A US submarine sank the Iranian frigate IRIS Dena off the southern coast of Sri Lanka on Wednesday, killing 87 people.

The submarine struck the ship with a torpedo, US Defense Secretary Pete Hegseth said, resulting in “quiet death”.

The event marks the first time a US submarine has launched a torpedo in combat or engaged and destroyed a ship since the second world war.

Why is this old weapon reappearing now? And for that matter, what are torpedoes? Can ships defend against them, or even see them coming?

What are torpedoes, and how do they work?

Torpedoes have changed little over the years in terms of their concept and their operation. Simply put, a torpedo is effectively a small, unmanned submersible – a kind of hybrid of a mini-submarine and a missile, designed to attack both submarines and surface ships.

However, they have been modernised to an extent. Torpedoes today are generally “dual purpose”, designed to attack and destroy both ships and submarines. Single-purpose torpedoes are less commonly seen, but given their specialised nature are often more lethal.

Torpedoes are initially connected to the launching submarine by a wire or fibre-optic cable transmitting targeting data. These wires are designed to be “cut” as the torpedo gets close to its target, with the torpedo then switching on its own active sonar to steer the rest of the way.

Exactly how far torpedoes can be fired is highly classified information, but it may be tens of kilometres. It depends........

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