In 10 days time, NASA's Parker Solar Probe will embark on an unprecedented journey to the center of the solar system. The probe, destined for the sun, will skim through the sun's atmosphere in the closest flyby around a star to date, marking a historic encounter with the sun's corona.
Scheduled to launch August 11th, 2o18, the Parker Solar Probe will become humanity's first ever mission to visit a star. Inevitably, the probe will endure some of the most extreme conditions ever experienced by a spacecraft; protecting the onboard scientific equipment from the hostile conditions are some of the most sophisticated and robust cooling systems which will prevent the star exploring Parker Solar Probe from melting in the sun's atmosphere.
On its journey, the Parker Solar Probe will provide the first ever close up view of a star as it plunges through the Sun's atmosphere. From a distance of just a few solar radii away (the length of the Sun's radius), the probe will be able to directly observe the sun's atmosphere and grant the opportunity to investigate solar anomalies impossible to witness from the earth.
It will be one of the most autonomous systems ever to be launched and will undoubtedly provide profound and perplexing information about the star at the center of our solar system. But the greatest challenge will be keeping it cool.
“Our solar arrays are going to operate in an extreme environment that other missions have never operated in before,” said the Johns Hopkins Applied Physics Lab’s Mary Kae Lockwood, spacecraft system engineer for Parker Solar Probe.
The probe has a notable narrow profile and situated in front of most of the craft is a massive 2.5-meter wide heat shield which will keep a majority of the spacecraft out of direct line from the sun. Furthermore, the probe will follow a highly elliptical orbital path which will ensure the spacecraft will not be exposed to the sun long enough to drastically heat it up.
Keeping NASA's Parker Solar Probe Cool in the Sun's Atmosphere
Over the next few months, the Parker Solar Probe will be en route to the sun. The mission is set to last nearly 7 years, and in that time the probe will make 24 close range passes around the sun. It will come closer to the sun than any previous aircraft, and to do so it will employ a combination of thermal protection and cooling systems to keep onboard equipment at nominal operating temperatures.
"Parker Solar Probe will swoop to within 4 million miles of the sun's surface, facing heat and radiation like no spacecraft before it. To get there, it takes an innovative route," claims NASA.
At its nearest perihelion (closest orbit point to the sun), the probe will come seven times closer than any spacecraft before.
The solar probe will enter the sun's coronal plasma - an atmospheric layer comprised of an ionized gas of electrons, protons, and heavy ions. At its perihelion, the Parker Solar Probe will enter a region of space where temperatures can reach over 500,000 degrees Celsius. However, the atmosphere 6 million kilometers above the sun's surface is rather thin and so a minimal amount of energy will be transferred into the probe. While temperatures may be in excess of half a million degrees Celsius, the probe's heat shield will only have to endure temperatures of up to 1,400 degrees Celcius.
Understanding how the probe does not melt from the sun is a matter of differentiating between heat vs temperature. It is the difference between the measure of how fast a particle is moving (the temperature) versus the total energy particles can transfer (heat). While the temperatures surrounding the probe may be hundreds of thousands of degrees, there are few particles to actually transfer much heat at all. Inside the corona, despite having a high temperature, minimal heat will be transferred into the spacecraft itself.
Parker Solar Probe Heat Shield
Taking on the blunt heat of the sun and acting as the first line of defense is Parker Solar Probe's innovative heat shield. It is comprised of an 11 cm thick carbon foam core which is sandwiched between two panels of superheated carbon-carbon composite. On Earth, the foam core is 97% air, making it extremely light while minimizing heat conduction. In its entirety, the heat shield weighs in at approximately 73 kg - about the weight of the average woman.
Since only a minimal amount of heat will be absorbed through the atmosphere, the greatest concern is mitigating the amount of sunlight which is absorbed as heat by the spacecraft.
The sun-facing side of the heat shield features a highly reflective carbon material intended to deflect as much energy as possible back towards the sun. It is painted with a specially formulated white coating capable of reflecting most of the suns rays, specifically ultraviolet, infrared, and most of the visible spectrum of light.
The material is designed not to deteriorate or crack in the extreme heat of the sun on its low pass by, and on the contrary, it is also minimally affected by the extreme cold experienced at the opposing end of its orbit.
“We learned a lot about solar array performance from the [APL-built] MESSENGER spacecraft, which was the first to study Mercury,” said Lockwood. “In particular, we learned how to design a panel that would mitigate degradation from ultraviolet light.”
Despite the heat shield reaching temperatures in excess of 1000 degrees Celcius, the rest of the spacecraft situated behind the shield will remain at nearly room temperature.
"The eight-foot-diameter heat shield will safeguard everything within its umbra, the shadow it casts on the spacecraft. At Parker Solar Probe’s closest approach to the Sun, temperatures on the heat shield will reach nearly 2,500 degrees Fahrenheit, but the spacecraft and its instruments will be kept at a relatively comfortable temperature of about 85 degrees Fahrenheit," claims NASA.
Preventing heat from reaching sensitive equipment requires extensive consideration not only of the material used but also how it should be mounted to the spacecraft.
Each point of contact between the heat shield and the rest of the probe creates a pathway for heat to travel; it is imperative to minimize connection points to prevent the transfer of heat through the truss while ensuring the craft remains structurally sound.
To prevent heat from transferring through the probe, the thermal protection system connects to the truss of the Parker Solar Probe at only 6 points, minimizing the transfer of heat to the rest of the spacecraft.
The thermal shield is a phenomenal defense against the extreme light of the sun, however, it is not designed to protect the whole of the craft. Some other systems, like the probe's solar arrays, must be designed in a way which can withstand direct sunlight from just a few solar radii away.
How the Solar Panels will Survive the Inferno
NASA's Parker Solar Probe is entirely dependent on the sun. While its focal mission is to investigate a star at an extremely close range, conducting such an investigation will require power, power which will also be entirely derived from the sun.
For most of the journey, the Parker Solar Probe's solar array will extend in front of the spacecraft directed towards the sun. But on its close approach, the panels will recess behind the heat shield leaving only a small portion of the outermost edges of the solar array to extend beyond the protection of the thermally protective shield. A small portion of the solar panels will always be exposed to the sun to provide the necessary power to the onboard systems.
"Parker Solar Probe is powered by two solar arrays, totaling just under 17 square feet (1.55 square meters) in area. They are mounted to motorized arms that will retract almost all of their surface behind the Thermal Protection System – the heat shield – when the spacecraft is close to the Sun." describes NASA.
The nature of the extreme range of sunlight the probe will be exposed to introduces problems typically not experienced by ground-based solar operations.
With a fixed solar array, on the close approach the sun, the solar panels would generate enough electricity to destroy themselves with heat, not to mention the heat absorbed from the sun itself.
“Unlike solar-powered missions that operate far from the Sun and are focused only on generating power from it, we need to manage the power generated along with the substantial heat that comes from being so close to the Sun,” said Andy Driesman, project manager from the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. “When we’re out around the orbit of Venus, we fully extend the arrays to get the power we need. But when we’re near the Sun, we tuck the arrays back until only a small wing is exposed, and that portion is enough to provide needed electrical power.”
Parker Probe Cooling
Without any cooling systems, the solar panels mounted on the Parker Solar Probe would become severely damaged at its closest orbit in the direct light of the sun. Keeping the panels cool is a relatively simple cooling system; a heated tank which keeps the coolant from freezing during launch, two radiators to dissipate heat during close orbit, aluminum fins to maximize the cooling surface, and pumps to circulate the coolant through the system.
The water will be housed separately in a heated container to prevent it from freezing during launch. One in orbit, the water will return to the cooling systems where it will begin to circulate.
In total, the probe will only require a little less than four liters of deionized water to keep the solar panels cool at all times. In operation, the water will be exposed to temperatures varying between 10 C and 125 C - a range beyond what most liquids are capable of handling as effectively as water.
To prevent the water from boiling at its high range peaking of 125 C, the system will be pressurized to raise the boiling point beyond 125 C.
“Part of the NASA technology demonstration funding was used by APL and our partners at UTAS to survey a variety of coolants,” said Lockwood. “But for the temperature range we required, and for the mass constraints, water was the solution.”
The solar panel systems have already proven their competency to withstand the extreme temperatures expected on its journey through the sun's outer atmosphere.
The panels were tested earlier this year by passing a large current through the array, drastically heating them up to simulate what would happen if the panels were to absorb and generate too much energy.
During testing, each solar cell began to glow red, allowing scientists to examine each individual solar cell for potential defects. Fortunately, the array passed the tests and are expected to survive a near direct confrontation with the sun.
Parker Solar Probe Autonomous Control
On Earth, light travels almost instantaneously, making it a rather convenient tool to use in communications and control. Radio controlled devices act almost immediately once a controller is given an input. However, a few million kilometers away, the speed of light begins to create quite an issue.
The Parker Solar Probe will largely be alone for most of its journey. On the far side of the sun, besides a gigantic fusion ball blocking all chance of a signal getting through, any command given to the probe would take as long as 8 minutes to reach the spacecraft. Effectively, if engineers were required to manually control the probe, each input would take 8 minutes before a signal is read. Worsening the problem still is the fact that if the probe is to relay any information back, it will also take 8 minutes to reach the engineers. In effect, it will take 8 minutes for an issue to be reported, and another 8 minutes before it could be manually corrected. In a dire situation in an environment as hostile as the sun, waiting 16 minutes for each command is not an option. So NASA engineers decided to automate all of it.
Several sensors of about the same size of a cellphone are mounted around the body of the spacecraft, as well as along the edge of the heat shield. Should any detector sense direct sunlight, an alert is sent to the central computer of the spacecraft before subsequent corrections are made to realign the craft and keep the rest of the probe safe from the heat of the sun.
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." says NASA.
Keeping the Parker Solar Probe from melting in the sun's atmosphere is a daunting challenge backed by a slew of innovative technologies to bring humanity closer to a star than ever before. While it may raise more questions than it answers, scientists are excited about the journey and the knowledge which will undoubtedly be gained. There is still a long way to go, but there is certainly light at the end of the tunnel, and NASA plans on capturing all of it.