The Solar Probe: So Simple a Thing As a Star
On the solar probe, and our long effort to understand the Sun
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It is reasonable to hope that in a not too distant future we shall be competent to understand so simple a thing as a star.
Arthur Eddington, 1920
Usually, you have to be dead before anyone will even consider naming a spacecraft after you.
Einstein had to wait twenty years for the honour. Hubble had been buried four decades before he got his famous telescope. In Europe things can be far worse: Euclid was in the ground for twenty-three centuries before he got anything, and Plato – dead in 347 BC – will only get one next year.
But Eugene Parker somehow broke all the rules. On August 12, 2018, the man – then still very much alive – stood and watched as the Parker Solar Probe soared into the heavens. Perhaps it seemed normal to him. He was, after all, no stranger to having things named in his honour: Parker has an Equation, a Model, a Limit, and an Instability. The Parker Spiral describes the way the Sun’s magnetic field twists in space; the Parker Theorem talks about how magnetic fields move through plasmas.
All this – the probe, the equations, the instabilities – is a tribute to a man who tried harder than almost anyone else to understand the Sun. He arrived on the scene in the 1950s, a time when some of the big questions about our star – notably what it was made of, and how it burned – had been solved. But science still viewed the Sun as a static object, one with limited influence on the rest of the Solar System.
Parker initially turned his attention to the solar corona, the mysterious and very hot layer surrounding the Sun. This region, he realised, had to be unstable. His calculations showed that a stream of particles must be constantly blowing off it, forming a fluctuating breeze he dubbed the solar wind. The idea was instantly ridiculed – a reviewer of his paper called it “nonsense” and suggested Parker spend more time in the library before trying again.
But Parker was right. In 1959, the Soviet probe Luna 2 detected particles streaming off the Sun. Three years later, Mariner 2 confirmed the discovery as it flew towards Venus. The solar wind was very real, and the Sun was shown to be far more dynamic and influential than anyone, bar Parker, had dared to believe.
In the years after, he made other important discoveries. He looked at how the Sun’s magnetic field behaved, studied its mysteriously hot corona, and pondered the question of magnetic monopoles. When he died in 2022, he was hailed as the father of heliophysics, a man who had written his name in the stars.
Yet, and despite all his work, we still struggle to understand the Sun. It is, as the physicist Arthur Eddington pointed out early in the twentieth century, very much an average star. It is of an unremarkable size, an unremarkable age, and appears little different to countless others scattered across the galaxy. It should be easy to understand.
But it has proven surprisingly complex. The mystery of how and why the corona is so hot – at times it can reach temperatures of ten million degrees, far higher than those of the surface of the Sun – remains unsolved. So too do questions about the origins of the solar wind, and the reasons why enormous flares occasionally burst out from its surface.
The only sensible solution, NASA realised, was to try something utterly insane. To properly grasp the workings of our star, we needed to fly as close to it as possible. The goal of the Solar Probe was to do exactly this: to approach the Sun, to penetrate its outer layers, and to plunge through the soaring heat of the corona itself.
“Wow!”, Eugene Parker said, as he watched his namesake lift off the launch pad and begin this perilous and unforgiving voyage. “Here we go!”.