The flight of Icarus is one of the most infamous tales in Greek mythology. In it, Daedalus and his overly-proud son Icarus are imprisoned on the Greek island of Crete. Ever the master craftsman, Daedalus constructed two sets of wings made of wax so that he and Icarus could escape the island prison. However, Icarus ignored his father’s warnings not to fly too close to the Sun and he tumbled into the sea as his wings melted.
Although a lesson in the dangers of hubris, the tale of Icarus illustrates the ancient desire within humans to know more about the mighty star that we call the Sun. Human understanding of the Sun had advanced since then, but many questions remain regarding the star that is our home. In an attempt to answer some of these questions, a team from NASA’s Goddard Space Flight Center and Johns Hopkins University Applied Physics Laboratory launched a mission in 2018 called the Parker Solar Probe to travel closer to the Sun than any human-made object before.
While technology has developed exponentially since the days of ancient Greece, the team responsible for the Parker Solar Probe still faced the same problem as Icarus — how does one get close to the Sun without burning under the extreme heat? The answer is the revolutionary Thermal Protection System that uses a four-inch thick heat shield to protect the car-sized Solar Probe.
Similar to how beachgoers will use umbrellas to shield themselves from the Sun, the heat shield is part of the Thermal Protection System that protects the instruments on the Parker Solar Probe. When closest to the Sun, the Solar Probe will be traveling through the Sun’s corona, or it’s outermost layer. The corona can reach temperatures up to 3 million degrees Fahrenheit.
However, the particles of plasma that make up the corona are spread so far apart that the Solar Probe will actually be experiencing temperatures up to 2500 degrees Fahrenheit. This may pale in comparison to 3 million degrees, it is still hotter than the temperature of lava as it erupts from a volcano.
An example of how this works can be found in your kitchen. For example, if you stick your hand in a hot oven, you will not actually feel the full heat of the oven. However, as soon as you hit a tray or the side of the oven, you will quickly feel the burn. In a similar fashion, the Parker Solar Probe is only touching a limited number of plasma particles, so it does not feel the full temperature of the solar corona.
The actual shield is built similarly to a sandwich. However, only if your sandwich was made of two panels of superheated carbon-carbon composite with a carbon foam core for a filling. The carbon-carbon composite is similar to the material used to attach the heads of golf clubs to the shaft, only it has been superheated. If you are planning on building a spacecraft, this would be the material that you would want to use because it allows for the creation of structures that are lightweight but require high stiffness and high loads.
Even the color of the panel that faces the Sun plays an important role in protecting the Solar Probe. The panel is painted a specially-designed shade of white that was uniquely made for the Solar Probe and assists by reflecting as much light as possible. It works the same way as wearing a white shirt will keep you cooler on a hot day than a black one does.
“We obviously would like to reflect as much light as possible, thus, limiting the amount of heat deposited,” said Dr. Adam Szabo, the Parker Solar Probe Mission Scientist at NASA Goddard Space Flight Center, “but it is unavoidable that quite a bit of heat will be heated up.”
When working in the solar corona, “quite a bit of heat” can still spell disaster for fragile instruments and create a temperature of up to 600 degrees Fahrenheit. This is where the meat of the sandwich, or the porous foam core, plays its role in absorbing the heat that manages to evade the front panel of the shield. The foam core is 97 percent air, so when heat hits it, it has very little room to continue spreading. Instead, it undergoes a process called conduction where it travels through the foam and then radiates off from the edges and back into space.
Underneath that core lies a final layer that connects the shield to the bus of the spacecraft that holds most of the instruments. Software on the Solar Probe senses its position about the Sun so that the instruments onboard follow the important rule that all children are taught — never look directly into the Sun. Finally, with all of these protective measures in place, the instruments on the Parker Solar Probe comfortably enjoy their trip to the Sun and only experience temperatures just above room temperature at 85 degrees Fahrenheit.
Editor’s Note: Vicky Woodburn is an intern at NASA Goddard Space Flight Center (GSFC). The research and interviews for this article were collected for a class while she was a student and prior to her employment with NASA GSFC. All views and opinions presented in this article belong to the author alone, and do not necessarily reflect the official policy or position of any agency of the U.S. government or any institution to which the author is or was affiliated.