Step aside, Icarus: NASA has made a spacecraft that can fly through the Sun’s atmosphere without melting.
On 4 August, if all goes to plan, the US$1.5-billion Parker Solar Probe will lift off from a launch pad at Florida’s Cape Canaveral. Just three months later, it will whiz far closer to the Sun than any spacecraft has ever come, to take the first-ever direct measurements of the star's maelstrom of energy.
But that's just the beginning. Over the next 7 years, the craft will loop around the Sun 23 more times, passing nearer and nearer — ultimately flying about 6.2 million kilometres above the surface, well within the solar corona. That’s nearly seven times closer than the record mark set by the German Helios 2 spacecraft in 1976.
The Parker Solar Probe aims to answer some of the biggest outstanding questions about the Sun, such as how its corona is heated to millions of degrees while the surface beneath it stays relatively cool1. The spacecraft will also visit the birthplace of the solar wind, a flood of energetic particles that streams outward into the Solar System at speeds of up to 800 kilometres a second. When the solar wind slams into Earth, it generates beautiful polar aurorae, but it can also disrupt satellite communications and navigation systems.
“We’re going to be right where all the interesting stuff happens,” says Nicola Fox, a solar physicist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, and the mission’s project scientist.
Data from the deep-diving probe should allow researchers to better understand the complex picture of how particles, magnetic fields and energy combine in the Sun. “This is going to be such a game-changer,” says Nicholeen Viall, a solar physicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Surfing the solar wind
Space physicists have dreamed of a mission that would fly through the solar corona, or at least travel inside the orbit of Mercury, the innermost planet, since 1958. That same year, Eugene Parker — the University of Chicago physicist for whom the probe is named — first proposed the existence of the solar wind2.
After decades on the drawing board, the mission is finally approaching launch. Eight weeks after lift-off, it will fly past Venus, using the planet’s gravity to slow down and slip into a tighter orbit around the Sun. Five weeks after that, on 3 November, the probe will make its first close approach — at more than 24 million kilometres, or 35 times the solar radius, from its surface.
From there, the spacecraft will loop around the Sun, drawing gradually closer as it flies past Venus 6 more times. That trajectory will give the probe ample time to gather data, says Yanping Guo, an engineer at APL who designed the mission trajectory.
Somewhere between the first close approach (at 35 solar radii) and its final ones (within 10 solar radii) the probe will encounter the Alfvén surface, a boundary where the solar wind becomes supersonic. Inside the Alfvén surface, the Sun’s magnetic field dominates; outside, the solar wind is more detached and streams away on its own.
Crossing that boundary with a spacecraft will be similar, symbolically, to the moment when the Voyager 1 probe entered interstellar space in 2012, says Justin Kasper, a physicist at the University of Michigan in Ann Arbor who has studied the Alfvén transition3. The moment will mark humanity’s passage to another realm in the Solar System. “I’m confident that something special will happen,” he says.
The boundary might be more complicated than previously thought. A recent analysis of images of the outer corona taken by the STEREO spacecraft in 2014 reveals that the Alfvén surface might be more of a broadly, poorly defined zone that contains complicated magnetic structures. That suggests that the Parker Solar Probe will have the chance to measure a new and previously unexpected border zone. “It’s far more wild and woolly than we would have expected,” says Craig DeForest, a solar physicist at the Southwest Research Institute in Boulder, Colorado. He led the team behind the analysis, which was published on 18 July in the Astrophysical Journal4.
The Parker Solar Probe bristles with an array of instruments designed to sample the corona directly. Protecting them is a 2.4-metre-wide heat shield made of 11-centimetre-thick carbon foam sandwiched between layers of carbon composite. It can withstand temperatures of nearly 1,400 °C.
Solar panels power the spacecraft, but to keep them cool they have a water-tubing system similar to a car’s radiator. During the searing conditions of close approach, most of the solar panels will fold back to shelter in the heat shield’s shade.
Staring at the Sun
Mission scientists hope that the Parker Solar Probe will kick off a new era of studying the Sun. In 2020, the European Space Agency plans to launch its Solar Orbiter spacecraft, which will study the Sun at higher latitudes and from a more distant point in space than the Parker Solar Probe will. Also by 2020, the Daniel K. Inouye Solar Telescope will come online in Hawaii, where it will make daily maps of the solar corona.
For his part, the 91-year-old Parker says that he is looking forward to seeing the waves and turbulence in the solar wind — which he predicted — measured by the probe that bears his name. “I expect to find some surprises,” he says.
Nature 559, 452-453 (2018)