Download PDF Fall Issue of The Bridge on Space Exploration September 1, 2021 Volume 51 Issue 3 Close collaboration between engineering and science has enabled marvels of space exploration over decades. Eight exemplary missions are described in this issue, conveying the excitement, challenges, and breakthroughs involved in efforts to better understand the wonders and mysteries of this solar system. Parker Solar Probe: A 60-Year Journey to Reach a Star Wednesday, September 15, 2021 Author: Nicola J. Fox and Andrew S. Driesman The story of Parker Solar Probe shows how engineers and scientists navigated constraints to develop the invaluable scientific powerhouse now flying by the Sun. In July 1958, three months before NASA was created, the National Research Council issued a report with recommendations for future space missions, including three focused on heliophysics (NRC 1958). Among them: a mission to fly inside the orbit of Mercury to study the particles and field environments near the Sun. Sixty years after release of the NRC report, in the early morning hours of August 12, 2018, NASA’s Parker Solar Probe lifted off from Cape Canaveral Air Force Base in Florida. Mounted atop a Delta IV Heavy rocket was a science and engineering marvel, the culmination of 6 decades of preparation. During the decades between the 1958 report and the launch, scientists and engineers worked to make a solar probe mission a reality. The historic challenge required engineering solutions with no precedent and testing under conditions difficult to simulate. The mission—the first close-up study of a star—would face an environment unlike any spacecraft before it. In this article we analyze the history of Parker Solar Probe in terms of solutions to three linked engineering challenges and their interplay with the mission’s science objectives: Trajectory design: Develop a trajectory that would bring the probe close enough to the Sun to capture measurements to answer the science questions Thermal protection: Protect the spacecraft with its sensitive scientific instruments from intense heat on close solar passes Power: Power the spacecraft across a range of environments, both close and far from the Sun. The story of Parker Solar Probe shows how engineers and scientists navigated these constraints with available technology and uncertain funding environments to develop the invaluable scientific powerhouse currently flying by the Sun. Scientific Motivations: Why Go to the Sun? The scientific dividends of a mission to the Sun were clear before 1958; later developments only increased the urgency of the mission. The first mystery to be addressed by a solar probe emerged with the 1869 total solar eclipse, when American astronomers Charles Augustus Young and William Harkness observed a novel green emission line with a wavelength of 530.3 nanometers in the solar corona. At the time, scientists suspected they were observing the chemical fingerprint of an undiscovered element; they dubbed it Coronium. Advances in laboratory spectroscopy led to the surprising discovery in the 1940s (Edlén 1941) that the spectral lines came from ionized iron, heated to such an extreme temperature that it had lost 13 of its 26 electrons. Such a highly ionized species of iron implied a coronal temperature of over 1.7 million degrees Celsius—several hundred times hotter than the Sun’s visible surface below it. The question of how and why the corona is so much hotter than the surface of the Sun became known as the coronal heating problem, and solving it was a key goal for a solar probe. Another solar mystery was soon recognized. In 1958 a young University of Chicago physicist, Eugene Parker, published a paper predicting the existence of a supersonic plasma flow (i.e., the solar wind)—remnants of a hot corona too energetic to remain trapped by the Sun that would continuously escape to space. In 1962 NASA’s Mariner 2 spacecraft, on its way to Venus, observed this wind and clocked it at speeds up to 800 km/s, consistent with the Parker prediction. The solar wind was real, complex, and incredibly fast and hot. These two questions—how the corona became so hot, and how the solar wind is heated and accelerated—became foundational in heliophysics. Answering them would require exploring the Sun’s atmosphere relatively close to the surface, beneath the Alfvén critical point, beyond which the plasma ceases to corotate with the Sun (i.e., where the magnetic field loses its rigidity to the plasma). Estimates of the location of the Alfvén critical point varied dramatically from a few to tens of solar radii (Rs) from the Sun. A proper solar probe had to get within that distance in order to provide answers (NRC 2003). Early Designs: 1978–2005 Before 2008, the solar probe concept underwent five major design revisions. Each addressed the engineering challenges described above in different ways, often resolving problems in earlier iterations and sometimes creating new ones. The question of how and why the corona is so much hotter than the Sun’s surface was a key focus for a solar probe. Trajectory Design The choice of trajectory is key to mission success. A well-engineered trajectory not only allows the probe to reach a targeted orbital position close to the Sun but also sets up the geometries for Earth communications, minimal fuel usage, radiation exposure, timing for gravitational assists, and generation of power, among other requirements. All these engineering and science factors generate a complex trade-space to be optimized. Failure to satisfy all conditions leads to an unviable design option. The first solar probe science definition study emerged from a 1978 workshop at the California Institute of Technology. The papers from that symposium (Neugebauer and Davies 1978) mostly focused on the science that a solar probe might accomplish, but they triggered efforts to develop and refine a solar probe concept that culminated in the 1982 Starprobe concept (figure 1). FIGURE 1 1982 Starprobe. RTG = radioisotope thermal generator; UV = ultraviolet. Image source: Randolph (1982). Starprobe would carry a huge suite of instruments for gravity, optical, and plasma experiments to within 4 Rs of the Sun. To reach this perihelion, it would require an extremely high energy trajectory: launching out toward Jupiter, it would use the planet’s intense gravitational field to slingshot itself out of the ecliptic plane. In some variations of the trajectory, several Earth or Venus gravity assists would be used along with several propulsive maneuvers (figure 2), necessitating additional onboard power requirements and engineering constraints. When it got close to the Sun the Starprobe would turn the tip of its protective conical heatshield toward it to protect the delicate spacecraft electronics and scientific instruments. FIGURE 2 Trajectory option studied for the 1982 Starprobe. AU = astronomical unit. Image source: Randolph (1982). The mission as proposed was too costly and complex at the time. But the trajectory—using a Jupiter gravity assist, which would put the solar probe in a polar orbit about the Sun—remained the primary focus of subsequent solar probe missions until scientists and engineers brought to bear the latest scientific understanding and technology. While a Jupiter gravity assist would take the probe extremely close to the Sun, well inside the Alfvén critical point, it had costs and engineering complexities. Jupiter’s radiation belts posed risks to the spacecraft’s electronics. The probe would also face extremely cold temperatures so far from the Sun, expanding the range of thermal environments in which it would have to operate. For similar reasons, the trajectory limited the options for powering the spacecraft. Finally, the trajectory allowed for only one or two close, very fast, and short passes of the Sun. Thermal Protection System Design A solar probe’s survival depends on its thermal protection system. The intense thermal environment at 4 Rs meant that the probe’s thermal shield would need to operate at about 2500 K (3500°F), while—about a meter behind the shield—sensitive spacecraft electronics had to be kept below 50°C (122°F) at the extreme. An effective shield would have to reject a maximal amount of heat out to space. But at these temperatures the heat shield would sublimate, creating a possible noise source for some of the science instrumentation. An initial constraint was that the chosen material could shed no more than 2.5 mg/s. Early studies investigated ceramic shields, which absorb low amounts of thermal energy at room temperature and are good insulators, but they were too brittle to withstand the launch conditions and degraded severely under intense UV light. Metals such as tungsten were also explored but found to be too heavy unless reduced to a thin shell, which was similarly brittle. By 1982 the community had settled on carbon-carbon composites, which had low mass, high strength, and high operating temperatures, although their surface properties were not as good as ceramics. This issue was later solved with a custom coating (coating materials available at the time faced the same UV degradation problem as ceramics). The heat shield’s shape was also critical to its function. The 1982, 1989, and 2005 designs all used a conical shape for the outer heat shield, backed by a secondary flat heat shield connected to the spacecraft bus. The conical shape minimized the incidence of solar flux per a given area and maximized the area to radiate heat back out to space. The conical heat shield design was massive and single-purpose. Two “small solar probe” designs in 1994 and 1999 tried another tack, using an elliptical shield that doubled as a high-gain antenna. This design improved signal transmission in what was expected to be an electrically noisy outer corona, while opening up room in the shadow of the heat shield previously devoted to an antenna. By 2005 the conical heat shield’s design had reached its most advanced level. A new low-emissivity ceramic coating resolved issues with the carbon-carbon surface properties and reduced the shield’s mass. Still, the design faced challenges. Launching a spacecraft on a high-energy trajectory to the Sun via Jupiter required reducing mass wherever possible. The heat shield’s conical design, with a 15-degree half angle required to keep solar incident at the appropriate levels, meant it had to be approximately 5 meters long. And with the secondary shield it weighed about 133 kg, more than 15 percent of the spacecraft’s total mass at launch. Powering the Spacecraft Solar power might seem the obvious choice for a solar probe. But several aspects of the early mission designs posed serious challenges for the use of solar arrays. The Jupiter gravity-assist trajectory would take the probe both extremely close to the Sun where solar flux would be abundant, and extremely far where it would be scarce. At perihelion the solar arrays would have to be stowed almost completely behind the heat shield. But to power the spacecraft at aphelion near Jupiter, they would have to be extremely large—too large to adequately tuck behind the heat shield. Their required size would also present challenges with stowing them safely during launch. For these reasons solar probe concepts through 2005 relied on radioisotope thermal generators (RTGs), converting heat from the nuclear decay of plutonium into power for the spacecraft. An elliptical heat shield that doubled as a high-gain antenna improved signal transmission while opening up room previously devoted to an antenna. By 2005 the power system and the mission as a whole were considered technically feasible, albeit extremely complicated and constrained. At a projected cost of $1.1 billion (in FY07 dollars), however, the concept was deemed too expensive in the existing funding environment. NASA’s Science Mission Directorate requested a new study for a solar probe mission with two primary constraints: a cost cap of $750 million (in FY07 dollars) and an ability to power the spacecraft without RTGs. These constraints forced the solar probe community to rethink the mission wholesale, culminating in the 2008 report of the Solar Probe Plus Science and Technology Definition Team (STDT) and a complementary mission concept study report (JHU APL 2008) outlining the design for what ultimately became the Parker Solar Probe mission. 2008 Design: Solar Probe+ With the prohibition on RTGs, the search was on for a way to meet the solar probe’s science objective using solar power. The resulting adjustments in both the trajectory and heat shield design produced a mission concept not only significantly less expensive but also scientifically superior, prompting the name Solar Probe+. Trajectory: From 4 Rs to 9.5 Rs In previous mission designs, the probe’s trajectory was designed to pass within 4 Rs of the Sun. But with the Jupiter gravity assist and the extreme proximity to the Sun, attaining this perihelion constrained virtually every other aspect of the mission, from the power source to the heat shield to the number of potential perihelion passes. FIGURE 3 The Parker Solar Probe trajectory. Rs = solar radii. Image credit: NASA/Johns Hopkins APL. The proposed Solar Probe+ design would not go as close to the Sun: its closest approach would be 9.5 Rs, just below the estimated location of the Alfvén critical point. What the mission design gave up in proximity it more than recovered in duration in the regions of interest: the total amount of time the spacecraft would spend within 20 Rs increased tenfold (figure 3). Cooperation between the science and engineering communities allowed this reimagining of the scientifically relevant observation points, both easing and enabling the engineering solutions for the mission. Parker Solar Probe would not have been possible without this. The new perihelion distance no longer required a Jupiter gravity assist. With a suitably powerful launch vehicle, the probe could launch opposite Earth’s orbital velocity and use repeated gravity assists from Venus to slow the spacecraft so that it gradually “fell” toward the Sun. Instead of one or two solar passes, the new mission would have 24, 17 of which would be within 20 solar radii. The gradual lowering of the orbit reduced risk and allowed operators to learn how to manage the spacecraft as it approached the Sun and prepare for upcoming solar encounters. Heat Shield: From Cone to Flat Panel The new perihelion at 9.5 Rs reduced the maximum solar flux expected by almost a factor of 6, greatly opening up options for thermal protection. A simpler, flat heat shield design based on the 2005 STDT report’s secondary shield would suffice; it would be a sandwich of carbon-carbon and carbon foam with a ceramic coating. At 11.4 cm thick and 2.4 m across, the heat shield’s mass was cut by almost half, to 70.5 kg. Power: Solar Panels Because the trajectory of Solar Probe+ would remain within 1 astronomical unit (AU) for the duration of the mission, solar power became viable. Still, conventional solar arrays used for Earth-orbiting spacecraft would not suffice. Solar Probe+ employed two solar arrays. Primary arrays would be used outside .25 AU and then, as the probe approached the Sun, retract behind the heat shield, when smaller secondary arrays would deploy. These secondary arrays would be actively cooled by a closed-loop system of liquid ammonia. Developments since 2008 The 2008 STDT report and 2008 mission concept study report (JHU APL 2008) were accepted by NASA, and soon the engineering team at the Johns Hopkins University Applied Physics Lab began working on technology development. Over the next 5 years, testing and design improvements led to a few changes in the spacecraft design. The final design placed the secondary solar arrays at the end of the primary solar arrays on panels that could be independently articulated. To cool the arrays, deionized water was used instead of ammonia, as it has much higher critical temperature and so could better survive extreme temperatures. Much has been said about the design of the spacecraft, but scientific observations would not be possible without the exquisite instrumentation on board (table 1). For the most part, instruments in the heatshield’s umbra are kept at manageable temperatures. But to directly sample the solar plasma and measure the electric field in the corona, it was necessary to place two instruments—the FIELDS antennas and a Faraday cup—outside the heatshield, where they experience the full intensity of the Sun. To withstand this environment, the instruments are made of high-temperature materials including niobium, tungsten, and sapphire. Both instruments must also control their own thermal environments, including protecting sensitive electronic amplifiers. The development and test programs for both these instruments were substantial. During the design phase for Solar Probe+, scientists studied the solar wind processes using data from the Helios mission, which reached a perihelion of 0.31 AU, just inside the orbit of the planet Mercury. Extrapolations inward from Helios data and outward using photospheric field measurements, together with radio data, enabled scientists to constrain the Alfvén speed at various distances from the Sun’s surface. This new analysis showed that the Alfvén critical point is typically about 20 Rs, farther out than the historical predictions of 10 Rs. This meant that the probe would spend much more time inside the Alfvén point than previously expected. Using advanced remote sensing techniques, scientists were also discovering more about the workings of the Sun, the intricacy of sunspot regions, and in particular the complexity of the Sun’s magnetic fields and their relevance to solar wind acceleration. FIGURE 4 (left) NASA’s Parker Solar Probe lifts off from Cape Canaveral, August 12, 2018. Image credit: NASA/Johns Hopkins APL. (right) Eugene Parker watches the launch. Image credit: NASA/Johns Hopkins APL. The mission, already steeped in history, had one more milestone before launch (figure 4). In 2017 it was renamed Parker Solar Probe in honor of Eugene Parker, whose theoretical work on the solar wind helped spur the mission. It is the first NASA mission to be named for someone during their lifetime and the first launch to be watched by its namesake (figure). Conclusion Through the many design changes, the significance of meeting the solar probe’s scientific goals has only grown. As the history of design iterations reveals, there is no single way to send a spacecraft to the Sun; instead, countless design decisions, each with advantages and drawbacks, strike a balance to create a successful mission. The path from the first ideas of a solar probe to implementation 60 years later required continued interplay between scientific ambitions and engineering possibilities. Since its launch August 12, 2018, Parker Solar Probe has not disappointed. On the very first orbit, during solar minimum conditions, the probe returned measurements that shed new light on the Sun and its atmosphere. Critical data provided tantalizing clues as to the cause of coronal heating and powering of solar wind. As the history of design iterations reveals, there is no single way to send a spacecraft to the Sun. As the mission has progressed, it has revealed a dust-free region close to the Sun and a Venus orbital dust ring. It is also providing dramatic new insights into the location of the Alfvén critical point, enhancing understanding of how the Sun’s rotation slows over time and of the lifecycle of stars, including the Sun. Images have shown new features in the solar wind known as switchbacks, rapid polarity flips in the Sun’s magnetic field. These switchbacks have sparked a flurry of studies and scientific debate as researchers try to explain how these unexpected magnetic pulses form. Do they originate from a dramatic magnetic explosion that happens in the Sun’s atmosphere? Are they a kind of magnetic structure (a flux rope)? Do they form in the solar wind as a byproduct of turbulent forces, or when fast and slow streams of solar wind rub against one another? Questions abound. The research community will continue to debate these and other exciting questions as Parker Solar Probe blazes a trail through the Sun’s corona. The probe has exceeded all NASA expectations and will continue to provide scientists with spectacular data that will fuel more mysteries and questions to be answered for many years. References Bale SD, Goetz K, Harvey PR, Turin P, Bonnell JW, Dudok de Wit T, Ergun RE, MacDowall RJ, Pulupa M, Andre M, and 74 others. 2016. The FIELDS Instrument Suite for Solar Probe Plus. Space Science Reviews 204:49–82. Edlén B. 1941. An attempt to identify the emission lines in the spectrum of the solar corona. Arkiv för Matematik, Astronomi och Fysik 28(1):1–4. JHU APL [Johns Hopkins University Applied Physics Laboratory]. 2008. Solar Probe Plus Mission Engineering Study Report. Laurel MD. Kasper JC, Abiad R, Austin G, Balat-Pichelin M, Bale SD, Belcher JW, Berg P, Bergner H, Berthomier M, Bookbinder J, and 69 others. 2016. Solar Wind Electrons Alphas and Protons (SWEAP) Investigation: Design of the solar wind and coronal plasma instrument suite for Solar Probe Plus. Space Science Reviews 204:131–86. McComas DJ, Alexander N, Angold N, Bale S, Beebe C, Birdwell B, Boyle M, Burgum JM, Burnham JA, Christian ER, and 41 others. 2016. Integrated Science Investigation of the Sun (ISIS): Design of the energetic particle investigation. Space Science Reviews 204:187–256. NRC [National Research Council]. 1958. Recommendations of the Space Science Board for space experiments: Letter report. Washington: National Academy Press. NRC. 2003. The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics. Washington: National Academies Press. Neugebauer M, Davies RW. 1978. A Close-Up of the Sun. JPL-PUB-78-70, NASA-CR-157734. Pasadena CA: Jet Propulsion Laboratory. Parker EN. 1958. Dynamics of the interplanetary gas and magnetic fields. Astrophysical Journal 128:664–76. Randolph JE. 1982. Starprobe: The mission and system options. AIAA paper 82-0041, Aerospace Sciences Mtg, Orlando FL. STDT [Science and Technology Definition Team]. 2008. Solar Probe Plus: Report of the Science and Technology Definition Team. NASA/TM—2008-214161. Washington: National Aeronautics and Space Administration.  The Alfvén critical point is where solar wind physics changes because of the multidirectionality of wave propagation (waves moving sunward and antisunward can affect the local dynamics such as the turbulent evolution, heating, and acceleration of the plasma). This is also the region where velocity gradients between the fast and slow speed streams develop, forming the initial conditions for the formation, farther out, of corotating interaction regions. About the Author:Nicola Fox is director of the Heliophysics Science Division in NASA’s Science Mission Directorate. Andrew Driesman is managing executive of the Space Exploration Sector at the Johns Hopkins University Applied Physics Laboratory.