Effective way of scanning sky for asteroids -problem
While it may seem simple enough in theory, as it is a simple theory to just scan the sky for asteroids heading for earth, the practicality of it is that it doesn't take for a 'mega-huge' rock to wipe out tens of millions of people, but only a relatively small rock of 100m in diam with the correct composition to explode with the force of a 100M ton thermonuclear bomb after being heated to 5,000 deg C in the falling through the atmosphere towards the ground. Secondly, the sky is VERY, VERY big. At any one point, we can only scan a EXTREMELY small percentage of the sky at one time, to have it be far enough out in trying to spot an object, that is again, only has to be a couple hundred meters wide to be devastating to the earth if it falls into the earth's gravity field and by chance falls towards a heavily populated city. If the 1908 Tunguska meteor had fallen instead onto London, NYC, Paris, Mumbai, Cairo, Alexandria, Jerusalem, Baghdad, Mexico City, etc, etc: any of the other hundreds of cities, even then with populations of over 5million + people in them, well over 90% of that city would've been obliterated by the impact of the meteorite slamming into the ground. An asteroid of 2-3 km diameter hitting the earth atmosphere & then ground like the Yucatan Penninsula meteorite, well, need i say more on it's potential effects? To spot a 3km wide rock hurtling our way at a meandering pace of 5 km/sec, at a distance of 1-2 million miles out is not easy to say the least, and we have usually only an approximated 8% chance of spotting it in time to work out it's size, it's likely composition,how close it will come to the earth, & if that is determined, how to alter it's course or destroy it & then launch 'something' in time to do just that. Sure, no problem at all.