Terrorism's new weapon: mines at sea

The mining of the Red Sea is a new, unfortunate step in international terrorism that will bring added costs and inconvenience to all of us. Mines are a unique weapon, in that the terrorist can be many miles away before a ship passes over them and they explode. By the same token, they are indiscriminate weapons that cannot be aimed at a particular victim, only at a type of ship. There are two types of mines and three methods of planting them.

There are ''contact'' mines which explode only when a ship physically hits them, and ''influence'' mines which are detonated by the sounds, magnetism, or pressures a ship generates as it moves through the water. An acoustic mine waits until the noise rises to a peak indicating that the ship has come close and then detonates. If the ship is close enough, it will damage it. The mine can have a threshold of noise set into it. A low threshold will make it detonate when a small, quiet ship passes by; with a high threshold the mine will wait for a big, noisy ship. Similarly, magnetic mines sense the magnetic field that surrounds the metal of a ship; and a pressure mine feels the pressure created as a ship compresses the water it passes through.

Mines can be ''planted'' - anchored to the bottom with the mine floating somewhere below the water's surface, or laid right on the bottom. Drifting mines are usually contact mines. They have a limited life because winds and currents carry them in odd directions and eventually wash them ashore. It's more common for contact mines to be moored below the surface so that they will strike the hull of the type of ship they are intended to damage. Influence mines are usually laid on the bottom, but can be moored below the surface when the water is too deep for an explosion from the bottom to be effective.

Mines can be delivered by almost any ship, by bomber, or cargo aircraft. I estimate that those in the Red Sea are acoustic-influence mines laid on the bottom and delivered by a ship. The fact that they have done relatively little damage is because they are likely in waters too deep for the size of the mines.

How do we go about countering mines once they've been laid? Ships can just avoid passing near them. One way to do that is to stick to very deep waters whenever possible. Most of the Red Sea is too deep for mines. Only its extremities, the Gulf or Suez at the north, and the Strait of Bab el Mandeb at the south, can be mined. Naval ships can use sonar to find the mines and tell ships where not to go. Navies can also try to sweep mines. For moored mines, either a helicopter or a ship tows underwater cables which snag the mine's mooring line reaching down to its anchor. A cutter device on the towed cable severs the anchor line, the mine pops to the surface, and someone has to shoot it to make it sink or explode. For bottom mines, the helicopter or ship tows a device that makes a noise like a ship or creates a magnetic field like one. When the mine senses these artificial signals, it thinks it has a ship and detonates.

The United States Navy relies heavily on helicopter mine sweepers for just the reason that they can get places quickly, as they have to the Red Sea in the past few days. Mine-sweeper ships from the US would have taken almost three weeks to get there. Mine-sweeping by helicopter is also safer, as the copter is above, rather than in, the mine field when it tows its mine-sweeping equipment.

Over the longer run, the way to stop this form of terrorism is to prevent the laying of mines in the first place. We will have to do at sea much of what we do in airports to deter hijackers; that is, inspect ships before they pass through narrow, shallow, minable waters to ensure that they do not drop mines as they go and prevent suspicious aircraft from passing over such waters. Those precautions will be costly. If the mining of the Red Sea encourages other terrorists to mine other places one way or another, each one of us will pay those costs of inspections, delays, and higher insurance rates, because the international shipping that is affected is essential to our economic well-being.

In the US one of the best deterrents to terrorism has been an alert public that does not hesitate to report suspicious activities. Now the world faces the challenge of reporting suspicious movements of ships at sea and in ports. We all have a stake in defeating this new challenge to world order.

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