Traveling to the sun: Why won’t Parker Solar
Video: NASA’s Parker Solar Probe flies to the sun. Why won’t the spaceship melt? Thermal protection systems engineer Betsy Congdon (Johns Hopkins APL) explains why Parker can withstand the heat. Download this video in HD formats: https://svs.gsfc.nasa.gov/12867
Photo credit: NASA’s Goddard Space Flight Center
NASA’s Parker Solar Probe will launch this summer to travel closer to the sun, deeper into the solar atmosphere than any other mission before. With the earth on one end of a meter stick and the sun on the other, Parker Solar Probe can get within four inches of the sun’s surface.
Within this portion of the solar atmosphere, a region known as the corona, Parker Solar Probe will provide unprecedented observations of what powers the wide range of particles, energy, and heat flowing through the region – particles out into and into the solar system the distance fling past Neptune.
Of course, it is unimaginably hot inside the Corona. The spacecraft will travel through material with temperatures in excess of a million degrees Fahrenheit while bombarded with intense sunlight.
So why won’t it melt?
Parker Solar Probe is designed to withstand the extreme conditions and temperature fluctuations required for the mission. The key lies in its bespoke heat shield and an autonomous system that protects the mission from the intense light emission from the sun, but allows the coronal material to “touch” the spacecraft.
The science behind why it doesn’t melt
One key to understanding what is keeping the spacecraft and its instruments safe is understanding the concept of heat versus temperature. Contrary to intuition, high temperatures do not always result in another object actually being heated.
In space, the temperature can be thousands of degrees without any particular object heating up significantly or feeling hot to the touch. Why? Temperature measures how fast particles are moving, while heat measures the total amount of energy they transfer. Particles can move quickly (high temperature), but when very few of them are present, they don’t transfer much energy (low heat). Since space is mostly empty, there are very few particles that can transfer energy to the spacecraft.
For example, the corona that Parker Solar Probe flies through is extremely high in temperature but very low density. Think of the difference between putting your hand in a hot oven and putting it in a saucepan of boiling water (don’t try this at home!) – your hand can withstand significantly hotter temperatures longer in the oven than in the water where it comes with many other particles must interact. Similarly, the corona is less dense compared to the Sun’s visible surface, so the spacecraft interacts with less hot particles and does not receive as much heat.
This means that while the Parker Solar Probe will travel through space with temperatures of several million degrees, the surface of the heat shield facing the sun will only be heated to about 2,500 degrees Fahrenheit (approx. 1,400 degrees Celsius).
The shield that protects it
Of course, thousands of degrees Fahrenheit are still fantastically hot. (To compare, lava from volcanic eruptions can range from 700 to 1200 ° C.) To withstand this heat, Parker Solar Probe uses a heat shield known as the Thermal Protection System (TPS) and 8. tall is feet (2.4 meters) in diameter and 4.5 inches (about 115 mm) thick. Those few inches of protection mean the body of the spacecraft sits right on the other side of the shield at a comfortable 85 F (30 C) .
The TPS was developed by the Johns Hopkins Applied Physics Laboratory and built at Carbon-Carbon Advanced Technologies using a carbon composite foam sandwiched between two carbon plates. This lightweight insulation is accompanied by a white ceramic paint on the sun visor to reflect as much heat as possible. Tested to withstand up to 3,000 F (1,650 C), the TPS can handle any heat the sun can send in its path and will keep almost all instruments safe.
The cup that measures the wind
But not all Solar Parker Probe instruments will be behind the TPS.
The Solar Probe Cup protrudes over the heat shield and is one of two Parker Solar Probe instruments that are not protected by the heat shield. This instrument is a so-called Faraday cup, a sensor that measures the ion and electron flows as well as the flow angles of the solar wind. Due to the intensity of the solar atmosphere, unique technologies had to be developed to ensure that not only could the instrument survive, but that the electronics on board could also send back accurate readings.
The cup itself is made of sheets of titanium-zirconium-molybdenum, a molybdenum alloy with a melting point of around 4,260 F (2,349 C). The chips that create an electric field for the Solar Probe Cup are made of tungsten, a metal with the highest known melting point of 6,192 F (3,422 C). Lasers are typically used to etch the grid lines in these chips – however, due to the high melting point, acid had to be used instead.
Another challenge was the electronic cabling – most cables would melt from the heat radiation in such close proximity to the sun. To solve this problem, the team grew sapphire crystal tubes to hang the wiring and made the wires from niobium.
To ensure that the instrument is prepared for the harsh environment, the researchers had to mimic the intense heat radiation of the sun in a laboratory. To create a test-worthy level of heat, the researchers used a particle accelerator and IMAX projectors – which were manipulated by a jury to raise their temperature. The projectors mimicked the heat of the sun while the particle accelerator exposed the cup to radiation to ensure that the cup could measure the accelerated particles under the intense conditions. To be absolutely sure that the Solar Probe Cup can withstand the harsh environment, the Odeillo Solar Furnace – which concentrates the solar heat through 10,000 adjustable mirrors – was used to test the cup against the intense solar radiation.
The Solar Probe Cup passed its tests with flying colors – in fact, it performed better and better and delivered clearer results the longer it was exposed to the test environments. “We believe the radiation cleared any contamination,” said Justin Kasper, lead researcher for the SWEAP instruments at the University of Michigan at Ann Arbor. “It basically cleaned itself up.”
The spaceship that stays cool
Several other space probe designs protect the Parker Solar Probe from the heat. Without protection, the solar panels that use the energy from the star under study to power the spacecraft can overheat. With every approach to the sun, the solar cells retreat behind the shadow of the heat shield and only leave a small segment exposed to the intense solar radiation.
But more protection is needed this close to the sun. The solar systems have a surprisingly simple cooling system: a heated tank that protects the coolant from freezing during start-up, two coolers that protect the coolant from freezing, aluminum fins to maximize the cooling surface, and pumps that circulate the coolant permit. The cooling system is powerful enough to cool an average sized living room and keeps the solar cells and instruments cool and functional while standing in the heat of the sun.
Which coolant is used for the system? Approximately one gallon (3.7 liters) of deionized water. Although there are many chemical coolants, 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 these areas like water. To prevent the water from boiling at the high end of temperatures, it is pressurized so that the boiling point is above 257 F (125 ° C).
Another problem with protecting a spacecraft is figuring out how to communicate with it. Parker Solar Probe will be largely alone on its journey. It takes an easy eight minutes to reach Earth – that is, if engineers had to control the spaceship from Earth, it would be too late if something went wrong to correct it.
So the spacecraft is designed to keep itself safe and on course to the sun. Several sensors, about half the size of a cell phone, are attached to the body of the spacecraft along the shadow edge of the heat shield. When one of these sensors detects sunlight, they alert the central computer and the spacecraft can correct its position to safely protect the sensors and the rest of the instruments. All of this has to be done without human intervention, so the central computer software has been programmed and extensively tested to ensure that all corrections can be made in the blink of an eye.
Start towards the sun
Once launched, Parker Solar Probe detects the position of the sun, aligns the heat shield with it, and continues its journey for the next three months to embrace the sun’s heat and protect itself from the cold vacuum of space.
Over the course of seven years of planned mission duration, the spacecraft will make 24 orbits around our star. Every time it approaches the sun, it will sense the solar wind, examine the solar corona, and provide unprecedented close-ups around our star – and armed with its many innovative technologies, we know it stays cool all the time.