Meteorites enter Earth's atmosphere every day. Luckily, the majority are so small #(<2cm)# that they burn up in Earth's atmosphere before ever hitting the surface. Even meteorites that are big enough to not burn up completely are usually only a few metres wide and don't have enough mass to destroy anything big, although if you got hit by one you would probably die.
However, once meteorites start climbing up to #100m# wider or more, that's when things get serious. At this size, meteorites are large enough to start causing some serious trouble around the area they are in, including small earthquakes and and a short nuclear winter within a hundred miles of the impact. Thankfully metorites this large only hit Earth roughly every 10,000 years.
By the time a metorite gets to #1km# wide or larger, it constitutes as a global catastrophe as its effects can start to be felt worldwide. At #10km# wide, it gets classified as an extinction event. The effect of a meteorite this large hitting Earth includes medium-large earthquakes, tsunami's, the starting of fires within a hundreds of kilometres of the impact site, and the ejection of so much dust and and ejecta that it could start a much longer and much more deadly nuclear winter globally.
http://www.ehso.com/climatechange/climatechangecauses-meteorites.php
To understand how this can happen we must look at the energy released at the time of impact. We do this using this nifty formula:
#E=1/2MV^2#, where #E# is the energy released upon impact in joules, #M# is the mass of the object in kilograms and #V# is the velocity of the object in metres per second. Now consider the facts that even the tiny meteorites have velocities of up to #40km//s#, and also that meteors only 17-20m across have masses of 11,000 tonnes, and you can already see how massive the nergy release would be for a meteor only #1km# wide.
https://en.wikipedia.org/wiki/Meteoroid#Frequency_of_impacts
If a meteor was #1km# wide with a mass of just 1 million tonnes (which is relatively small), and a velocity of only #40km//s#, this would convert into the energy formula as
#E= (10*9kg)*(40,000m//s)^2#
#E=10*9*1,600,000,000#
#E=1,600,000,000,000,000,000# joules
#E=1,600,000,000,000# megajoules
#E=1.6*10^11 megajoules. In comparison, the entire nuclear arsenal of the world is only equivalent to #60,000# megajoules.
http://www.tulane.edu/~sanelson/Natural_Disasters/impacts.htm
I hope I helped!
