If the ship is at a stop, a=0....therefore, regardless of how much it weighs, F=0. Any force applied to that object would move it. And as I stated above, and what you simply seemed to restate in your post, the amount of force applied to the object determines it's acceleration, not whether it could be moved or not. As long as the ship is not accelerating (changing velocity), a force upon that object of even the slightest amount would cause the object to have an acceleration.... I'm not trying to say opening a valve on a compressed oxygen tank will take you to a different galaxy. I'm just saying that even the tiniest amount of force on the object will, in fact, move it.