1 the original vid of that was set to Pink Floyd's Great Gig in the Sky, much better imo
http://www.youtube.com/watch?v=-zvCUmeoHpw2 according to the Op2 storyline, the asteroid was small enough to be fragmented by a nuke, that puts an upper limit on the size of the asteroid, and its destructive capability's.
The size limit of the asteroid appears to be the same as the one that caused the extinction of the dinosaurs 65 million years ago (Op1 states that it was a million megatons, which is roughly the same as the Chicxulub impact).
Such an impact wouldn't have would not have reduced earth to a molten sphere as in the video. The area around the impact would be sterilized by pyroclastic events, and tsunami's would be extremely devastating, and there would be other geological effects. But, the main killer would be the dust that got kicked up into the air from the impact, it would linger for years choking off the suns rays. This stops the food chain at its source, the plants die, the herbivores die, the carnivores die, and that is what causes the great extinctions.
That also leads to an interesting thought, according to the Op2 storyline, at the time of the impact humanity had the technology for cryogenic stasis, and cool fusion reactors to power them for long periods of time. Due to that fact, do you really believe that they would save only 200 people, and the other 6-7 billion would just accept their fate? Hell no, you can bet your ass that every single bomb shelter across the globe would have these stasis pods packed like sardines. The humans wait for the dust to settle, reawaken, plant their stored crops, and rebuild society.
It took around 100 years for the Conestoga to arrive at New Terra, the dust would only last 1/20 of that time, by the time the Conestoga reached its detonation humanity has probably rebuilt itself a good portion back to its former glory (or it could be one of those post apocalyptic settings that's been done to death).
Regardless, the Colonists the colonists of New Terra would NOT be the last surviving humans, but that loophole kinda takes away a good portion of the Zing of the storyline, which is probably why it was left out.
http://www.lpl.arizona.edu/impacteffects/ is a good guide to the effects of different impacts.
