Five years ago Monday, the space shuttle Challenger exploded 73 seconds after liftoff in a disaster rivaling the sinking of the Titanic as a symbol of tragedy and misplaced trust in the infallibility of technology.

It began within seconds of liftoff when an unseen but insidious cloud of sooty black smoke began puffing from the side of a fatally flawed rocket that never should have been launched."All right!" astronaut Judith Resnik said as the Challenger thundered away from the Kennedy Space Center after a string of frustrating delays.

"Here we go," added co-pilot Michael Smith just one second after the ship's twin solid-fuel boosters ignited with a ground-shaking roar to kick off the 25th shuttle mission.

It was 11:38 a.m. EST on Jan. 28, 1986, and unknown to Challenger's seven-member crew, jets of hot gas were swirling through a critical joint holding two sections of the shuttle's towering right-side booster together, eating through the wall of the rocket like a 5,000-degree blow torch.

Telltale puffs of black smoke shooting from the flawed joint went unnoticed, disappearing a few seconds later as the sooty remains of charred rubber O-ring seals, grease and other debris apparently lodged in the joint, temporarily staunching the fatal breach.

On board were Smith, Resnik, commander Francis "Dick" Scobee, Ellison Onizuka, Ronald McNair, civilian satellite engineer Gregory Jarvis and New Hampshire high school teacher Christa McAuliffe, the first private citizen to win a seat on a space shuttle.

"Go you mother," Smith said into the ship's intercom 11 seconds after liftoff as Challenger wheeled about to line up on the proper trajectory. Despite a frigid night and trouble with ground equipment, all systems finally appeared to be "go" for flight.

But the shuttle was buffeted by higher-than-normal winds aloft and as the ship thundered past the speed of sound and the region of maximum aerodynamic pressure, the weakened booster joint finally gave way once and for all just under one minute into the flight. The crew remained unaware of anything amiss.

"Feel that mother go," Smith said over the ship's intercom as the shuttle rocketed toward orbit.

It was exactly 11:39 a.m., and Challenger was 13 seconds away from destruction.

At 64.660 seconds into the flight, the jet of flame from the booster O-ring joint "burn through" ate through the thin skin of the shuttle's external fuel tank. Two seconds after that, pressure in the tank began to drop as liquid hydrogen spewed out through the ever enlarging hole.

"Challenger, go at throttle up," astronaut Richard Covey radioed from mission control in Houston, telling the crew that Challenger's engines were running properly at full power.

"Roger. Go at throttle up," Scobee calmly replied.

Two seconds later, the jet of flame from the booster rupture burned through a strut holding the base of the rocket to the external tank. The bottom of the 14-story booster then pulled away, causing the nose to rotate into the top of the fuel tank, spilling a cloud of liquid oxygen into the sky.

Less than one second after that, the bottom of the tank gave way and the shuttle disappeared in a conflagration as propellants mixed and burned in the upper reaches of the atmosphere.

"Uh oh," Smith said over the intercom in the crew's last recorded utterance. A moment later, the astronauts were subjected to a bone-jarring but survivable jolt as the ship's nose section ripped away and arced through the frigid sky 48,000 feet above Cape Canaveral.

For an instant, the astronauts must have heard, or felt, the violent metal-on-metal rip as the fuselage tore away behind them. Scobee may have tried to radio mission control out of reflex, but the shuttle's crew module no longer had electrical power.

Back at the space center, McAuliffe's shocked family and relatives of the other astronauts watched in horror as billowing streams of smoke-trailing wreckage arced away from the still-ballooning cloud, plummeting to the ocean below.

"Flight controllers here are looking very carefully at the situation," said NASA commentator Steven Nesbitt in Houston. "Obviously a major malfunction."

A few seconds later Nesbitt confirmed the worst fears of thousands of spectators: "We have a report from the flight dynamics officer that the vehicle has exploded."

Inside Challenger's crew module the instruments were dead, and it was deadly quiet: the crackling roar of the solids, the dull throbbing of the main engines were suddenly and forever gone.

At least some of the shuttle fliers apparently activated emergency air supplies after the initial breakup of the orbiter, but it is not known how long they remained conscious as the crew module shot up to 65,000 feet before arcing over and plunging back to Earth.

Finally, less than 3 minutes after the explosion, the crew cabin slammed into the Atlantic Ocean in a steep left bank at more than 200 mph, instantly killing any of the seven shuttle fliers who still might have been alive.

Like the sinking of the Titanic, Challenger's destruction on that cold January day will be remembered for decades as an icon of disaster, the worst accident in the history of the space program and one that has come to symbolize the fallibility of America's vaunted high technology.

And like the aftermath of the Titanic disaster, Challenger's demise prompted sweeping changes to improve safety, changes that grounded NASA's shuttle fleet for nearly three years but left America with a safer if less adventurous space program.