Solar Probe Plus: A Scientific Investigation Sixty Years in the Making

Abstract:

The in situ measurment of the conditions near the Sun's corona, responsible for coronal heating, solar wind acceleration, and energetic particle production and transport has been a high priority, but elusive, scientific goal since the beginning of the Space Age. The first proposal for a solar probe was from the six-man Fields and Particles Group (Committee 8 of the Space Science Board (SSB)) chaired by John Simpson of the University of Chicago. In their interim report of 24 Octobr 1958, the Group suggested a variety of missions, including "a solar probe to pass inside the orbit of Mercury to study the particles and fields in the vicinity of the Sun...". The exteme thermal and propulsive requirements were immediately recognized. Following initial trajectory studies using a variety of gravity-assist stategies, in the mid-1970's detailed mission and engineering studies for such a mission were carried out in the U.S. by the Jet Propulsion Laboratory (JPL) and in Europe by the European Space Agency (ESA). The mission rationale did not change substantially since the 1978 workshop at which Harold Glaser, then head of NASA’s Solar Terrestrial Program office asked the attendees “What can Solar Probe do that no other mission can do?” Answers provided at the workshop included solar energetic particle propagation effects, acceleration of the solar wind, and testing "the validity of the many models now in use for interpretation of remotely observed solar phenomena and interplanetary phenomena observed near 1 AU.” Studies in the 1980's emphasized a comprehensive payload passing to within 4 solar radii of the Sun's center. Budgetary concerns led to a "minimal mission" concept in 1995, followed by a more robust concept studied in 1999. A renewed study in 2005 was followed by a non-nucelar "Solar Probe Lite." The requirment to use solar power eliminated the use of a Jupiter gravity assist and a polar pass as close as 4 solar radii. However, the substitute of using multiple Venus gravity assists to "walk" the spacecraft perihelion in to 10 solar radii in Venus's orbital plane, also enabled far more time close to the Sun. This trade was found actually to enhance the science return and "Solar Probe Plus" was born, with a launch now set to occur just months shy of the 60th anniversary of the issuance of the Simpson committee interim report.