Satellites are assembled in clean rooms, which have about 50% relative humidity to avoid electrostatic discharge. The multilayer thermal insulation or MLI recaptures 1% of its own weight after bake-out, within 24 hours in a normal atmosphere. In the vacuum of space at cryogenic temperatures, it can take years and decades for the MLI to dry up. It's a problem for all satellites, but Euclid is particularly sensitive. The Falcon9 fairing was purged with gaseous nitrogen for 4 hours prior to Euclid's launch, to at least minimize the water that could freeze out directly from the atmosphere traveling into space with the satellite.