  The performance of the Planck instruments in space is enabled by their low
operating temperatures, 20K for LFI and 0.1K for HFI, achieved through a
combination of passive radiative cooling and three active mechanical coolers.
The scientific requirement for very broad frequency coverage led to two
detector technologies with widely different temperature and cooling needs.
Active coolers could satisfy these needs; a helium cryostat, as used by
previous cryogenic space missions (IRAS, COBE, ISO, Spitzer, AKARI), could not.
Radiative cooling is provided by three V-groove radiators and a large telescope
baffle. The active coolers are a hydrogen sorption cooler (<20K), a 4He
Joule-Thomson cooler (4.7K), and a 3He-4He dilution cooler (1.4K and 0.1K). The
flight system was at ambient temperature at launch and cooled in space to
operating conditions. The HFI bolometer plate reached 93mK on 3 July 2009, 50
days after launch. The solar panel always faces the Sun, shadowing the rest of
Planck, andoperates at a mean temperature of 384K. At the other end of the
spacecraft, the telescope baffle operates at 42.3K and the telescope primary
mirror operates at 35.9K. The temperatures of key parts of the instruments are
stabilized by both active and passive methods. Temperature fluctuations are
driven by changes in the distance from the Sun, sorption cooler cycling and
fluctuations in gas-liquid flow, and fluctuations in cosmic ray flux on the
dilution and bolometer plates. These fluctuations do not compromise the science
data.
