Tom,

We can plan years in advance for colonizing Mars. We can't for long period comets. The long period comets are the ones we are least likely to be able to deal with due to low warning time, large size, and high speed. Years of planning and 6 month journeys are reasonable for colonization, but not cometary diversion.

There is also the matter of increasing size. The kinetic energy of an incoming object is proportional to the cube of its diameter and there are potential impactors far requiring much more advanced technology than that needed for colonization.

I don't believe the technology needed to colonize Mars is as far beyond our current capabilities as stopping long period comets. The biggest barrier to colonization currently is cost. Colonization should be a private not government effort and much lower costs are needed to allow that. Even if it was considered a logical government priority it would be prohibitively expensive currently. Development of more powerful/efficient launch vehicles would benefit either Martian exploration or Planetary protection as well as other options. Choosing power over cost reduction would be more beneficial to protection than colonization, and I'd be agreeable to that approach, but it overlooks other reasons for reduced launch costs.

While we don't yet have other important colonization technology most seem to believe it is simply a matter of spending the time and money for development. The matter of launch cost may be similar, but much effort has already been put into that, and it appears to me to be the most difficult technical barrier.

Regards,
Mark