  Solar Orbiter, the first mission of ESA's Cosmic Vision 2015-2025 programme
and a mission of international collaboration between ESA and NASA, will explore
the Sun and heliosphere from close up and out of the ecliptic plane. It was
launched on 10 February 2020 04:03 UTC from Cape Canaveral and aims to address
key questions of solar and heliospheric physics pertaining to how the Sun
creates and controls the Heliosphere, and why solar activity changes with time.
To answer these, the mission carries six remote-sensing instruments to observe
the Sun and the solar corona, and four in-situ instruments to measure the solar
wind, energetic particles, and electromagnetic fields. In this paper, we
describe the science objectives of the mission, and how these will be addressed
by the joint observations of the instruments onboard. The paper first
summarises the mission-level science objectives, followed by an overview of the
spacecraft and payload. We report the observables and performance figures of
each instrument, as well as the trajectory design. This is followed by a
summary of the science operations concept. The paper concludes with a more
detailed description of the science objectives. Solar Orbiter will combine
in-situ measurements in the heliosphere with high-resolution remote-sensing
observations of the Sun to address fundamental questions of solar and
heliospheric physics. The performance of the Solar Orbiter payload meets the
requirements derived from the mission's science objectives. Its science return
will be augmented further by coordinated observations with other space missions
and ground-based observatories.
