I can't help but theorize that the same beings/life forms that designed the dome were also responsible for that comet impact, probably by altering its trajectory.
I know I'm probably biased, since I wanted Dale to hit Eva so hard for hiding that key, and sentencing those folks who had survived the initial impact to a fiery death. I mean, I can sort of sympathize not having enough rations to feed those extra people, but who says that they couldn't compromise by telling the folks to either bring their own food (well, if they could find anything that would survive getting soaked), or to not expect to get feed much, if at all.
What really pissed me off is that she dismissed those banging on the walls of the shield as "part of the old world", as if their lives didn't matter one bit.
Which makes me think that the E.T.s inhabiting and influencing the Chester's Mills folks had intended from the start to wipe out the humans, wait out that comet strike nice in snug in those cocoons, under their shield, and then use the Chester's Mills folk's bodies as vessels to take over Earth for themselves when the dust settled.
I mean, why try to settle on a planet that they obviously knew was going to get smacked? Aren't there other inhabitable planets to inhabit, like that planet whose name I forgot that we discovered recently? Or was that their homeworld that got smacked by a comet (must've made the planet uninhabitable for them to want to leave), and Earth was the only inhabitable planet they could find, which they knew would survive that comet, albeit not without being bruised?
Of course, another question is what the hell happened to their bodies? Or could they only travel to Earth in the form of energy, not unlike Sargon and his buddies?