First off, its a myth that a large asteroid or comet can not be dealt with, and dealt with with existing technologies. It takes a long lead time and (gasp!) extremely large nuclear charges (150 megatons) and nuclear (double gasp!) thermal propulsion. But this is only for the largest bodies - the other smaller ones (the ones that hit most ofter) can be dealt with much more easily -

Second, it does not take a large impactor to kill large numbers of people - smaller ones will raise enough dust to cause climate collapse, and with a population of 6,000,000,000, a large number will starve to death - unless they resort to canibalism -

Third, these smaller ones have been hitting recently at a VERY HIGH RATE

Nick, the way you calculated survival chances stinks.

At most we could put maybe 100-200 people on Mars. That leaves nearly all of the remaining 6,000,000,000 on Earth. Multiply the chances times the populations -

Second,if we spend the money required for a manned Mars flight first on planetary defense the chances of either population dying falls to 0.

I need to get back to work -
EP