NASA says an asteroid (or fragment) needs to be “one to two kilometers” to do widespread damage.
Nice of NASA to be so reassuring, but an object one mile in diameter would end civilization and kill 99 percent of humans.
The rock that produced 3/4 mile wide Barringer Crater in AZ is estimated to have been 150-160 feet across. If a similar object struck any city today, it would not only create such a crater, the energy from it would knock down everything for miles and very likely kill everyone even further away.