Sometimes Size Does Matter

The problem with living on a planet that is careering through space is that we are not the only ball of rock zinging around the place and as the dinosaurs found out, occasionally one of those which comes crashing through the atmosphere is big enough to cause us a problem and every now and then we hear of one coming uncomfortably close to us such as the 1km wide asteroid (7482) 1994 PC which is currently skimming by us.
There is no standard measurement for asteroids so this one is described as being two and a half times the height of New York’s Empire State Building but how large must an Asteroid be to exterminate all life on Earth?
Scientists estimate that the object that did for the Dinosaurs was between 11 and 12 km wide and created a huge dust plume that completely covered the globe and blocked out the sun which gives us a benchmark of how big it would need to be before we start panicking but even smaller ones could still cause massive damage.
NASA scientist say that a space rock just 0.8 km wide would pulverize all man-made structures up to a kilometer of where it landed while a larger rock (three or four Eiffel Tower's for example) would flatten everything within 8 km from the point of impact.
So while objects hitting us less than 6km would cause massive destruction, anything above that size would mean game over for us but thankfully, NASA are keeping an eye on all potential space rocks zooming around us and say there are none they know of in our vicinity of a size that would completely annihilate life on Earth and the closest will be in October 2028 when a 2km Asteroid will pass a million km or two and a half times the Moon’s distance from Earth.
Us Earthlings can sleep easy for now then.