THE MISSION

PLANCK is a satellite pointed sunward with small manoeuvrability. To avoid the stray light from the Earth, the chosen orbit is a small Lissajous orbit at L2 Lagrange point of the Earth-Sun system, with a Sun-Satellite-Earth angle limited to 15 degrees. This constraint imposes an insertion manoeuvre at the arrival to the L2 point to reduce the orbit amplitude, which must be taken into account in the determination of the launch window.

Advantages of an L2 orbit:

due to the constant distance of the Earth and the Sun, the satellite thermal environment is very stable. The thermal radiative effects of the Earth are low and lead to a cold environment favourable to cryogenic satellites such as PLANCK and HERSCHEL, the radiative environment is very low compared to excentric orbits, or even to GEO, the solar panels directed towards the Sun and the Earth, shielding the payload from the solar thermal radiation and the stray light from the Earth, also favour the satellite-Earth communications.

In orbit, PLANCK will sistematiquely sweep the sky with a rotation axis oriented antisunward, a rotation speed of 1 turn/minute and a telescope line of sight angle at 85° from the rotation axis enabling a sweeping by big cercles of 85°. To even enable to sweep celestial polar areas, a 10° depointing from the rotation axis from the Sun is ensured by the satellite.
PLANCK turns around the Sun in 1 year. The rotation axis must stay pointed sunward during this periode by regular accurate manoeuvres while maintaining the observation strategy, imposing constraints on the rotation axis angle (-XS) and the satellite-Earth line which must be kept at 15°.

PLANCK observation strategy and attitude constraints (rotation axis - Sun angle lower than 10°)