What happens if you fire a bullet from a plane?
What happens if you fire a two bullets from an aircraft moving at 1000mph? One is fired forwards at 1000mph and the other backwards at 1000mph.
This is moving relatively slowly so this isn't complicated by the problems of relativity. All you have to do is add up the speeds. The one going forwards will be going at 1000mph plus 1000mph so 2000mph. The one going backwards will be going at 1000 minus 1000, so not at all it will fall straight down.