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.
On supporting science journalism
If you’re enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
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 387 km above the surface of Venus. Since then, it has been on its trajectory that will take it closer to the sun than any of its previous 21 passes over the last six years; its last solar flyby in September was about a million km, or 10.4 solar radii, farther out.
Parker carries four instruments to study the sun. This includes a camera to image our star and its surroundings, a tool that measures electric and magnetic fields and two others to investigate the solar particles and plasma showering the spacecraft. Over the course of the mission so far, Parker has helped to discover that magnetic fields on the sun’s surface might drive heat into the corona and has found more particles coming from the sun than expected. Such results come in large part from Parker’s successful piercing of and passage through the sun’s corona, which first occurred in April 2021.
These latest flybys will bring the spacecraft deeper within the corona, although short of a supersonic threshold in the vicinity of four solar radii, where the solar wind is thought to reach the speed of sound. Even from Parker’s comparatively remote view about 10 solar radii out, scientists are hoping to learn more about the solar wind, specifically what causes the difference between its “fast” (up to 800 km per second) and “slow” (down to 300 km per second) varieties. “We think fast wind comes from coronal holes, and slow wind might come from the boundaries of these holes,” says Steph Yardley, a solar scientist at Northumbria University in England. “But it’s something we’re still debating.” This process can also tell us more about how space weather is produced on Earth as the solar wind travels to our planet. “The closer we get into that source region of particles that creates space weather, the more we learn,” says Joseph Westlake, director of the heliophysics division in the Science Mission Directorate at NASA headquarters in Washington, D.C.
There is also hope that Parker will fly past the sun during an eruption. The sun is right now in its so-called solar maximum, a period of peak tumult in our star’s 11-year activity cycle. This boosts chances for fortuitously timed eruptions bathing the spacecraft, something that has happened on at least one previous occasion. “We’re hoping we will gather more of these events very close to the sun,” says Nour Rawafi, an astrophysicist and project scientist for Parker at JHUAPL, “because we need to understand how events like flares and coronal mass ejections accelerate particles to relativistic speeds.” Parker may also encounter a dust-free zone, a hypothesized region close to the sun where debris drifting inward from around the solar system “is vaporized,” says John Wirzburger, systems engineer for Parker at JHUAPL. “We’ve been seeing inklings of that as we’ve been getting closer and closer.”
The action this time around begins on December 20, when the spacecraft reaches 0.25 times the Earth-sun distance, about 37 million km or 53 solar radii from the sun. Here, in preparation for its plunge, the spacecraft will first send a short beacon tone to Earth, confirming its good health. To survive the close passage and the intense conditions that follow, Parker must hide most of its hardware behind a carbon-composite heat shield. This heat shield is so effective that, despite temperatures reaching about 1,000 degrees Celsius, the spacecraft’s instruments behind it remain at “basically room temperature,” Wirzburger says.
The positioning of the spacecraft and its heat shield, and the desire to maximize data returns, means the spacecraft can’t communicate with Earth during this pass. It flies entirely autonomously, changing its position ever so slightly to track the motion of the sun, keeping the heat shield precisely pointed in the star’s direction to produce a conelike shadow that entirely encompasses its precious instruments. The only other part of Parker that will see the sun during the flyby will be a tiny sliver of solar panels, tucked into the spacecraft’s sides, to generate power from our star’s immense radiance.
The entire encounter with the sun will last about a week, with the spacecraft reaching its closest point on Christmas Eve at about 6:40 A.M. EST. Were you able to survive here and avoid instant blindness from the sun’s overwhelming light—500 times more intense than seen from Earth—our star would loom in your view as an immense disk 22 times larger than the full moon in our planet’s sky. “It would fill a huge part of space in front of you,” Rawafi says. Parker’s camera, pointed to the side, will watch for tracks of particles flowing through the surrounding corona, while its other instruments gather their vital data. But what exactly they’ll see is anyone’s guess. “We don’t really know,” Zurbuchen says.
The team won’t know if the flyby has been successful until December 27, when the spacecraft reaches 35 solar radii on its way back out and beams another beacon tone back home to announce its survival. The team will then prepare to receive the spacecraft’s invaluable data starting on New Year’s Day, which will trickle back in the coming weeks and months. In March, Parker will do it all again when it flies past the sun at close proximity once more, before another, final close pass in June. The gravitational pull of far-off Jupiter will make these two later flybys technically ever so slightly nearer to the sun—about 100 km closer each, a mere rounding error on the multimillion-kilometer distance between Parker and our star. Practically speaking, the Christmas Eve flyby will be as close to the sun as Parker ever gets.
The conclusion of these close encounters will also bring the spacecraft’s primary mission to an end, but it might then be extended. Parker could, for instance, be left in its current orbit to monitor the effects of the waning solar maximum. “It would be amazing to watch this decline,” Westlake says, because many large solar events are predicted to happen in this period. The spacecraft could also start moving into a more inclined orbit with its remaining fuel, kicking itself out of the ecliptic plane where most planets twirl to get a slightly different view of the sun and peer toward its polar regions, although at the considerable expense of flying slightly farther away. “We want to stay very close to the sun,” Rawafi says.
Whatever becomes of the mission, the data Parker collects will be studied for years. “This is the closest humanity has ever gotten to a star,” Westlake says—a record unlikely to be beaten for the foreseeable future.
