Sun worship is a popular theme in human history, for good reason. Our local yellow dwarf star is the head of our solar family, the most influential body in our cosmic vicinity, and the midwife of all life on Earth. It's the biggest cheese for light years around, and it's earned its share of reverence.
Yet our Sun remains one of the most unexplored bodies in the solar system. After all, it is tough to study a massive fusion reactor that will burn out your retinas if you even look at it the wrong way, let alone send a spacecraft to brave the inferno up close.
Enter: Solar Probe Plus (SPP), a NASA mission in development at the Johns Hopkins University Applied Physics Laboratory. This robotic explorer will venture closer to the Sun than any other probe before it, flying through its corona—the searing atmosphere surrounding the star—for the first time in history. It will brave both fiery and freezing temperatures, travel faster than anything ever made by humans, and deliver the most intimate glimpse of our star—and the forceful solar wind it emits—in spaceflight history.
Scheduled for launch in the summer of 2018, the mission has been a major goal in space science since before NASA was even formed, according to SPP project scientist Nicola Fox.
"We've been waiting an awfully long time to go touch the Sun," Fox told me over the phone. "It's the last major region in the solar system to be visited by a spacecraft, and it's an important region, because the Sun is the center of the solar system, and our life depends on it. All the planets get affected by it in some way or another."
SPP is designed to swoop through the outer corona at altitudes of 8.5 solar radii (5.9 million kilometers or 3.67 million miles) above the Sun's "surface," or photosphere. This beats out the next-best distance—27 million miles set by NASA's Helios 2 probe in 1976—seven times over.
If all goes according to plan, the Sun-kissing robot will also become the fastest human-made object in history, with an estimated orbital velocity surpassing 450,000 miles per hour, triple the 165,000 mph record recently established by NASA's Juno orbiter. To put that in context, if you were to accelerate to the SPP's breakneck pace, you could scoot from Philadelphia to Washington, DC in one second. (The speed of light, meanwhile, would take you around our planet's equator almost eight times in a single second.)
The spacecraft will moderate these mind-boggling speeds with the help of seven Venus flybys over the course of seven years, which will cumulatively propel it into an ever-tightening orbit around the Sun.
"It's almost like we surf around the Sun like a surfer on a wave," Fox said. "Surfers don't fall into the ocean as long as they're going fast enough. That's kind of what we do. We use successive flybys of Venus to slow us down a little bit, and take some energy off. So, we've got to gracefully step closer and closer into the Sun over seven steps."
These repeated swan dives through the corona will expose SPP to scalding temperatures of 2,500 degrees Fahrenheit (1,377 degrees Celsius), which would normally fry electronics and equipment.
That's where the probe's sophisticated Thermal Protection System (TPS) comes into play. Measuring eight feet in diameter but only 4.5 inches in depth, the solar heat shield is constructed from lightweight carbon-carbon foam wedged between two facesheets.
"We like to call it a giant Frisbee," said TPS lead engineer Elizabeth Congdon in an interview with Motherboard. "We had some strict constraints about how heavy everything could be, so one of the drivers was to figure out not only how to design something that could get really hot, but also to make sure that it was lightweight enough that we could actually get off the ground." Building the fastest spacecraft ever, she said, is "really a mass game."
The interior foam, which is 97 percent air, helps the shield meet the weight restrictions of 160 pounds. The foam also provides a load-bearing layer for the facesheets and a thermal buffer for the probe's instruments. The extraordinary heat on the Sun-facing side is efficiently dissipated into a comfortable room temperature environment on the shaded side.
On top of that, the TPS is flexible and durable enough to withstand the extreme pendulum shifts in temperature that SPP will experience as it crosses through the solar atmosphere, then hurtles back into the freezing environment out in Venus' orbit.
Animation of SPP approach. Video: JHU Applied Physics Laboratory/YouTube
"We actually test hot and cold," Congdon said, describing the process of inspecting the TPS. "Because of the Venus flybys, we have a cycle that we go through of hot-cold, hot-cold in 24 cycles. Not only do we get very hot, we also get down to about minus 200 Fahrenheit. That happens 24 times over seven years. That's just a lot of life on something."
The spacecraft's suite of instruments will remain blissfully unhindered by these punishing conditions, freeing them up to focus on measuring and observing the Sun's magnetic fields, plasma and energetic particles, and the dynamics of the solar wind, which is the powerful flow of charged particles emitted by the Sun.
Because the probe will dip into the never-before-explored outer corona, the mission is expected to solve lingering enigmas about the solar wind. Space weather, auroras, comet tails, life on Earth, and the performance of our civilization's electronics are all deeply shaped by the solar wind, so it's imperative that we better understand its whims.
"There are a few major mysteries with the Sun and the solar wind," Fox told me. "One is that the corona—the atmosphere that you see around the Sun during a solar eclipse—is actually hotter than the surface of the Sun. So, that kind of defies the laws of physics. It just shouldn't happen."
This inexplicably hot region is also where the solar wind becomes so energized that it's no longer constrained by the gravitational pull of the Sun, which is a second unsolved riddle.
"It's like [the solar wind] gets a big injection of caffeine, and off it flies and breaks away from the Sun," Fox said. "These two big questions have been around for more than 100 years, since the solar wind was first discovered, and no one's been able to explain them."
SPP aims to demystify this tantalizing environment near our star, while shattering several spaceflight records in the process. "We're going to a region we've never been before," Fox said.
"Magic happens in that area."
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Correction: This piece initially identified Elizabeth Congdon as leader of the TPS materials testing team. She is in fact the TPS lead engineer.