NASA’s Solar Probe Plus mission — which will fly closer to the Sun than any spacecraft has before — reached a major milestone last month when it successfully completed its Critical Design Review (CDR).
An independent NASA review board met at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, from March 16 to 20 to review all aspects of the mission plan; APL has designed and will build and operate the spacecraft for NASA. The CDR certifies that the Solar Probe Plus mission design is at an advanced stage and that fabrication, assembly, integration and testing of the many elements of the mission may proceed.
It will approach to within 8.5 solar radii (0.034 astronomical units or 5.9 million kilometers or 3.67 million miles, roughly 1/8 of the perihelion of Mercury) to the ‘surface’ (photosphere) of the Sun
The mission is designed to survive the harsh environment near the Sun, where the incident solar intensity is approximately 520 times the intensity at Earth orbit, by the use of a solar shadow-shield. The solar shield, at the front of the spacecraft, is made of reinforced carbon-carbon composite. The spacecraft systems, and the scientific instruments, are located in the umbra of the shield, where direct light from the sun is fully blocked. The primary power for the mission will be by use of a dual system of photovoltaic arrays. A primary photovoltaic array, used for the portion of the mission outside 0.25 AU, is retracted behind the shadow shield during the close approach to the Sun, and a much smaller secondary array powers the spacecraft through closest approach. This secondary array uses pumped-fluid cooling to maintain operating temperature.
As the probe passes around the Sun, it will achieve a velocity of up to 200 km/s (120 mi/s) at that time making it the fastest manmade object ever, almost three times faster than the current record holder, Helios II.
Artist rendering of Solar Probe Plus, solar panels folded into the shadows of its protective shield, as it gathers data on its approach to the Sun. (Credit: NASA/JHUAPL)
Solar Probe Plus is scheduled to launch aboard a United Launch Alliance Delta 4-Heavy rocket with an upper stage from Cape Canaveral Air Force Station, Florida. The launch window opens for 20 days starting on July 31, 2018. Over 24 orbits, the mission will use seven flybys of Venus to reduce its distance from the Sun. The closest three will be just 3.8 million miles from the surface of the star.
Scientists have long wanted to send a probe through the Sun’s outer atmosphere, or corona, to better understand the solar wind and the material it carries into our solar system. The primary science goals for the Solar Probe Plus mission are to trace the flow of energy and understand the heating of the solar corona and to explore the physical mechanisms that accelerate the solar wind and energetic particles.
To meet those objectives, Solar Probe Plus will carry four instrument suites into the Sun’s corona and study the solar wind and energetic particles as they blast off the surface of the star. These instruments will study magnetic fields, plasma, and energetic particles, and will image the solar wind. The instruments are: the Fields Experiment from the University of California Space Sciences Laboratory, Berkeley, California; the Integrated Science Investigation of the Sun (ISIS) from the Southwest Research Institute, San Antonio; the Solar Wind Electrons Alphas and Protons Investigation (SWEAP) from the Smithsonian Astrophysical Observatory, Cambridge, Massachusetts; and the Wide Field Imager for Solar Probe Plus (WISPR) from the Naval Research Laboratory, Washington, D.C.
The spacecraft and instruments will be protected from the Sun’s heat by a 4.5-inch-thick carbon-composite shield. During the closest passes around the Sun, temperatures outside the spacecraft will reach nearly 2,500 degrees Fahrenheit.
SPP will be subjected to up to 475 times the solar irradiance experienced at Earth