There are thousands of large objects -- asteroids, mostly -- that could one day collide with Earth with enough impact to render our dear blue sphere inhabitable to a demanding species such as ours. Fortunately, space is large and can contain thousands of objects quite safely for quite a long time. About 500,000 years or so. On average, that's about the amount of time between catastrophic impacts, like the one that may have turned the dinosaurs into the extinction cliche they have become.
NASA, however, has been trying to keep an eye on them for us. For the past years, they have been watching for and cataloging as many of the larger objects as they could. In fact, they are closing in on identifying at least 90% of all objects in near-Earth orbits that are larger than a kilometer across. If one of those hits the Earth -- anywhere -- humanity's lights will be turned out permanently. The agency was planning to catalog 90% of smaller objects, too, those that are a minimum of 140 meters across. Though much smaller, they could still leave a nasty mark.
Unfortunately, budget cuts have meant this goal will likely not be met. Which means one morning we could wake up to find a giant rock hurtling toward us.
I'm all for watching out, but if we see one coming, can we really do anything about it? NASA has a plan to try and deflect such an object, if it can be detected early enough, but do we really have the capability to turn a giant asteroid from its course? You can find a link to NASA's deflection plan here.
Here's the Cliff Notes: "Unless there are decades of warning time, hazardous NEOs (those are Near-Earth Objects) larger than a few hundred meters in diameter may require large energies to deflect or fragment. In these cases, nuclear explosions, either stand-off or surface blasts, might provide a suitable response. For the far more numerous objects that are smaller than a few hundred meters in diameter, and provided there is a sufficient warning time, a kinetic energy (KE) impactor spacecraft might be sufficient to deflect the hazardous NEO so that it would miss the Earth at the time of a predicted impact."
Key word in both those instances: might.
So wear a hat.