Solar Orbiter aims to make significant breakthroughs in our understanding both of how the inner heliosphere works, and of the effects of solar activity on it. The spacecraft will take a unique combination of measurements: in situ measurements will be used alongside remote sensing close to the Sun to relate these measurements back to their source regions and structures on the Sun's surface. It will operate both in and out of the ecliptic plane. Solar Orbiter will measure solar wind plasma, fields, waves and energetic particles close enough to the Sun to ensure that they are still relatively pristine.
The in situ instruments will operate throughout each orbit, whilst remote sensing will be confined to 30 days per orbit, in particular periods when the spacecraft is at its greatest angles to the solar equator, and during the closest approach to the Sun. During the nominal mission, Solar Orbiter will view the Sun from latitudes of up to 25°. This will enable the instruments to image the polar regions of the Sun clearly for the first time and make key measurements that will advance our understanding of the solar dynamo and the polarity reversal of the global magnetic field. After around seven years, Solar Orbiter will view the poles from solar latitudes higher than 30°, compared with 7° at best from the Earth.
The spacecraft will make a close approach to the Sun every five months. Around closest approach, when travelling at its fastest, Solar Orbiter will be positioned for several days over roughly the same region of the solar atmosphere, as the Sun rotates on its axis. Just as geostationary weather and telecommunications satellites are stationed over particular spots on the Earth's surface, so the spacecraft will seem to 'hover' for a while over the Sun. Solar Orbiter will therefore be able to watch magnetic activity building up in the atmosphere that can lead to powerful flares and eruptions.
Researchers will also have the chance to co-ordinate observations with NASA's planned Solar Probe Plus mission which will make in situ measurements in the Sun's extended corona (down to approximately 9.5 solar radii).
Solar Orbiter will set about answering four top-level science questions:
What drives the solar wind and where does the coronal magnetic field originate from?
Each of these questions requires both remote-sensing and in situ data to help form the answer, and almost every instrument on board Solar Orbiter will be used in some manner for each top-level question. Please see the links to the right for information about each question and what Solar Orbiter will do to answer them.