We start our “Gravity Assist” virtual tour of the solar system with – where else – the Sun! How hot is the Sun, what are solar flares, and how does space weather affect us here in Earth? Jim is joined by Project Scientist Dr. Nicky Fox of the Johns Hopkins University Applied Physics Lab to talk about our fascinating star and NASA’s upcoming Parker Solar Probe—a mission to “touch the Sun.”
To watch an introductory video on Gravity Assist click here.
Jim Green:Our solar system is a wondrous place with a single star, our Sun, and everything that orbits around it. Planets, moons, asteroids and comets.What do we know about this beautiful solar system we call home? It’s part of an even larger cosmos, with billions of other solar systems.
Hi, I’m Jim Green, Director of Planetary Science at NASA, and this is Gravity Assist. I’m here with Doctor Nicky Fox from the Johns Hopkins University Applied Physics Lab, and she is the project scientist for a fabulous mission called the Parker Solar Probe, and that’s the mission that will touch the Sun, and that’s what we’re going to talk about today. It’s all about the Sun.
Nicky, what do we know about the Sun? How does the Sun really work? I mean, it’s so important for life here on Earth.
Nicky Fox: Yes, indeed. So the Sun, of course, is in the center of our solar system. It provides light and heat to us here on Earth. We’re in that perfect location in the solar system orbit where the Sun allows us to live and breathe and have a wonderful life.
But of course, the Sun is just a very average, very basic star. Even though it’s the most important thing to us, it is just an average star when you compare it with all of the others. And so it works just like any other star. In the center, there was a core which is kind of a boiling, roiling mass of gas very much like a nuclear furnace and nuclear reactions go on in the center there.
Hydrogen combines to make helium, etc., just like you would have in a nuclear reactor. And as that heat moves out towards the surface of the Sun, it gradually cools. So the core of the Sun is about 27 million degrees Fahrenheit. By the time we get to the visible surface, it’s cooled to about 10,000 degrees Fahrenheit. From there, the atmosphere of the Sun lifts off and moves away from the Sun and bathes all of the planets.
So we here on Earth, we actually live in the atmosphere of the Sun.
Jim Green: Yeah. You know, the analogy I use for that is it’s like the Sun is exhaling in all directions.
Nicky Fox: It absolutely is, and as it exhales, sometimes it sneezes. And when the Sun sneezes, the Earth catches a cold.
Jim Green: So let’s talk about some of those sneezes. We call them space weather events, but you know, the Sun can be really violent at times.
Nicky Fox: The Sun can indeed be very violent, and one of the ways that to measure the activity on the Sun is you can actually count sunspots. Not that you should ever look at the Sun with your naked eye, but, hey, you pull up a web browser and you look at the SDO (Solar Dynamics Observatory), spectacular images, you see those dark spots if you’re looking in visible.
If you look at them in ultraviolet, they suddenly become very bright, very active spots. The more spots there are, the more active the Sun is, and that’s how we characterize the solar cycle: fewest number of spots, solar minimum; most number of spots, solar maximum. It takes about 11 years for a full solar cycle.
During that 11 years, we see all kinds of different stuff coming from our Sun. As you mentioned, there are these very intense events that we call coronal mass ejections, literally a mass of coronal material ejected fast from the Sun. You know, we see the flare, it’s followed by the very high-energy particles. They travel at about half the speed of light.
They can cause issues for astronauts, which is why, you know, at the space station we make sure that the crew area is very well-protected. So NASA watches for any big events before they allow for spacewalks. They can also smash into solar panels and cause damage to spacecraft. But behind that comes the main event, the big blob of plasma.
When that enters into our Earth’s atmosphere, it does power the beautiful aurora, and that’s a gorgeous, gorgeous effect. However, the aurora is basically a big current in the sky and a current always needs somewhere to close, and the ground is not conducting, they will look through for long structures that they can flow through, like a power grid, like undersea pipelines, like any kind of long, straight conducting surface.
And they will go through those and cause problems. They can cause catastrophic power outages like we saw in 1989 when the Hydro-Québec system went down, leaving 6 million people without power. It also pumps up our radiation belts where the Van Allen Probes are working and taking data for us of these events all the time, causing issues for spacecraft, either damaging with the particles.
It can also cause the atmosphere to become much more dense, slowing down spacecraft and changing its orbit, which has to be mitigated. Many, many effects of space weather. It causes problems for GPS. It can cause outages or even just, you know, problems with how accurate it is and we all rely on our GPS to get us everywhere, every day.
So we really do rely on technology, and technology is very much affected by the Sun.
Jim Green: Well, you know, that’s a fascinating topic, space weather. We just need to know much more about it and have the ability to predict it. So as humans to take off and leave low Earth orbit and go beyond the moon and out into the solar system, perhaps to Mars next, understanding space weather will be really critical.
Now, Mars doesn’t have a magnetic field, and the solar wind which hits us, and we are pseudo-protected I would say because of our magnetic field, Mars and many of the other objects, even Venus, really can get hit by the solar wind. So what happens when those things occur?
Nicky Fox: Well, so then you’re really looking at the radiation effects, because as you say you don’t have the magnetic field to protect you. Also, with such a very weak atmosphere, there’s really no way for it to absorb the heat from the Sun and keep it.
So you’re really looking at direct radiation effects. Yes, we can certainly travel to these other planets. We just want to make sure we understand both the cosmic rays that are coming from outside our solar system and also the solar wind particles and the very energetic stuff coming from the Sun. We need to understand both of those and make sure that we correctly shelter our explorers.
Jim Green: As a magnetic physicist, I love anything with a magnetic field and even those things that have lost their magnetic fields in the past. And so it’s been clear from the research that’s been going on for the last several decades that when there’s that coronal mass ejection that hits the Earth, we always get aurora. And so indeed, those events are intimately linked.
Nicky Fox: They certainly are. So when this material, or as you put it, when the sun is exhaling and it puts all that material out, as the material is accelerated, it grabs the solar magnetic field with it and it carries it out towards the Earth.
Just as you remember from high school, like poles repel, opposite poles attract. So if that field in this solar wind is pointing in the opposite direction to the Earth’s magnetic field, they will attract. They will connect and they will let all of that solar plasma come into our atmosphere.
There’s a process that occurs. We call it magnetic reconnection, but it’s basically all of this excess magnetic field getting moved back around to the sun side. When that happens, all of these energetic particles come streaming down the magnetic fields and they impact our Earth’s atmosphere.
Earth’s atmosphere is predominantly nitrogen and oxygen. When it gets struck by these really high-energy particles, it actually excites the molecules and atoms in our own atmosphere, and that causes them to glow. That glow is what we see as the Aurora. It’s kind of like giving a two-year-old sugar.
You dump a whole bunch of energy in, it gets totally excited and then it goes right back to how it was before you gave it the sugar. It’s just like that with the aurora. You dump a whole bunch of energy in, it lets off a tremendous amount of light and then it goes right back to the atmosphere that we had before the event.
Jim Green: Well, you know that sugar event, as you said, that really starts in the tale of the Earth when there’s a reconnection that occurs that accelerates the particles that create the aurora.
But reconnection as a physical phenomenon, it doesn’t happen just at the Earth. The coronal mass ejections, there’s also reconnection that occurs with that, but there’s another phenomenon on the Sun not like a coronal mass ejection, but equally important, and that is solar flares.
Nicky Fox: Absolutely, yes. They often occur right before a coronal mass ejection leaves the Sun. Those magnetic field lines that you see, the Sun has a magnetic field very similar to the Earth. It’s a lot more complex, but it’s still got a magnetic field. These little loops can pop out, and if you look at a really nice SDO (Solar Dynamics Observatory) images, you can often see these long loops of sort of illumination.
And as they get stretched and stretched like an elastic band here, if you stretch it and stretch it, at some point it will break. When it breaks, you always feel pain in your hand, because you snapped an elastic band. That pain is actually the heat coming from the band into your fingers.
So as you stretch these magnetic field lines of the sun, they become more and more and more energetic. You’re putting more into it. At some point, they will burst. When they burst, they let off a whole bunch of energy and all kinds of different forms, and one of them is visible light, and that’s when we see a flare.
Jim Green: You know, really, the study of the Sun that we do has all been very remote, and some of the things that we’ve been talking about, you know, have been observed and then assumed that these kind of things occur, you know, and how particles get accelerated through magnetic reconnection. But in reality, we’ve really got to go there and we’ve really got to make some measurements right where some of this action happens.
And you’re involved in a fabulous mission called the Parker Solar Probe. Can you tell us a little bit about it?
Nicky Fox: Yes. Well, Parker Solar Probe is the coolest, hottest mission under the Sun. And for the very first time we are going to go into this region. We’re going to go and visit to this area where kind of all of the magic happens. So we’ve done so much with remote sensing, we really have. We’ve done a lot with a measuring even the solar wind out around Mercury.
But everything happens in that coronal region, in that hazy atmosphere around the Sun. In that atmosphere, first of all, we have two mysteries. One, is that that material itself is actually hotter than the visible surface of the Sun, about 300 times hotter with temperatures around 3 million degrees, and yet the surface is about 10,000 degrees Fahrenheit.
So, you know, there’s a big mystery. If you walk away from a campfire, you get colder, not hotter. But if you walk away from the Sun, you would suddenly get into a much hotter region. Where this big temperature happens, the plasma itself gets so energized that it can actually break away from that huge gravitational hold of the Sun and form this solar wind; it bathes all of the planets.
So these are these two mysteries we have. Why is the solar wind born there? Why is it so hot? And why is it accelerated so much? We’ve done an awful lot. We’ve postulated theories. We’ve actually learned an awful lot about our star, but that’s the kind of final piece of the puzzle. Going up and visiting is the key here.
You know, if you’re looking out of the window and you see it sunny, great, it’s sunny. But you have no idea how hot it actually is. You don’t know if the wind is blowing. You really need to go into that atmosphere and experience it. And so for the very first time, Parker Solar Probe will journey into the Sun’s corona and take those key missing in situ measurements.
Jim Green: Now, you know, when it gets launched, it’s going to fly not directly to the Sun, but it’s going to make some encounters with some of the planets. What’s the plan?
Nicky Fox: Yeah, so we launch–our launch window opens July 31, 2018. We launch out of Cape Canaveral, and our first encounter is with the planet Venus. So we launch and were moving so fast when we launch, we’re on a Delta-4 heavy, which is the biggest rocket currently in NASA’s arsenal, and we also have an upper stage on top of that to really make blistering speeds as we’re leaving the Earth.
So it just takes us six weeks to get to the planet Venus, and we do our first gravity-assist around Venus. Now Jim, I know that you’re very used to missions using gravity assists to speed up. New Horizons went to Jupiter, shot around there and came out much faster.
So we’re actually very generous. We are giving energy to Venus. We’re not using Venus to speed up, we are actually using Venus to trim our orbit very slightly and very carefully to allow us to move around the Sun in very big petal orbits.
We actually encounter Venus seven times during our seven-year mission, and each time, we just trim our orbit just a little bit to allow us to gradually come closer and closer to the Sun with our closest approaches until, in our final configuration, we are just under 4 million miles away from the Sun’s surface.
I realize when I say 4 million miles you think, “Well, that doesn’t sound particularly close,” but if you put the Earth and the Sun 1 meter apart, Parker Solar Probe will be at 4 centimeters from the solar surface, and that is close.
Jim Green: We just ended Cassini by having it plow into the atmosphere of Saturn, and that ended it. Is the plan that Parker Solar Probe will end by plowing into the Sun?
Nicky Fox: I hate that question. I really do. It’s such a sad end for us. We go seven years, as I mentioned, is our prime mission. After that, the final orbit is totally stable, so we can stay in that orbit and continue taking science as long as we still have fuel on the spacecraft. So we basically use the momentum wheels to keep our spacecraft oriented so our wonderful heat shield remains between us and the Sun at all times.
However, these wheels do need to be de-spun. Every now and again, we do need to fire thrusters. And so at some point we will run out of fuel and we won’t be able to keep that heat shield where it needs to be. At that point, the spacecraft will start to turn.
That means the Sun is now hitting parts of the spacecraft that are not designed to have the Sun on it, so the spacecraft at that point will start to break up and eventually will join the dust cloud around the Sun and it will be a very sad day.
Jim Green: Yes, it is. So many people work on our missions and really devote a significant part of their career to make them happen. The return, of course, is the data and the observations that they make will live on forever and will be archived and studied and we’ll learn an enormous amount. That’s why we build these spacecraft, to do these things.
Nicky Fox: That’s true. And of course with Parker Solar Probe, people have devoted decades and decades of work to this, because Parker was first proposed in 1958. Around the time that Gene Parker put out his seminal paper really postulating what was happening around the Sun and in fact named the solar wind.
And so, you know, since 1958 it’s been the highest priority mission in many decadal surveys and many NASA roadmaps, and it’s taken us nearly 60 years to finally have the technology available to us to really make our dreams come true.
So, you know, talk about pioneering spirit. People have donated six decades of their life to making this happen, so we’re all very proud of this mission.
Jim Green: Yeah, I think it’s aptly named. You know, Eugene Parker was really a fantastic scientist and took a lot of heat, no pun intended, when he actually proposed that the Sun should be outgassing, it should be losing matter through a process called the solar wind and wasn’t really originally believed.
It really took our first successful planetary mission, which went by Venus and also measured the solar wind for the first time, to then validate that his theories and ideas were, indeed, correct and the start of what we now know about the space weather.
Nicky Fox: Yes, that’s right. And of course the big thing was showing that it was going much faster than people would expect. You know, if you pump the gas on a car and then you let it just slow down, it slows down fairly quickly.
If you just think of it dumping a bunch of energy into the coronal material and letting it accelerate, you would expect it to slow down very quickly. You would also expect it to cool down very quickly. But of course those first measurements showed us that it was moving faster than you would predict if you just used basic adiabatic equations, and it’s way hotter than we would expect, even out around us.
Jim Green: I’m here with Nicky Fox and we’re talking about the Sun. You know, one of the things that happened most recently was the eclipse. It now looks like that may be 85 or more percent of the entire U.S. population had an opportunity to see it.
Now, with the Parker Solar Probe going into the corona and really tasting it, that solar eclipse gave us an opportunity to see the corona for our own eyes. Where were you when that happened?
Nicky Fox: So this was actually my first solar eclipse, and I was in Beatrice National Park in Nebraska with about 10,000 people supporting a NASA event.
Jim Green: A beautiful area, I might add.
Nicky Fox: It is a beautiful area. The only problem for me was I’d brought my friends and family with me, and then it was cloudy and, in fact, after the beginning of the eclipse it even started to rain. We were very, very lucky that right at about totality, the clouds parted and we actually saw the most amazing corona, the most fantastic diamond ring.
It was a truly spectacular, truly momentous occasion. And not only that, I actually could see the part of the sky where Solar Probe will be living and working, so it was very exciting.
Jim Green: Yeah, I know that connection. I received it too. I was in Idaho Falls at the Idaho Falls Museum, and this was my first eclipse. As it was really kicking off and getting very dark, all I could say is, “Oh, wow.” And then I saw Venus. I guess as a planetary scientist, I couldn’t not comment on where Venus was.
Nicky Fox: You should have seen all four planets.
Jim Green: Well, Venus popped up right away and then, unfortunately, the streetlights came on. (laughs)
You know, as the Sun’s magnetic field is moving out with the solar wind, that’s got to stop at some point. What do we know about that?
Nicky Fox: We do know that our solar system has a finite boundary. We call it the interstellar boundary, and it’s where we moved from the space that our Sun is a dominant force into the space where the Sun no longer has any real influence. And the Voyager spacecraft crossed that boundary and is now out in interstellar space.
We continue to learn more and more about the boundary with our great observations, and particularly our IBEX (Interstellar Boundary Explorer) spacecraft that has actually managed to image the heliopause, the sort of final boundary from our solar system out into interstellar space. And it found a ribbon structure along there and has actually promoted a new mission to be proposed in the decadal survey called IMAP, which is going to take it kind of to the next stage.
With IBEX we’ve found all the questions we want to answer, and now with IMAP (Interstellar Mapping Probe) we’ll take it one step further and actually bring answers to those questions.
Jim Green: You know, it was also thought at one time that the heliopause, like the Earth’s magnetosphere, had a very blunt front end in the direction that the stars moving around the center of our galaxy, and then a long tail.
But some recent observations, particularly those from Cassini, an instrument on Cassini, have indicated that it’s a little different than that. What have we found out?
Nicky Fox: So we’ve found that it almost breathes. The boundary is not a fixed place. It’s sort of moving in and out, and we think it’s actually to do with our solar cycle, as the Sun’s influence kind of changes with the solar cycle. And so this boundary is not a fixed boundary as we first thought it was, but it’s changing in time just like everything else in our solar system. As the Sun changes, everything feels an effect, even the boundary with interstellar space.
Jim Green: With Parker Solar Probe getting gravity assists from Venus and allowing it to meet its objective, I always ask my guests, “What was your gravity assist that really propelled you into this field and kept you going?”
Nicky Fox: My gravity assist happened at a very early age, and it was my dad, because he was fascinated by everything NASA, everything space. You know, instead of bedtime stories, I would get how the Sun was in the center and the Earth went round the Sun and the Moon went around the Earth.
And I remember, you know, glasses being used to show me how the orbits worked. And he was totally– followed every single thing that NASA did and absolutely loved it, and instilled in me the idea that the best possible thing I could do would be to work for NASA.
I think that even, you know, as a girl in England where you don’t think that really is possible, there was still this sort of magical a thought that stayed with me that, “Okay, I’ll probably end up doing something different, but, gee, it would be great to work for NASA.”
And so you know, I went to college and I did space plasma physics, and then a very good friend of mine, Dr. Jim Slavin, came up to me and said, “Could I interest you in a job at NASA?” And that was probably my final gravity assist.
Jim Green: Yeah, Jim will do that. He’s a good friend of mine, too. Well thanks so much, Nicky. It’s just been a delight. Join us next time as we continue our virtual tour of the solar system. I’m Jim Green, and this is your gravity assist.
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