If a big comet hit the Earth, it would cause great devastation. It might cause huge tidal waves and fires and put so much dust and soot in the air that most of the sunlight would be blocked, and plants would die and animals would have nothing to eat. Such events are very rare, however. While very small meteorites hit the Earth every day, the last big impact was 65 million years ago, when it is believed that a comet hit the Yucatan peninsula in Mexico. It caused enough damage to the environment to kill all the dinosaurs and many other kinds of animals. Such events seem to happen surprisingly regularly throughout the history of the Earth. The fossil record shows that roughly every 130 million years, most of the species alive on Earth are wiped out fairly suddenly. This is believed to be due to the impact of comets or large asteroids. While the chances of something happening even in the next 1000 years is small, astronomers have begun to chart the paths of as many of the asteroids and comets which come near to the Earth as they can. Because it is possible to predict very accurately the motion of these objects, they should be able to provide a warning well in advance. I don't know what the plan is if they were to find such an object; it might be possible to nudge the asteroid or comet (using a nuclear bomb) so that it would just miss the Earth. Again, the NASA website might have more information on this topic, or you could probably write to them and ask.

I wouldn't worry too much about it if I were you.The chances of a "doomsday rock" hitting the earth are better than winning the lottery (~1:18,000,000) but you are more likely to be killed in a car accident than either the rock or the lottery. So we worry more about car accidents than doomsday rocks.

But if you really want to know what would happen...The impact would probably be like a few hundred thousand atomic bombs blowing up at once. The doomsday rock is more likely to hit the ocean than land. The area of immediate effect would only be a few thousand miles across but the after effects would affect the entire earth. The impact would have blown so much dust, dirt, and water into the atmosphere that the earth would be cold and cloudy year round for several years. During that time, crops would fail and we would have to grow food in lighted greenhouses to survive. Many animals would die and many other people would starve.

The upside is that we can detect when a meteor or comet is going to pass close by months in advance. There are no formal systems in place to nuke the thing right now, but there are weapons systems that could be employed if it ever happens. You can bet that we would be willing to pay A LOT of money to build and deploy a system to handle the problem if it ever arose.

Your question is a good one and one that has made good material several thriller movies (Armagedon, Deep Impact, Lucifer's Hammer). Although these movies may present an unrealistic picture of such an event, scientists are pretty sure that large comets or asteroids are responsible for many (possibly all) of the major extinctions through Earth's history (the best known example is the dinosaurs). We know from smaller events that have occurred in recorded history that even relatively comets can have big effects. One example is the Tunguska event in the early part of this century when a comet hit a remote part of Siberia and flattened many trees and had effects for miles. Dust from the impact caused brilliant sunsets around the world. Of course, Tunguska was only a fraction of the size of the object that caused the extinction of the dinosaurs. A really big comet or asteroid like the one that killed the dinosaurs would not only kill everything for miles around the blast, but would send a cloud of dust into the atmosphere that would block sunlight for weeks or years creating an artificial winter that would lower global temperatures which would also kill many species. The other possibility is that the object would strike the ocean (more of the Earth's surface is water than land so this is more likely) and instantly vaporize a huge amount of water into the atmosphere while also causing devastating tsunamis (really big waves) that would swamp coastal cities (much of the Earth's population lives in coastal cities and towns). You can imagine that either of these possibilities would be very bad for humanity. It's possible that people would survive but life would be drastically changed and the artificial winter effect would make farming and obtaining food much more difficult for years. The encouraging aspect of this situation comes from considering the fact that a major impact occurs only every several hundred million years and the fact that humanity as an agricultural society has been around for thousands. The chances that a large object would hit in the near future, or even the next ten thousand years, are astronomically small (if you'll excuse the pun). At this time, there is little we can do to stop one of these large objects. Our best hope is to identify it early (there are programs to find and track big objects crossing Earth's orbit), because the earlier we find such an object, the less energy would be necessary to divert its orbit slightly so that it wouldn't run into the Earth. Changing the trajectory of an asteroid or comet is another difficult problem and at this point, most people believe the best option is to use nuclear weapons to change the course of such an object or break it apart so more of the object would break up while entering the atmosphere.