Notice how the metallic orbs are only ever captured from moving aircraft, generally looking down on them. That's because they are balloons, and the aircraft's m

Notice how the metallic orbs are only ever captured from moving aircraft, generally looking down on them. That's because they are balloons, and the aircraft's motion give them the appearance that they're moving quickly relative to the ground behind them, when in fact they're moving quite slow, as balloons do. Either that, or the aliens made their craft transparent on the bottom but forgot to make them transparent on the top.