I know the sentence means something like "This suddenly came out of the sky...".
"これが" means "this is" (side question: why add "が" instead of just keeping it as "これ"?)
"突然空" means "suddenly" and "sky"
"から" means "from"
But I'm not sure what "ふってきて" means. Is it a variation of "ふる"?, if so, why alter it?