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Solar Orbiter is a joint ESA-NASA collaboration that will address the central question of heliophysics: How does the Sun create and control the heliosphere? This primary, overarching scientific objective can be broken down into four interrelated scientific questions:
- How and where do the solar wind plasma and magnetic field originate in the corona?
- How do solar transients drive heliospheric variability?
- How do solar eruptions produce energetic particle radiation that fills the heliosphere?
- How does the solar dynamo work and drive connections between the Sun and the heliosphere?
These are outstanding fundamental questions in solar and heliophysics today. By addressing them, Solar Orbiter will make major breakthroughs in our understanding of how the inner solar system works and is driven by solar activity.
To answer these questions, it is essential to make in-situ measurements of the solar wind plasma, fields, waves, and energetic particles close enough to the Sun that they are still relatively pristine and have not had their properties modified by subsequent transport and propagation processes. This is one of the fundamental drivers for the Solar Orbiter mission, which will approach the Sun to 0.28 AU.
Solar Orbiter is a three-axis stabilized spacecraft equipped with instruments for both in-situ measurements and remote-sensing observations. It will be placed into an elliptical orbit about the Sun with perihelion as close as 0.28 AU. After an in-ecliptic phase of perihelion passes where it is nearly corotating with the Sun, Solar Orbiter will use multiple Venus gravity assist maneuvers to move the inclination of its orbit to progressively higher heliolatitudes, reaching ~34° by the end of its extended mission.
NASA is contributing two instruments to the collaboration, as well as an expendable launch vehicle. The two U.S. contributions are SoloHI, a wide-field heliospheric imager provided by the Naval Research Laboratory; and a Heavy Ion Sensor (part of the Solar Wind Analyzer) provided by Southwest Research Institute. ESA is providing the spacecraft, built by Astrium, U.K.; and European nations are providing the remainder of the ten instrument payload. The launch date from Kennedy Space Center is currently scheduled for 2017.
Relating these in-situ measurements back to their source regions and structures on the Sun requires simultaneous, high-resolution imaging and spectroscopic observations of the Sun in and out of the ecliptic plane. The resulting combination of in-situ and remote-sensing instruments on the same spacecraft, together with the new, inner-heliospheric perspective, distinguishes Solar Orbiter from all previous and current missions, enabling breakthrough science which can be achieved in no other way.