The ESA Astronomy Missions at L 2 : FIRST and Planck
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The ESA Astronomy Missions at L 2: FIRST and Planck 1 F. Felici,2 M. Hechler,3 and F. Vandenbussche! Abstract FIRST and Planck are approved ESA Astronomy missions with different objectives but with similar requirements of a highly stable thermal environment and sky viewing conditions unobstructed by Earth and Sun. A class of orbits near the second Lagrangian point L 2 in the Sun-Earth system has been selected for these projects. FIRST will investigate a region of the electro-magnetic spectrum still unexplored (85670 /-Lm) i.e. the far infrared. These observations are expected to reveal very cold and distant building blocks of the universe. Planck is a third-generation tool for the mapping of the cosmic background radiation anisotropy. The study of this "relic" radiation will allow a study of unprecedented accuracy of the universe as it was only 300,000 years after the Big Bang. Both scientific payloads require sub-Kelvin temperatures at the limit of today's cooling technology for space. The two spacecraft will be launched from Kourou by a single Ariane 5 in early 2007, into a nearly equatorial and nearly parabolic orbit. This orbit can be chosen to be on the stable manifold of a large amplitude Lissajous orbit around L 2 • The choice of the target Lissajous orbit will depend on the launch date. FIRST will then remain on this orbit, without any further, deterministic maneuvers. Planck will perform an "amplitude reduction maneuver" onto the stable manifold of another Lissajous orbit which at the end has a maximum Sunspacecraft-Earth angle of less than 10° as required for the Planck mission design. The scientific observations at L 2 will last a minimum of three years (FIRST) and one year (Planck).
Project Evolution FIRST (Far InfraRed and Submillimetre Telescope) and Planck (named after Max Planck) are two approved missions of the European Space Agency (ESA). As such they have been gaining momentum through a complex development program which faces novel programmatic challenges imposed on ESA by the evolution of European Space policies. FIRST's Evolution
FIRST is the observatory-type mission chosen in November 1993 as the fourth cornerstone of the Horizon 2000 Science Program to explore for the first time the 'This paper was invited by Robert Farquhar and is based mainly on references [19-20]. 2ESA/ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands. 3ESA / ESOC , Robert-Bosch-str. 5, 64293 Darmstadt, Germany.
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85-670 J.Lm wavelength range. The detectors of its instruments need to be cooled at a cryogenic temperature in the range of 0.3 K to 2 K. At the time of selection the first stages of the cool-down of the two instruments was expected to be achieved by a series of mechanical Stirling coolers, under development by European Industry. Another major technology challenge of FIRST was expected to be the tight wavefront error required for the 3 m Cassegrain telescope. A highly eccentric 24 h-orbit (perigee 1000 km, apogee 70500 km) was chosen for FIRST at that time. The
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