Here's What You Need to Know: France and Britain's most powerful warships avoided what could have been a total disaster.

Late at night on February 3, 2009 the crew of the French nuclear submarine Triomphant, experienced something of a shock. The 138-meter-long submarine, the lead boat of four serving today as a key part of France’s nuclear strike force, was returning to port submerged under the heavy seas of the East Atlantic when something impacted violently upon its bow and sail.

On February 6 the French Ministry of Defense reported that the submarine had suffered a collision with an “an immersed object (probably a container).” The same day the Triomphant returned to its base in Ile Longue, Brest escorted by a frigate.

Curiously, the HMS Vanguard, a Vigilant-class British Royal Navy nuclear submarine also experienced a collision that evening. The first of her class, the Vanguard measures 150 meters long and displaces 16,900 tons when submerged.

At some point, the two navies compared notes. On February 16 they announced the two submarines “briefly came into contact at a very low speed while submerged.” Fortunately, no crew members were harmed in the accident, though repairs were estimated to cost a minimum of 50 million pounds.

When the Vanguard returned to its base in Faslane, Scotland, it was visibly badly mangled around its missile compartment and starboard side.

“The French submarine had took a massive chunk out of the front of HMS Vanguard and grazed down the side of the boat,” later claimed William McNeilly, a whistleblower who served in the U.K.’s nuclear submarine program. “The High Pressured Air (HPA) bottle groups were hanging off and banging against the pressure hull. They had to return to base port slowly, because if one of HPA bottle groups exploded it would've created a chain reaction and sent the submarine plummeting to the bottom.”

On the French side of things, official statements indicated the damage to the Triomphant was confined to its Thales active sonar dome on the tip of the starboard bow. However, a regional newspaper later reported that its conning tower and the starboard sail plane attached to it were both deformed, implying multiple impacts.

Of course, particularly alarming was that both ships were designed to carry nuclear missiles: sixteen M45 ballistic missiles on the Triomphant and the same number of Trident II missiles onboard the Vanguard, each carrying 4 and 6 nuclear warheads respectively. Losing such apocalyptic firepower on the ocean floor would have been a catastrophe. However, nuclear warheads are not susceptible to “going off” as a result of a collision.

The same cannot be said of the nuclear reactors powering the two ships. A sufficiently serious collision could have breached the containment of the reactors, irradiating the crew and the surrounding expanse of oceanic waters. Fortunately, the British defense ministry assured “there was no compromise to nuclear safety.”

So, who was at fault for this potentially catastrophic brushing of cold, watery steel? In a way, what’s most alarming may be that the crew did not make any mistakes and that the error may truly lie with secretive ballistic missile submarine strategy that may be difficult to change.

While an attack submarine is always on the lookout for other ships and submarines and often seeks to shadow those of foreign nations a ballistic missile submarine just wants to be left alone and undetected under the ocean. Such submarines serve as a stealthy guarantor that any deadly attack on its home country could be reciprocated with a nuclear strike from a Submarine Launched Ballistic Missile (SLBM) launched from underwater. While a hypothetical aggressor might hope to take out a nation’s ground and air-based nuclear forces with a preemptive strike, submarines concealed deep underwater across the globe would be impossible to reliably track down and destroy—at least not all of them, and only as long as they don’t broadcast their presence.

However, one might think that two submarines passing close enough to scratch each other’s backs should be able to detect each other’s presence. However, modern subs have become very quiet, benefitting from tear-drop shaped hulls, superior propellers, and sound-absorbing anechoic tiles, among other technologies. As French defense minister Hervé Morin humble-bragged, “We face an extremely simple technological problem, which is that these submarines are not detectable.”

A submerged submarine can use either active or passive sonar to detect other subs. Passive sonar basically entails using audiophones to listen to the surrounding water, but that might not be adequate to detect a slow-moving modern submarine. A submarine could employ its active sonar to create sound waves which reflect off of other undersea objects, improving its detection power. However doing so would also broadcast the submarine’s position to anyone else who is listening. Because a missile sub’s chief priority is to avoid detection, both the Triomphant and Vanguard were relying purely on passive sonar—and neither submarine detected the other with it.

Submarine collisions are hardly unknown. Usually these involved one submarine shadowing another just a bit too closely, such as happened in the collision of the Russian K-407 and the USS Grayling in 1993. This has led to speculation that the Triomphant was chasing after the Vanguard. However, these kind of cat and mouse games are the province of attack submarines, not missile submarines.

It may seem vastly improbable that two submarines bumped into each other randomly across the vast volume of the ocean. However, the explanation may be that submariners are inclined to operate in certain common undersea regions—increasing the still remote chance of collision significantly. “Both navies want quiet areas, deep areas, roughly the same distance from their home ports,” nuclear engineer John Strong remarked in an interview with the BBC. “So you find these station grounds have got quite a few submarines, not only French and Royal Navy but also from Russia and the United States.”

The solution to avoiding further collisions would be to coordinate sub patrols between nations to avoid operating in the same place at the same time—but that runs counter to the paranoid logic underlying ballistic missile patrols. After all, even information shared between allies could theoretically be obtained by a hostile nation to help track down the missile submarines and take destroy them. While France was singled out for criticism for not sharing its patrol routes with NATO, in reality even the water space management information shared between the United Kingdom and United States did not include ballistic missile submarines according to the New York Times.

The Triomphant-Vanguard collision suggests that what seemed extraordinarily unlikely event—a collision between nuclear submarines in the middle of the ocean doing their best to remain discrete—may not be so in fact. Sharing more data between allies to mitigate the risks of future collisions would likely enhance, not weaken, the security of both those submarines and the nations they defend.

Sébastien Roblin holds a Master’s Degree in Conflict Resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring.

This article first appeared in December 2016.

Image: Reuters