Thought I would start a thread to for a discussion of the NASA star ship. 

http://www.worldswithoutend.com/index.asp?view=plink&id=344

 What Questions need to be asked before planing this can even start?

 1. What size population does the ship carry to prevent birth defects due to inbreeding?

 Q1. Would determine the minimum size of the space ship required.

 2. What are the inhabitants supposed to do in the case of a population explosion on board?

 I would think that the boredom of only having the limited activities might play a role in this. 

 Let the lively debate start.

I'll play along.

1. What size population does the ship carry to prevent birth defects due to inbreeding?

I'm guessing that by time a generation ship could be built we'd have enough understanding of genetics to prevent any mutations  In fact we'd probably be manipulating genes to make humans more able to deal with life aboard the ship.  That's a pretty common theme in SF and it makes sense that if we could tweak the inhabitants of the ship we would.

I have no earthly idea how many people would be needed to man the ship and ensure a stable colony at their destination though.

2. What are the inhabitants supposed to do in the case of a population explosion on board?

I'm guessing there would be safeties in place to prevent a population spike: contraception, sterilization (reversible of course), planned parenthood, birth limits etc. as well as precautions for a downturn in population like frozen embryos or artificial insemination and the like.