jahy astra-teja unnaddham
astra-jño hy astra-tejasā
For the atomic bombs there is no counterweapon to neutralize the effects. But by subtle science the action of a brahmāstra can be counteracted, and those who were expert in the military science in those days could counteract the brahmāstra. The son of Droṇācārya did not know the art of counteracting the weapon, and therefore Arjuna was asked to counteract it by the power of his own weapon.