*Spoiler* Question about Cortana's "fate"...

Ok, so like everyone else I was blown away by the ending of Halo 4. However, after i played through it something about the scene stuck out to me. During the scene where Cortana is saying goodbye to John she mentions that most of her is “down there.” Yet when the scene ends debris starts falling from above where the chief is standing. Now i’ll admit I’m not an expert one nuclear physics, but i would assume that if the ship is below Master Chief and a nuke detonates on that ship wouldn’t it send debris upward? Actually now that i think about it do we have any real confirmation that the didact’s ship/composer was destroyed? Is it possible that maybe the rampant cortana copies that she injected into the ship deactivated the nuke and jumped into slipspace in order to preserve themselves? Or am I missing something?

Was Chief flipped right-side up when he was saved by hard-light mode Cortana?

I get the feeling she may be referring to “down there” as if the bulk of the ship’s debris had already fallen into Earth’s atmosphere.

Unless she was referring to the slipspace portal that was below Chief when he was battling Didact. That most of her memory had gone through the portal and all that was left was a small echo that used up all of its remaining life to hold that hard-light bubble together.