In American AEGIS systems, the main AN/SPY-* radar would detect the missiles at distance and surface-to-air missiles would be fired at them. If failed, CIWS sys

In American AEGIS systems, the main AN/SPY-* radar would detect the missiles at distance and surface-to-air missiles would be fired at them. If failed, CIWS systems such as this Phalanx, SeaRAM, Goalkeeper, AK-630... would be activated, and hopefully it autonomously notices incoming supersonic object and engages it. These systems aren't designed to be left on all the time just in case, but has to be put into standby mode at the beginning of a combat and un-leashed as needed. Moskva wasn't paying attention to the situation and all these equipment were visibly in stored, powered down state in leaked photos. Among suspected reasons were that the sailors were lazy, the Neptune was technically out of engagement range but upper atmospheric phenomenon allowed acquisition by luck, therefore the ship wasn't expecting it, etc.