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Correction to this article

THE Perseus, a 900kg (2,000lb) bomb made in Greece, incinerates almost everything in an area larger than a dozen football fields. Farther out, oxygen is sucked from the air and people may be crushed by a pressure wave. The inferno is similar to that caused by napalm—a jellied-petrol explosive heavily restricted by a United Nations weapons convention.

Modified with new technologies, however, the Perseus is increasingly considered legitimate. Mark Hiznay, a bombs-control expert at Human Rights Watch, a humanitarian group based in New York, has gone so far as to say it has become a necessary weapon. With a stronger steel casing and backup shock-resistant triggering mechanisms, the Perseus can smash through several metres of reinforced concrete and detonate only after it has gone into a bunker. This makes the bomb a good way to destroy and sterilise germ- and chemical-warfare laboratories while limiting damage nearby, says Mr Hiznay.

A new generation of advanced ordnance, including the Perseus, is making bombing campaigns safer for civilians. During the first Gulf war, in 1991, American warplanes had to drop an average of six 450kg satellite-guided bombs to destroy a tank or a small building. During the second war, 12 years later, a similar attack required bombs half that size, and fewer of them. Today 100kg bombs would suffice, because guidance systems are so good that individual rooms, as opposed to entire buildings, can be aimed at.

Greek fire

Much of this revolution, as Mr Hiznay terms it, is due to guidance kits that can be attached to existing “dumb” bombs. An upgraded bomb, when falling, uses data from the global-positioning system in combination with laser and infra-red sensors to adjust a set of fins that work like aeroplane flaps. This steers the bomb towards its target—even if that target is moving. The AASM, a French navy and air-force guidance system, has fins that can guide and glide bombs for 50km (31 miles) and hit a target within a metre of the bullseye. The LJDAM, a system made by Boeing and first exported in 2008, can land a bomb on a vehicle that is travelling at 110kph.

The market for add-on guidance systems is booming. More than a dozen countries, including South Africa, make them. Two dozen—including India, Pakistan and Turkey—buy them. They are not cheap: $23,000 per bomb will get you one at the bottom of the range. It is not just a more effective weapon, but also a safer one for the bomber. He can fly higher, meaning that he is at less risk from ground fire.

Moreover, these bombs continue to be clever even after arriving at the target. Their fuses can set off explosions at precisely the right moment. One defence contractor, Israel Military Industries, makes a 225kg bomb, the MPR-500, that can hammer through several storeys of a building and explode on a chosen floor. This feat means triggering the detonation about two milliseconds after the bomb hits the ceiling above the doomed storey. The bomb can be programmed to do this just seconds before it is dropped. Such precision means it is sold as a replacement for ordnance two or more times its size.

Bomb-makers are also finding cleverer ways of destroying deeply buried bunkers. Almost five years ago, America's Congress cut research funding for a controversial bunker-busting nuke called the Robust Nuclear Earth Penetrator. Today the bulk of effort to develop bunker-smashers in Western countries employs conventional weaponry. In a classic attack, a succession of big bombs is dropped on the same spot. Such “drilling”, however, may require numerous warplanes and inflict great damage on the surrounding area.

Souping up bombs with rockets that speed up their impact might provide an alternative. Bunker-busters work best if they detonate after burrowing into the ground. This helps “couple” the explosion to the ground so that shock waves designed to collapse a bunker travel deeper. Israel Military Industries is studying a rocket that would ignite just before the bomb hit, digging it deeper than ever before exploding.

That, according to Meir Geva, head of aerial munitions at Israel Military Industries, can be very deep indeed. His firm makes a bunker-buster which weighs about as much as a small car. “To my great sorrow”, Mr Geva says, its shockwave ranges cannot be revealed.

Whatever the bunker-buster's destructive power, the next generation of bombs will dwarf it. The Massive Ordnance Penetrator, an American bunker-buster scheduled for deployment at the end of the year, weighs 15 times as much.

On April 2nd 2003, during the second Gulf war, a hundred or so Iraqi armoured vehicles approached a far smaller American reconnaissance unit south of Baghdad. Responding to a call for help, a B-52 bomber attacked the first 30 or so vehicles in the column with a single, historic pass. It dropped two new CBU-105 bombs, and the result shocked the soldiers of both sides—and, soon enough, military observers everywhere.

While falling, the CBU-105 bombs popped open, each releasing ten submunitions which were slowed by parachutes. Each of these used mini rockets to spin and eject outward four discs the size of ice-hockey pucks.

The 80 free-falling discs from the pair of bombs then scanned the ground with lasers and heat-detecting infra-red sensors to locate armoured vehicles. Those discs that identified a target exploded dozens of metres up. The blast propelled a tangerine-sized slug of copper down into the target, destroying it with the impact and the accompanying shrapnel. The soldiers in the 70 vehicles farther back in the column surrendered immediately.

A kinder, gentler future

The CBU-105, however frightening, may actually point the way toward less violent warfare. Cluster munitions—which release bomblets to cover a wide area—are banned or tightly restricted by an international convention. But the CBU-105 and its cousins, known as sensor-fused weapons, are considered legal because very few discs remain unexploded on the battlefield. Those that fail to detect a target are supposed to self-destruct in the air. The trigger batteries of those that do not will quickly die, so duds are unlikely to kill civilians later.

Crucially, the manufacturer of the CBU-105, Textron Defense Systems, of Wilmington, Massachusetts, is improving sensors to allow the weapon to distinguish the heat signatures of cars, buses and homes from those of military hardware. If there is such a thing as a humanitarian bomb, this might be it.

By contrast, consider another sort of new weapon. The explosion of Russia's “Father of All Bombs” approaches that of a small nuclear weapon; it would flatten many city blocks. In 2007, the government showed it off proudly on prime-time television. To most military men, such a bomb is not a PR coup, but an embarrassment.