The design of the Planck spacecraft allows it to cool down passively to about 45 K (-228 °C) by radiating heat towards the cold space environment. However, to reach the required operational temperatures for the two instruments (20K for LFI and down to 0.1 K for HFI), Planck is equipped with a sophisticated active cooling system. This system comprises three different coolers that act on different stages of the two Planck instruments, respectively down to 20K, 4K and 0.1K above absolute zero. The passive cool down of the spacecraft in the space environment started immediately after launch on May 14th. The two coldest coolers (0.1K dilution cooler and 4K Joules-Thompson cooler) were switched on very shortly after launch, but at a low level. This was done to allow the gas that these stages use for cooling to flow through the system's pipes, to prevent any potential clogging of these pipes that might be caused by the rapid passive cool-down.
Two days after launch the coldest of the spacecraft's three V-grooves — panels that isolate thermally the payload from the hot spacecraft and help to cool the spacecraft by passively radiating excess heat — was already at 75 K. However, the Focal Plane Units (FPUs) of the HFI and LFI as well as the telescope were actively kept at a higher temperature of about 170 K (-103 °C) for two weeks, using dedicated heaters. This was to prevent any contamination, from the spacecraft that is outgassing early in the mission, to freeze onto the surface of these elements.
On June 1st, the anti-contamination heaters were switched off and the focal plane and telescope were allowed to resume the passive cool down. Three days later, on June 4th, when the LFI FPU had passively cooled to 100K, the sorption cooler (20 K stage) was switched on. The sorption cooler is responsible for actively cooling the LFI Focal Plane Unit to its operational temperature of 20K and pre-cools the HFI Focal Plane Unit to 18K. Its coldest operational level was reached after nine days, on 13 June, with a temperature of ~17.5K. As the next steps in activating the cooling system, the remaining two coolers (4K and 0.1K coolers), which had been switched on at a low level shortly after launch, were set to full power one after the other on June 28th. With all coolers of the active cooling chain in operation, the detectors in the HFI Focal Plane Unit finally reached the 0.1K level on July 3rd. For the two instruments HFI and LFI, the next and last phase of their commissioning before the start of nominal science operations is the Calibration and Performance Verification (CPV) phase. This phase will now last until mid-August, during which they will carry out specific activities targeted to calibrate both the HFI and LFI. Upon completion of the CPV phase, Planck will start its scientific mission, surveying the whole sky twice over within about one year.