The Low Frequency Instrument (LFI) and High Frequency Instrument (HFI) on Planck will operate at extremely low temperatures; 20 K for LFI and 0.1 K for HFI. To achieve these low temperatures, the satellite employs a three-stage active cryogenic cooler. The last cooling stage makes use of the cooling effect of mixing 4He with the rare isotope 3He at low temperatures (around -272°C).
Both helium isotopes are stored under very high pressure in special tanks and the helium is depleted slowly during the mission. The supply cylinders containing the pressurised helium were inspected and filling activities started on Monday 9 March.