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From a project’s smallest steps to humanity’s greatest leaps, NASA’s technical workforce embodies the spirit of Neil Armstrong’s immortal words from the surface of the Moon, boldly pushing the envelope of human achievement and scientific understanding. In our podcast, Small Steps, Giant Leaps, APPEL Knowledge Services talks with systems engineers, scientists, project managers and thought leaders about challenges, opportunities, and successes. New episodes are released bi-weekly on Wednesdays. 

Parker Solar Probe Mission Scientist Adam Szabo discusses NASA’s journey to the Sun.

Inching closer to the Sun than any spacecraft before it, NASA’s Parker Solar Probe is untangling long-standing mysteries of the complex solar environment and ushering in discoveries that are shaping scientists’ understanding of Earth’s star. Launched in August 2018, the spacecraft uses Venus’ gravity during seven flybys over nearly seven years to gradually bring its orbit closer to the Sun. Parker Solar Probe performs scientific investigations in a hazardous region of intense heat and solar radiation and makes critical contributions to the ability to forecast changes in Earth’s space environment that affect life and technology on Earth.

In this episode of Small Steps, Giant Leaps, you’ll learn about:

  • How Parker Solar Probe is revolutionizing our understanding of the Sun
  • Why it’s important to study the Sun and the solar wind
  • How the mission team has responded to surprises and engineering challenges

 

Related Resources

Parker Solar Probe

Parker Solar Probe Team Earns Aerospace Award from the National Space Club and Foundation

Parker Solar Probe Videos

APPEL Courses:

Science Mission & Systems: Design & Operations (APPEL-vSMSDO)

Science Mission & Systems: Design & Operations Lab (APPEL-vSMSDO-LAB)

Space Mission Operations (APPEL-vSMO)

Space System Verification and Validation (APPEL-vSSVV)

 

Adam Szabo Credit: NASA

Adam Szabo
Credit: NASA

Adam Szabo is the Parker Solar Probe Mission Scientist and the Chief of the Heliospheric Physics Laboratory at NASA’s Goddard Space Flight Center. Szabo specializes in heliospheric and magnetospheric shocks and discontinuities. His work includes the extension of the magnetohydrodynamic (MHD) Rankine-Hugoniot shock jump condition fitting technique. He has extensively analyzed interplanetary shocks. Szabo has also investigated the structure of the heliospheric current sheet and its evolution from the Sun. He has extensive experience calibrating and analyzing plasma and magnetic field data from the Voyager, IMP-8 and Wind spacecraft. Szabo was the project scientist for the Wind mission from 2007 to 2016 and currently serves as the DSCOVR project scientist. He has a bachelor’s and doctorate in physics from the University of Chicago and the Massachusetts Institute of Technology, respectively.


Transcript

Adam Szabo: The only star we can study in situ, that is locally, is our own — the Sun. So, when we are studying the solar wind, we are in fact studying the whole universe.

Indeed, we will be going really, really fast — faster than any other manmade object in space.

Deana Nunley (Host): Welcome back to Small Steps, Giant Leaps, a NASA APPEL Knowledge Services podcast where we tap into project experiences to share best practices, lessons learned and novel ideas.

I’m Deana Nunley.

Humanity’s first visit to a star – the Parker Solar Probe mission – is revolutionizing our understanding of the Sun. And here to fill us in on NASA’s journey to the Sun is Parker Solar Probe Mission Scientist Adam Szabo.

Adam, thank you for being our guest.

Szabo: Thank you for having me.

Host: Could you give us a brief overview of Parker Solar Probe and perhaps toss in some of the fun facts and firsts of this mission?

Szabo: Would love to. Parker Solar Probe is a flagship mission in the NASA’s Heliophysics Division. Its primary objective is to understand the physical processes that accelerate and heat the solar wind and accelerate solar energetic particles. To accomplish this, the mission needs to go as close to the Sun as possible. So, I like to use an analogy in describing just how close are we going to the Sun. If we imagine the inner heliosphere between the Sun and Earth as a one football field, then the planet Venus, would be at the Earth’s 28-yard line. Mercury, the closest planet, would be the Sun’s 35-yard line.

The mission that got closest to the Sun before — the European Helios 2 spacecraft — got to the 29th yard line of the Sun. The largest corona loops of the Sun, can extend to the 15th yard line and we are going to the four yard line. So really, really close to the Sun. If you want it in terms of kilometers, that’s 6.12 million kilometers. Yes, it’s a really big number, but compared to the distance between the Sun and Earth, we are really, really close.

Now, needless to say, when we get that close to the Sun, things get a little bit hot. So, the surface of the, the somewhat basic surface of the spacecraft, will reach 1400 centigrade or 2500 Fahrenheit. Now, how do we get there? People tell me that, ‘Well, that should be an easy one. You just fall right into the Sun. It’s very easy to get toward the Sun. It’s hard to get away.’ Well, the reality is that we are starting from Earth. And Earth is safely staying away from the Sun because we are zooming around.

We are starting at 30 kilometers per second, sitting on the surface of Earth. So in order to fall into the Sun, what we have to do, is step on the brakes. So, we launched on the biggest rocket there is, the Delta IV Heavy launch vehicle. And rather than speeding it up, as soon as it left the surface of the Earth, it went backwards relative to the Earth’s motion, braking as hard as we could. This allowed us to get to Venus where we use Venus gravity, not to speed up, but further slowing down the spacecraft.

It takes seven separate encounters with Venus to get down to our final perihelion, closest approach to the Sun’s distance. We’d like to use a solar radii as a unit of distance, simply because those are smaller numbers, rather than millions of kilometers. So, our ultimate objective is to go below 10 solar radii that close to the Sun. After the first Venus gravity assist we got to 35, then 27, then 20. Right now we are at 16. That’s our closest approach. We are coming up to the next Venus gravity assist in a couple of weeks, that will put us down to 13.

And after that, two more encounters that will finally allow us to get to our ultimate objective of below 10 solar radii. Now, as we get closer and closer to the Sun, at closest approach, as those who remember Kepler’s laws, when you are at closest point, that’s when you go the fastest. And indeed, we will be going really, really fast — faster than any other manmade object in space. For reference, let me start out that a speeding bullet, the fastest bullets can cover one and a half kilometers every second or a mile a second.

The fastest rocket launching from Earth, they can get maybe six, seven times more like 10 kilometers per second. As I mentioned, Earth, itself going around the Sun is 30 kilometers every second. Not hour, every second. Parker will reach 190 kilometers per second. Just to illustrate how fast that is, if Parker used that speed to go out on the Earth, it would take four-and-a-half minutes to completely circumnavigate the Earth.

Now, once we get there, what do we do there? Obviously, we want to make critical measurements. So we have four instruments suites. They focus on different types of observations. We have one suite that measures the magnetic fields of the solar, wind and the Sun. We have another one that measures the solar wind particles. Protons, electrons, alpha particles. Then we have a third one that measures energetic particles.

And finally, we do have one instrument which takes pictures, but we cannot take pictures toward the Sun because no camera could withstand the heat. We are taking pictures sideways, as we try to understand and take pictures of different structures in the solar wind. So how is that for introduction?

Host: That is an introduction. What an amazing mission. This is going to be so much fun to hear about.

Szabo: Great.

Host: How has Parker Solar Probe revolutionizing our understanding of the Sun?

Szabo: So, the acceleration of the solar wind and of energetic particles, takes place very near the Sun, travels into orbit at Mercury. And by the distance of Mercury, the solar wind reaches its full speed. Previously in the 1970s as I mentioned, the European Helios mission, got to this point just below Mercury’s orbit. And they returned unprecedented observations, but they were not close enough to really understand the fundamental question. Why is the solar wind traveling so fast? Why is the solar wind so hot? And how are energetic particles that most of us worry about, astronauts worry about for health’s sake, technologies, like typical spacecraft worry about energetic particles, because they can negatively impact their operation. How do these energetic particles get into being? How are they born, generated, accelerated? And if we want to get to the point that we can forecast reliably, the behavior of the solar wind transients and energetic particles, first, we need to understand how are they generated? So Solar Probe is going to revolutionize this understanding because it takes measurements where really, all these events take place. Location, location, location, just like in real estate. You have to be at the right place where the action is taking place, collect the data and then we are very positive and optimistic that this then will allow us to understand and therefore develop forecasting capabilities.

Host: What are the primary science goals of the mission?

Szabo: So, let’s start at the Sun. The solar wind, is the supersonically expanding outer atmosphere of the Sun. As the ionized gases of the solar wind, which is mostly hydrogen and an ionized hydrogen is really just a proton, leave the visible surface of the Sun, which we call the photosphere and travels through the outer atmosphere of the Sun, which we call the corona. They suddenly, that is the solar wind, suddenly accelerates by many factors, reaching speeds that are much faster than local wave speeds.

So, the solar wind starts out at the lowish sort of 100 kilometers per second. And all of the sudden, as it leaves the surface of the Sun, rather than slowing down or just continuing at its original speed, it metrically speeds up to four to 800, sometimes over 1000 kilometers per second. This is very important, because these speeds are so high that the local electromagnetic waves cannot travel nearly as fast.

That is, information cannot catch up from the Sun to the solar wind nor the solar wind can talk back to the Sun. It becomes independent. It’s sort of like, you cannot shout after a supersonic airplane because the sound of your voice will never catch up to it because airplane is traveling faster than the speed of sound. So is the solar wind. It’s traveling faster than any information, any wave can travel from the Sun away.

So, the big underlying question, what physical mechanism generates this huge acceleration? We do not fully understand such a fundamental thing, simply because we have never been at the location where this acceleration takes place. It’s not that we have no ideas. The problem is that we have too many ideas and they are different ideas and they all cannot be right. So, we are looking forward to observational or measurement techniques to differentiate between the various theories and determine who is right, or maybe nobody. And we had to come up with a brand-new set of theories.

Similarly, as the solar wind flows away from the Sun and travels through the corona, its temperature also jumps. From near the surface of the Sun, visible surface, the photosphere, temperatures measure four, 5,000 Kelvin. When it reaches the corona as it’s moving away, all of the sudden the temperature jumps to a million degrees. From 5,000 to a million, quite a bit of heating up. Some physical mechanism is dumping an enormous amount of energy into the system.

This question is probably related to the previous one. That is the acceleration of the solar wind, that somehow energy from the Sun, which is probably some form of magnetic energy, gets to deposit energy in the corona, into the particles. Some of it goes into heating, some of it goes into speeding up their flow.

Now, there is a third scientific objective for Parker Solar Probe. That is energetic particle acceleration. So energetic particles are harmful to systems and astronauts as I said before. They are accelerated just about everywhere in the heliosphere. They usually have their origin — they are ordinary particles like protons, electrons, or heavier elements, but some of them, all of the sudden get kicked up into enormous energies, traveling near the speed of light. So, while this acceleration happens just about anywhere or everywhere in the heliosphere, the most energetic, therefore the most dangerous particles, have their origin exclusively near the Sun.

So, if we want to be able to forecast their timing and fluxes, we need to first understand the physical processes that are in play, or that are at play for this acceleration process. By going close to the Sun, Parker will provide key measurements to answer all of these three main science objectives.

Host: Adam, have there been surprises with this mission?

Szabo: Yes. Which is my experience working for almost 30 years at NASA. That wherever we go, we of course plan very carefully what sort of observations we want to make and why and how are we going to answer very specific questions. But invariably, we find perplexing observations that we didn’t even dream of before the launch of the mission. So one of these is as we were getting closer and closer to the Sun, normally the magnetic field of the Sun that is embedded in the solar wind, discharged particles of the solar wind, they carried the magnetic field of the Sun with it and they stretch it out in a relatively smooth direction, wrapping it up into a spiral very much similar to the pattern that a garden sprinkler would have throwing out parcels of water and rotating around and also forcing a spiral direction.

But as we were getting closer and closer to the Sun, we notice that the local magnetic field started to make 180-degree U-turns for just a short period of time. Then it resumed its normal direction after sometimes a few seconds. Sometimes after a few minutes. We term these structures as switchbacks, very descriptive way of just talking about two U-turns in the magnetic field. But their numbers just skyrocketed as we got closer and closer to the Sun.

So, this we did not expect. And so it must mean something. The only problem is different theoreticians interpreted these switchbacks rather differently. They could be signals that the solar wind is not as homogeneous as we are used to it at 1 AU. Maybe little jetlets are the origin so it’s rather clumpy at its birthplace. And then it just morphs into a uniform solar wind, as it propagates away from the Sun.

Others argue that no, what we are clearly seeing here is oscillations on the surface of the photosphere sort of shaking the bottom of the magnetic field lines like a jump rope. And so, what we see here is that the jump rope, the wave structure on it is just steepened. That then diminutes as we travel away from the Sun. We have a cool bunch of ideas. Again, we are still debating what is the meaning behind them. This was completely unexpected.

Another one, is the dust-free zone around the Sun. Now there is quite a bit of dust. The heliosphere is dusty. Nobody cleaned for a million years now in the heliosphere. Mostly comets that stray close to the Sun, they evaporate. And of course, comets such as basically dirty pieces of ice. So once the ice melts, the dust remains and stays in orbit around the Sun. We see this as the zodiacal light, light diffracted by these dust particles around on the Sun.

But it has been postulated decades ago that maybe very close to the Sun, the light pressure is so high, that these dust particles are literally blown away so that there should be a region near the Sun, that is completely dust-free. No need to sweep there. But we cannot see this from 1 AU, from Earth. because of course, we always look through the dusty regions. Sort of like when you are driving on a foggy day and you turn on your beams, you cannot see through the fog, the end of the fog.

So, Parker, as it’s getting closer and closer to the Sun and remember we have one imaging instrument which takes pictures sideways. What we have noticed that we expected this reflective light, the zodiacal light to be getting stronger and stronger as we get close to the Sun. Because well, there is more light to be deflected by the dust particles, but it’s not increasing nearly as much as theory would predict.

This is a possible indication that there is a dust free zone that we are approaching. We are not in it yet. And we probably will never be in it completely, because it’s just that much closer to the Sun. But if these trends continue as we get closer to the Sun, it might be enough to prove that indeed there is a dust free zone. So those are just two unexpected developments are at the early in the mission. And remember that we still have years to reach our ultimate objective.

Host: In addition to the surprises or unexpected developments along the way, there must be a lot of challenges with a mission to the Sun. What are some of the engineering challenges?

Szabo: Well, clearly one of them was temperature. How do you keep a spacecraft from melting? As I indicated, it’s rather hot that close to the Sun. And most instrumentation is rather sensitive electronics, designed to operate well, in room temperature because well, most engineers work in room temperature conditions. So how do you keep these instruments working? So we needed to develop a thermal protection system, which is just a fancy name for a heat shield, in front of the spacecraft.

But first, let’s talk about exactly how much this thermal protection system needs to protect from. As I indicated, the solar corona where we are flying now during closest approach, the temperature of the particles a million degree Kelvin. Why is it that I said that we are only at around 1400 centigrade? We need to distinguish between temperature and heat. So, while the particles, the solar wind particles have indeed a 1 million Kelvin temperature, that just indicates how much their individual velocity or speed differs from the average bulk speed.

Temperature is really nothing else than just how much particles stream relative to the bulk of the solar wind or the gas. Heat is the amount of energy that these particles then deposit in any surface or object. To illustrate the difference that, for example, if one opens a hot oven, if you reach in with your hand to take out a dish, even if it’s even at 450 degrees, you’re not going to immediately burn your hand, unless you touch something for a very short period of time.

Take the same temperature water, which is well above boiling, put your finger in there and you would immediately burn yourself. Now, please don’t try this. But that’s an illustration that, what matters is how many particles are available. Air has much fewer particles than water. Liquid versus gas. So that’s exactly the situation with the Parker Solar Probe, that instead of protecting against a million degrees, if we can protect against two and a half thousand Fahrenheit, that’s good enough.

Now, before people say, ‘Oh it’s only 2,500 degrees. What is to worry about?’ Well, to illustrate it, a typical lava, that is a volcanic lava flowing out red and molten rock, that temperature is between 1300 to 2200 Fahrenheit. So yes, our system has to be able to withstand that type of temperatures all the time, not just for a short period of time. So how did we do it? So we have an eight foot diameter, big shield in front of the spacecraft, which is four and a half inches thick. And you can think of it as a sandwich.

The middle of it is carbon composite foam, in-between two carbon plates. And it’s actually a good enough insulator that on one side of it, is we have 2,500 degree Fahrenheit and on the other side of it, only 85. It’s also a fairly light structure. We had to get something that it’s not too heavy because otherwise we couldn’t launch it into space. So, if we were able to test it up to 3000 Fahrenheit, it would still stand. For added good measure, the front of it is painted white with a ceramic paint, so that to maximize the amount of reflective light.

So that was just hard to build one of these, nobody built one of these before. It had to be specifically developed for Parker Solar Probe. The next challenge was well, how to make electricity? We are relying on solar panels. And so, one might say, ‘Hey, that should be easy. One thing you have plenty of, is Sunlight when you are near the Sun.’ The problem is that we have too much of it. You stick a solar panel out in the Sun and it would melt right off.

So how do I use a solar panel? It’s sort of like if you are thirsty and somebody says, ‘Oh, here is a fire hydrant. Why don’t you drink out of it?’ So, the solution was actually somewhat perplexing. So one, is that we are bending back the solar panels to run. Normally the solar panels are set up just like on the roofs of buildings that you want the sunlight to hit it perpendicular to its surface to maximize the amount of light that is absorbed and converted into electricity.

We wanted to minimize that so we actually feather back the solar panels, so that it’s at a very shallow angle relative to the sunlight so only a portion of it is deposited into the solar panels. Again, we have plenty of it so that’s not a problem that we wasted some sunlight. But this may be also reduce the amount of heat deposited into the solar panels. This wasn’t enough. So, we realized that we will have to do active cooling.

So next question was that, what coolant should be used? Sort of like a car’s radiator. So how do you cool? What sort of exotic liquid did NASA come up with, to cool the solar panels? So, we lined up a whole bunch of exotic liquids with different properties. And to our surprise, the best one that fared the best, is plain water. Nothing exotic. H2O turned out to be the best coolant we could have. It doesn’t oxidize things, it carried lots of heat away and it works just perfectly.

So, we are circulating water. It’s the first water-cooled spacecraft in space. It comes ready with radiators, just like a car. So, then you’ll see pictures of it. There’s that heat shield in front and behind of it, it looks like that there is a skirt. It’s not because we are modest and we don’t want to display what is behind there. Those skirts are the radiators. So, they are just pipes going up and down. And behind the heat shield where there is sunlight, it’s actually freezing cold.

So, the pipes radiate away the heat from the water and then we pipe it out, circulate it out to the backside of the solar panels, where they pick up heat from the solar panels and then they keep on circulating. And that’s the only way we could use solar panels to actually generate electricity. So interesting thing by the way, is that once you put everything behind the heat shield, one of our biggest problem is, that the instruments tend to freeze.

If you take away the Sun, it doesn’t matter how close you are to the Sun. There is no air, there’s no conduction of temperature. Behind the heat shield is just as cold as if you fly beyond the orbit of Pluto. It’s the exact same radiation environmentSo, most of our instruments, at 10 solar radii from the Sun, they are number one worry is that they are freezing. So, we actually have to use heaters to keep it at room temperature, even though the heat shield is running at 2,500 degrees. So, one of these mixtures of problems.

Finally, when we are so close to the Sun, that is typically closer than a quarter AU. We cannot stick out our high gain antenna to talk to Earth because well, it would melt right off. So, we retracted behind the heat shield, but now we cannot talk to the spacecraft. So, for a large portion of the orbit, the spacecraft has to fly itself. Now, you would say that ‘Well, it’s in an orbit so where is it going to go?’ The problem is that the heat shield has to be pointed precisely toward the Sun. Any slight movement and any portion of the backside of the spacecraft gets illuminated. How you would know it, is that when you get out, it’s not there anymore because it melted away. So, we can’t allow that.

So, the spacecraft is actively controlling its orientation, relative to the Sun. So, it has little sensors on the edge that said, ‘Oh, I start to see too much brightness, turn just a little bit.’ Automation became an absolutely essential feature of the spacecraft that it had to be able to solve, not just the normal operation, but if something went wrong, something had a hiccup. It has to be able to solve all these problems on its own without any input from ground controllers. Otherwise, there is no mission.

So far, it’s worked marvelously well. So, no problem whatsoever, but again, now there is computer science too not just thermal engineering, electrical engineering, but computer science was required too. Just three examples. There are many, many more, but let’s leave it at that.

Host: Wow. Those are so fascinating. Are there lessons learned in the development and execution of this mission, that you could share with other NASA scientists and engineers?

Szabo: Yes. Integration of the science / instrument teams and the project that is, the engineering teams building the spacecraft, was absolutely essential from day one. And continuously continuing on. One of the big problems of Parker Solar Probe, was that in order to get to this orbit, we really needed to use the biggest rocket there was and at the same time, minimize the mass of the spacecraft. That’s the only way we could reach the appropriate speed necessarily to get into orbit.

So that meant that every gram was counted literally. Every gram. Not kilos. So, we had 500 kilo spacecraft. We were looking at that, well, if I can just reduce 10 grams here, I might be able to add a function to an instrument. Or allow something else to work. So, it was absolutely essential that all the teams had to work constantly with each other, compromises had to be made. Obviously, the scientists wanted more measurements, more capabilities. The engineers of course, wanted to make sure that we have spacecraft and instruments still operating when we got there. So we wanted to reduce the risks.

So that interplay was intense for years and years. And this I think was absolutely a key to the success of this mission. That both sides, both scientists and engineers, were understanding each other’s point of view and they were willing to compromise and work with each other.

Host: Why is it important to study the Sun and the solar wind?

Szabo: Well, of course, if you ask a space scientist, it’s because it’s fun. It’s the spirit of exploration of unanswered questions. I want to study it because nobody understands it. Therefore, it’s sort of like, why do you want to climb that mountain? Well, because it’s there. That level of, that’s the sort of my personal answer. But from a societal point of view, heliophysics is actually very applicable for societal needs particularly. We call this space weather.

We live in an increasingly space-based society, where OK, astronaut health, yes we care deeply about health of astronauts. But the space systems that we grew accustomed to from GPS, from communications systems, beeper systems, they are all based on space-based assets. And space weather significantly impacts their operations. Space weather also impacts power generation on the ground. One of the easier things to understand about space weather, is that when a big solar wind transient comes, it cannot reach the surface of the Earth because Earth has a protective magnetic field, a big bubble that keeps all the solar wind and its structures away.

But as a Corona Mass Ejection comes, what these structures can do, it can compress a little bit, not to the surface of the Earth, but compress down to Earth’s magnetic bubble. And then when it passes, then it expands back out. So basically, what we have on the Earth is changing magnetic fields. The Earth is all magnetic field. When it’s compressed, the field goes up, when it decompresses then the field strengths goes down.

Now, from basic electricity and magnetism physics, we know that if you have a long piece of wire and then you have changing magnetic field, what do you have? You just made an open-air generator. You are making current. So where do we have long pieces of wire on Earth unshielded? Well, those are called power lines. They are really long and they go in all directions. So, start wiggling the Earth’s magnetic field and these power lines just became the world’s largest generators.

So, we have extra current flowing in them. But wait, current is already flowing in them by design. In fact, they are designed to carry what they can normally maximally carry. So, all of the sudden, extra current shows up and goes into the transformer stations. What will happen is that, they are overloaded and they blow up. Then you lose electricity in that particular region. But this became a much more serious issue in recent history, because in order to alleviate blackouts, different power systems are connected together.

So, let’s say that there is a power outage in one area, then a second area can distribute electricity into this affected area, so that we minimize the blackout time period. But now that we connected all these power generation systems, we just generated an even longer piece of wire, so the impacts can like domino effects spread across whole continents. We had such an event in the late ’70s in Quebec where the whole territory, lost electricity due to a coronal mass ejection that bum, bum, bum, knocked out one another. And it’s not very localized, it happens all at once on large portion of the surface of the Earth.

So this is just an illustration that obviously, nobody can stop a CME, Coronal Mass Ejection, coming from the Sun, but we can forecast it. And then if we separate power systems, for example, we can limit its impact. If we know that there’s a space weather event coming, maybe spacecraft can be temporarily turned off or reoriented or your operation modified to minimize the negative impact. So, there is a very economic and practical application of the studying of the solar wind.

Finally, this is the only place where we can study hands-on astrophysics. So, if we are interested in stars. And most stars have solar winds or stellar winds, they form their own heliosphere, well, we call them astrospheres. But at the moment we cannot get to them. The only star we can study in situ, that is locally, is our own — the Sun. So, when we are studying the solar wind, we are in fact studying the whole universe.

Host: That is so fascinating. With this mission exceeding expectations and producing amazing science return, what would you highlight as your favorite discoveries so far?

Szabo: Well, I would answer that we have a bet ongoing. That when we reach what we call the Alfvén surface — that is the point in the solar wind when its speed is below the local wave speed. Below this level or altitude from the Sun, we are expecting that the physics will be drastically different. All of the sudden, the solar wind is in communication with the photosphere back and forth, waves can travel back and forth. We are expecting very different behavior than what we are normally used to.

We know that it has to be below at least theoretically below 20 solar radii. We are down at 16. We had inklings that we had short time periods when it looked like that we are below this critical speed. And so, but they lasted only for minutes, maybe an hour or two. So very, very special short time periods. So, we have a bet ongoing, who can predict the precise time, when we will be finally below the Alfvén surface. Even when that happens, this will be historic, this will be the first ever. We will have new scientific understanding, that I bet we will have to rewrite textbooks based on those observations. So even though it hasn’t happened yet, that’s my favorite discovery.

Host: Very interesting. Adam, thank you so much for taking time to talk with us today. This really has been fun.

Szabo: Well, thank you very much for having me. I really enjoy working with this mission. And I find it that indeed its measurements are revolutionary.

Host: Adam’s bio and links to topics covered during our conversation are available at APPEL.NASA.gov/podcast along with a show transcript.

If you want to listen to more interviews and get information about what else is happening at NASA, we encourage you to check out other NASA podcasts at nasa.gov/podcasts.

As always, thanks for listening to Small Steps, Giant Leaps.