Chronos said:
Since Chronos is getting picky about the destructive power of my asteroid, some clarifications are due:
It is metallic, iron-nickel, so it packs a heavy punch.
It is coming fast as hell, adding momentum and hence more energy to the impact detonation.
Its incoming path is perpendicular to the point of impact, thus it will traverse the minimum amount of atmospheric distance possible for an earth-entering object. Of course, this type of trajectory minimizes the amount of mass loss generated by atmospheric friction and enhances the kick-ass potential of the asteroid.
Now, regarding size:
I thought about sticking to the Armageddon script and let it be “about the size of Texas,” but that would imply that it had to be Pallas, Vesta or another asteroid around that sizeable neighborhood. In the name of scientific congruency I arbitrarily assigned to it the “measly” 10 Km value.
UPDATE: according to my recent calculations, ground zero will be located in the Atlantic Ocean. Oceanic impacts are the most catastrophic so we are really, really fucked.
O.K. now, what IMMEDIATE effects could this asteroid–which, in the absence of better suggestions, I’m naming Chronos :D–have upon our lifestyle?
----A tsunami, possible several kilometers in height, devastates the Eastern US Coast.
----Reentering ejecta raises the global temperature above the threshold for global forest ignition, so expect most of the planet to be on fire, literally… Much of the existing plant life disappears. The remaining fauna will soon follow that path as it will be unable to undergo photosynthesis due the darkness derived from the Cosmic Winter occasioned by the atmospheric debris blocking out the sun.
----Volcanism, earthquakes, This could go for millennia.
----300 mile/hour winds.
—Crops obliterated.
----Sizeable fragment of the population dead.
----Civilization destroyed.
Onto more long term effects
----Bye, bye,food chain! Unless animals undergo a rather quick an unprecedented adaptation, they are doomed and so are those who feed on them–us.
----Cosmic winter. No sun for months or maybe even years. Freezing cold dudes!
----Ozone layer depletion in a matter of decades.
Summarizing
Those who survive wished they didn’t. They essentially go back to living like cavemen, under the worst climatic conditions and without the essential benefit of any food source to feed on–no animals, no plants. It would be just a matter of time before our species becomes extinct.
I say that is pretty much a doomsday asteroid. But, just to make Chronos happy, I will use my brand new ** Acme electromagnetic protonic mass enhancer** to make it increase its size to 100 km and ensure an instantaneous annihilation of the human race. 
BTW, Chronos, according to renowned space researcher Duncan Steel (my source for most of this info), the Chicxulub Crater could have been generated by a 20-40 Km impactor, instead of the traditionally accepted 10 Km offender. This due to the fact that consideration of gravitational slumping of the crater (a phenomena that causes it to be filled by “gravitational accretion” of the surrounding terrain, making it smaller than it originally was) was omitted from initial estimations of its size.
As to how we can manage to avoid this seemingly certain doom, I am hoping that Demo’s assertions regarding Silo’s gigantic penis are true. If so, a clean hit could possibly destroy the asteroid and save mankind. Of course, that may diminish his penis’ size and upset the ladies, but heck, saving humanity implies certain sacrifices. 
