With its 100th mission this week, NASA's space shuttle is going where no spacecraft has gone before. The venerable, winged spaceship has flown more people and more cargo for more miles (kilometers) than any space vehicle ever built -- earning the shuttle a unique spot in the record books.

Even more impressive is the fact that the shuttle -- now nearly 20 years old -- remains the most complicated piece of machinery ever built.

It has more than 2.5 million parts, 230 miles of wire, 1,060 plumbing valves and connections and 1,440 electrical circuit breakers. Not surprisingly, it requires the utmost care and patience to prepare a shuttle for flight. Failure of any one of some 2,000 "Criticality 1" items on the shuttle could mean disaster for the spacecraft or its crew.

Yet NASA managers, along with their unique contractor-partner United Space Alliance, have kept the shuttle fleet busy for 100 missions spanning almost two decades. NASA's four shuttles -- Atlantis, Columbia, Discovery and Endeavour -- together have logged a cumulative time in space of about two and a half years.

Those laps around Earth have covered some 350 million miles (560 million kilometers), the equivalent of about 700 round trips to the moon or roughly three round trips to Mars. And yet the shuttle fleet is only a quarter of the way through its design life.

That the shuttle has flown so far so long is a testament to perseverance. Conceived just months after the first moon landing in 1969, the $2 billion shuttle has something no other U.S. rocket can claim -- reusability. It was that unique element that appealed to Congress and the White House at a time when the nation could no longer afford single-use rockets like the mighty Saturn 5 of Apollo.

What NASA really wanted -- both a shuttle and a space station for it to visit -- was out of the question. But it managed to get the shuttle on the wildly optimistic idea that such a spacecraft could fly 60 times a year. President Nixon agreed to support the project as long as the Defense Department was on board. He announced it to the nation on January 5, 1972.

It took nine frustrating years, $10 billion in development costs and seemingly endless tests before Columbia finally was launched on April 12, 1981.

Columbia came too late to save America's first space station, Skylab, which had fallen from orbit two years earlier. But the shuttle plays a crucial role today in helping assemble and service the International Space Station (ISS), where astronauts from 16 partner nations will live and conduct experiments in orbit this decade.

Though it hasn't lived up to the early rhetoric, the shuttle still is the most versatile, reliable and impressive space vehicle in the world.

About the same size and weight as a DC-9, it blasts off like a rocket, operates in orbit like a spacecraft, returns to Earth as a heavy glider and lands like an airplane. In eight and a half minutes after launch, the 100-ton shuttle goes from zero to 17,500 miles (28,165 kilometers) per hour -- about nine times as fast as a bullet -- to reach orbit.

That thrust comes from the shuttle's twin solid-fuel rocket boosters and its three main liquid-fuel engines. The 149-foot- (45-meter-) long boosters, burning a rubbery solid fuel like that of a pencil eraser, fire at liftoff for two minutes with 6.6 million pounds of thrust. It would take 32 Boeing 747 jumbo jets, all applying maximum takeoff thrust, to get the same power yield. The boosters account for three-fourths of the shuttle's thrust at launch.

Once they peel away two minutes after launch, it's up to the shuttle's three main engines to get the spacecraft safely into orbit. Clustered at the rear of the shuttle orbiter, they begin firing six seconds before the shuttle stack lifts off the pad and don't shut down until the shuttle is in orbit. Each is 14 feet (4.3 meters) long, 8 feet (2.4 meters) in diameter at the nozzle and weighs 7,000 pounds (3,175 kilograms).

The engines burn liquid hydrogen and liquid oxygen, fed by the shuttle's enormous rust-colored external fuel tank. They gulp fuel at a prodigious rate -- fast enough to drain an average size swimming pool in 25 seconds -- and together produce nearly 1.2 million pounds of thrust. That's the power equivalent of 23 Hoover Dams.

Built by Rockwell International, now part of Boeing, the shuttle orbiters have logged more than 600 human ticket rides. More than 250 people from a dozen nations -- many of them more than once -- have strapped themselves into a shuttle seat for a ride to the high frontier.

But along with people, the shuttles also carry lots of cargo. The fleet has hauled into orbit more than 3 million pounds (1.3 million kilograms) of cargo -- the equivalent weight of about 1,000 automobiles. That's more than half the mass that the United States has put into orbit in 40 years of human spaceflight.

Out of that shuttle tonnage came more than 50 civilian and military satellites that astronauts deployed in space with help of the shuttle's 50-foot- (15.2-meter-) long robotic arm. Crews also have retrieved or repaired a dozen more satellites in the cargo bay. At 60 feet (18.3 meters) long and 15 feet (4.6 meters) wide, the bay is big enough to hold a city bus.

Among the more famous payloads carried by shuttles were three interplanetary explorer missions -- the Magellan probe to Venus and the Galileo probe to Jupiter, both in 1989, as well as the Ulysses probe to the sun in 1990. Shuttles also have deployed three of NASA's four "Great Observatories" -- the Hubble Space Telescope in 1990, the Gamma Ray Observatory (later renamed Compton) in 1991 and the Chandra X-ray Observatory in 1999.

The shuttle enjoys an envious launch success rate of 99 percent -- a higher rating over a longer period of time than any other rocket system in the world. By comparison, the French Ariane rocket, the only other vehicle that was designed in the 1970s and flown in the 1980s, had five failures in its first 40 flights.

Though the shuttle space has flown for 20 years, NASA expects to continue flying it for at least 12 more. After all, each orbiter was built to last 100 flights and NASA's four birds have just notched 100 flights among them. That's only a quarter of the projected lifetime of the fleet. Even the most frequent flier -- Discovery -- has just 27 flights in its logbook.

The shuttles already are flying with more than 200 improvements put in after the 1986 Challenger disaster that claimed the lives of seven astronauts. A new series of shuttle upgrades, the agency says, easily can keep the venerable spacecraft going until 2012.

Coming in the next five years are new changes to the shuttle's cockpit, main engines, hydraulic equipment, landing gear, solid-fuel rocket boosters and external tank. All of those, NASA hopes, should double the launch safety of the shuttle by 2005.

The world got a preview this past May of how good some of those improvements will be.

On Atlantis' trip to the International Space Station, the shuttle had a new "glass cockpit" similar to those flying on modern commercial airliners. The cockpit had 11 full-color, flat-panel display screens in place of the old gauges and cathode-ray tube displays of the past. The new model also was 75 pounds (34 kilograms) lighter and used less power and its color displays were easier on the pilot's eyes. All shuttles will sport the new cockpit by 2002.

Beyond the next decade, it is unclear what might develop as the next-generation space vehicle to carry humans into orbit. Several single-stage-to-orbit proposals are under study by a handful of private space companies. Any of those -- or perhaps a NASA-designed spacecraft -- could emerge as the next great explorer.

But even then the shuttle's place in history as the ultimate workhorse is assured. After all, no other U.S. spacecraft has carried more humans, more cargo or more hopes into space while spanning two centuries in the bargain.