“The scorching rays of the nearer Sun
softened the fragrant wax which held his wings.
The wax melted; his arms were bare as he beat them up and down,
but, lacking wings, they took no hold on the air.”
— Ovid, “Metamorphoses,” Book VIII (8th century AD)
One of the most well-known myths of ancient Greek mythology recounts the story of Icarus, who, in his quest for freedom and the thrill of adventure, flies perilously close to the Sun, only to meet his doom after the latter’s intense heat melts the young man’s wax wings. In real life, NASA is aiming to conduct a similar daring close approach to the Sun with the Solar Probe Plus mission, which is scheduled to launch in July 2018. Currently under development, Solar Probe Plus will ultimately approach the Sun closer than any other spacecraft in history by flying into its outer atmosphere, or corona, in its quest to answer a series of fundamental questions in the science of heliophysics. Unlike the mythological Icarus, however, the well-shielded spacecraft will be designed to brace the extreme conditions near the vicinity of the Sun, leading to breakthrough insights into the mechanisms that drive the activity of our home star.
Being the most luminous object in the sky, the Sun has been studied extensively by many different cultures throughout history. Yet the realisation of the Sun’s true nature as a typical star among many arose slowly in the last few centuries with the advent of modern science. The 20th century, in particular, saw many major breakthroughs in heliophysics research. Notable scientific findings include the discovery of the solar wind, the continuous flow of charged particles that emanates at supersonic speeds from the Sun’s atmosphere, the corona, and the unexpected discovery that the latter is actually millions of degrees hotter than the solar surface below it, which has a mean temperature of only 5,800 K. The advent of the space age has further revolutionized our understanding of the Sun’s activity and its effects throughout the entire Solar System. In the last 50 years, a plethora of space missions has observed our home star in multiple wavelengths throughout the entire electromagnetic spectrum, allowing us to study the properties and overall structure of the solar wind and the solar magnetic field in great detail. These missions have allowed us to map many of the physical processes that govern the Sun’s activity from far below its surface to the outer reaches of its atmosphere, which extends all the way to the heliopause, well beyond the orbit of Pluto.
Yet, for all the advances made in the field of heliophysics during the last decades, a series of puzzling mysteries have remained unsolved. In particular, the exact mechanism by which the solar wind is generated inside the Sun’s atmosphere and then propelled outward at supersonic speeds remains unknown to this date. Furthermore, the solution to the famous “coronal heating probelm” regarding the corona’s inexplicably high temperatures of several million Kelvins has equally eluded scientists. Observational evidence by various space-based observatories in recent years have suggested that the corona’s inexplicable heating may be caused by magnetic waves that travel along the lines of the solar magnetic field that emanates from deep within the Sun and extends all the way through the corona into interplanetary space, while carrying energy in the form of heat from the solar interior to the atmosphere in the process. Despite these tantalising evidence, however, current solar observatory missions lack the imaging resolution needed to provide a definite answer to this puzzle.
Scientists had long advocated for a space mission that would travel inside the heart of the Sun’s atmosphere where all these processes take place, in order to provide the answers to these long-standing questions, but fiscal and technical realities prevented such a mission concept from materializing. Nevertheless, the National Research Council’s first heliophysics decadal survey, which was published in 2003, identified such a mission as the highest priority for the next decade, requesting NASA to implement it “as soon as possible.” Answering to this call by the science community, the space agency’s Solar Probe Science and Technology Definition Team published a report in 2005, after an 18-month study, showing how such a mission to the vicinity of the Sun, named Solar Probe, could be implemented in a cost-effective manner within NASA’s already constrained budget. The Solar Probe mission concept, as was originally outlined in the report, envisioned a close flyby of Jupiter by a spacecraft equipped with a Radioisotope Thermoelectric Generator, or RTG, for a gravity assist that would propel it into a polar orbit around the Sun, allowing it to make two close passages of our home star in the course of five years, at a minimum distance of 4 solar radii, or 0.018 Astronomical Units—well inside Mercury’s 0.39 AU-wide orbit.
Yet, with an estimated cost of $1.1 billion, the Solar Probe concept was ultimately found to be too expensive by NASA, with the space agency requesting the Solar Probe team to come up with an alternative baseline mission that would not exceed a price tag of $750 million and would be also powered by solar panels instead of RTGs. Responding to this challenge, the team went back to redesign the mission in order to meet NASA’s requirements. To its surprise, the team soon discovered that the new revamped concept, which was named Solar Probe Plus, offered significant advantages to the original while allowing for a more scientifically productive mission, leading to its official selection by NASA in 2009 as part of the agency’s Living With a Star program.
Under the new mission concept, Solar Probe Plus will orbit the Sun a total of 24 times along the plane of the ecliptic during its seven-year mission, gradually spiraling inward toward the Sun with the help of seven flybys of Venus, with each consecutive orbit being closer to our home star than the previous one. During its last three orbits, the 1,350-pound spacecraft will have approached the Sun at a distance of 8.5 solar radii, or 0.039 AU—10 times closer than Mercury’s orbit. These final close passages will bring Solar Probe Plus a mere 3.7 million miles (5.9 million kilometers) from the Sun’s infernal surface, positioning it inside the outer layers of the solar corona, where it will have to withstand temperatures as high as 2,500 degrees Farenheit (1,371 degrees Celsius) and survive a solar radiation intensity more than 520 times that experienced by spacecraft in low-Earth orbit. Throughout the duration of its mission, Solar Probe Plus will spend a total of 961 hours inside 20 solar radii, 434 hours inside 15 solar radii, and 30 hours inside 10 solar radii.
In order to survive these extreme conditions, the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., which will design, build, and operate the mission, will develop a 8-foot-diameter, 4.5-inch-thick sunshield made of reinforced carbon-carbon composite material. This specialized sunshield, which is based on the one onboard the MESSENGER spacecraft that is currently exploring Mercury, will protect Solar Probe Plus’ scientific instruments from the scorching-hot temperatures in the vicinity of the Sun, while an active cooling loop system will keep them at safe operating temperatures. In addition, the spacecraft will use two sets of solar arrays for maintaining electrical power at all times depending its distance from the Sun. The primary legacy arrays will be used only when the Solar Probe Plus will be further than 0.25 AU from the Sun. Once inside 0.25 AU, the primary arrays will be folded inside the sunshield’s shadow, and the spacecraft will switch to its secondary arrays, mounted on two moveable, liquid-cooled base plates that will adjust their position relative to the Sun accordingly, as Solar Probe Plus moves inward so as to maintain power as well as proper heat rejection at all times.
As outlined by its science definition team, Solar Probe Plus will aim to address four primary science objectives:
- Determine the structure and dynamics of the magnetic fields at the sources of both fast and slow solar wind.
- Trace the flow of energy that heats the corona and accelerates the solar wind.
- Determine what mechanisms accelerate and transport energetic particles.
- Explore dusty plasma phenomena near the Sun and its influence on the solar wind and energetic particle formation.
To that end, the spacecraft will be equipped with a set of five science instruments:
— The Solar Wind Electrons Alphas and Protons Investigation, which will specifically count the most abundant particles in the solar wind—electrons, protons, and helium ions—and measure their properties. The investigation also is designed to catch some of the particles in a special cup (known as a Faraday cup) for direct analysis.
— The Wide-field Imager, a telescope that will make 3-D images of the sun’s corona, or atmosphere. The experiment actually will see the solar wind and provide 3-D images of clouds and shocks as they approach and pass the spacecraft. This investigation complements instruments on the spacecraft providing direct measurements by imaging the plasma the other instruments sample.
— The Fields Experiment, which will make direct measurements of electric and magnetic fields, radio emissions, and shock waves that course through the Sun’s atmospheric plasma. The experiment also serves as a giant dust detector, registering voltage signatures when specks of space dust hit the spacecraft’s antenna.
— The Integrated Science Investigation of the Sun, or ISIS, which consists of two instruments that will take an inventory of elements in the Sun’s atmosphere using a mass spectrometer to weigh and sort ions in the vicinity of the spacecraft.
— The Heliospheric Origins with Solar Probe Plus, which will provide an independent assessment of scientific performance and act as a community advocate for the mission.
In addition to their immense value in the field of heliophysics research, Solar Probe Plus’ findings will be invaluable to future human space exploration efforts beyond low-Earth orbit as well. Presenting one of the main health hazards for astronauts travelling beyond the Earth’s magnetic field, showers of solar energetic particles, consisting of helium and other heavy ions that originate from solar flares and coronal mass ejections in the surface of the Sun, wash through interplanetary space at near light speeds in a matter of hours. Since these events are sporadic and episodic, they are extremely difficult to predict. The in-situ measurements of these energetic particles, and their properties that will be taken inside the Sun’s corona by Solar Probe Plus, will greatly advance the accuracy of space weather forecasting models, allowing astronauts on future deep space missions to be better warned of solar radiation events in advance. “A number of major research initiatives are now under way to improve our understanding of the biological effects of [space] radiation and to develop effective shielding materials and other mitigation strategies,” writes the Solar Probe Plus Science and Technology Definition Team in its report. “Another important aspect of space radiation risk reduction and management is the development of the capability to forecast the radiation environment, and here Solar Probe Plus has a unique and significant contribution to make to human exploration … Solar Probe Plus will thereby provide the ‘ground truth’ for models that eventually will be run in real-time to make global predictions, and it will thus play a truly enabling role in the human exploration of the Moon and Mars.” “The answers to these questions can be obtained only through in-situ measurements of the solar wind down in the corona,” says Nicky Fox, Solar Probe Plus project scientist at the Johns Hopkins University Applied Physics Laboratory. “Solar Probe Plus gets close enough to provide the missing links, with the right complement of instruments to make the measurements. For the first time, we will be able to go up and touch our star.”
Making steady progress toward meeting its scheduled July 2018 launch date, the Solar Probe Plus mission passed several major key milestones in its development during this year. Having successfully completed its Preliminary Design Review in January, Solar Probe Plus passed its Key Decision Point C Confirmation Review during March, clearing the mission to move toward construction. Despite this solid progress, the mission hadn’t been without its occasional hiccups. The Solar Probe Plus spacecraft, originally planned to launch onboard a United Launch Alliance Atlas V rocket utilizing a custom-built upper stage by ATK, was eventually found to be too heavy, requiring a larger heavy-lift vehicle, as reported recently by the SpaceNews website. The leading contenders to fill that role are now ULA’s Delta IV Heavy and SpaceX’s still unflown Falcon Heavy rocket. NASA hopes that the latter, which is currently under development, will have completed its required three certification missions by 2017, to be considered as a viable launch option by the agency.
Whatever the launch vehicle may be, however, to loft Solar Probe Plus into its path toward the Sun, this historic and ambitious heliophysics mission promises to usher in a new era in our overall understanding of our home star and the effects of its activity throughout the Solar System. “Solar Probe Plus is a pathfinder for voyages to other stars and will explore one of the last unexplored regions of the Solar System, the solar corona, where space weather is born,” says Lika Guhathakurta, the mission’s program scientist at NASA Headquarters in Washington, D.C.
In a few years, we will have the chance to navigate the infernal seas of the Sun for the first time. No matter what we’ll find in this journey, it will definitely be one of the most spectacular ones we have ever undertaken.