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(From article)
Poseidon is an ‘Intercontinental Nuclear-Powered Nuclear-Armed Autonomous Torpedo’. It is a giant torpedo which can hit coastal cities with devastating results. Compared to an intercontinental ballistic missile it is very slow, but possibly unstoppable.
Russia maintains that it can also be used as a tactical nuclear weapon against warships. High-value targets would include aircraft carriers. This is harder to rationalize than the second-strike nuclear deterrence role, but it is a constant theme. Ever since it was first revealed in November 2015, then known as Status-6. it has been described as a multirole system.
The weapon’s expected speed, around 70 knots, is fast enough to make it realistically uncatchable to existing torpedoes. And its operating depths, perhaps as deep as 1,000 meters (3,300 feet) puts it beyond reach. Western planners will have to develop new weapons to intercept it. And that will take considerable time and investment.
A nuclear reactor gives the weapon essentially unlimited range. This gives it new levels of operational flexibility in terms of launch and target locations. Although it is restricted to at-sea or coastal targets, such as New York, Los Angeles. It can be launched from under the protection of the ice cap, or from coastal waters.
https://www.navalnews.com/naval-news/2022/03/russias-new-poseidon-super-weapon-what-you-need-to-know/