SB 1.7.27
sri-bhagavan uvaca
vetthedam drona-putrasya
brahmam astram pradarsitam
naivasau veda samharam
prana-badha upasthite
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The Supreme Personality of Godhead said: Know from Me that this is the act of the son of Drona. He has thrown the hymns of nuclear energy [brahmastra], and he does not know how to retract the glare. He has helplessly done this, being afraid of imminent death.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The brahmastra is similar to the modern nuclear weapon manipulated by atomic energy. The atomic energy works wholly on total combustibility, and so the brahmastra also acts. It creates an intolerable heat similar to atomic radiation, but the difference is that the atomic bomb is a gross type of nuclear weapon, whereas the brahmastra is a subtle type of weapon produced by chanting hymns. It is a different science, and in the days gone by such science was cultivated in the land of Bharata-varsa. The subtle science of chanting hymns is also material, but it has yet to be known by the modern material scientists. Subtle material science is not spiritual, but it has a direct relationship with the spiritual method, which is still subtler. A chanter of hymns knew how to apply the weapon as well as how to retract it. That was perfect knowledge. But the son of Dronacarya, who made use of this subtle science, did not know how to retract. He applied it, being afraid of his imminent death, and thus the practice was not only improper but also irreligious. As the son of a brahmana, he should not have made so many mistakes, and for such gross negligence of duty he was to be punished by the Lord Himself.