Traveling to the Sun -- Why the Parker Solar Probe Won’t Melt
The Parker Solar Probe circling the Sun -- At closest approach, Parker Solar Probe will hurtle around the Sun at approximately 430,000 mph (700,000 kph). That's fast enough to get from Philadelphia to Washington, DC, in one second. At closest approach to the Sun, the front of Parker Solar Probe's solar shield will face temperatures approaching 2500 degrees F (1377 degrees C), yet he spacecraft's payload will be near room temperature. On the final three orbits, Parker Solar Probe will fly to within 3.8 million miles of the Sun's surface, more than seven times closer than the current record holder for a close solar pass, the Helios 2 spacecraft, which came within 27 million miles in 1976. For comparison, Mercury is, on average, about 36 million miles from the Sun. (Credit: Susannah Darling, NASA Headquarters - Washington DC) (Image Credit: NASA, Johns Hopkins APL, Steve Gribben)
Traveling to the Sun -- Why the Parker Solar Probe Won’t Melt
This summer, NASA’s Parker Solar Probe will launch on a United Launch Alliance Delta IV Heavy rocket from Cape Canaveral Air Force Station in Florida. It will travel closer to the Sun and deeper into the solar atmosphere than any mission before it. Over the course of seven years of planned mission duration, the spacecraft will make 24 orbits of the Sun. On each close approach, it will sample the solar wind, study the Sun’s corona, and provide unprecedented close-up observations. On the final three orbits, Parker Solar Probe will fly to within 3.8 million miles of the Sun's surface. For comparison, Mercury is, on average, about 36 million miles from the Sun and Earth’s average distance is 93 million miles. Inside the part of the solar atmosphere known as the corona, the spacecraft will travel through material with temperatures greater than a million degrees Fahrenheit while being bombarded with intense sun light. Yet it won’t melt… Here’s why.
So, Why Won’t It Melt?
Parker Solar Probe has been designed to withstand the extreme conditions and temperature fluctuations for the mission. The key lies in its custom heat shield and an autonomous system that helps protect the mission from the Sun’s intense light emission, but does allow the coronal material to “touch” the spacecraft.
One key to understanding what keeps the spacecraft and its instruments safe, is understanding the concept of heat versus temperature. Counter-intuitively, high temperatures do not always translate to actually heating another object.
In space, the temperature can be thousands of degrees without providing significant heat to a given object or feeling hot. Why? Temperature measures how fast particles are moving, whereas heat measures the total amount of energy that they transfer. Particles may be moving fast (high temperature), but if there are very few of them, they won’t transfer much energy (low heat). Since space is mostly empty, there are very few particles that can transfer energy to the spacecraft.
The corona through which Parker Solar Probe flies, for example, has an extremely high temperature but very low density. Think of the difference between putting your hand in a hot oven versus putting it in a pot of boiling water (don’t try this at home!) — in the oven, your hand can withstand significantly hotter temperatures for longer than in the water where it has to interact with many more particles. Similarly, compared to the visible surface of the Sun, the corona is less dense, so the spacecraft interacts with fewer hot particles and doesn’t receive as much heat.
That means that while Parker Solar Probe will be traveling through a space with temperatures of several million degrees, the surface of the heat shield that faces the Sun will only get heated to about 2500 degrees Fahrenheit (about 1400 degrees Celsius).
The Shield that Protects It
Of course, thousands of degrees Fahrenheit is still fantastically hot. (For comparison, lava from volcano eruptions can be anywhere between 1300 and 2200 F (700 and 1200 C) And to withstand that heat, Parker Solar Probe makes use of a heat shield known as the Thermal Protection System, or TPS, which is 8 feet (2.4 meters) in diameter and 4.5 inches (about 115 mm) thick. Those few inches of protection mean that just on the other side of the shield, the spacecraft body will sit at a comfortable 85 F (30 C).
The TPS was designed by the Johns Hopkins Applied Physics Laboratory, and was built at Carbon-Carbon Advanced Technologies, using a carbon composite foam sandwiched between two carbon plates. This lightweight insulation will be accompanied by a finishing touch of white ceramic paint on the sun-facing plate, to reflect as much heat as possible. Tested to withstand up to 3000 F (1650 C), the TPS can handle any heat the Sun can send its way, keeping almost all instrumentation safe.
The Cup that Measures the Wind
But not all of the Solar Parker Probe instruments will be behind the TPS. Poking out over the heat shield, the Solar Probe Cup is one of two instruments on Parker Solar Probe that will not be protected by the heat shield. This instrument is what’s known as a Faraday cup, a sensor designed to measure the ion and electron fluxes and flow angles from the solar wind. Due to the intensity of the solar atmosphere, unique technologies had to be engineered to make sure that not only can the instrument survive, but also the electronics aboard can send back accurate readings.
The cup itself is made from sheets of Titanium-Zirconium-Molybdenum, an alloy of molybdenum, with a melting point of about 4260 F (2349 C). The grids that produce an electric field for the Solar Probe Cup are made from tungsten, a metal with the highest known melting point of 6192 F (3422 C). Normally lasers are used to etch the gridlines in these grids — however due to the high melting point acid had to be used instead.
Another challenge came in the form of the electronic wiring — most cables would melt from exposure to heat radiation at such close proximity to the Sun. To solve this problem, the team grew sapphire crystal tubes to suspend the wiring, and made the wires from niobium.
To make sure the instrument was ready for the harsh environment, the researchers needed to mimic the Sun’s intense heat radiation in a lab. To create a test-worthy level of heat, the researchers used a particle accelerator and IMAX projectors — jury-rigged to increase their temperature. The projectors mimicked the heat of the Sun, while the particle accelerator exposed the cup to radiation to make sure the cup could measure the accelerated particles under the intense conditions. To be absolutely sure the Solar Probe Cup would withstand the harsh environment, the Odeillo Solar Furnace — which concentrates the heat of the Sun through 10,000 adjustable mirrors — was used to test the cup against the intense solar emission.
The Solar Probe Cup passed its tests with flying colors — indeed, it continued to perform better and give clearer results the longer it was exposed to the test environments. “We think the radiation removed any potential contamination,” Justin Kasper, principal investigator for the SWEAP instruments at the University of Michigan in Ann Arbor, said. “It basically cleaned itself.”
The Spacecraft that Keeps its Cool
Several other designs on the spacecraft keep Parker Solar Probe sheltered from the heat. Without protection, the solar panels — which use energy from the very star being studied to power the spacecraft — can overheat. At each approach to the Sun, the solar arrays retract behind the heat shield’s shadow, leaving only a small segment exposed to the Sun’s intense rays.
But that close to the Sun, even more protection is needed. The solar arrays have a surprisingly simple cooling system: a heated tank that keeps the coolant from freezing during launch, two radiators that will keep the coolant from freezing, aluminum fins to maximize the cooling surface, and pumps to circulate the coolant. The cooling system is powerful enough to cool an average sized living room, and will keep the solar arrays and instrumentation cool and functioning while in the heat of the Sun.
The coolant used for the system? About a gallon (3.7 liters) of deionized water. While plenty of chemical coolants exist, the range of temperatures the spacecraft will be exposed to varies between 50 F (10 C) and 257 F (125 C). Very few liquids can handle those ranges like water. To keep the water from boiling at the higher end of the temperatures, it will be pressurized so the boiling point is over 257 F (125 C).
Another issue with protecting any spacecraft is figuring out how to communicate with it. Parker Solar Probe will largely be alone on its journey. It takes light eight minutes to reach Earth — meaning if engineers had to control the spacecraft from Earth, by the time something went wrong it would be too late to correct it.
So, the spacecraft is designed to autonomously keep itself safe and on track to the Sun. Several sensors, about half the size of a cell phone, are attached to the body of the spacecraft along the edge of the shadow from the heat shield. If any of these sensors detect sunlight, they alert the central computer and the spacecraft can correct its position to keep the sensors, and the rest of the instruments, safely protected. This all has to happen without any human intervention, so the central computer software has been programmed and extensively tested to make sure all corrections can be made on the fly.
Journey to the Sun
After launch, Parker Solar Probe will detect the position of the Sun, align the thermal protection shield to face it and continue its journey for the next three months, embracing the heat of the Sun and protecting itself from the cold vacuum of space.
Over the course of seven years of planned mission duration, the spacecraft will make 24 orbits of our star. On each close approach to the Sun it will sample the solar wind, study the Sun’s corona, and provide unprecedentedly close up observations from around our star — and armed with its slew of innovative technologies, we know it will keep its cool the whole time.
In order to unlock the mysteries of the Sun's atmosphere, Parker Solar Probe will use Venus’ gravity during seven flybys over nearly seven years to gradually bring its orbit closer to the Sun. The spacecraft will fly through the Sun’s atmosphere as close as 3.8 million miles to our star’s surface, well within the orbit of Mercury and more than seven times closer than any spacecraft has come before. (Earth’s average distance to the Sun is 93 million miles.)
Flying into the outermost part of the Sun's atmosphere, known as the corona, for the first time, Parker Solar Probe will employ a combination of in-situ measurements and imaging to revolutionize our understanding of the corona and expand our knowledge of the origin and evolution of the solar wind. It will also make critical contributions to our ability to forecast changes in Earth's space environment that affect life and technology on Earth.
Parker Solar Probe will perform its scientific investigations in a hazardous region of intense heat and solar radiation. The spacecraft will fly close enough to the Sun to watch the solar wind speed up from subsonic to supersonic, and it will fly though the birthplace of the highest-energy solar particles.
To perform these unprecedented investigations, the spacecraft and instruments will be protected from the Sun’s heat by a 4.5-inch-thick (11.43 cm) carbon-composite shield, which will need to withstand temperatures outside the spacecraft that reach nearly 2500 F (1377 C).
The Science of the Sun
The primary science goals for the mission are to trace how energy and heat move through the solar corona and to explore what accelerates the solar wind as well as solar energetic particles. Scientists have sought these answers for more than 60 years, but the investigation requires sending a probe right through the 2500 degrees Fahrenheit heat of the corona. Today, this is finally possible with cutting-edge thermal engineering advances that can protect the mission on its dangerous journey. Parker Solar Probe will carry four instrument suites designed to study magnetic fields, plasma and energetic particles, and image the solar wind.
Teaming for Success
Parker Solar Probe is part of NASA’s Living With a Star program to explore aspects of the Sun-Earth system that directly affect life and society. The Living With a Star flight program is managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland, for NASA’s Science Mission Directorate in Washington. The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, manages the mission for NASA. APL is designing and building the spacecraft and will also operate it.
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