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UPI Context: Second-by-second to the shuttle disaster

Here is a chronology of the final moments of shuttle Challenger's countdown, its brief climb toward space and the agonizing minutes that followed its explosion 13 seconds past 11:39 a.m. EST Tuesday.

Each time entry is EST, showing the hour, minutes and seconds.

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Times, commentary and observations are based on a stopwatch analysis of NASA TV tracking camera documentation and air-to-ground conversations between ground controllers and Challenger's crew, except for the time of shuttle data loss, which was supplied by Mission Control in Houston.

11:36:30 -- Hugh Harris, launch control spokesman: ''90 seconds (to launch) and counting. The (Challenger) mission ready to go. The liquid hydrogen tank now at flight pressure and all three engines ready to go.''

11:37:00 -- Harris: ''Coming up on the one-minute point in our countdown ... T-minus one minute and counting. Sound supression water system now armed. The hydrogen burn igniters have been armed. These igniters will fire at T-minus 10 seconds to burn off any residual hydrogen gas. T-minus 45 seconds and counting. The solid rocket booster flight instrumentation recorders have gone into the record mode. Coming upon the 30-second point in our countdown.

11:37:29 -- Unidentified launch control technician: ''We have a go for auto sequence start (the computer-controlled final moments before launch).''

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11:37:30 -- Harris: ''And we've had a go for auto sequence start. The SRB hydraulic power units have started. T-minus 21 seconds and the solid rocket booster engine gimbal now underway. T-minus 15 seconds ...''

11:37:50 -- Unidentified technician: ''T-minus 10 seconds, GLS (ground launch sequencer) go for main engine start.''

11:38:00 -- Harris: ''... 10, 9, 8, 7, 6, we have main engine start, 4, 3, 2, 1 and liftoff! Liftoff of the 25th space shuttle mission, and it has cleared the tower.

11:38:07 -- TV tracking camera: Challenger, climbing smoothly, rolls over on its back in a pre-planned maneuver to improve aerodynamic forces on the ship and position the craft for jettisoning its external fuel tank and rocket boosters.

11:38:08 -- Challenger commander Francis ''Dick'' Scobee: ''Houston, (this is) Challenger. Roll program.''

11:38:09 -- Spacecraft communicator Richard Covey, Mission Control: ''Roger roll, Challenger.''

11:38:16 -- Mission Control spokesman Steven Nesbitt: ''Good roll program confirmed. Challenger now heading downrange.''

11:38:28 -- Nesbitt: ''Engines beginning throttling down now at 94 percent. Normal throttle (setting) for most of the flight is 104 percent. We'll throttle down to 65 percent shortly.

11:38:45 -- Nesbitt: ''Engines are at 65 percent. Three engines running normally. Three good fuel cells. Three good APUs (auxiliary power units).''

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11:38:52 -- Nesbitt: ''Velocity 2,257 feet per second (1,539 mph), altitude 4.3 nautical miles, downrange distance 3 nautical miles. Engines are throttling up. Three engines now at 104 percent.''

11:39:08 -- Covey: ''Challenger, go at throttle up.''

11:39:09 -- Scobee's last transmission: ''Challenger. Go at throttle up.''

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