I was thinking that Phobos would be the comet dlc. The cabal launch Phobos at earth to destroy the traveller, and we have to go there to blow it up, kind of like Armageddon actually. The cabal have loaded the comet with troops and defenses to make sure we can't get to its core to lay the nukes, and there would be a strike or a raid to stop it or blow it up.