1) Why do SETI fans think searching for extraterrestrial signal is worth the effort?

2) Why do SETI critics think the effort is wasted?

3) Suppose that astronomers announce that an asteroid big enough to destroy the united states will strike Ohio in ten years. What could we do about it?

4) Given the inevitability of an eventual asteroid impact, how important is it that we plan ahead? How much should we spend per year on preparations for warding off the impact or recovering afterward?