Key points from article :
Asteroids and meteors are a prime candidate for wiping out humanity.
In 1998 film, Armageddon, Bruce Willis blows up the asteroid before it hits Earth.
John Hopkins University scientists suggests this would be much harder than thought.
Blasted away fragments would be likely to reform with the asteroid because of gravity.
Computer modelling examined how rock breaks apart.
Asteroid core exerted a strong gravitational pull on the fragments blasted away.
