I’m happy to announce today that for the first time ever, a spacecraft has “touched” the Sun. Three years after launch, our Parker Solar Probe has now flown within the Sun’s inner corona, sampling particles and fields still bound to the Sun’s atmosphere.
This monumental achievement is more than 60 years in the making, the goal of a mission concept and dreams of scientists that predate NASA itself. Just as the Moon landings revolutionized our ability to study the Moon and our solar system, our first close encounter with our star marks the beginning of a new phase in solar science, one where we can ask and answer questions that had previously been out of reach.
This milestone is even more meaningful considering that this technologically-advanced spacecraft was named in honor of astrophysicist Dr. Eugene Parker. It is the first and only time a NASA spacecraft has a living individual as its namesake. I chose to advocate for naming this spacecraft after Dr. Parker as a testament to the importance of his entire body of work, work that I felt had been overlooked by many – even though it’s hard to find a scientist with a bigger or broader impact on space science.
At the heart of this work is a story of pioneering science with much perseverance: In 1958, Dr. Parker, a humble but somewhat stubborn mid-westerner, published an article sharing his theory that high-speed matter and magnetic fields were constantly – and supersonically – escaping the Sun. He predicted that this constant torrent of what came to be known as solar wind affected all the planets and space throughout our solar system. This important prediction, and eventual confirmation, ultimately informed our understanding about how stars and other astrophysical objects throughout the whole universe interact with the worlds and space around them. Not only did Dr. Parker’s work introduce a new field of science, he inspired my own research as well as crucial science questions that NASA continues to study to this day. More than 20 missions in heliophysics, planetary sciences, and astrophysics currently focus on scientific fields he significantly affected. Adding Parker Solar Probe to Dr. Parker’s legacy is among my proudest accomplishments and one that is meaningful to me even today.
What it means to “touch” the Sun requires some defining, since the Sun doesn’t have a solid crust like Earth. But it does have an invisible boundary where solar material stops being “stuck” to the Sun, and instead is free to decouple from its source and escape outward as the constantly streaming solar wind. We call this boundary the Alfvén critical surface, named after the Swedish scientist and Nobel Prize winner who made many notable contributions to plasma physics. At the Alfven surface, the solar wind begins to travel faster than the speed of the waves that can couple the wind to the surface of the Sun – which implies a speed where the solar material is traveling fast enough that it can decouple from its source in the solar atmosphere. Freed, the solar wind can now escape into space.
We have long known the Alfvén critical surface exists, but not exactly where it was located or what was within it. Based on remote images of the corona, as well as solar wind measurements in space, estimates had put the boundary somewhere between 10 to 20 solar radii (4.3 to 8.6 million miles) from the photosphere, or solar surface. But we’ve never had a spacecraft close enough to confirm those estimates and we have never gotten close enough to see what’s on the other side.
During Parker’s eighth flyby of the Sun on April 28, 2021, the spacecraft passed within 18.8 solar radii (8.127 million miles) of the photosphere when it detected the conditions scientists had long awaited. Parker was passing through what’s known as a pseudostreamer, a giant magnetic loop that extended from the corona, when the magnetic field intensified and particle speeds slowed. A combination of measurements from multiple sensors revealed that Parker had indeed crossed the Alfvén critical surface and was sampling particles that were not part of the supersonic solar wind, but the slower-moving solar atmosphere itself.
So what are we seeing on the other side? For one thing, an answer to a question that Parker Solar Probe identified soon after it launched: What is causing mysterious hairpin bends in the solar wind called “switchbacks”? We have known about switchbacks since the NASA/ESA Ulysses mission in the mid-90s, which observed S-shaped kinks in the solar wind where the Sun’s magnetic field abruptly reversed direction like a magnetic zig-zag. Due to assumptions that the solar wind was fairly stationary, we had suspected these switchbacks were relatively rare phenomena restricted to the Sun’s polar regions.
In 2019, Parker upended those assumptions when it revealed that switchbacks were plentiful in the solar wind, even in regions far from the solar poles. The new observations suggested that switchbacks would tell us more about the Sun than we had expected – but how and where they formed remained unknown.
Parker’s close pass within the solar atmosphere got us close enough to find out. On its sixth flyby, Parker measured clusters of switchbacks and found that the percentage of helium in them matched the composition of solar material at the photosphere, the solar surface. During the same flyby, a different analysis showed that the switchbacks were aligned with magnetic “funnels” in the photosphere. Together, these facts suggest that the switchbacks start near the solar surface, a dynamic, roiling region of solar material and magnetic field that looks somewhat like a searing frying pan of oil at home.
Parker will continue orbiting even closer to the Sun on upcoming flybys, reaching as close as 8.86 solar radii (3.83 million miles) from the surface at its closest approach. The next perihelion in January 2022 will likely pass through the solar atmosphere, the corona once more – meaning we are now in a very exciting time for solar science, a time when we can directly sample the Sun itself. This opens a whole new realm of solar and heliospheric science!
Finally, I want to thank Dr. Eugene Parker for the time he took with me and so many other early career scientists over 20 years ago. May we all take his intellectual courage and steadfast perseverance as an example for us as we move into the new era of science enabled by these new data. And let’s also learn from Parker’s senior colleagues who were less than welcoming of new thought when as an early career scientist, Parker brought forward his novel ideas about the solar wind and its embedded magnetic field. Instead, let’s celebrate new thought and insights, especially if they come from new community members!