The cause of the catastrophic explosion that destroyed Challenger on Tuesday remains a mystery, but television pictures point to the fuel-laden external tank as a prime clue.
The 15-story, apricot-colored tank, which holds 528,000 gallons of liquid oxygen and hydrogen, appeared to explode first, shattering the fragile spaceplane and killing its seven crew members.
The tank was attached to the underside of Challenger. It normally supplies fuel to a shuttle's three main engines for the first 8 1/2 minutes of flight and then is jettisoned to fall into the ocean.
Two solid-fuel rocket boosters attached to the tank also provide thrust. They normally fall away at 2 minutes, 7 seconds into flight and drop into the Atlantic by parachutes. Unlike the fuel tank, they are recovered, cleaned and reused.
''I think the tank just came apart,'' said Sam Beddingfield, who retired in November as deputy director of space shuttle management at Kennedy Space Center.
He watched the launch from his Titusville home and studied slow-motion replays on television.
''The whole orbiter appeared to disintegrate,'' he said. ''The first two minutes and four seconds of the launch are very dangerous because the solid rocket boosters are burning. If anything happens during this time, it's a catastrophe,'' Beddingfield said.
Other experts speculated that freezing temperatures at the pad and the rocket boosters may be linked to the explosion.
Marc Vaucher, program manager for the space station project at the Center for Space Policy in Cambridge, Mass., said, ''It sure looked to me like I saw a flare-up coming out of the external tank somewhere near one of the solid rocket boosters.''
He wondered whether a chunk of ice might have fallen and somehow dented or damaged the tank or the rocket boosters.
Bert Zin, a Georgia Tech University professor who specializes in fuels and propulsion, said that a bonding agent that binds the solid fuel to the inside of the boosters could have become unstable if it contracted or expanded in the freezing temperatures at the pad.
''Maybe it broke or separated,'' he said.
NASA shuttle chief Jesse Moore saw the television pictures too but warned against early conclusions.
''It will take all the data and a careful review of the data before we can draw any conclusions,'' Moore said.
To assure that nothing would be lost, Moore immediately set up an interim investigative board to secure all data related to the liftoff and flight, including any technicians' notes.
As in plane crashes, the electronic data will be compared with debris and visual records to piece together a scenario of the failure.
Before the explosion, there was no indication of trouble on monitors at Johnson Space Center in Houston or at Kennedy Space Center, Moore said.
Although temperatures had dipped to 27 degrees a few hours before the launch and some ice had formed at the pad, weather was not a factor in the explosion, Moore said.
Skies at liftoff were clear.
The shuttle rose from the pad and headed on a trajectory to the southeast.
At 9 seconds after liftoff, it began a normal rolling motion, and at 35 seconds the three main engines throttled down to 65 percent of regular power to reduce stress on the shuttle from gravity and other forces in the first minute of flight.
''Roll program confirmed,'' reported mission control.
''Engines are at 65 percent. Three engines running normally. Three good fuel cells. Three good APUs auxiliary power units.''
Challenger was traveling 2,257 feet per second and was flying 5 miles high, 3 1/2 miles downrange of the launch pad.
''Challenger, go with throttle up,'' mission control said, referring to the increase of the engine power to 104
percent after the earlier throttle down. It was about a minute after liftoff.
''Roger,'' said shuttle pilot Mike Smith. ''Go with throttle up.''
At 1 minute, 15 seconds, the shuttle was about 10 miles high and about 8 miles downrange.
Television pictures showed a normal flight, and suddenly tragedy.
Close-up pictures from a tracking camera showed a small ball of orange fire bloom near the bottom of the fuel tank.
A second, larger ball of fire appeared higher on the other side of the tank. The fireballs merged into a bright yellow and red mass of flame that engulfed the spaceplane.
Then it exploded.
From the ground, spectators saw a flash of fire and a cloud of smoke. The rocket boosters ejected outward, spinning with flames coming from their nozzles.
All telemetry from the shuttle went dead.
Debris fell, trailing white smoke.
Challenger contained a variety of volatile solid and liquid propellants.
The fuel tank is divided into two portions, one containing 385,000 gallons of supercold liquid hydrogen at launch and the other containing 144,000 gallons of liquid oxygen.
The shuttle also carries volatile fuels for its large orbital maneuvering system thrusters at the tail of the spaceplane and its smaller reaction control thrusters at the front and rear.Copyright © 2014, Los Angeles Times