Flying at over 70,000 feet above sea level, a lone pilot of an American U-2 spy plane scoured the rugged Afghan mountains near the southern city of Kandahar during a classified mission in mid-September. While the plane's high-tech camera was sending back detailed photographs of the Taliban strongholds below, coalition soldiers operating in the area got embroiled in a firefight with insurgents. The U-2, which flies too high to be heard or seen on the ground, was dispatched to relay images of the battle, locate any targets, and identify possible escape routes—all in close to real time. Soon after, the plane headed up to eastern Afghanistan to sweep the area for any electronic communications between Taliban fighters. The U-2's sensor picked up several suspect transmissions, and the plane was sent to take high-resolution images of possible targets. After nine hours over Afghanistan, the U-2 returned home to its base at a secret location in southwest Asia.

This mission, typical of the almost daily flights over Afghanistan and Iraq, is vastly different from the U-2's maiden mission 51 years ago. In that first operational flight on June 20, 1956, pilot Carl Overstreet flew a carefully planned route behind the Iron Curtain to provide valuable glimpses of military targets inside Czechoslovakia and Poland. It took more than two days for the film to be developed and delivered to analysts in Washington.

Workhorse. Implausibly enough, the gliderlike U-2, whose mere existence was once one of America's most prized secrets, has been adapted to the age of al Qaeda and has emerged as an indispensable workhorse in the skies today. In just the past two years, the number of U-2 missions flown has increased by 20 percent, taking its operational pace to an all-time record. "It's busier than ever," says George Zielsdorff, the U-2 program director for Lockheed Martin, the defense contractor that built the original plane in a mere nine months under a CIA contract.

In fact, demand for the plane's sophisticated set of cameras and eavesdropping equipment is so high that some are questioning the U.S. Air Force's plan to retire the legendary aircraft in the coming years. Military planners are eager to bring on the U-2's successor—an unmanned high-altitude plane called the Global Hawk. But there is one problem: A version of the Global Hawk drone that can match the U-2's capabilities is still at least two years away from deployment.

Even with the U.S. intelligence community's array of spy satellites, surveillance aircraft, and other reconnaissance tools, the U-2 still boasts unique capabilities that fill a crucial gap. While satellites steadily orbit the globe on a predictable schedule, providing only momentary glimpses of any particular scene, the U-2 can fly over a target area for hours—a trait referred to as "persistence" by Air Force strategists. "We're talking now about a strategic environment where you're not so much tracking large army formations or hard targets, but you're talking about individuals and a network," says Col. Charles Bartlett, director of the Air Force's unmanned aerial systems task force. "To do that effectively, you need to have persistence, and you need to build patterns of behavior." While the better-known Predator drone can provide similar coverage when the weather is decent, it can be audible from the ground, unlike the higher-flying U-2.

In its first life, the U-2 was a straightforward reconnaissance plane, offering revolutionary peeks inside forbidden places like the Soviet Union and Communist China. Former CIA Director George Tenet once called the U-2 one of "the CIA's greatest intelligence achievements." It was still cloaked in secrecy when it made 24 daring flights over the Soviet Union from 1956 to 1960, helping to shatter alarming myths that Moscow was building significantly more missiles and bombers than the U.S. military. The danger became all too real in 1960 when the Soviets fired a missile that exploded just behind a U-2 piloted by Francis Gary Powers, who was captured by the Soviets after he parachuted to the ground. The incident turned into a prime-time Cold War drama, and the CIA was forced to end its flights over the Soviet Union. Powers was, after a show trial, later returned to the United States in a prisoner exchange.

The U-2 took center stage again in 1962, when a flight's photographs detected Soviet long-range missiles in Cuba, sparking the Cuban missile crisis. Two years later, after China's first nuclear test, U-2 missions took air samples to assess the aftermath. "It had probably some of the most strategic impact of any aviation operation ever," says Tom Ehrhard, an Air Force veteran now at the Center for Strategic and Budgetary Assessments. "We never bought enough of them." U-2s have also flown over most of the conflict zones in recent years, including Vietnam, the Persian Gulf, Bosnia, and Afghanistan.