Why the Spacecraft That Touched the Sun Didn’t Melt
The Parker Solar Probe was the first to enter the Sun’s corona, where temperatures are well over a million degrees. It emerged unscathed.
On April 28, 2021, after 8 passes through the solar system at more than 350,000 mph, the Parker Solar Probe made history by being the first manmade object to officially cross the Alfvén Critical Surface, the official boundary of the Sun’s atmosphere, and emerge intact to transmit data back to Earth. Temperatures in the outermost layer of the Sun are certainly enough to melt the probe, but its sensitive internal electronics were kept at a cool 85 degrees Fahrenheit.
This mission marked a turning point for science, as the data collected has broadened our understanding of the solar wind, the energetic stream of particles emanating from the Sun, which was first seriously studied by Eugene Newman Parker, professor emeritus at the University of Chicago, for which the probe is named. It might also illuminate other solar mysteries, such as why the Sun’s corona is so much hotter than the layers below it, something that seemingly shouldn’t be possible. The Parker Solar Probe has plunged into the Sun several times since April, with many more missions scheduled out to its 25th in December 2025, each one putting it deeper into the solar atmosphere than before.
How is any of this possible? How did the probe and its sensitive electronics survive such extreme temperatures?
Heat is Different Than Temperature
First of all, heat and temperature are not the same thing. Although we tend to use the terms interchangeably, temperature is merely a measurement of the average kinetic energy of particles, while heat is the actual transferring of energy. In other words, temperature is a concept invented by humans to explain how hot or cold something is, but heat is a physical phenomenon, and for something to experience a temperature change, energy needs to be transferred through heat.
For example, if I go outside on a hot day, air particles collide with me and transfer their energy to me, causing me to feel hotter. It’s only through interacting with the air particles does my temperature go up, and if the particles don’t hit me, then my temperature doesn’t change. Of course, this is impossible for a human in Earth’s atmosphere near the surface.
However, in the Sun’s corona the story is a bit different. Although we can measure it’s temperature to be in the millions of degrees, the highly energetic solar particles aren’t very dense. This means that when the Parker Solar Probe passed through, it didn’t interact with many solar particles and not much energy was transferred, leaving its temperature far short of the corona’s temperature. In other words, if I walk outside on a hot day but the atmosphere is as sparse as the Sun’s corona, I wouldn’t feel the heat because there wouldn’t be enough particles to transfer the energy.
Despite this, the outside of the probe still reached temperatures of 2,500 °F (1,370 °C), roughly the melting point of steel, prompting NASA to devise some clever methods to help the probe survive.
Reinforced Carbon-Carbon Heat Shield and Artificial Intelligence
Carbon is amazing. Because of its 4 valence electrons, it tends to form strong covalent bonds with other carbon atoms, allowing long, stable chains to form. This is why it’s the basis of life on Earth, and it’s why the space industry and many other cutting-edge industries love it.
NASA has been using something called reinforced carbon-carbon for years, most notably on the nose cone of the space shuttle and now the heat shield of the Parker Solar Probe. This substance is made of tiny carbon filaments, chains of carbon atoms, suspended in a matrix of graphite, a crystalline form of carbon. This unique composite has low thermal expansion, meaning it can retain its properties in high temperatures; can withstand up to 700 MPa, making it 5 times stronger than steel; has low thermal conductivity so it doesn’t transfer heat quickly; is an excellent shied of electromagnetic radiation; and has a low weight, with a density somewhere between 1.6–1.98 grams per cubic centimeter.
Because of all these reasons, the heat-shield (also known as the Thermal Protection System), with a coating of white ceramic paint, only needs to have a diameter of 8 feet (2.4 meters) and a thickness of 4.5 inches (about 115 mm) is enough to absorb the few energetic particles beaming out of the Sun.
NASA is also making use of advanced artificial intelligence to orient the Parker Solar Probe so that the heat shield is always protecting the probe. Behind the heat shield are solar sensors. If these get illuminated, the AI system is able to understand the current position of the probe, it’s optimal position, and how to correct it. NASA engineers are unable to interact the with probe in real time, as the Sun is roughly 8 light-minutes away.
The Cup and Antennae in the Wind
Not everything is behind the heat-shield and is directly subjected to whatever the Sun can dish out. For example, the Solar Probe Cup is what is known as a Faraday Cup, so it can’t hide because its job is to measure the flow of ions and electrons in the solar wind as they pass through an electric field. To overcome the extreme heat, the cup itself is composed of sheets of a titanium-zirconium-molybdenum alloy, which has a melting over 4,200 degrees Fahrenheit, and the grid producing the electric field is composed of tungsten, which can withstand temperatures well over 6000 degrees Fahrenheit. To protect the wiring, NASA made them out of niobium and then grew sapphire crystals to encase them.
Likewise, the FIELDS instrument suite can’t bask in the shadow of the heat-shield. 4 of its 5 antennas stick out past the heat-shield to measure the Sun’s complicated electric fields, while the 5th is perpendicular to the others to create a 3D image. These are made out of a niobium alloy with a melting point well above that needed to survive.
To test both the cup and the antennae, researchers used particle accelerators to mimic the stream of particles coming out of the Sun and IMAX projectors tweaked to increase their output and the Odeillo Solar Furnace’s 10,000 focused mirrors to replicate the Sun’s heat output.
With missions scheduled out to 2025, the Parker Solar Probe to set to complete 25 passes through the Sun. Let’s hope NASA designed it well enough to survive all of them and fulfill its purpose of delving into the Sun’s many mysteries.
Help support independent journalism by clicking the links below:
