> This icing issue is not uncommon, and, as a result, part of the telescope’s commissioning phase included an “outgassing campaign.” This process involved Euclid turning to face the Sun for a total of 96 hours. This process was implemented specifically to reduce the risk of ice build-up after the agency’s Gaia mission faced a similar issue.
It's not going to be in a vacuum inside the payload shroud. There's air, and that air contains some amount of moisture. And Florida isn't known for its dry air (it was launched from Cape on a Falcon 9 – but French Guiana would hardly have been better in that respect).
> It was always expected that water could gradually build up and contaminate Euclid’s vision, as it is very difficult to build and launch a spacecraft from Earth without some of the water in our planet’s atmosphere creeping into it.
Satellites are transported in very specialized containers that will often be purged with dry nitrogen. Then after encapsulation in the faring, conditioned air of whatever sort is pumped into the faring from rollout to launch. [Example air conditioning duct attached to a Thor-Agena](https://space.stackexchange.com/questions/39440/why-use-an-a...)
