One of the greatest mathematicians and scientists of all times, Henri Poincaré proved that given enough time any three (or more) body dynamic system like our solar system will turn at the end unstable.
Taken further the gravitational pull of planets in our solar system may actually cause the internal balance in our sun fluctuate following the gravitational interaction of the planets. And while at it the projected paths of all asteroids and alike may get some new directions even is altered only so slightly. And our solar system has the time to wait the inevitable to come true some day!The meteorites, large asteroids and comets can always pose a threat to us. Their sizes vary from river pebbles to giant mountains with diameters as large as up to 70 kilometers - perhaps even more. Whenever these travelers pass close by to us, other planets or the Sun the gravitational pull will alter their future travel. This brings up uncertainty and this is a problem. Don't believe, then check the surface of the moon.
The comets form a visible group of "space ships" as they carry a visible tail that keeps on pointing away from the sun. The most famous of them is the comet Halley, which is a 16x8x8 km rock visiting us every 69 to 77 years, depending on it's previous path through the our planetary system. It's presence has been noticed and feared throughout the known human history.
It was just proven that thee solar wind will force the asteroids to rotate at ever increasing speed like a wind mill but this time there is no resisting force. The proof was explained in a recent TV newscast by the discoverer. This is both good and bad. The good part is that one day this spin may provide a solution to guide an asteroid away from earth and the bad news is that a piece of rock may rotate too fast and just "explodes" as a normal rock has no real strength against tension. It just splits and disintegrates sending it all flying away on a 360 degree plane. Just compare the figure skaters when they accelerate and decelerate their spin on the ice. The rocks are no different.
We have learned recently that our near miss encounters are more frequent than we ever have realized. The most recent larger hit happened in 1908 in Russia - and got a name "Tunguska". This Russia bound meteorite had a diameter of only 20 meters and it is believed to have exploded about 10 km above the ground.
It still caused an earthquake size 5 on Richter scale and destroyed everything on 2100 square km area on ground. The power of this encounter was comparable to 1,000 Hiroshima nuclear bombs. The bombs in Hiroshima had a power of 1,000 tons of TNT or as shortened 1 Mt of TNT and they killed 140,000 people. In comparison the largest nuclear device that the USA has ever tested was called "Castle Bravo" and it had the explosion power of 15 Mt. The largest bomb Russia ever tested was named "Emperor Bomb" or "Ivan" by it's developers. Ivan was originally planned to be a 100 Mt bomb but was scaled down to 50 Mt after the leaders became worried of the likely huge radioactive material fallout on Russian territory.
Before "Tunguska" our written history mentions large meteorites only rarely. And when brought up, they were believed to be divine signs and no leader ever left this opportunity unused.
A large asteroid called Apophis is currently on it's way for at least to a close encounter with earth. Of course we can let this "rock" to run its course and if something goes wrong we just let the least capable to suffer. Fortunately at it's final arrival time up to five Nations will master the technology to avert the problem during it's second encounter only 7 years later.
This potentially serious incident will be 29 years from now on April 13th, 2036. Apophis appears to have at that time a razor thin margin when it passes the earth on it's twice a year 323 day path across our orbit around the sun. Apophis is actually a small 320 to 415 meter long rock weighing about 46 million tons. The change that it hits the earth at this time is one in 45,000. However, for this to happen it must pass through a specific "keyhole" in space during it's "near miss" path during 2029. If it passes and we just wait it will land 7 years later. The landing into ocean is no better than landing on dry land as the tidal waves would be colossal destroying all adjacent coastal regions.
Nasa estimates the blast caused by Apophis would be a 100,000 times more powerful than the bomb dropped on Hiroshima at the end of WW II.
However, Apophis is nothing but a precursor that may already be heading towards us somewhere in the space within a few million years or so. But so far we have not found any evidence of a real threat. However, if a really big like the one that killed the dinosaurs the destruction on earth will be enormous - nothing that is exposed will survive. It may also take years before the planet come back to life again.