The sun, for all its glitz, tends to hide behind a veil of mystery. For instance, its outer atmosphere, the corona, appears to be hotter than its surface — like, way hotter — and scientists aren’t sure why. There’s also a big puzzle concerning how the solar wind, which is a stream of charged particles emanating from the sun, gets accelerated while zooming through space. What exactly drives those particles? And soon, NASA intends to tackle yet another solar conundrum that has to do with both the corona and the solar wind.
The agency wants to know how one turns into the other — and learn how this connection ultimately creates the solar wind chamber we live in, known as the heliosphere.
On Feb. 27, aboard a SpaceX Falcon 9 rocket, NASA plans to launch the sun-studying PUNCH mission to low Earth orbit. It will ride alongside the agency’s SPHEREx observatory, which is sort of like a wide-angle version of the James Webb Space Telescope, as part of the agency’s Launch Services Program that works to make space missions more cost efficient.
“PUNCH is the first mission specifically designed to unify two particular major fields within heliophysics: solar physics and solar wind physics,” Craig DeForest, PUNCH’s principal investigator from the Southwest Research Institute, told reporters on Feb. 4.
PUNCH, which stands for Polarimeter to Unify the Corona and Heliosphere, is made up of four small satellites that will work in tandem to create 3D observations of the inner heliosphere specifically. The “heliosphere” refers to a region (akin to a bubble) around the sun. This region actually encapsulates our entire solar system, and is formed as a result of solar wind particles streaming from the sun and eventually pressing into the interstellar medium. That creates a barrier between the part of space dominated by the solar wind and the rest of the universe, leaving our cosmic neighborhood in this sort of solar wind chamber.
“Our heliosphere is this massive, gigantic object that spans so far out from the sun, and PUNCH is going to connect from the closest end of the sun to the farthest out,” Joe Westlake, the director of NASA’s Heliophysics Division at the agency’s headquarters, told reporters during the call.
But, again, what scientists really hope to discover is how the corona becomes the heliosphere. “PUNCH is just going to be a really great unifier,” Nicholeen Viall, PUNCH mission scientist at NASA’s Goddard Space Flight Center in Maryland, said during the call.
This could have far-reaching implications, most fascinatingly to do with how well we can forecast bittersweet space weather events that simultaneously provide us with gorgeous auroras and threats to the power grid.
How PUNCH works
There are a few key aspects to the way PUNCH will operate. First off, as mentioned, it’s not one spacecraft but rather a constellation of four that’ll be stationed in different areas of low Earth orbit. The reason for this quartet is very interesting, and in fact not an unheard of concept in space exploration.
“We have to have two kinds of instruments, DeForest said, “one that looks close to the sun, where it’s bright, and one that looks farther from the sun where it’s fainter — but the ones that are looking farther away have a problem: There’s a planet in the way, in the form of the Earth.”
There is a solution, however.
“We had to make the instrument either very far from the Earth, which would be cost prohibitive,” DeForest said. “Or, we needed to make it as large as the Earth.”
The team went with the latter. Obviously, though, it isn’t easy — and maybe impossible? — to make an Earth-sized spacecraft with the technology we have. So, the scientists went with the next best thing. “All we had to do was break it into three parts and mount it on three different spacecraft,” DeForest said. “So that’s why we have three wide-field imagers.”
The concept is similar to how the Event Horizon Telescope (EHT) team took the first-ever (and second-ever) black hole image captured by humanity. To achieve such a feat, EHT researchers realized they’d need a telescope as big as the Earth — so, they scattered a bunch of telescopes across different sections of the planet, then later stitched all the data together.
As for the final part of PUNCH — the fourth member of the satellite constellation — DeForest explained that it’s a narrow-field imager built by the Naval Research Laboratory in Washington, D.C. This one works differently than the other three; it involves a camera situated such that an artificial total solar eclipse can be seen. Recall what the natural 2024 total solar eclipse looked like, for instance: a delicate white halo around a dark circle. The white halo was the sun’s corona, and the dark circle was the moon’s silhouette. It’s the same idea.
“PUNCH is going to see a total solar eclipse; see that corona; see it as it extends outwards, but it’s even better than an eclipse — because you get to see it not just for the couple of minutes that the moon blocks out the main body of the sun, allowing you to see the corona, but because PUNCH makes an artificial eclipse, we get to watch it all of the time, and we get to watch it in high definition,” Viall said.
“If you imagine getting to see the eclipse on an actual sky, what PUNCH gets to see extends all the way from straight overhead, all the way to the horizon,” she added. “That’s an enormous volume of the solar system.”
But this is just half of the story. The second important thing about PUNCH is that it’s a polarimeter that takes advantage of a property of light called polarization.
“A polarimeter is a camera that can measure polarized light like you might see through a pair of polarized sunglasses, and we use that to measure what we’re imaging in three dimensions,” DeForest said.
“Everything we see around us, except for the light source itself, is scattered off of a surface or off of an object that’s in space,” he added. “So as we look at the solar wind, we’re looking at sunlight that’s scattered off the actual material that we’re seeing. That scattering process polarizes the light, and the degree of polarization tells us where the object was in three dimensions.”
Welcome side effects
PUNCH, if all goes to plan, should be able to reveal things about the sun that go beyond the unification of the corona and heliosphere. For example, the team says that this could very well change the game for space weather forecasts, meaning we may be able to know when a solar storm will hit with greater accuracy than we currently can. This is good for aurora chasers, sure, but it’s also good in case a major storm with the power to harm spacewalking astronauts, for example, is headed for us.
“We expect that we will revolutionize how space weather is forecasted,” DeForest said. “We are the first mission to be able to track space weather events in three dimensions routinely across the solar system.”
PUNCH will also manage to work with other solar probes, including the trailblazing Parker Solar Probe that literally plunges into the sun’s corona every so often, most recently on Christmas Eve of last year.
“Together, the two missions will provide the first complementary data sets, showing how the sun gives rise to the solar wind and space weather that span a factor of one billion in scale,” DeForest said. “That’s like tracking and measuring simultaneously the anatomy of a sprinting track star and also the individual motions of molecules of glucose in that runner’s muscles.”
And finally, DeForest calls out a special “side effect” of PUNCH’s main science: “We will enable new astronomy by producing the world’s most comprehensive polarimetric star map of just over three quarters of the visible sky.”