Zero G wouldn't be a problem. They have developed a means of generating self-contained gravitational fields inside their vehicles and operating independently of outer gravitational and inertial effects. At least that's what I would assume, given the maneuvers I saw that AFV do back in the late 60s. If they weren't entirely independent of the Earth's gravitational field, the ship would have been destroyed and everyone in it killed (assuming it had a crew on board, which seems very likely to me).