Cutaway view of Planck, with the temperatures of key components in flight. The solar panel at the bottom always faces the Sun and the Earth, and is the only part of the flight system illuminated by the Sun, the Earth, and the Moon. Temperature decreases steadily towards the telescope end, due to low-conductivity mechanical connections and aggressive use of radiative cooling. The focal plane detectors are actively cooled to 20 K and 0.1 K.