The Planck spacecraft's active cooling system builds on the thermal environment provided by the passive cooling system. It comprises three different coolers and ultimately reaches a temperature close to absolute zero to bring the instrument detectors to their final sensitivities:

1. a hydrogen sorption cooler that cools the LFI FPU to 20 K and pre-cools the HFI FPU to 18 K
2. a Joule-Thomson cooler that cools the HFI FPU to 4 K
3. a ³He/⁴He dilution cooler that cools the bolometers of the HFI FPU to 0.1 K

20 K cooler

Planck cooling system with the 20 K cooler elements highlighted.

The hydrogen leaves the heated compressor element and flows through piping to the cold end, located inside the focal plane. The piping runs past the three V-grooves that efficiently pre-cool the gas through heat exchange down to about 45 K. At the cold end the gas undergoes a Joule-Thomson expansion, producing liquid droplets whose evaporation provide the cooling power. After cooling the FPU the hydrogen is guided back to the compressors to be re-absorbed. Cycling through the six compressor elements (switching from one compressor to the next when it has released its hydrogen) achieves a constant massflow and cooling power.

To provide full redundancy to the mission, two identical 20K coolers are installed on the Planck spacecraft. The 20K coolers were built by the Jet Propulsion Laboratory (Pasadena, California, US), a member of the LFI and HFI Consortia.

4 K cooler

Planck cooling system with the  4 K cooler elements highlighted.

At the cold end the ⁴He gas undergoes a Joule-Thomson expansion leading to small amounts of liquid helium that provides the cooling power and cools the HFI FPU to about 4 K. The 4K cooler was built by the Rutherford Appleton Laboratory (Didcot, UK), a member of the HFI Consortium.

0.1 K cooler

Planck cooling system with the 0.1 K cooler elements highlighted.

The working principle of the 0.1 cooler is based on the thermo-dynamical properties of ³He/⁴He mixtures at a temperature below ~1K. A phase transition induced by over-saturating the mixture of these two helium isotopes with ³He causes the mixture to cool, drawing heat from its surroundings.

Three high pressure tanks filled with ⁴He and one high pressure tank filled with ³He are housed in the service module. From each tank a pipe runs to the flow regulation control unit. From here the helium isotope gases flow separately towards the HFI FPU. Along the way the gases progressively cool down through heat exchangers with the spacecraft's three V-grooves and with the 18K and 4K stages of the active cooling system. Within the HFI FPU the ³He and ⁴He are mixed, after which the mixture cools and the detectors reach 0.1 K.

The 0.1 K cooler is an open system: at the end of the cooling process in the FPU the gas mixture is guided out through piping and vented into space. Over the course of the mission the helium from the four tanks will slowly be depleted. The required gas flow rates, however, are very low and the amount of ³He and ⁴He that Planck carries on-board is enough for about 3 years of operations.

The 0.1K cooler was built in collaboration by the Institut Néel (Grenoble), the Institut d'Astrophysique Spatiale (Orsay, FR), both members of the HFI Consortium, and DTA Air Liquide (Grenoble).