Episode 76: PUNCH: Imaging the Solar Wind

Lisa Peña (LP): PUNCH prepares for launch. NASA's polarimeter to unify the corona and heliosphere mission will capture images of the sun's corona transitioning to the solar wind, making this invisible system visible for the first time. Mission principal investigator Dr. Craig DeForest discusses what PUNCH will uncover about the heliosphere, space weather, and its impact on Earth. That's next on this episode of Technology Today. 

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Hello, and welcome to Technology Today, I'm Lisa Peña. NASA's polarimeter to unify the corona and heliosphere, or PUNCH mission, will achieve a first imaging the solar wind as it streams out of the sun. Four suitcase sized satellites equipped with special instruments and designed and built by SwRI will produce the first 3D images of the inner heliosphere, shedding light on the sun, solar wind Earth system. How will PUNCH instruments make the invisible visible? What will we learn from this groundbreaking mission?

PUNCH principal investigator Dr. Craig DeForest of SwRI's Solar System Science and Exploration division joins us for a pre-launch discussion. He's here at SwRI San Antonio headquarters for one day from Boulder, Colorado. So thank you so much for being here during this especially busy time, Craig, and spending time with us on your one day in town.

NASA's Goddard Space Flight Center Conceptual Image Lab

The PUNCH constellation of satellites will launch into a polar orbit along the day-night line. The spacecraft will remain in the sunlight with a clear view in all directions. Three PUNCH satellites will carry SwRI-developed Wide Field Imagers, and one will carry the Narrow Field Imager.

Craig DeForest (CD): Thank you for having me down. It's wonderful to be here. 

LP: So we'll get to your preparations for launch day, a huge day coming up, and we'll talk about that in just a bit, but first, PUNCH is set to explore the inner heliosphere and uncover never before seen images and data. So let's get into it. I want to start with understanding some of the terminology associated with the mission. So during this conversation, we'll be talking about the corona, heliosphere, the solar wind. So will you define these sun-related terms for us? 

CD: Absolutely. So if you've been here in San Antonio for a while, you probably saw the total solar eclipse last spring. The sun's corona is that white halo of material around the sun itself that we can't normally see because the sky is too bright, but it's there all the time. It is a very, very hot atmosphere around the sun. 

The corona material is about a million degrees Centigrade. That's 1.5 million Fahrenheit, much hotter than the surface of the sun itself. It's so hot that material is constantly streaming away from the star and filling the entire solar system, with a very, very tenuous wind that we think of as empty space, but scientists know is called the solar wind. It actually fills the solar system. 

LP: All right. So the solar wind is rushing out from the corona. As you mentioned, during that eclipse last year, we caught a glimpse of that corona when the moon passed right in front of the sun. So let's talk about the heliosphere, something else that's going to come up during this conversation. 

CD: So the heliosphere is the region of space around our star that's full of the solar wind. The solar wind fills a finite volume of space. It's much larger than the orbit of pluto. And then there's an interface in the very outest, coldest reaches of the solar system, between that and the interstellar medium. So crossing that is something that very few objects have done. Three of them have done it, the Voyager 1 and 2 probes and also the New Horizons mission that was built right here at Southwest Research Institute. 

LP: So PUNCH is next up, and we want to talk about that mission. So what is the PUNCH mission's purpose? What does your team want to accomplish with this mission? 

CD: Well, up until now, the fields of solar physics and of solar wind physics, or space physics, have been separate. We've been viewing these two parts of our solar system with very different equipment. People study the solar wind by launching spacecraft into the solar wind and sampling it, whereas we study the sun by looking at it with telescopes or other kinds of remote-sensing instruments. And this has led to very different understanding of these two different systems, except they're part of the same system. 

So it's very much like the parable of the blind man and the elephant, that these two fields have been very separate because they see the world in different ways. So by bringing imaging science outward from the sun to capture the structures in the solar wind, we hope to be able to understand the sun and the solar wind that surrounds us as parts of the same system, as a unified whole. 

LP: Okay. And why is it valuable to understand how the sun's corona shifts into the solar wind? 

CD: The solar wind is constantly washing over the Earth. In fact, we wouldn't even have an atmosphere if it weren't for our own protective magnetic field that diverts the solar wind around the planet. So it's important to understand the wind itself, both from the standpoint of intellectual interests and also the practical standpoint of understanding something called space weather that affects our technology here on Earth. 

So to understand that, we need to understand its origins near the sun. But the space weather effects that form on the sun and streak out across the solar system are changed as they pass through this medium of the solar wind. And that void between the sun and the Earth is an area we don't understand very well because it's very poorly studied right now. It's just very hard to either mount a probe that will fly in that region or to sense that material remotely. So PUNCH will bring imaging to this space between the star and ourselves. 

LP: So since you mentioned it, I want to talk more about this magnetic field around Earth, this protective magnetic field that is essentially shielding us from the solar wind but at the same time just lets just enough through heat, light from the sun to enable life on Earth. 

CD: Sure. We normally think of the sun as the source of all light and energy on Earth. And in fact, it's the only star in the universe that's been proven to grow vegetables. But light isn't the only thing that comes from the sun. 

This material, the solar wind that's constantly streaming away from the star, impacts literally everything in the solar system. And when that fast moving material, individual atoms moving at hundreds of miles per second, impacts the top of an atmosphere, it spurs off, it knocks off little bits of the atmosphere, one molecule at a time, and over billions of years, that can strip a planet entirely of its atmosphere. We believe that's what happened to Mars. 

However, the Earth, fortunately, has a strong magnetic field. And so the fact that your compass works as you are walking around in the forest is deeply tied to the fact that you can breathe. 

LP: Huh, that took me a minute, to let that sink in. Okay, so I want to talk more about how the solar wind affects Earth. What are we talking about when we're talking about space weather and the impact it can have on Earth? 
 

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SwRI Solar System Science and Exploration Division staff pose with the shipping containers holding the Polarimeter to Unify the Corona and Heliosphere (PUNCH) observatories set to launch into polar orbit. SwRI leads PUNCH, a NASA Small Explorer (SMEX) mission, that will examine how the corona transitions into the solar wind.

CD: So if you've ever seen an aurora-- in last spring, many of us did, even here in San Antonio-- that's caused by material from the sun impacting the Earth and changing the balance of the magnetic field in the near Earth space, the top of geospace. We have a mission from Southwest that's studying that in situ right now. That's the Magnetospheric Multiscale Mission, or MMS, that was led in part by Jim Burch here at Southwest. 

But when the solar wind impacts the Earth and changes that structure, it excites the aurora and other effects here on the Earth. It changes the radiation environment in Earth orbit, which affects astronauts and satellites. It also alters the magnetic field of the Earth momentarily, which induces electric currents in any long wires that may be lying around, such as our power distribution grid or telephone lines or anything else long, such as oil pipelines. 

Space weather arguably was first noticed in the mid 19th century, when this nation's nascent telegraph system caught fire from the influence of a solar storm impacting the Earth, which induced currents in telegraph stations and set them alight. 

LP: So there are real impacts from the solar wind. It's not just floating around up there, minding its own business. We can definitely feel its effects. So PUNCH is a constellation of satellites and imagers designed and built by SwRI. So tell us about these PUNCH instruments. How will they work together to capture images of the inner heliosphere? 

CD: PUNCH is fundamentally a coronagraph. Now, a coronagraph is a regular camera, usually looks at visible light, that forms its own eclipse. We have several of them in outer space, where they're above Earth's atmosphere and can see the black sky of space in that artificial eclipse that they create. 

The difficulty that we faced with PUNCH is that we wanted to capture an extremely wide field of view. PUNCH is not a telescope. It's a wide field imager to watch this material as it leaves the sun and streams over us. It's part of our immediate environment. 

So we found that we had to divide that field of view into two different kinds of instrument, a conventional coronagraph, similar to ones that have flown before, and a wide-field imager that captures out to 45 degrees away from the star itself. So the entire field of view is 90 degrees wide, centered on the sun. That would extend from the horizon to the zenith if you were looking at it relative to the Earth. 

LP: Let's talk about these four satellites. Why four? Is that an important number? 

CD: So four is an important number. First of all, we had to divide that field of view across two different types of instrument. The narrower instrument, the narrow field imager, the coronagraph itself, can look all around the sun, close to the sun, but the other three, the wide field imagers, had a problem that we had to get them close to a radio station so we could beam the images down to the Earth. But all the radio stations are on a planet, so there was a large planet in the way. So the only solution we could find was to make the instrument as large as the Earth. 

Now, you can't build one spacecraft as large as the Earth, but you can build one instrument onto three separate spacecraft and allow them to spread out around the Earth, which is what we did. So we have four identical spacecraft, one instrument that's separate, and three instruments that are identical, all spread out around the Earth to work together as a single instrument, even though they're on four separate spacecraft. 

LP: And how will the spacecraft utilize polarization? I know that's a big part. 

CD: So that's right in the title, is we are the polarimeter to unify the corona and heliosphere. Now, we use polarized light. We have a polarizing filter in front of each camera to measure the amount of polarization. Now, you may be familiar with Polaroid sunglasses, which cut glare because light that reflects off of a horizontal surface becomes polarized. The vibrations of the light wave itself happen in one direction only and not in all directions across the light. 

So by blocking out polarized light, your sunglasses allow you to cut glare while allowing ordinary light through. PUNCH uses a similar effect. The light that's scattered in the corona, that we use to see the corona with the solar wind, is polarized by the process that diverts it down toward the earth. And so the degree of polarization tells us where each feature is in three dimensions. 

Objects that are far off to the side near the plane of the sky, if you will, tend to be highly polarized. Objects that are in the space directly between us and the sun that are coming almost right at us are not very polarized. And so we can use that measurement to determine where everything is in three dimensions, from one vantage point. 

LP: All right. And you've already touched on this a little bit, but as we've said, PUNCH will make the invisible visible. Really cool. So how will it accomplish that? What does that mean exactly? 
 

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SwRI space scientist and heliophysics expert Dr. Craig DeForest is principal investigator of NASA’s PUNCH mission. He says PUNCH will capture a never-before-seen view of the solar wind, complementing other ground and space missions currently exploring the Sun.

CD: One of the most exciting things about this mission for me is that we will be looking at ordinary objects that in principle you could see with your own eye, like the corona. You can see the corona with your own eyes if you happen to be at the place where there's a total solar eclipse. But the material of the solar wind is so faint. It's actually too faint to detect with your eye. It's there. It's an ordinary object. You can capture it with an ordinary camera, but it's 1,000 times fainter than the galaxy in the night sky. 

And so the real challenge of this mission was not building the spacecraft themselves. That was hard enough. It was building the ground pipeline to merge and combine these images with exquisite precision so that we could remove 99.9% of the light from every image and reveal only the moving features of the solar wind against a much, much brighter star field. 

LP: So the PUNCH constellation of satellites, as you've mentioned, will be in low-Earth orbit. What does that look like, and what else can you tell us about where the satellites will be positioned? 

CD: PUNCH is launching into low-Earth orbit. That means we'll be about 400 miles above the surface of the Earth, just above the atmosphere, so that we can move fast enough to remain in orbit. In order that we can see the sun all the time, we're in something called a sun synchronous dawn dusk orbit. 

Now, when you think on a planetary scale and you think about orbits, sunset is not a time. Sunset is a place. So we are always right over that sunrise-sunset line on the planet as we orbit around in a polar orbit. So we go from north to south, skimming that line so that every spacecraft is always in the sun, looking sideways out across the Earth toward the star itself. 

LP: I may never forget that. Sunset is not a time. Sunset is a place. And that's where PUNCH is going. Amazing. Okay. So how do this is going to work? How do you know that you can go up there and get these images of the solar wind? What here on Earth tells you that this is the way to do it, this is going to work? 

CD: Like every mission, we are standing on the shoulders of giants. This type of measurement has been tested before. There's a mission called STEREO that prototyped an imaging system similar to our wide-field imager. 

STEREO was a first cut, so it wasn't able to produce the kind of detail or the three dimensional images that we will be seeing, and it didn't look all the way around the sun to view the solar wind as it washes over us. But we were able to demonstrate, using the STEREO data, that we would be able to extract the images that we plan to with PUNCH. 

LP: We talked about PUNCH being in low-Earth orbit, but how close will the PUNCH satellites actually be to the sun, and how do they withstand that intense heat of the solar wind coming off the sun? 

CD: Low-Earth orbit is called that because it is inside the magnetosphere. So the PUNCH spacecraft themselves will be protected from the solar wind that they're viewing. And in fact, they won't be much closer to the sun than we are right here in the studio. So they do have to deal with the intense sunlight of outer space. And we do have a thermal management system that maintains the right temperatures on the spacecraft. 

But unlike Parker Solar Probe, which is actually flying to the sun itself three times a year, we don't have to deal with the intense heat. We do have to deal, in our instruments, with the intense brightness of the sunlight. The wide-field imagers in particular, are exposed to the eternal noontime of space, and they're photographing something that's 1,000 times fainter than the Milky Way. 

So the baffle system that we use to block out the sunlight is exquisitely engineered, and it removes all but one million billionth of the sunlight that impacts the spacecraft, all in the space of less than a yard. 

LP: One million billionth. Okay. That's a new number for me. You mentioned Parker Solar Probe mission. Can you tell us a little bit more about that and how it connects, if it connects to PUNCH at all? 

CD: Parker Solar Probe and PUNCH are counterpart missions in many ways. I mentioned at the beginning that solar wind physics and solar physics have been divided by the technologies that we use to make the measurements. So PUNCH is working to image the solar wind and bring large-scale and cross-scale measurements to the understanding of that material. 

Parker Solar Probe is going the other way, bringing in-situ sampling inward into the corona itself. That's a much more technically challenging process. But Parker Solar Probe is flying through the actual corona of the sun periodically, sampling the material directly and getting the ground truth of the physics that we produce images of with PUNCH. 

You need both kinds of measurement. If all you have to measure the material around the sun and the corona itself is in-situ measurements, that's Latin for on site sampled measurements of the material, that's sort of like trying to understand human anatomy and biology if the only tool you have is an electron microscope. You could learn everything there is to know about one place, but it's very hard to see how sinews or muscles connect to one another to form a body. 

Likewise, if all you had is images, you wouldn't understand the biology of how each part of the anatomy works. You would only understand the large scale structure. You really need both those views. PUNCH and Parker Solar Probe will be seeing some of the same systems in the outer solar corona. 

Joint studies between the two missions will achieve an unprecedented factor of a billion in scale. So imagine if you were studying the anatomy of a sprinter running down a track. That would be like understanding the gross anatomy of the biophysics of how the sprinting is happening, at the same time that you're tracking individual glucose molecules in the muscles of the runner. That is a phenomenal contrast in scale, and I'm really looking forward to it. 

LP: Yeah, that's exciting and a really great analogy too. So you mentioned that prior to PUNCH, the way to find out more about the solar wind was to sample it. How do you get a sample of the solar wind? You just dip your hand out with a cup or-- 

CD: So yes, it is as simple as that. You bring an instrument out into outer space outside of Earth's protective bubble, and it will be directly exposed to this material that's coming from the sun. You can sample it. 

LP: Okay. I want to get into launch now. That's the big event coming up. So PUNCH will lift off with SPHEREx, the Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer. So PUNCH is taking a ride on this explorer, and launch is set for no earlier than February 27. So tell us about this rideshare plan. How does that work and how will the launch date be determined? 

CD: So the launch is scheduled for a particular date, but many things can delay a launch, everything from weather to issues with the rocket, of course, to other launches elsewhere in the nation can cause us to wait a day or more. 

Ride sharing SPHEREx is a cost-saving measure. It's a wonderful thing. Rockets have become much more powerful and spacecraft have become much smaller than they used to be. And as a result, there's often excess capacity. And so making use of the same rocket to carry multiple missions is smart. It saves taxpayer dollars and saves fuel and all the good things that we associate with carpooling. 

LP: Yeah, exactly. This is just a space carpooling. All right, I like that. How are you preparing for launch? What is your launch day plan? Where will you be? 

CD: So on launch day, I plan to be in the control room for most of the countdown. There comes a point where all of our systems are ready to go and our spacecraft are switched off for launch. They switch on automatically when they're deployed. So I will go out and watch the rocket go up because who wouldn't? 

LP: And this is happening in California? 

CD: We're launching from Vandenberg Space Force Base in California. Vandenberg is on the westernmost corner. If you look at Southern California, it starts out the coast, runs north south, but it bends outward around Los Angeles, and there's a little point before it goes north again. That point is Vandenberg Space Force Base. And it's there because it's surrounded on three sides by ocean. 

LP: So what happens prior to this launch day that's fast approaching this tentative launch day, fast approaching? Is there last minute test going on? What are you running through before actual launch day? 

CD: So there's a tremendous amount that has to happen in order for launch to occur and for all of these spacecraft to come together. PUNCH has already been, as we talk, through its post-delivery tests. We know that all the spacecraft work. They've been through their paces. We've tried exercising all the mechanisms, make sure they work, and we're just getting ready to bolt them to the deployer ring, which will itself be bolted to the rocket. 

So the stack is being assembled over the next week or so. We'll do final electrical checks and then truck the whole unit from the assembly building at Vandenberg, across the Space Force Base, to the launch pad to be integrated to the rocket. 

LP: And once launched, how soon will you start collecting data or be able to see images from PUNCH? 

CD: So all four PUNCH spacecraft launch on one rocket underneath SPHEREx after launch, we'll get in the right orbit and then deploy SPHEREx. And then a little bit of time goes by and the four PUNCH spacecraft will be spring deployed away from the rocket. So they're kicked out by little springs that will launch each one out at a walking pace relative to the others. 

Now, if you started walking right now and just never, ever stopped, it would take you about 90 days to get a third of the way around the world. And that's exactly what happens with PUNCH. We spread out all the way around the world. So the three, WFIs, Wide Field Imagers, are 120 degrees apart, and that takes 90 days to occur. 

So during that time, we check out the spacecraft. We check out the instruments. We start testing out the ground pipeline. and then 90 days after, we use little, thimble-sized rocket engines on each spacecraft to just give a slight kick and settle them down so they're exactly the right distance and stay there, then we move into science mode. 

LP: And do they immediately start feeding back data or is that also a little bit of a wait, till you start seeing the first images and information? 

CD: So the spacecraft can only talk to the ground when they're close to a radio station, from a ground network that we use to communicate between the spacecraft and the mission operation center in Boulder. Shortly after launch, I think 80 minutes after launch, we'll get our first radio contact over Sweden. That's when we'll check that all the systems are working correctly. But over the next few days, we'll check out the spacecraft and get them situated, pointed at the sun, and ready to operate. 

We wait 30 days before we open the doors on the instruments. That's to make sure any dust that came up with the spacecraft is dissipated. After that, we'll take test images until we're properly calibrated, and then we'll start taking science data after that. So it does take a couple of months to get everything ready. 

LP: All right. But fairly quickly, considering all the planning that has gone into it, and then in a couple of months, you're going to start seeing some good info come back. So that's really neat. How long will PUNCH be deployed? 

CD: PUNCH has a nominal two-year mission. However, the orbit will be stable for many years, and we hope to operate for up to a decade. 

LP: So we've talked about it a little bit already, PUNCH coordinating with other missions and building on past missions, but how is this particular mission contributing to the field of heliophysics as a whole? 

CD: So PUNCH is part of, if you will, a balanced breakfast of observations. We have our own science that we're doing, and that's focused on unifying these two different parts of the field. However, we also can provide context for many other observatories and missions that are operating. So all these parts on the ground and in space come together to form a large observatory for humanity that can observe the sun in many more ways than we could with any one of them. And all of them are important. 

LP: So I want to learn a little bit more about you. You're obviously excited and passionate about your work. So what drives you to study the sun, our star? What is your motivation? What inspires your work? 

CD: The sun is a fascinating system. It is the only star we can see clearly, but more importantly, it's our star. It controls our environment. It is a part of where we live. And understanding the area around us is important. We live here, not somewhere else in the universe. And understanding the universe as a whole is important. That's why we have things like the James Webb Telescope, but understanding here where we live is important because after all, we live here. 

LP: Is there a favorite sun fun fact or favorite sun tidbit that you'd like to share? 

CD: There are lots of things that are counterintuitive about stars and about the sun. My favorite fact about it is the sun is not hot because it produces a lot of energy. It's hot because it's very large. Pound for pound, a cow produces about 10,000 times more heat than the sun. 

LP: What? How? 

CD: It turns out it's very hard to conduct nuclear fusion. And so it takes the entire weight of a star to push together atoms in the core enough to produce energy. If the sun were made of cows and they could somehow stay alive, that means it would be 10 times hotter and 10,000 times brighter than it is. 

The only reason that cows don't shine brighter than the sun is they're very small. So they can cool themselves very effectively. So the sun is hot because it's very hard for heat from the core to get all the way out. It has to travel half a million miles to get to the surface and become sunlight. Heat from the core of a cow only has to go something like a yard. 

LP: All right. That's definitely a fun fact you delivered there. Why is it important for humankind to understand these processes? Part of our mission here at SwRI is to conduct research and development and create technology to benefit humanity. So why is it important for us to move forward with these studies of the sun? 

CD: Well, there are practical reasons because we'd like to understand space weather. And in fact, we think PUNCH will revolutionize space weather the same way that geosynchronous satellites revolutionized weather forecasting when we could track storms, but there's a balance between these immediate practical effects and also understanding our environment around us. 

LP: Okay. Well, Craig, thank you so much for stopping in today and for being here as you prepare to launch PUNCH, a mission in the making since 2015. And here we are a decade later, and you're just days out now from getting those four suitcase-sized satellites up in low-Earth orbit. So I know it's an exciting time for you and your team, and it's really an exciting time in heliophysics and an important moment for all of us to better understand the sun-earth connection, again, the very source of light and life for our planet. 

CD: Thank you for having me. It's always a pleasure to speak, and I'm a big fan of the podcast, so it's a real treat to be on it. 
 

And thank you to our listeners for learning along with us today. You can hear all of our Technology Today episodes, and see photos, and complete transcripts at podcast.swri.org. Remember to share our podcast and subscribe on your favorite podcast platform.

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Ian McKinney and Bryan Ortiz are the podcast audio engineers and editors. I am producer and host, Lisa Peña.

Thanks for listening.

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