From "Death by Black Hole and Other Cosmic Quandaries" by Neil DeGrasse Tyson

In this game of gravity, by far the scariest breed of impactor is the long-period comet, which, by convention, are those with periods greater than 200 years. Representing about one-fourth of Earth's total risk of impacts, they fall toward the inner solar system from great distances and achieve speeds in excess of 100,000 miles per hour by the time they reach Earth. Long-period comets thus achieve more awesome impact energy for their size than your run of-the-mill asteroid. More importantly, they are too dim over most of their orbit to be reliably tracked. By the time a long-period comet is discovered to be heading our way, we might have anywhere from several months to two years to fund, design, build, launch, and intercept it. For example, in 1996, comet Hyakutake was discovered only four months before its closest approach to the Sun because its orbit was tipped strongly out of the plane of our solar system, precisely where nobody was looking. While en route, it came within 10 million miles of Earth (a narrow miss) and made for spectacular nighttime viewing.

And here's one for your calendar: On Friday the 13th of April, 2029, an asteroid large enough to fill the Rose Bowl as though it were an egg cup, will fly so close to Earth that it will dip below the altitude of our communication satellites. We did not name this asteroid Bambi. Instead, it's named Apophis, after the Egyptian god of darkness and death. If the trajectory of Apophis at close approach passes within a narrow range of altitudes called the "keyhole," the precise influence of Earth's gravity on its orbit will guarantee that seven years later in 2036, on its next time around, the asteroid will hit Earth directly, slamming in the Pacific Ocean between California and Hawaii. The tsunami it creates will wipe out the entire west coast of North America, bury Hawaii, and devastate all the land masses of the Pacific Rim.

If Apophis misses the keyhole in 2029, then, of course, we have nothing to worry about in 2036.

Should we build high-tech missiles that live in silos somewhere awaiting their call to defend the human species? We would first need that detailed inventory of the orbits of all objects that pose a risk to life on Earth. The number of people in the world engaged in this search totals a few dozen. How long into the future are you willing to protect Earth? If humans one day become extinct from a catastrophic collision, there would be no greater tragedy in the history of life in the universe. Not because we lacked the brain power to protect ourselves but because we lacked the foresight. The dominant species that replaces us in postapocalyptic Earth just might wonder, as they gaze upon our mounted skeletons in their natural history museums, why large-headed Homo sapiens fared no better than the proverbially pea-brained dinosaurs.