A couple of elite nuclear submarines, the British HMS Vanguard and the French Navy's Le Triomphant, collided in the Northern Atlantic on Feb. 4 while "conducting routine patrols," both countries acknowledged on Sunday. Both subs are armed with multiple-warhead nuclear missiles.
But the Vanguard and Le Triomphant are also equipped with some of the most sophisticated military sonar devices available, complete with various antennas and sensors that collect information that is analyzed by high-powered computers.
So how is it possible that two nuclear subs of allied countries, both carrying state-of-the-art sonar equipment, could bump into each other?
"Sonars come in two flavors," John Pike, director of the Globalsecurity.org, told Foxnews.com.
"There's passive sonar and there's active sonar. These ballistic subs spend most of their time hiding with a nuclear arsenal. They use passive sonar to remain as silent as possible."
Passive sonar, which listens without transmitting, is particularly helpful in stealth situations, but what happens when two ballistic submarines designed to hide and remain silent are within close range?
"Active sonar is like a flashlight in the dark. I can use that flashlight to see, but everyone can see where I am," said Pike. "Passive sonar accuracy becomes much more ambiguous at close range."
Submarine collisions are rare, but they do happen. In 2001, the USS Greeneville surfaced and collided with a Japanese ship, the Ehime Maru, off the coast of
During the Cold War, Soviet and American submarines often played cat-and-mouse, and near-collisions took on a dimension of international crisis.
Times have changed, but the danger is still enormous.
The HMS Vanguard is one of the Royal Navy's V-Class subs. It is armed with 16 ballistic nuclear missiles and forms a "Trident nuclear deterrent." Le Triomphant carries 16 M-45 class Sea-ground-Strategic ballistic missiles, also armed with thermonuclear warheads. An accident, even an unintentional one, could have had serious consequences.
The French and British navies said the collision was unavoidable, because the vessels were "running silently" to avoid sonar detection.
Le Triomphant has the British built DMUX 80, which provides passive target ranging and interception capability. The Vanguard carries the Thales Underwater Systems Type 2054 composite sonar — a multi-mode, multi-frequency multi-million-dollar detection system that failed to detect the 453-foot-long French sub until they butted.
The Vanguard has two periscopes, a CK51 search model and a CH91 attack model, both of which have a TV camera and thermal imager as well as conventional optics.
"But the periscopes are useless at that depth," Pike said.
"It's pitch black after a couple of hundred feet. In the movies like 'Hunt for Red October,' you can see the subs in the water, but in reality it's blindman's bluff down there."
"The crash could have been a coincidence — some people win the lottery — but it's much more possible that one vessel was chasing the other, trying to figure out what it was."
