Death from above: the 2,000-pound Mark-84 JDAM

WASHINGTON — It falls silently and unseen from the distant sky, a cigar-shaped steel capsule hurtling down at 300 mph with a single deadly purpose: to kill. In the final moments, there might come a brief, chilling whir as tiny gears adjust its tail fins to nudge it closer to its target.

At home, television viewers monitoring the air war over Baghdad see the Pentagon's familiar gun-camera footage: cross hairs on a blurred image of a building and, as a Pentagon officer narrates, the flicker of a shadow and a bright flash before the tape runs out. From afar, television cameras record a fireball.

On the ground, however, the work of the 2,000-pound Mark-84 JDAM (Joint Direct Attack Munition) bomb, the new workhorse of the U.S. military, is just beginning.

In nanoseconds it releases a crushing shock wave and showers jagged, white-hot metal fragments at supersonic speed, shattering concrete, shredding flesh and ripping away limbs in a maelstrom of destruction.

Along with the Tomahawk ship-launched cruise missiles, JDAMs have been the chief weapon in the airstrikes on Baghdad this week.

Defense Secretary Donald Rumsfeld yesterday praised the precision of the JDAM and "the humanity that goes into" the use of the weapon.

Physicians who treat trauma injuries point out the simple truth that weapons like JDAM are designed to kill.

The Mark-84 JDAM is a 2,000-pound "dumb" or unguided bomb of the type used by U.S. forces for decades. What is relatively new, however, is a strap-on kit consisting of an inertial navigation system that guides the bomb toward the target, a satellite receiver, and tail fins for small final corrections in the dive toward an aim point determined by Global Positioning System satellites.

No one in the Defense Department nor its research labs or weapons contractors would publicly discuss the actual effects of the munition as it detonates. Privately, however, engineers and weapons designers were eager to describe the mechanism.

As the Mark-84 JDAM strikes the ground, its fuse ignites a priming charge that detonates 945 pounds of Tritonal, a silvery solid of TNT mixed with a dollop of aluminum for stability.

The ensuing chemical reaction produces an expanding nucleus of hot gas that swells the Mark-84's 14-inch-wide cast-steel casing to almost twice its size before the steel shears and fractures, showering a thousand pounds of white-hot steel fragments at 6,000 feet per second and driving a shock wave of several thousand pounds per square inch.

Instantaneously, a fireball lashes out at 8,500 degrees Fahrenheit, and the explosion gouges a 20-foot crater and hurls off 10,000 pounds of rock and dirt debris at supersonic speed.

Trauma physicians confronting the human wreckage from a bomb like the JDAM divide casualties into four classes:

• One is injury from the blast itself, mostly caused by a pressure wave a hundred times or more the injury threshold of 15 pounds per square inch (psi). By comparison, a shock wave of 12 psi will knock over a standing person.

• A second class of injury is from the wind and debris that immediately follow the blast wave. A blast force of 4 psi — far below the force of these winds — can shatter glass and drive lethal fragments at 120 mph. Metal fragments will travel about 3,800 feet, nearly three-quarters of a mile. Bigger fragments of the bomb — heavy pieces of the thick metal nose cone, for instance — will sail out a mile and a half, a Defense Department engineer said.

• A third set of injuries results either as bodies are picked up and thrown against something, or as part of a stationary body is ripped away.

• A fourth class takes in everything else, including burns from the fireball and crush injuries from falling debris.

Almost no one survives primary blast injuries, experts say. The brutal shock wave smashes into and explodes lungs, colon, bowels, even the sinuses into the skull.

The overpressure can burst individual cells and rupture critical blood vessels, forcing air through them and on into the heart and brain, causing instant death.

"You really don't treat consequences of primary overpressure," said Dr. Michael McCalley, a physician and professor of public health and preventive medicine at Oregon Health & Science University in Portland.

Third-class injuries also are often lethal.

"The type of force that picks you up and throws you, where you get traumatic amputations, you're pretty much already dead from the blast," said Dr. Harry Severance, an emergency physician and associate clinical professor at the Duke University Trauma Center.

Crush injuries, from the pressure wave or from falling rubble, also can be lethal. Crushing can break open muscle cells, dumping the contents into the capillaries and clogging critical blood vessels, McCalley said. That can cause cardiac problems, kidney failure and other complications difficult to treat in a war zone.

"In a poor country like Iraq, you don't survive a crush injury," said McCalley, who recently returned from making a survey of public-health facilities there.

Surprisingly, many people who avoid the worst blast injuries of a munition like the Mark-84 survive its other effects — severe burns from the fireball, losing chunks of flesh to flying debris, and crushed limbs.

That's when emergency triage — sorting out the dying from those who can be saved — becomes critical.

"You may have 85 to 95 percent of the victims of a major blast who are walking — scared, covered with debris and dust, bleeding from lacerations, and wanting somebody to help them right now," Severance said. "Those walking you can green-tag for later medical care. Five to 10 percent, these are people who need an operating room right now, and they are red-tagged.

"Black tag? Today's your day to die," he said.

Medical care in Iraq, according to the United Nations and other organizations, is minimal and declining. Iraq does not have an operative burn unit anywhere in the country, McCalley said, and no broad system of civil defense or bomb shelters.

His survey found many towns have ambulances — but often they are not equipped with defibrillators, intubators or other common emergency medical equipment.

And with Iraq's hospital system in a shambles, McCalley said, "Where would the ambulances take people?"