This essay is an entry in our "Dear Spacecraft" series, where we ask writers, scientists, and astronomy enthusiasts to share why they feel personally connected to robotic space explorers.
Dear Solar Probe:
I think there's a point that’s not widely appreciated, but it's fundamental: The sun is an ordinary star of middling mass and middling brightness, but it’s a model for almost all stars—and the only one we’re going to see up close enough to do a whole lot of measurements. There are stars that are oddballs, the ones that interest the astrophysics types. But the fact that the sun supports life on one of its planets is already a unique designation.
I’m in love with the sun for that reason. Somehow, in many circles, solar physics is looked upon as old, dusty, dried-up problems that don’t really have new solutions. On the contrary, it’s the one star where we know what we’re talking about!
Like the sun, you also have a unique designation: You're named after me. I was sitting in the office in 2017 when Tom Zurbuchen, NASA's associate administrator, called one day and said, “NASA’s talking about putting your name on the Solar Probe. Can you think of any reason why we shouldn't name it after you?” I was sort of stunned; it took a little while to get it straight in my mind. I guess it makes me more conceited than ever.
After all, the people who built you were in a position to make a heck of a spacecraft. You're making many, many trips past the sun, each time getting a little closer because you've hooked around Venus. You plunge in, you get some data, and your trajectory takes you out again. Then on the next swoop in, you come a bit closer than the last time, monitoring the gas and magnetic fields streaming by and eagerly looking for new effects. On the way, the sun must appear mighty large in the sky.
It takes my breath away when I think about it, how far we’ve come. When I think back to 1955, we had a lot of concepts about how space worked, but most of them were wrong, or at least so vague that you couldn’t argue whether they were right or wrong. When John Simpson at the University of Chicago offered me a position at one of the first space science laboratories, I didn't have any experience in the field, really. This was in the days before we went into space and sent up spacecraft like you; whatever we did, we stood at the surface of the earth and looked up at the sky and wondered what was going on.
In those days, one of the outstanding questions was whether the space between planets should be considered absolutely empty, or full of loose electrons, magnetic fields, and stuff from the sun. So John had a brilliant idea. He built neutron monitors—sensitive cosmic-ray detectors—and distributed them from Peru to Colorado. By watching how the cosmic-ray intensity varied at each latitude, we could deduce variations in what's going on in space.
I was hired to interpret the data. Once I saw the sun was expelling particles and magnetic fields, I could see that this “solar wind” of freely moving particles was simply a problem in fluid mechanics. Its easily solved equation of motion showed that the wind starts from the sun with a very small velocity, but because of its million-degree temperature, it expands to supersonic speeds at large distances. The wind becomes more tenuous as it goes farther out, and as the sun rotates, the magnetic fields are stretched out by the wind and form a spiral in interplanetary space.
At that time, most people had a simpler picture, so the response to my paper on the subject was bitterly negative, except for a few acquaintances. I tended to shrug my shoulders: I had done the math correctly, I had a job and a salary, and I really didn’t give a damn what they thought.
Observations in space in 1962 verified the existence of the solar wind, and missions in later years blocked out the details of its varying structure, a record to which you will be adding fresh data. Everywhere around the sun—over the poles, over the equator—there's an outflow of particles at a few hundred kilometers a second, and it's going so fast because its observed temperature is on the order of a million degrees. The question now is, why so hot? It's not heated by magic.
In the 1980s, I published a theory on coronal heating that you're now putting to the test. You, Solar Probe, are the first serious effort to go into this mess and get some numbers, so we can have something explicit to argue about. My hope is you'll get in close enough to the sun to pick up the waves and turbulent magnetic fields whose dissipation is responsible for heating the corona. But if it's something else, it's just that much more exotic, and that much more fun.
Thirty years ago, I would never have dreamed of a successful mission like you, but you are well-armed with instruments to do the task. I've always felt that the people who don’t get the right credit are the teams that designed and built you. You don’t see their names anywhere, but you and many other spacecraft are remarkable technological achievements. Nicola Fox, who was your project scientist until recently, is quite an impressive person. She and the team really are outstanding people.
I was fortunate enough to be invited to your launch. We were back at some distance from the launchpad. We saw the rocket rising up and going faster and faster, and as we watched, you simply got fainter and fainter.
In seven years, you'll be abandoned, you know, and forever orbit the sun. We will never see you again. We can talk to you on the radio, but we’ll never see you. To see you leaving, I almost feel that you're an old friend, and I hate to throw you to the dogs this way.
Eugene Parker is the S. Chandrasekhar distinguished service professor emeritus for the Department of Astronomy and Astrophysics at the University of Chicago. He is the only living person for whom a NASA spacecraft has been named.