In an effort to explore what is perhaps the last salient region of our solar system yet to be visited by a spacecraft, NASA has announced Solar Probe Plus, a mission that will launch a probe directly into the sun's atmosphere. The mission will seek to answer some of the outstanding questions about the nature of our very own star, while helping to understand and forecast the radiation environment in which future space explorers will be living and operating.
Humans have been observing the sun for millions of years, and although much knowledge has been gathered in the past few decades, at least two outstanding questions keep puzzling scientists even today. The first enigma is the discovery, back in the 1940s, that our star's atmosphere (or corona) appears to be several hundred times hotter than the photosphere, the visible surface. The second is about the origin of the solar winds in the atmosphere, which travel at supersonic speeds and affect our planet as well as the rest of the solar system.
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Because a definite answer to these questions can only be provided by direct measurements in the solar atmosphere, we have been forced to wait since 1958, when a mission to provide these answers was first recommended even though we lacked the necessary technology. Since then, a solar mission has remained one of the agency's top priorities, and several studies of its possible implementations have been conducted.
As the spacecraft approaches the sun, it will be keeping a distance of "only" 6 million kilometers (3.7 million miles) from its surface. The extreme conditions in this region, where scientists expect to find temperatures in excess of 1400 degrees Celsius (2552F) and intense radiation, requires an ad-hoc structure for adequate protection. This function is performed by the innovative Thermal Protection System (TPS) – a large, flat carbon shield 2.7 meters (8.86 feet) in diameter protecting the spacecraft and instruments within its shadow during the solar encounters.
Power is provided by two separate solar array systems. The first is only intended to function as the probe approaches the star, while the second – consisting of two movable, liquid-cooled panels of high-temperature cells – is specifically designed to withstand the high temperatures of the Sun's corona. As the spacecraft moves even closer to the Sun, these secondary arrays will be partially retracted behind the TPS in order to maintain constant temperature and power output, while a lithium-ion battery will function as a backup power source.
The craft, which is roughly the size of a small car, will be guided by a system of three star trackers, an inertial measurement unit, as well as a solar horizon sensor. Four reaction wheels for attitude control and a monopropellant system for trajectory correction maneuvers are also part of the system.
Back in 2009, NASA invited researchers to submit science proposals that would be useful toward its goals of solving the outstanding questions about the solar atmosphere. Now that the project is being finalized, the five chosen projects (whose total cost is estimated at around US$180 million) have been announced.
- Solar Wind Electrons Alphas and Protons Investigation: the project will capture particles in the sun's atmosphere, such as electrons, protons and helium ions, into a specially designed cup and will directly measure their properties.
- Wide-field Imager: a telescope will take 3-D images of the solar corona, including three-dimensional images of clouds and radiation shocks as they approach and travel past the spacecraft.
- Fields Experiment: this experiment involves the direct measurement of electric and magnetic fields, radio emissions and shock waves that course through the sun's atmospheric plasma. The experiment will also serve as a giant dust detector, registering voltage signatures as space dust hits the spacecraft's antenna.
- Integrated Science Investigation of the Sun: two instruments 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.
- Heliospheric Origins with Solar Probe Plus: directed by the mission's observatory scientist Marco Velli, the aim of this project is to provide an independent assessment of the scientific performance of the overall mission.