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B. Fleck (ESA), E. Marsch (MPAe), R. Schwenn (MPAe), E. Antonucci (Univ.\,Torino), P. Bochsler (Univ.\,Bern), J.-L. Bougeret (Meudon), R.A. Harrison (RAL), R. Marsden (ESA), J.-C. Vial (IAS)
The scientific rationale of the Solar Orbiter (SO) is to provide, at high spatial and temporal resolution, observations of the solar atmosphere and unexplored inner heliosphere. The most interesting and novel observations will be made in the almost heliosynchronous segments of the orbits at heliocentric distances near 45\,R\odot and out-of-ecliptic at heliographic latitudes of up to 38\circ. By going to 45\,R\odot the SO will allow remote sensing of the solar atmosphere with unprecedented spatial resolution, and the almost heliosynchronous orbit segments will permit us to disentangle spatial and temporal variations in the solar wind in close linkage with the plasma and radiation conditions in the source regions of the Sun.
The strawman payload encompasses two instrument packages: Heliospheric Instruments --- high-res visible light telescope and magnetograph (<40\,km), high-res X-ray/EUV imager (<30\,km), high-res EUV spectrometer (<100\,km), EUV and visible-light coronagraphs, solar neutron and \gamma-ray detectors, radiometers. Heliospheric Instruments --- solar wind analyzer, magnetometer, energetic particle detectors, IP dust detector, plasma wave analyser, radio experiment, neutral particle detector.
Using solar electric propulsion (SEP) in conjunction with multiple planet swing-by manoeuvres, it will take SO two years to reach a perihelion of 45\,R\odot at an orbital period of 149 days, with an inclination ranging from 6.7\circ to 23.4\circ w.r.t. the ecliptic. During an extended mission phase of about 2 years the inclination will increase to 31.7\circ, leading to a maximum heliographic latitude of 38.3\circ.
The SO was one of the about 40 responses to the Call for Proposals for the next two "flexi-missions" (F2 and F3) within ESA's Scientific Programme. At its meeting on 1 March 2000, ESA's Space Science Advisory Committee recommended the Solar Orbiter among 5 other proposals for an assessment study. Launch is expected by the end of the decade.