Planets: Who Needs 'Em?

And thank you. Right now, I'm having a bit of trouble
convincing people that the "world-ship" wouldn't be any
more prone to destruction than a city on Earth. I've
gotten them to shut up about "an angry crewman could open
all the airlocks and kill everybody!" and "an angry
crewman could fly it into the sun!" Only if the engineers
who design the thing are idiots.

Now I'm getting talk of asteroids and supernovae. Well,
yeah, that would destroy the world-ship, but then a city
on the surface of a planet wouldn't survive that either.

Anybody care to jump in? http://tinyurl.com/2wkfbzs