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Dozens of asteroids and comets such as 2003 QQ47 (mentioned above) come within frighteningly close distances to Earth every year. Each of which has the ability to destroy a large percentage of life on this planet. Although 2003 QQ47 is no longer a threat (calculations prove it will come close - but not impact), it was the only threatening space object in history to be given a 1 on the Torino scale, which is maximum. The Torino scale measures the threat and possibility of an impact by an object. Until 2003 QQ47, all other objects were listed as 0 on the scale, which means "Requires Low to Medium monitoring". All other objects that do not pose a threat are not listed in the Torino scale.
In the late 20th Century, films such as "Deep Impact" and "Armageddon" made the public aware of such threats. In one of these films they mention the United States government only tracks 3% of the sky with their current budget. Of course, today NASA's SENTRY program has improved apon these statistics significantly.
The object that killed off the dinosaurs may have been a lump of iron the size of a mountain traveling at about 30 miles per second. Enormous quantities of heat are generated which vaporize most of the impacting object and a similar amount of the Earth. The effect of this is similar to a nuclear explosion, but much larger, with a hole punched right through the crust and a crater a 100 miles across. The fireball will burn everything for huge distances, blastwaves in the atmosphere and ground waves through the earth will go round the Earth several times. If you survive the first ones you will hear and feel the impact several times as it reverberates around the planet. The secondary effects will be tidal waves, freak winds, the sky darkened by dust, and heavy rain (if some of the sea was vaporized). The atmosphere may become close to unbreathable in some cases due to dust and volcanic gases - and the weather may be very cold for many years. Another big killer, that I have only recently become aware of, is the sky becoming red hot due to millions of tons of material ejected by the impact explosion falling back to earth from space. Needless to say, it would be Hell.
The most massive object known to have hit the Earth was the size of the planet Mars. We know it happened because after the impact some material splashed out to form a blob orbiting the Earth - and we now call it the Moon. This happened soon after the Earth first formed and must have completely liquified the Earth and made it somewhat larger. No one would survive an impact like that; but there are reasons to think that most of these very serious impacts happened soon after the planets first formed. The orbits of all the very large objects orbiting the Sun seem to have been stable now for 100s of millions of years and we can hope they will stay that way.
In the movies Armageddon and Deep Impact, the world's governments build spaceships and sent teams of cosmonauts and astronauts to the surface of these objects to plant a nuclear weapon and destroy it.
Most of us know, we are not capable of doing this. In Armageddon, they had 18 days (which is absurd, you couldn't plan a shuttle launch in that time, let alone travel around the moon and onto an asteroid). In Deep Impact they did it in just over a year, which would've been feasable if crews worked 24/7 (which they probably would) and if everything went perfectly to plan, with no mistakes.
In today's society, it would be near impossible to sent a team to the surface of a comet or asteroid, simply because of the risks involved, as well as the time frame. If NASA were to attempt this, it would take approximately 4-5 years to plan, prepare, and send a crew into space. NASA doesn't have any Saturn V rockets anymore, so going beyond high-Earth orbit is out of the question. Which means we cannot even go to the moon anymore. I guess you could say, the space program has de-evolved since the 1970's.
Keeping this in mind, if an asteroid or comet were on a collision course with Earth, there is a very slim chance of us having the time or posessing the technology to prevent it. This being the case, we'd be out of luck.
What would you do if you woke up tomorrow morning and saw on the news an asteroid was discovered, and was expected to impact Earth within a year? As with the alien invasion topic, there'd be a breakdown in social and civil services, looting, rioting, etc.
. My over all summry however: Load of Bull.
