that. Then I’ve just been at the right place at the right time ever since. I went to graduate school to do physics and was working with data from a NASA mission, Mars Global Surveyor, which naturally led to working as a post-doctoral research up at Goddard [Space Flight Center]. They couldn’t get rid of me, and I’ve been there ever since. It’s the right place at the right time and being interested in space I think is how I got to where I am.