Sun and heliosphere in ESA's scientific programme

Abstract

Two of the five missions in ESA's Scientific Programme that are currently operated in orbit are devoted to solar and heliospheric science: ULYSSES and SOHO. ULYSSES explores the heliosphere in a heliocentric out-of-ecliptic orbit - predominantly through in-situ sensing of its local plasma environment and by solar-wind and cosmic-ray composition measurements. The Solar and Heliospheric Observatory, SOHO, which has been placed in a halo orbit around the first Lagrange point, investigates the global Sun, i.e., its interior, atmosphere, and wind by both remote-sensing and in-situ techniques, the latter comprising composition (mass, charge and energy) measurements of solar wind and of other solar and cosmic particle radiation. A further mission devoted to aspects of space-plasma, solar-terrestrial and heliospheric science — the four-space craft CLUSTER mission, which was to be placed in an eccentric near-polar orbit — was destroyed on 4 June 1996 on the ill-fated Ariane flight 501: CLUSTER was going to regularly cross the boundaries of the magnetosphere and emerge into heliospheric plasma. Ways to recover the foreseen 4-point space-plasma measurements are currently being studied. The current roster of ESA missions in solar and heliospheric physics thus corresponds to the importance of the topics and the size and vitality of the associated community. In contrast, none of the ESA missions that are under study or development are devoted to solar or heliospheric objectives; in the relevant discipline area - solarsystem science - there is currently an exclusive focus on planetary and comet science. Considering this fact, the Survey Committee, in preparing ‘Horizon 2000 Plus’, the continuation of ESA's initial long-term programme in space science, ‘Horizon 2000’, made the following recommendation: “In view of the great interest expressed by the solar-physics community, the Survey Committee recommends that ESA should, at an appropriate moment within the time frame of Horizon 2000 Plus, participate in an international solar mission or take advantage of opportunities provided by the Space Station or the small or medium-class missions of Horizon 2000 Plus.” Concepts for this mission opportunity are examined.