Last summer people across North America were captivated by one of nature’s most spectacular phenomena: a total solar eclipse. Some traveled thousands of miles to witness the blotting out of the sun’s light, if only for a few moments. When the moon passes in front of our star, we can glimpse a rare sight: the outer atmosphere of the sun.
Known as the corona, this shimmering haze encircling our star is the target of NASA’s next space mission. In the predawn hours of August 11 the Parker Solar Probe will launch on a first-of-its-kind journey to reach out and touch the sun. The spacecraft will fly through our star’s outer atmosphere, scooping up particles and taking measurements that scientists hope will solve the mystery of how the sun’s corona gets so hot.
Perched atop a Delta 4–Heavy rocket, the Parker Solar Probe will race up from Earth and, with seven gravity assists from Venus (and a boost from the rocket’s special upper stage), become the fastest spacecraft ever flown, reaching a top speed of 430,000 miles per hour at its closest approach to the sun in December 2024. That would be fast enough to travel between New York and Los Angeles in just 20 seconds. As part of a carefully choreographed orbital ballet, the probe will complete 24 orbits around our star over some eight years, getting as close as a mere 3.8 million miles from the sun’s surface—seven times closer than any other spacecraft in history and well within the coronal boundary, which extends somewhere between 15 to 20 solar radii (or some 6.4 million to 8.6 million miles) out from its surface. From this vantage point the probe will use its set of four instrument suites to analyze solar particles and plasma as well as electric and magnetic fields within the corona.
Our host star is a fiery enigma: Heat is generated in the sun’s core and radiates outward, yet the sun’s outer atmosphere is nearly 300 times hotter than its surface. This fact seems to break the very laws of thermodynamics—the farther away you are from the fireplace, the colder you get, right? Not in the corona. “The sun is the primary puzzle in the universe,” says astrophysicist Eugene Parker, a professor emeritus at the University of Chicago and the spacecraft’s namesake. Because it is the one star we can study up-close, understanding it helps us better understand all stars.
The mission will aim to answer three questions that have befuddled astrophysicists for decades: The first is what mechanism is responsible for the corona’s heat. Scientists suspect the answer has to do with the sun’s magnetic field. The magnetic field lines store and stir up energy in the photosphere (which we perceive as the sun’s surface) and release it into the star’s atmosphere, says Nicholeen Viall, an astrophysicist at NASA Goddard Space Flight Center. But how and when this release happens remains a mystery. “If we can figure that out,” she says, “that’s a pretty fundamental physical problem we’ve solved about the universe in general.”
The probe's next task is to study what scientists call the solar wind—a slew of charged particles the sun spews into space. This outflow traverses the solar system, bathing every planet, moon, asteroid and comet in radiation. The spacecraft will fly right through the action, shining a light on the mysterious processes that generate the solar wind. First proposed by Parker in 1958, the wind still puzzles scientists with its counterintuitive behavior: Instead of fizzling out the farther it gets from the sun, the solar wind actually picks up speed and somehow transitions from a steady breeze to a supersonic outflow racing away from the corona at millions of miles an hour.
The solar wind quandary and mystery of the corona are connected, Viall says. “You don’t get a solar wind if you don’t have a hot corona in the first place.” The Parker Probe’s close-up observations of the corona and the solar wind are expected to revolutionize the field of heliophysics—the physics of the sun. An argument over the source of this extra heat has simmered for decades. But experts agree on one thing: It probably begins in the convection zone, just below the photosphere. As the sun’s plasma flows through magnetic field lines, it can create electrical currents that in turn create more magnetic fields. These lines sometimes get tangled up and eventually snap, releasing large amounts of energy into the surrounding plasma in the form of a solar flare. Coupled with a barrage of smaller explosions (called nanoflares, each as powerful as a 50-megaton hydrogen bomb) on the sun’s surface, they could account for the corona’s high temperature.
Finally, the probe will investigate a pesky phenomenon called space weather. From Earth the sun appears as a serene glowing orb, but in reality it is like a pouty child, continually throwing tantrums. During these fits of rage our star occasionally burps bursts of radiation and knots of plasma into space. The most extreme of these belches, known as coronal mass ejections (CMEs), can damage vital systems such as power grids and communications satellites, and also could zap astronauts in space with harmful doses of radiation. Although our planet’s magnetic field shields us from much of the sun’s fury, its protective effects can get overwhelmed at times. The probe’s measurements will show us where in the sun CMEs are born, potentially leading to improved forecasts for potentially dangerous space weather events. “We are going to see how what happens on the sun transitions into what we see and experience here on Earth,” says Nicky Fox, a Parker Probe project scientist at the Johns Hopkins Applied Physics Laboratory.
In exploring the full force of the sun’s searing heat and radiation, the spacecraft will help us understand our own stellar host as well as others throughout the universe. “The solar probe is going to a region of space that has never been explored before,” Parker says. “It’s very exciting that we’ll finally get a look.”