How big would an asteroid need to be to wipe out all life on Earth?

[bettysfetish]

Oh well, I guess your right; it was a whimsical thought at best I suppose. You guys have pointed out a couple things that may be prohibitive, although I dispute a couple of points as well, but a chain is only as strong as it's weakest link, and this idea, obviously has a couple very weak links. Thanks for your input. Thats what I came to you for. Hey - - - - Mabey a good movie; that's the ticket.
Thanks again.
L8R