Think about how the Polynesians colonized Hawaii and New Zealand. They didn't do it with hundreds of thousands of people travelling in Boeing 747s. They did it with hundreds of people in open outrigger canoes voyaging thousands of miles across open ocean with almost no navigation. Compared to the Polynesian achievement, colonzing Mars is almost trivial. Mars could be colonized with less that 1,000 people using a single cycler spacecraft (500 people is a large enough group against inbreeding so 1000 are sent assuming 50% mortality). This sort of Mars program is only a little bit more difficult than a pointless flag-and-footprints mission (I'd say $100 billion versus $50 billion for a well managed program not micromanaged by politicians).

As far as defending against rocks from space, Mark has already stated the punch line. Assuming near term technology, there is no real defense against long period comets. They'd be good sized rocks (5-10 km dia.) coming in at 30-80 km/sec with only a few months warning. Before we can deal with long period comets, we need an interplanetary transportation infrastructure such as would exist to support commerce between civilizations on Earth and Mars. When we have spacecraft the size of supertankers powered by nuclear fusion propulsion then we can deal with long period comets.

Gary