Board Thread:General Discussion/@comment-26120151-20150428233659/@comment-3263817-20150429000611

Mixels are a whole different species from humans and, from what we saw, can indeed breath in space. As for the dome? I don't know, just to be comical? Flamzer did point it out that they were just outside and could breath. Naut does say it protects them from the "hideous vacuum of space", which turns out to be a giant object in space after all (though just as a joke).

Point is, Mixels simply have the ability to breath in space, and the dome was likely just to be comical about that. We do see the dome shatter in Mixel Moon Madness though it likely went back to status quo, considering the dome is still there in static art of Orbitopia.