There's perhaps a small blessing in the sudden vacuum that swallows Seras's words, that being that Jan's undoubtedly raucous and obnoxious laughter is also silenced. He leaps immediately, nearly as soon as the door is open, springing out onto the surface like a maniacal deer. You look pretty stupid trying to move in 16% gravity, but Jan's had practice enough by now to at least make it effective.