When the dinosaurs went extinct some 65 million years ago, it is widely believed that a large impact between the earth and an asteroid was responsible. There is evidence for such an impact near the Yucatan Peninsula. In 1994, for the first time in human history we were able to watch the Shoemaker-Lavy comet impact Jupiter. The Jupiter impacts, where some of the impact areas were larger than the earth itself, and studies of moon crater rates, showed us that large impacts do still happen, even though the inner solar system is not as crowded as it was millions of years ago with asteroids in earth-crossing orbits around the sun. Most scientists who study these events agree that the earth will ultimately experience another large impact with an asteroid.
While this is popularly the stuff of science fiction, with Hollywood movies such as Armegeddon, it is a serious topic when considered by scientists. Yes, there could, and likely will, be an impact one day in the not so distant future, between earth and a larger asteroid. And if that happens, there would be catastrophic, global consequences.
In 2005, Congress assigned NASA the mission of identifying 90% of the estimated 20,000 larger earth-crossing asteroids by the year 2020. In this case, 'large' would be asteroids with a diamter of 460 feet or more. This sized rock is large enough to survive passage through the atmosphere and devastate a large region on the ground. Of course, as this size increases, so does the impact energy and devastation area. The goal is to plot orbits and try to identify any that are on a possible collision course with the earth in the near future (within several decades worth of time until impact). To date, some 6000 have been identified and catelogued.
However, NASA reports conclude that the funding Congress was supposed to be providing to carry out this mission has never been given to NASA. In order to find such close objects to earth, a different network of telescopes is needed to accurately track such objects. It is to NASA's credit that they have been able to find 6000 at this point without the telescopes and funds.
With limited funding, it is doubtful NASA will be able to accomplish this mission, and little is being done anywhere else in the world regarding this issue. With enough warning time, there are possible interventions humans could make to save the earth from an impact event, and this is the purpose for beginning the NASA asteroid hunting mission in the first place. This is a mission worth completing.