OVERVIEW: The Mk 57 “MOSS” (Mobile Submarine Simulator) is a 10‑inch (254 mm) active-passive submarine decoy deployed via torpedo tubes. It simulates submarine acoustic signatures to distract homing torpedoes and enhance underwater survivability.

DETAILS: MOSS entered U.S. Navy service in 1976 and was fielded on SSBNs and SSNs through the late 1990s. After launch from a submarine’s torpedo or dedicated decoy tube, it propels itself and emits prerecorded passive propulsion noise along with active sonar echoes mimicking the host submarine. Its purpose is to create a credible decoy that torpedoes home on instead of the actual submarine. MOSS was eventually phased out in favor of smaller, multi-role acoustic countermeasures such as the Mk 70.

FONCTION:
Launched underwater from a torpedo or decoy tube.
Activates internal propulsion and acoustic emission systems.
Draws acoustic-homing torpedoes toward the decoy instead of the submarine.

SOURCE: Wikipedia – “Mobile submarine simulator” article ; Navy‑Matters blog overview ; Indian Navy “Modern Torpedoes And Countermeasures” summary