Parker Solar Probe Touches the Sun

Parker Solar Probe Touches the Sun

For the first time in history, a spacecraft touched the sun. NASA's Parker Solar Probe, launched in 2018, made increasingly closer orbits around the sun over 3 years before flying through the corona, the sun's upper atmosphere, at the end of 2021, where it sampled particles and magnetic fields. This extraordinary achievement marks a major step forward for solar science. Touching the material that makes up the sun will help scientists uncover critical information about our closest star and its effects on the solar system. The interior of the corona is, of course, unimaginably hot. The spacecraft was bombarded with intense sunlight while being exposed to temperatures above one million degrees Fahrenheit.

So Why Doesn't It Melt?

Parker Solar Probe was designed to withstand harsh conditions and temperature fluctuations for its mission. The key lies in a special heat shield and an autonomous system that helps protect the mission from the sun's intense light emissions while allowing the coronal material to "touch" the spacecraft.

The Science Behind Coating Technology

One key to understanding what keeps the spacecraft and its instruments safe is understanding the concept of temperature versus heat resistance. Contrary to intuitive assumption, high temperatures do not always mean actually heating another object. In space, temperature can rise to thousands of degrees without providing significant heat to or warming a particular object. But how? Temperature measures how fast particles are moving, while heat measures the total amount of energy that particles transfer. Particles may be moving fast (high temperature), but if there are very few of them, they do not transfer much energy (low heat). Since space is mostly empty, there are very few particles to transfer energy to the spacecraft. For example, the corona that Parker Solar Probe passes through has an extremely high temperature but very low density. Think of the difference between putting your hand inside a hot oven versus putting it in a pot of boiling water (don't try this at home!) - your hand in the oven can withstand much higher temperatures for longer because it interacts with far fewer particles than boiling water. Similarly, compared to the sun's visible surface, the corona is less dense, so the spacecraft interacts with fewer hot particles and does not absorb much heat. This means that while Parker Solar Probe travels through a vacuum with temperatures of several million degrees, the surface of the heat shield facing the sun will only be heated to approximately 2,500 degrees Fahrenheit (approximately 1,400 degrees Celsius).

The Shield Protecting the Spacecraft

Of course, thousands of degrees is still an extraordinarily high temperature. (For comparison, lava from volcanic eruptions ranges between 700 and 1,200°C) and to withstand this heat, Parker Solar Probe has a shield known as the Thermal Protection System, or TPS, which is 8 feet (2.4 metres) in diameter and 4.5 inches (approximately 115 mm) thick. With this few inches of protection, the spacecraft body immediately behind the shield is maintained at 85°F (30°C). The TPS was designed by Johns Hopkins Applied Physics Laboratory and built by Carbon-Carbon Advanced Technologies using a carbon composite foam compressed between two carbon sheets. This lightweight insulation is accompanied by the finishing touch of white ceramic paint on the sun-facing plate to reflect as much heat as possible, which will be explained in greater detail below. Tested to withstand 3,000°F (1,650°C), the TPS can handle any heat the sun can send and keeps nearly all instrumentation safe. [caption id="attachment_134224" align="aligncenter"] On Wednesday, 27 June 2018, technicians and engineers used a crane to install the heat shield on NASA's Parker Solar Probe at the Astrotech processing facility in Titusville, Florida, near NASA's Kennedy Space Center. Image Source: NASA/Glenn Benson[/caption] For this purpose, APL (Applied Physics Laboratory) turned to the Advanced Technology Laboratory at Johns Hopkins University Whiting School of Engineering; and with the help of a fortunate coincidence, it was possible to assemble a remarkably fine heat shield coating team comprising high-temperature ceramic, chemistry and plasma spray coating specialists. After extensive engineering and testing, the team settled on a bright white aluminum oxide-based coating. However, this coating could react with the carbon of the heat shield at high temperatures and turn grey, so engineers added a tungsten layer thinner than a human hair between the heat shield and coating to prevent the two from interacting. They added nano-scale additives to make the coating whiter and prevent aluminum oxide grains from expanding when exposed to heat. The engineers then had to determine how best to formulate and apply the coating. Dennis Nagle, senior research engineer at the Center for Systems Science and Engineering, said, "The entire team was working to find a ceramic coating that both reflects light and emits heat." Nagle noted that when working with enamel, hard, non-porous coatings that would crack when struck with a hammer were typically preferred. However, at the temperatures Parker Solar Probe encounters, a smooth coating would shatter like a stone thrown at glass. Instead, the goal was a uniform, porous coating that could withstand harsh environments. In porous coatings, cracks that begin to form stop when they encounter a pore. As a result, the coating was composed of a series of ceramic grains, or several rough, granular layers that would reflect light missed by the other layer.

Device Measuring Solar Wind

However, not all of Parker Solar Probe's instruments are located behind the TPS. The Solar Probe Cup, which extends outward from above the heat shield, is one of two instruments on Parker Solar Probe that is not protected by the heat shield. [caption id="attachment_134227" align="aligncenter"] NASA's Parker Solar Probe completed final procedures in the clean room before being transported to the launch pad. Image Source: NASA/Johns Hopkins APL/Ed Whitman[/caption] This instrument is known as a Faraday cup, a sensor designed to measure ion and electron flows from the solar wind and flow angles. Due to the density of the solar atmosphere, it was necessary to design unique technologies not only to keep the device intact, but also to ensure that the spacecraft's electronics could send accurate readings back. The cup itself is made of layers of Titanium-Zirconium-Molybdenum, a molybdenum alloy, with a melting point of approximately 4,260°F (2,349°C). The grids that generate 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). Normally lasers are used to abrade the grid lines in these grids - but due to the high melting point, acid had to be used instead. Another challenge arose in the form of electronic wiring. Most cables would melt due to heat radiation at such close proximity to the sun. To solve this problem, the team produced sapphire crystal tubes to suspend the cables and made the cables from niobium. To ensure the instrument was ready for the harsh environment, researchers had to simulate the sun's intense heat radiation in a laboratory. To create a worthwhile heat level for testing, researchers used a temporary particle accelerator and IMAX projectors to increase temperatures. While the projectors mimicked the sun's heat, the particle accelerator exposed the cup to radiation to ensure it could measure accelerated particles under intense conditions. To be absolutely certain that the Solar Probe Cup could withstand the harsh environment, the Odeillo Solar Furnace, which concentrates the sun's heat through 10,000 adjustable mirrors, was used to test the cup against intense solar radiation. The Solar Probe Cup successfully passed its tests - in fact, the longer it was exposed to test environments, the better it performed and the clearer results it produced. Justin Kasper, principal investigator of the SWEAP instruments at the University of Michigan in Ann Arbor, explained, "We believe the radiation eliminated any possible contamination. Actually, it cleaned itself." [caption id="attachment_134228" align="aligncenter"] On Tuesday, 5 June 2018, technicians and engineers performed a light rod test on NASA's Parker Solar Probe at the Astrotech processing facility in Titusville, Florida, near NASA's Kennedy Space Center. Image Source: NASA/Glenn Benson[/caption]

The Spacecraft Maintaining its Coolness

Several other design features on the spacecraft protect Parker Solar Probe from heat. Without protection, the solar panels, which worked to power the spacecraft and utilize the sun's energy, could overheat. As the solar panels approach the sun with each pass, they are pulled behind the shadow of the heat shield, leaving only a small area exposed to the sun's intense rays. However, at such close proximity to the sun, even more protection is needed. The solar energy panel arrays have a surprisingly simple cooling system; a heated tank to prevent freezer coolant during launch, two radiators to prevent the coolant from freezing, aluminum fins to maximize cooling surface area, and pumps to circulate the coolant. The cooling system is actually powerful enough to cool an average-sized living room and will keep the solar panels and instrumentation cool and operational in the sun's heat. The cooling fluid used in the system is approximately one gallon (3.7 litres) of deionized water. Although abundant chemical coolants are available, the temperature range the spacecraft will experience varies between 50°F (10°C) and 257°F (125°C). Very few liquids can handle these ranges like water. At the higher end of the temperature scale, the water will be pressurized so that its boiling point is above 257°F (125°C). Another problem in protecting any spacecraft is figuring out how to communicate with it. Parker Solar Probe will travel largely alone on its journey. It takes eight minutes for light to reach Earth - meaning if engineers had to control the spacecraft from Earth, it would be too late to correct things if something went wrong. Therefore, the spacecraft was designed to keep itself safe autonomously and maintain its course to the sun. A few sensors, roughly half the size of a cell phone, are attached to the spacecraft body along the edge of the shadow cast by the heat shield. If any of these sensors detect sunlight, they alert the central computer and the spacecraft can adjust its position to safely protect the sensors and other instruments. All of this must happen without any human intervention, so the central computer software is programmed and extensively tested to ensure all corrections can be made instantaneously. [caption id="attachment_134229" align="aligncenter"] United Launch Alliance Delta IV Heavy rocket launched NASA's Parker Solar Probe on its mission to touch the sun from Launch Complex 37 at Cape Canaveral Space Force Station in Florida on Sunday, 12 August 2018. Image Source: NASA/Bill Ingalls[/caption]

Sources

• https://www.nasa.gov/content/goddard/parker-solar-probe • https://www.nasa.gov/feature/goddard/2021/nasa-enters-the-solar-atmosphere-for-the-first-time-bringing-new-discoveries • https://phys.org/news/2018-08-parker-solar-probe.html • Images: https://www.nasa.gov/content/goddard/parker-solar-probe-images