Near Earth Objects (NEO) pose a risk to mankind varying in scale from bad locally, think Haitian earthquake, to universally devastating, think of the end of the dinosaurs. Options for dealing with this risk range from doing nothing to last minute emergency measures, as in Balmer and Wylie's "When Worlds Collide".
The efficacy of any of these choices is an exercise in probability in which the values of many variables are guesswork. Some of the asymptotic factors are that significant meteor impacts happen. The Tunguska impact of 1908, a multi-megaton event, is fairly widely known. Was surprised to learn that a kiloton range impact occurred in Norway in 2006*. Sooner or later, then, there will be a meteor with your, my or our names on it. While ignorance may be bliss, it is also an element of certainty. If we don't observe NEOs, then we won't know what hit us. Another certainty is that if man lacks the technology to assemble some countermeasure in space, whatever we observe is simply going to hit us. In the last fifty years we have moved closer to an ability to affect outcomes. How do we proceed from here?
Timing and targeting are important variables. If even kiloton range impacts are rare, and given that only something on the order of 5% of the earth's surface is heavily populated, doing any advance preparation can seem excessive. Tunguska was so remote that it took years to actually survey its extent. People saw the Norwegian meteor pass overhead, felt and heard the impact, but none were near enough to be injured. At this point we have an identified 500 megaton impactor, Apophis, with a small measurable chance of hitting us in 2036. Even if it struck in the sea, a good bet, the effects would not be trivial**. Assessing the monetary affects of such a strike would be a good place to start on the feasibility of prevention.
The feasibility of observation is now proven with Apophis. But Apophis is pretty big, about a quarter of a kilometer in diameter, Tunguska was about a tenth that size. The Norwegian meteor was undetected until it was in the atmosphere over an inhabited area. Good news is that a 50 meter diameter meteor was detected on a potential collision course with Mars in 2007. It missed, but the advance observation is encouraging. A challenge, going forward, would be to forecast impact locations, at least to within a hemisphere. Evacuation and dispersion might then become a reasonable defensive scenario.
Prevention is another interesting topic with a time dimension. Early detection and intervention require minimum energy inputs. At too late a stage, our technology lacks the necessary power to affect the matter. A number of intervention technologies were discussed in "To Move An Asteroid", cited below. This is another place were time comes into play. Today any of these technologies would be astronomically costly. When, or if, we become a space-faring race they will simply be line item(s) on the budget of whatever we call the equivalent of the Coast Guard.
The desirability of moving toward the second situation seems overwhelming in the long run. The pace of that movement has disappointed some. Stanley Kubrick overestimated the pace of this trend in "2001, A Space Odyssey". Possibly Cordwainer Smith*** underestimated it. What is going to affect the rate of change? Well, obviously, the vast expansion in the use of orbital space is based, mostly, on simple economics. Telecommunications is a money-maker and satellite routed communication is cheap enough to be competitive. What might affect this in terms of things heavier than photons?
Unlike the development of worldwide shipping in the 16th to 19th centuries, there doesn't appear to be anything outstandingly valuable in space to justify voyaging there. Two lower priority demands might be bulk materials and establishment of a "last refuge" in the event of a dinosaur-killer sized impact.
The demand for materials is an interesting economic question. The more we use space, the more materials are needed. Materials lifted from Earth are hugely expensive because of the cost of the lift. Relatively speaking, though, they can be had pretty quickly, easily, and using present technology. The cost just has to be factored into the use. The flip side of this coin is that materials which have been lifted to orbit have a value. Possibly the first space business will be salvage and scrap. Mining the asteroids would involve similar technologies as deflecting meteors. It could have small energy requirements****, but would require large amounts of time. As Ben Franklin said, "Time is money." How much money is a matter of rate of return. Rate of return is, partly, a matter of the stability of the value of money. Arguably, then, an economic policy of very low inflation is a strategy for encouraging space travel and protecting the planet from asteroid/meteor collision.
As to establishing a human presence in space, the moon or Mars as a "last refuge" of mankind, that's likely either to be accidental, or the result of some future confluence of mass belief and private means. I have, somewhat facetiously, proposed a parasol in space at L1 to cool the earth. Could a catastrophic but not apocalyptic meteor strike be the catalyst?
*"To Move An Asteroid", Ailor, Planetary Report, January 2010
**"Footfall", Niven
***"The Boy Who Bought Old Earth", Smith
****"http://www.space-travel.com/reports/Planetary_Society_To_Sail_Again_With_LightSail_999.html"