in order to protect against such (or at least reduce their effects),NASA has developed way to monitor Near-Earth Objects (NEOs).
for example ,the near -earth asteroid rendezvous (near)is a spacecraft that monitors Eros, one of the largest asteroids being studied .Eros is scheduled for a near -Earth approach on January 31,2012.This huge flying rock is the shape of a potato ,and its size is 170 square miles (442 square kilometers).
Scientists believe that it might strike Earth in 1.5 to 5 million years .
The probability of one of these Near -Earth Objects hitting Earth over the next hundred years is virtually negligible.
However,scientist are concerned,because at present,there are about 900 asteroids circulating our solar system.
if an asteroid the size of Eros fell on Earth in the middle of the Atlantic,it would have catastrophic effects.