Thermal environment

The satellite design and its location at L2 provide an extremely stable thermal environment. The main temperature variation on long timescales is driven by the total radiative power absorbed by the solar panels, which varies depending on distance from the Sun and the solar aspect angle (i.e. the angle between the solar direction and the spin axis). On shorter timescales, temperature variations are driven by active thermal regulation cycles. Both seasonal and shorter-timescale variations are observed across the satellite’s service module (SVM), but are heavily damped and almost unobservable within the payload module (PLM).

Specific operations and deviations from the scanning strategy have a thermal influence on the satellite and payload. Some significant effects are clearly visible in Fig. 6 and listed below.

– The thruster heaters were unintentionally turned off between 31 August and 16 September 2009 (the so-called “catbed” event).

– As planned, the RF transmitter was initially turned on and off every day in synchrony with the daily visibility window, in order to reduce potential interference by the transmitter on the scientific data. The induced daily temperature variation had a measurable effect throughout the satellite. An important effect was on the temperature of the 4He-JT cooler compressors, which caused variations of the levels of the interference lines that they induce on the bolometer data (Planck HFI Core Team 2011a). Therefore the RF transmitter was left permanently on starting from 25 January 2010 (257 days after launch), which made a noticeable improvement on the daily temperature variations.

– A significant thermal effect arises from the (approximately) weekly adjustments to the operation of the Sorption Cooler.

The thermal environment of the payload module is – by design – extremely well decoupled from that of the service module. As a consequence, in spite of the significant thermal perturbations originating in the SVM, the thermal variability affecting the detectors is essentially completely due to the operation of the cryogenic cooling chain (described in detail in Planck Collaboration 2011b), which ensures their cold environment.