The Mystery at the Center of the Solar System
A spacecraft has finally gotten close enough to the sun to gather clues about some lingering questions.
For a little NASA spacecraft, the weather outside is frightful.
The Parker Solar Probe is on a mission toward the sun. The spacecraft has been exposed to scorching temperatures and intense sunlight as it draws closer with every loop around. Eventually, Parker will glide through the star’s outer atmosphere and feel the toastiness of nearly 2 million degrees Fahrenheit (more than 1 million degrees Celsius).
Parker is dressed appropriately for the journey. It wears a thick, custom-made shield to protect its scientific instruments and systems, and tubes with flowing water to cool itself down. Inside, it is a cozy 78 degrees Fahrenheit (26 degrees Celsius). Since it set out last summer, Parker has made three sweltering passes of the sun, with many more still to come in the next five years. And its findings are already surprising scientists back home.
One of the earliest scientific discoveries about the sun was also the most significant: The Earth was not the center of the universe, but orbited the sun. In the centuries since Copernicus redrew the cosmic map, scientists have traveled closer to the heart of the solar system, first with telescopes, then with satellites and spacecraft. They learned about the nuclear fusion that powers our star and countless others and, with the discovery of exoplanets, realized that many stars could be someone else’s sun. Today, our understanding of the sun and its subtle mechanisms is more sophisticated than ever before, but it remains incomplete.
To ponder the unknowns feels like sitting with an inquisitive toddler. Why is the sun’s outer atmosphere, the corona, so hot? Where does the solar wind come from? Why does it shoot out of the corona like that? What makes the sun flare up sometimes, shooting even more excited particles out into space? These are some of the questions that scientists hope Parker can answer before its mission ends in 2025, with a fiery plunge right into the sun.
NASA released the first batch of results this week, published across four papers in Nature. The findings come from measurements of the corona, which is, remarkably, hotter than the surface itself. The corona extends millions of miles from the surface into space. The region is only visible to the naked eye during a solar eclipse, when the moon casts a shadow on the Earth and blocks out the sun, leaving only a golden ring hanging in a darkened sky.
The corona unleashes powerful streams of high-energy particles, known as the solar wind, which can be felt all across the solar system, and far beyond Pluto. The data from the Parker probe show that the solar wind is far more turbulent near the sun than in our own vicinity, tens of millions of miles away. The wind drags the sun’s magnetic field out into space, and even bends the field enough for magnetic forces to completely flip around for a few minutes at a time, pointing back at the sun itself instead of into space. The researchers weren’t expecting the strength of this effect, as well as how often it seems to occur.
Scientists also found that shifts in the sun’s magnetic field speed up the particles flowing away from the sun much faster than any of their models had predicted. Astronomers have spent decades probing the depths of countless distant stars in the cosmos, some of them billions of light-years away, but their own still keeps secrets from them.
Scientists haven’t been able to make such close-up detections with instruments on Earth, or even with earlier missions to the sun, which never got as close. For studying the sun, proximity is everything. “Imagine that we live halfway down a waterfall, and the water is always going past us, and we want to know, what is the source of the waterfall up at the top?” says Stuart Bale, a scientist at the University of California at Berkeley, and the lead on a Parker instrument that examines the solar wind by measuring magnetic fields. “Is there an iceberg melting up there? Is there a sprinkler system? Is there a lake, a hole in the ground? And it’s very hard to tell from halfway down. So what Parker has done is got us closer than ever to the sun.”
At every close approach, the Parker probe will also get closer to pulling off one of the toughest feats of robotic space exploration. It sounds counterintuitive, but it’s actually harder to reach the sun than it is to leave the solar system altogether. The sun’s gravity is always tugging at everything around it, from giant planets to tiny moons, but those objects are also looping around the sun at great speeds, which keeps them from falling toward it. “To get to Mars, you only need to increase slightly your orbital speed. If you need to get to the sun, you basically have to completely slow down your current momentum,” Yanping Guo, the mission-design and navigation manager for the Parker Solar Probe, explained to me.
No existing rocket technology is powerful enough to cancel out the Earth’s motion like that, so the Parker probe is getting an assist from other planets. The spacecraft has been flying way out to Venus and looping around, trimming its orbit each time to shed some of the Earth’s momentum and bring itself closer to the center of the solar system.
The mission is named for Eugene Parker, the American astrophysicist who first described the dynamics of solar wind in 1958. Few believed his theory until NASA started sending robotic spacecraft deep into the solar system a few years later, and their instruments felt the breeze. There’s no doubt now, and robotic missions have followed the wind as far as they can go. In 2013, one of the Voyager probes, which have traveled farther than any other spacecraft, detected particles in the solar wind mixing with cooler particles of interstellar space—where the weather is a different kind of frightful.