How NASA Probe Beat the Heat to Become First Spacecraft to Touch the Sun

NASA's Parker Solar Probe has become the first spacecraft to "touch the sun," passing through the blisteringly hot atmosphere of our star. These outer layers of the sun—known as the corona—are actually hotter than its surface, a 50-year-old mystery that the craft has been tasked with solving.

Launched in 2018, the Parker probe has this week reached the part of the corona known as the Alfvén critical surface, the edge of the sun's atmosphere where the solar winds begin.

This brought the craft to within 3.83 million miles of the sun's surface and meant the probe required protection against its intense heat, delivered by a relatively straightforward shield system.

The outermost layer of the corona has a temperature of around 2 million degrees Fahrenheit, whereas what this ball of hot plasma has as a surface—known as the photosphere layer—has a temperature of "only" 10,000 F. The photosphere is significantly cooler than the sun's core, estimated to have a temperature of around 27 million F.

Despite being hotter than the surface of the sun, the corona is usually not visible because the light from the photosphere drowns out its light.

Shining mainly in ultraviolet light and at extreme ultraviolet wavelengths, the corona of the sun is only visible during solar eclipses or using devices that block light from the photosphere.

"Flying so close to the sun, Parker Solar Probe now senses conditions in the magnetically dominated layer of the solar atmosphere – the corona – that we never could before," said Nour Raouafi, Parker project scientist at the Johns Hopkins Applied Physics Laboratory in Maryland. "We see evidence of being in the corona in magnetic field data, solar wind data, and visually in images.

"We can actually see the spacecraft flying through coronal structures that can be observed during a total solar eclipse."

Any mission that intends to move through the corona requires significant heat protection. The key to resisting the extreme conditions lies in the probe's heat shield and an automated system that protects it against powerful light emissions.

Heat Shield Parker Solar Probe
A schematic of the heat shield of the Parker Solar Probe. The shield has been put to the test as the craft it protects became the first to "touch the sun." Johns Hopkins/GREG STANLEY / OFFICE OF COMMUNICATIONS

Known as the Thermal Protection System, the shield is 8 feet wide and just 4.5 inches thick. Despite its relative thinness, the carbon composite foam sandwiched between two carbon plates keeps the spacecraft body at a safe temperature of 85F. The sun-facing side heat shield is coated with a white ceramic paint to reflect much sunlight as possible.

The Parker Solar Probe also has a cooling system composed of a gallon of deionized water. This was chosen as a chemical coolant thanks to its ability to handle a wide range of temperatures. This water is kept highly pressurized, raising its boiling point to over 257 F.

These allow the craft to weave through the atmosphere of our star, sampling the particles that exist there and the powerful magnetic field that many researchers believe is responsible for coronal heating.

In addition to revealing more information about our own sun and solar system than any other mission in the past, the Parker Solar Probe could also help astronomers and astrophysicists understand other stars.

"Parker Solar Probe 'touching the sun' is a monumental moment for solar science and a truly remarkable feat," said Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate. "Not only does this milestone provide us with deeper insights into our sun's evolution and its impacts on our solar system, but everything we learn about our own star also teaches us more about stars in the rest of the universe."

Parker Solar Probe
A illustration of the Parker Solar Probe as it approaches the sun. This week the craft became the first to "touch the sun," braving the heat of the corona—the sun's outer layer. Johns Hopkins