HERSCHEL, PLANCK AND GAIA ORBIT DESIGN

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HERSCHEL/PLANCK (double launch in 2007 on ARIANE) and GAIA (launch in 2010 on SOYUZ/FREGAT) are Astronomy missions in the ESA Scientific Program with different objectives but with quite common requirements of a highly stable thermal environment and sky viewing conditions unobstructed by Earth and sun. A class of orbits near the L2 libration point (outside Earth) in the sun-Earth system has been selected for these projects. Not differentiating in the conventional way between Halo or Lissajous orbits, a family of non-escape orbits around L2 has been classified solely by their property of neither falling towards the Earth nor to the sun within the limits of numerical precision of the initial conditions. The stable manifolds of some of the Lissajous orbits in this family (generally with large amplitudes) touch perigee conditions which can be naturally injected into by a launcher, e.g. a low perigee altitude, and specifically for ARIANE a line of apsides near the equator plane. Thus a ’free’ transfer to some of these orbits which requires no manoeuvres after perigee, except stochastic orbit corrections, exists. Starting from the free transfers to the large amplitude non-escape orbits, transfers to small amplitude (e.g. maximum sun-spacecraft-earth angle below 10◦) have been constructed combining the linear theory for orbits in the restricted circular three body problem with the numerical algorithm. The linear theory defines directions of escape (e-term) and non-escape in the velocity subspace. Manoeuvres along the non-escape direction which are optimum in the linear theory to change the orbit amplitude, are numerically corrected along the escape direction. By the same approach also optimum eclipse avoidance strategies could be derived, which guarantee a mission of at least 6 years without eclipse with a manoeuvre of typically 15 m/s. The eclipse avoidance manoeuvres are performed during the last revolution before the eclipse occurs near one of the maximum amplitudes, and essentially revert the track of the motion in the plane orthogonal to the sun Earth line. Herschel and Planck will share a launcher, either an ARIANE 5 ESV (upper stage with delayed ignition after coast arc) or an ARIANE 5 ECA (cryogenic upper stage) launched from Kourou. Herschel will remain on the large amplitude orbit to which the launcher naturally delivers, whereas Planck will perform an amplitude reduction manoeuvre. GAIA will be launched by a Soyuz/Fregat launch from Baikonur, with or without lunar gravity assists and the necessary phasing orbits. In all cases the combination of the orbit amplitudes (in ecliptic and orthogonal to it) for a given size (maximum sun-spacecraft-Earth angle) and the initial phase in the orbit will depend on the launch date. Launch windows will be given. Finally the navigation and orbit maintenance aspects of the mission have been studied for the transfer and for the phase in the Lissajous orbit comparing several maintenance strategies. The preferred strategy is using a constant manoeuvre direction (along the escape line in the linear theory) which allows a dedicated spacecraft design.