The launch felt more dangerous to Professor Justin Kasper than touching the Sun, in terms of instrument survivability. His instrument on the solar probe contains ceramic balls that rattled during launch to absorb excess vibrations, preventing electronics from turning to dust. Photo: NASA/Bill Ingalls
This is an article from the spring 2019 issue of LSA Magazine. Read more stories from the magazine.
The Sun is a dangerous place. Charged particles burst from its molten plasma at millions of miles per hour and millions of degrees Fahrenheit. Those solar winds, flares, and coronal mass ejections can tear through outer space, all the way to Earth, and fry our global electrical grid, GPS navigation systems, and long-distance communications.
Up close, the danger is even more immediate. But up close is exactly where Justin Kasper wants to be.
A professor in the Applied Physics Program in LSA and the Department of Climate and Space Sciences and Engineering, Kasper has a seat on the executive committee of the Michigan Institute for Research in Astrophysics. He’s well aware of the intractable problems that have prevented space missions from approaching the giant fiery center of our solar system. But he’s always wanted to send a probe to the Sun, anyway. And now he and an international team of researchers finally have figured out how to do it.
Last August, they launched Parker Solar Probe, a NASA spacecraft that will carry four sturdy instruments close enough to the Sun to extract secrets about how it works. They’ve sent the probe in the second-largest rocket that exists, the Delta IV Heavy. Only such a large rocket could give the car-sized probe enough speed to bust out of Earth’s orbit and veer in the direction of the Sun’s certain danger.
During its seven-year mission, the probe will revolve around Venus for seven gravity assists, which will boost the probe into closer and closer orbits around the Sun. All told, the probe will circle the Sun 24 times, with a top speed of about 430,000 miles per hour and a closest approach less than four million miles from the Sun’s surface—about double the speed and seven times closer than the prior records set by the Helios 2 spacecraft in 1976.
All things considered, the solar probe is pretty small—about 1,300 pounds and the size of a small car. Some of its instruments individually could fit inside a shoe box. Photo: NASA/Johns Hopkins APL/Ed Whitman
The solar probe itself is well equipped for protection against the Sun’s destructive powers, even at that close a range. A heat shield protects much of it from direct contact with the Sun’s strong rays. Mostly carbon composite, the thick shield bears a coating of white ceramic paint and crushed sapphire to reflect light and heat. Amazingly, instruments in the shadow of the shield don’t get much warmer than room temperature.
A cooling and heating system gives the solar probe consistent conditions even within a wildly fluctuating temperature range—from the beyond-boiling heat of the Sun to the frigid emptiness of outer space. Circulating water sheds extra heat through radiators, and the spacecraft can rotate to thaw instruments in the warmth of a distant Sun. Kasper says that the probe thus regulates its own temperature “just like a warm-blooded animal.”
The spacecraft flies with an autonomous correction system on board, designed to detect and respond immediately to any predicaments. Urgent problems would take mission control on Earth too long to even notice on a probe so far away, making the option of manual corrections impossible.
Keep It Together
Four sets of scientific instruments attached to the probe have been specially designed for the task of sending data back to Earth without becoming a blobby mess. One takes photos, a second measures electric and magnetic fields, and a third tracks energetic particles flung from the Sun.
As for the fourth set of instruments on the probe, Kasper leads the team in charge of scooping particles directly from the Sun’s roiling atmosphere. The instruments include what they call a Faraday cup, about the size and shape of a roll of packing tape, which can measure the speed, temperature, and direction of particles in the solar wind.
An early prototype of the Faraday cup. Amazingly, Kasper found through experiments that contact with the Sun will clean the cup, not destroy it; hot temperatures bake out the metal’s impurities, which improves how the instrument functions. Photo: Elizabeth Wason
A more primitive model of the Faraday cup has floated near Earth since its launch in the mid-1990s to intercept solar wind samples. By collecting the particles with the two Faraday cups stationed at two different points in space, Kasper and his lab hope to witness how the solar wind starts at the Sun before striking the Earth.
“We can take those measurements right at the source and compare them with what we see close to Earth, to learn something about how the wind evolves along the way,” says applied physics Ph.D. student Ben Alterman.
Data from the Faraday cup and the other instruments on the solar probe will be the first of their kind, incredible for their novelty. The hope is that this new information will give a better idea of how to spot worrisome wind with more advance warning, and to help solve other lingering mysteries about the Sun’s atmosphere and its whipping wind.
Test It Out
Earlier designs of the Faraday cup led to the hardier, more advanced instrument that Kasper hopes will survive the Sun’s punishing atmosphere. He has reason to believe that the Faraday cup on the solar probe will stay intact and take good data throughout the mission: He and the team hunted down the strongest materials on Earth to build the instrument, and they subjected it to brutal stress tests years in advance of the probe’s launch.
“Gold, steel, aluminum, or copper would vaporize at much lower temperatures,” Kasper says about materials commonly used in spacecraft. Nearer the Sun, “It’s not like, ‘Uh-oh, my instrument’s gotten goopy, and it’s not staying together.’ It’s more like the instrument is just gone.”
Kasper noted that high-heat objects, like rocket engine nozzles and nuclear reactor fuel rods, have materials in common, which come from a narrow range of refractory metals in obscure parts of the periodic table. Those materials informed the new design of the Faraday cup: An exotic alloy of molybdenum, titanium, and zirconium forms the cup. Synthetic sapphire crystals support other pieces in the instrument. Niobium metal tubes shield wires that run from the scoop of the Faraday cup to its electrical box.
Satisfied with the prototype, Kasper took it to a custom facility of giant mirrors in the French Pyrenees. All the reflective surfaces focused the Sun’s distant rays at the target, producing a blindingly hot beam to mimic a close approach.
But simulating the Sun nearer to home would be more convenient. The team bought old IMAX film projectors on eBay to stock their special lab in the United States. Improbably, xenon bulbs in the old projectors can heat up to about the temperature of the Sun’s surface (9,800 degrees Fahrenheit) and emit light that resembles the natural solar spectrum. Four IMAX projectors, all focused on the Faraday cup, could approximate the scorching heat and light that would bombard the probe as it plunges through the solar atmosphere. Pumping out all the oxygen in a chamber to form a vacuum, and shooting particles at the Faraday cup with an ion gun, completed the effect of contact with the Sun in outer space.
Four old IMAX projectors simulated the sun to help Kasper stress-test the Faraday cup. On the probe in outer space, the cup will change in temperature from more than 1,500 degrees Fahrenheit to 100 degrees below zero. Photo: Levi Hutmacher/Michigan Engineering
“We developed a facility that worked really well,” Kasper says, “but it took a ton of trial and error,” with plenty of broken glass and mangled equipment along the way.
After years of exhaustively applying stress tests, creative solutions, and contingency plans, the team launched Parker Solar Probe safely to space after just a few false starts at Cape Canaveral. What started out decades ago as pipe-dream plans finally became real. The probe will—and already has—set new historical records for closest encounter with the Sun and fastest object ever created.
As the probe completes its mission in the coming decade, researchers hope to have the data they need to figure out how to shield technology on Earth from solar-flare shock waves, why the solar atmosphere is so scalding hot, how solar wind accelerates to supersonic speeds, and what it takes to build a craft that won’t blister in the Sun.
When the time came to wake up the probe and watch for the first bits of data, the team wasn’t expecting to see anything interesting the very first time they switched on the sensors. “But then the instrument scientists suddenly said, ‘Hold on—it’s reporting that it’s tracking on the solar wind!’
“We looked more closely, and the signal got even stronger.” Which was crazy, Kasper says, because the spacecraft was still far from the Sun and facing away from its rays.
“So the fact that we saw the solar wind was very exciting,” he says, especially because the instruments had not even come close to their most sensitive vantage point. “And we knew that eventually we’d reach a point near the Sun where signals will be 16 times stronger.
“Clearly, this is a very well-functioning instrument,” Kasper says. “Now we just need it to, you know, not melt and all that.”