There are some places in the solar system no human will ever go. The surface of Venus, with its thick atmosphere and crushing pressure, is all but inaccessible. The outer worlds, such as Pluto, are too remote to presently consider for anything but robotic exploration. And the sun, our bright burning ball of hydrogen and helium, is far too hot and tumultuous for astronauts to closely approach. In our place, one intrepid robotic explorer, the Parker Solar Probe, has been performing a series of dramatic swoops toward our star, reaching closer than any spacecraft before to unlock its secrets. Now it is about to perform its final, closest passes, skimming inside the solar atmosphere like never before.
“It’s a big moment,” says Yanping Guo, a space mission designer at the Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Maryland. “Across 60 years of space exploration, the sun has been the most difficult destination to reach.”
On Christmas Eve, December 24, Parker will fly just 6.1 million kilometers above the surface of the sun, or 9.86 solar radii from the sun’s center, ten times closer than Mercury orbits the star and the first of three of these extremely close flybys. It will do so at an astonishing speed of 690,000 km per hour, faster than any spacecraft in history (albeit still reaching just 0.064 percent of the speed of light). During its flyby, Parker will be moving fast enough to travel from London to Paris in less than two seconds; its speed will be so great that relativistic effects such as time dilation and frame dragging may register on the spacecraft’s instruments.
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The spacecraft will fly through the sun’s atmosphere, its corona, where some of the biggest questions about our star remain, including why the corona is so much hotter than the solar surface, and how the solar wind is accelerated. While other spacecraft have studied the sun before, only Parker has come so close. “There is no precedent,” says Thomas Zurbuchen, former associate administrator for the Science Mission Directorate at NASA. “It is truly an exploratory mission.”
It was Zurbuchen who gave the spacecraft its name, after the late U.S. solar physicist Eugene Parker, who in the 1950s predicted the solar wind’s existence. The mission launched in 2018, the culmination of decades of study on how to “touch” the sun. Getting close to our star is surprisingly difficult, because to fall toward it you need to “kill off the orbital speed of Earth,” says Ralph McNutt, chief scientist in the space department at JHUAPL. Scientists had long thought the best way to do that was to fly out to Jupiter, and then use the gas giant’s gravitational pull to dive in toward the sun. Such a mission would get you very close, just four solar radii away, but at the expense of being extremely difficult and time-consuming, giving you maybe just one or two close passes of the sun with an orbital period of nearly five years.
In 2007 Guo proposed instead that multiple flybys of Venus could be used to bring a spacecraft into similar if slightly more remote proximity, but with the added benefit of dozens of passes over several years with an orbital period of just under three months. “The requirement was to be close enough to take samples inside the solar corona,” Guo says. “I found you could use seven Venus flybys.” The last of those flybys occurred on November 6, with the spacecraft swooping…
