# Timeline of the STS-51-L mission

The STS-51-L mission started with the ignition of Challenger's main engines until the remote destruction of the two Solid rocket boosters (SRBs), and includes a transcript of crew conversations from the cockpit voice recorder on board the orbiter. STS-51-L was the twenty-fifth flight in the American Space Shuttle program, and marked the first time a civilian had flown aboard the Space Shuttle. The mission used Space Shuttle Challenger, which lifted off from launch pad 39B (LC-39B) on January 28, 1986, from Kennedy Space Center, Florida. The mission ended in disaster following the destruction of Challenger 73 seconds after lift-off, because of the failure of an O-ring seals on Challenger's right solid rocket booster, which led to the rapid disintegration of the Space Shuttle stack from overwhelming aerodynamic pressures. The seven-member crew was killed when the crew compartment hit the Atlantic Ocean at 333 km/h (207 mph), after two and a half minutes of freefall.

## Summary timeline

Following several days of lengthy delays, Challenger finally lifted off at 16:38:00 UTC on January 28, 1986. Her three main engines were ignited at T-6.6 seconds, and at T-0 the solid rocket boosters were ignited, lifting the shuttle stack off launchpad LC-39B at Kennedy Space Center. Almost immediately, cameras recording the launch registered the presence of smoke at the field joint next to the attachment strut on the right-hand SRB, indicating the failure of the O-rings that were supposed to seal the joint against the "blow-by" of hot gases from the boosters. However, sometime at around T+2 seconds, a piece of solid fuel from inside the booster moved inside the joint and provided a temporary seal against the blow-by, allowing the launch to proceed normally for around forty seconds.
However, at around T+36 seconds and an altitude of just over 3,000 m (9,800 ft), Challenger experienced the strongest wind shear ever felt during a Space Shuttle launch. The pitch and yaw commanded by the shuttle's computers in order to counter this wind caused the solid fuel plug to become dislodged from the field joint on the right SRB.
At around T+58 seconds, cameras noted the creation of a plume on the aft attachment strut on the right-hand SRB, as ignited gas began to force itself through a rapidly growing hole in the field joint. Within a second, the plume became well defined and intense. Internal pressure in the right SRB began to drop because of the rapidly enlarging hole in the failed joint, and at T+60 seconds there was visual evidence of flame coming through the joint and impinging on the external tank (ET). As the mission clock passed up through T+64 seconds, the plume suddenly changed shape, showing that it had burned a hole in the liquid hydrogen tank in Challenger's ET, causing the tank to leak. The pressure in the tank began to drop, and Challenger's onboard computers began to pivot the nozzles of the SSMEs to counter the now-unbalanced thrust between the two SRBs.
At this stage, the situation still seemed normal both to the astronauts and to flight controllers. At T+68, the CAPCOM informed the crew – "Challenger, go at throttle up", and Commander Francis R. "Dick" Scobee confirmed the call. His response, "Roger, go at throttle up", was the last communication from Challenger on the air-to-ground loop.
At around T+72 seconds, the right SRB apparently pulled away from the aft strut attaching it to the external tank. Later analysis of telemetry data showed a sudden lateral acceleration to the right at T+72.525 seconds, which may have been felt by the crew. The last statement captured by the crew cabin recorder came just half a second after this acceleration, when Pilot Michael J. Smith said, "Uh oh". Smith may also have been responding to onboard indications of main engine performance or to falling pressures in the external fuel tank.
At T+73.124 seconds, the aft dome of the liquid hydrogen tank failed, producing a propulsive force that pushed the hydrogen tank into the liquid oxygen tank in the forward part of the external tank. At the same time, the right SRB rotated about the forward attach strut, and struck the intertank structure.
The breakup of the vehicle began at T+73.162 seconds, at an altitude of 14,600 m (47,900 ft). With the external tank disintegrating, Challenger veered from its correct attitude with respect to the local air flow and was immediately torn apart by aerodynamic forces, resulting in a load factor of up to 20g – well over its design limit. The two SRBs, which could withstand greater aerodynamic loads, separated from the ET and continued in uncontrolled powered flight for another 37 seconds. The SRB casings were made of 12.7 mm (0.50 in) thick steel and were much stronger than the orbiter and ET; thus, both SRBs survived the breakup of the Space Shuttle stack, even though the right SRB was still suffering the effects of the joint burn-through that had set the destruction of Challenger in motion. The boosters were destroyed by the range safety system at around 110 seconds after launch.

## Detailed timeline and transcript

The following timeline provides a detailed list of the major events of the launch of STS-51-L, culminating in the destruction of Challenger. The list also contains a transcript from the shuttle's Cockpit Voice Recorder (CVR), from ignition of the main engines to T+73 seconds. Acronyms used in the timeline are as follows:
* APU – Auxiliary Power Unit
* CAPCOM – Capsule Communicator (Richard O. Covey)
* CDR – Commander (Francis R. "Dick" Scobee)
* CVR – Cockpit Voice Recorder
* DPS - Data Processing Systems
* ET – External Tank
* FIDO – Flight Dynamics Officer
* GLS – Ground Launch Sequencer
* GPC – General Purpose Computer
* HPFT – High-Pressure Fuel Turbopump
* LH2 – Liquid Hydrogen
* LO2 – Liquid Oxygen (same as LOX)
* LVLH – Local Vertical Local Horizontal
* MCC – Mission Control Center
* MEC – Main Engine Controller
* MPS - Main Propulsion System
* MS1/MS2 – Mission Specialist (Ellison S. Onizuka/Judith A. Resnik)
* PAO – Public Affairs Officer
* PIC – Pyrotechnics Initiator Controller
* PLT – Pilot (Michael J. Smith)
* psf – pounds per square foot
* psi – pounds per square inch
* RCS – Reaction Control System
* SGTC – Shuttle to Ground Telemetry Channel
* SRB – Solid Rocket Booster
* SSME - Space Shuttle Main Engine
* TVC - Thrust Vector Control
| Time (UTC) (h:min:s) | Mission Elapsed Time (MET) (s) | Event | Source |
| 16:37:53.444 | -6.566 | SSME-3 ignition command. | GPC |
| 16:37:53.564 | -6.446 | SSME-2 ignition command. | GPC |
| 16:37:53.684 | -6.326 | SSME-1 ignition command. | GPC |
| 16:37:54 | -6 | CDR: "There they go guys!" MS2: "All right!" CDR: "Three at a hundred." | CVR |
| 16:38:00.010 | 0.000 | SRB ignition command. | GPC |
| 16:38:00.018 | 0.008 | Holddown Post 2 PIC firing. | E8 camera |
| 16:38:00 | 0 | MS2: "Aall riight!" | CVR |
| 16:38:00.260 | 0.250 | First continuous vertical motion. | E9 camera |
| The O-rings fail in the right-hand SRB field joint. ||||
| 16:38:00.688 | 0.678 | First confirmed puff of smoke appears above SRB/ET attachment ring field joint on right-hand SRB. | E60 camera |
| 16:38:00.846 | 0.836 | Eight puffs of smoke appear above field joint, lasting from T+0.836 to T+2.5 seconds MET. | E63 camera |
| 16:38:00.900 | 0.890 | Ground launch sequence computers begin post-liftoff "safing" of launch pad structures and equipment. | GLS |
| 16:38:01 | 1 | PLT: "Here we go." | CVR |
| 16:38:02.743 | 2.733 | Last evidence of smoke from field joint. | CZR-1 camera |
| 16:38:03 | 3 | PAO: "Liftoff of the 25th space shuttle mission, and it has cleared the tower." | NASA TV |
| A solid-fuel 'plug' seals the gap formed by the O-ring "blow-by". ||||
| 16:38:03.385 | 3.375 | Last evidence of smoke. | E60 camera |
| 16:38:04.349 | 4.339 | SSMEs throttled up to 104% | SGTC |
| 16:38:05 | 5 | DPS: "Liftoff confirmed." Flight Director: "Liftoff..." | MCC |
| 16:38:05.684 | 5.674 | Right-hand SRB pressure 11.8 psi above normal. | SGTC |
| 16:38:07 | 7 | CDR: "Houston, Challenger – Roll program." | CVR |
| 16:38:07.734 | 7.724 | Roll program initiated. | SGTC |
| 16:38:10 | 10 | CAPCOM: "Roger roll, Challenger." FIDO: "Good roll, flight." Flight Director: "Rog, good roll." | MCC |
| 16:38:11 | 11 | PLT: "Go you Mother!" | CVR |
| 16:38:14 | 14 | MS1: "LVLH" | CVR |
| 16:38:15 | 15 | MS2: "(Expletive) hot!" CDR: "OK." | CVR |
| 16:38:16 | 16 | PAO: "Good roll program confirmed. Challenger now heading downrange." | NASA TV |
| 16:38:19 | 19 | PLT: "Looks like we've got a lotta wind here today." | CVR |
| 16:38:19.869 | 19.859 | SSMEs throttled back to 94% | SGTC |
| 16:38:20 | 20 | CDR: "Yeah." | CVR |
| 16:38:21.134 | 21.124 | Roll program completed. | SGTC |
| 16:38:22 | 22 | CDR: "It's a little hard to see out my window here." | CVR |
| 16:38:27 | 27 | BOOSTER: "Throttle down to 94." Flight Director: "Ninety four..." | MCC |
| 16:38:28 | 28 | PLT: "There's ten thousand feet and Mach point five." | CVR |
| 16:38:28 | 28 | PAO: "Engines beginning throttling down, now at 94%. Normal throttle for most of the flight is 104%. We'll throttle down to 65% shortly." | NASA TV |
| 16:38:30 | 30 | [Garble] | CVR |
| 16:38:35 | 35 | CDR: "Point nine." | CVR |
| 16:38:35.389 | 35.379 | SSMEs throttled back to 65%. | SGTC |
| 16:38:37.000 | 36.990 | Roll and Yaw Attitude Response to wind shear (36.990 to 62.990 seconds). | SGTC |
| The solid fuel 'plug' is dislodged. ||||
| 16:38:40 | 40 | PLT: "There's Mach one." | CVR |
| 16:38:41 | 41 | CDR: "Going through nineteen thousand." | CVR |
| 16:38:43 | 43 | CDR: "OK, we're throttling down." | CVR |
| 16:38:45.227 | 45.217 | A flash is observed downstream of the shuttle's right wing. | |
| 16:38:48.128 | 48.118 | A second flash is seen trailing the right wing. | |
| 16:38:48.428 | 48.418 | A third unexplained flash is seen downstream of the shuttle's right-hand wing – a brilliant orange ball of flame appears to emerge from under the right wing and quickly merges with the plume of the solid rocket boosters, a phenomenon noted on previous flights. | 70mm camera |
| 16:38:49 | 49 | BOOSTER: "Three at 65." PAO: "...Three good fuel cells. Three good APUs..." Flight Director: "Sixty-five, FIDO..." FIDO: "T-del confirms throttles." Flight Director: "...Thank you." | MCC and NASA TV |
| 16:38:51.870 | 51.860 | SSMEs throttled up to 104% | SGTC |
| 16:38:52 | 52 | PAO: "Velocity 2257 feet per second (1539 mph), altitude 4.3 nautical miles, downrange distance 3 nautical miles..." | NASA TV |
| 16:38:57 | 57 | CDR: "Throttling up." | CVR |
| 16:38:58 | 58 | PLT: "Throttle up." | CVR |
| 16:38:58.798 | 58.788 | First evidence of flame on right-hand SRB | E207 camera |
| 16:38:59 | 59 | CDR: "Roger." | CVR |
| 16:38:59.010 | 59.000 | Reconstructed Max Q (720 psf) | Best estimated trajectory |
| 16:38:59.272 | 59.262 | Continuous well-defined plume of flame on right-hand SRB | E207 camera |
| 16:38:59.763 | 59.753 | Flame from right-hand SRB in downwards direction (seen from south side of vehicle) | E204 camera |
| 16:38:60 | 60 | PLT: "Feel that mother go!" "Woooohoooo!" | CVR |
| 16:39:00.014 | 60.238 | Pressures in right- and left-hand SRBs begin to diverge. | B47P2302 |
| 16:39:00.248 | 60.238 | First evidence of intermittent plume deflection | E207 camera |
| 16:39:00.258 | 60.248 | First evidence of SRB plume attaching to ET ring frame | E203 camera |
| 16:39:00.998 | 60.988 | First evidence of continuous plume deflection | E207 camera |
| 16:39:01.734 | 61.724 | Peak roll rate in response to wind shear | SGTC |
| 16:39:02 | 62 | PLT: "Thirty-five thousand going through one point five." | CVR |
| 16:39:02.094 | 62.084 | Peak TVC response to wind shear | SGTC |
| 16:39:02.414 | 62.404 | Peak yaw response to wind shear | SGTC |
| 16:39:02.494 | 62.484 | Right-hand outboard elevon actuator hinge moment spike | SGTC |
| 16:39:03.934 | 63.924 | RH outboard elevon actuator delta pressure change | SGTC |
| 16:39:03.974 | 63.964 | Start of planned pitch rate maneuver | SGTC |
| The plume of flame burns through the LH2 tank in the ET. ||||
| 16:39:04.670 | 64.660 | Change in anomalous plume shape (LH2 tank leak near 2058 ring frame) | E204 camera |
| 16:39:04.715 | 64.705 | Bright sustained glow on sides of ET | E204 camera |
| 16:39:04.947 | 64.937 | Start SSME gimbal angle large pitch variations | SGTC |
| 16:39:05 | 65 | CDR: "Reading four eighty six on mine." | CVR |
| 16:39:05.174 | 65.164 | Beginning of transient motion due to changes in aero forces due to plume | SGTC |
| 16:39:06 | 66 | BOOSTER: "Throttle up, three at 104." Flight Director: "CAPCOM, go at throttle up." | MCC |
| 16:39:06.774 | 66.764 | Start ET LH2 ullage pressure deviations | SGTC |
| 16:39:07 | 67 | PLT: "Yep, that's what I've got, too." | CVR |
| 16:39:08 | 68 | PAO: "Engines are throttling up. Three engines now at 104 percent." CAPCOM: "Challenger, go at throttle up." | NASA TV and MCC |
| 16:39:10 | 70 | CDR: "Roger, go at throttle up." (last transmission on air-to-ground voice loop) | CVR |
| The flame burns through the lower attachment strut on the right-hand SRB, causing the SRB to move away from the ET. ||||
| 16:39:12.214 | 72.204 | Left- and right-hand SRB yaw rates begin to diverge | SGTC |
| 16:39:12.294 | 72.284 | Left- and right-hand SRB pitch rates begin to diverge | SGTC |
| 16:39:12.488 | 72.478 | SRB major high-rate actuator command | SGTC |
| 16:39:12.507 | 72.497 | SSME roll gimbal rates 5°/second | SGTC |
| 16:39:12.535 | 72.525 | Vehicle max +Y lateral acceleration (+.227 g) | SGTC |
| 16:39:12.574 | 72.564 | SRB major high-rate actuator motion | SGTC |
| 16:39:12.574 | 72.564 | Start of H2 tank pressure decrease with two flow control valves open | SGTC |
| 16:39:12.634 | 72.624 | Last state vector downlinked | Data reduction |
| 16:39:12.974 | 72.964 | Start of sharp MPS LOX inlet pressure drop | SGTC |
| 16:39:13 | 73 | PLT: "Uh-oh..." | CVR |
| 16:39:13.020 | 73.010 | Last full computer frame of TDRS data | Data reduction |
| 16:39:13.054 | 73.044 | Start of sharp MPS LH2 inlet pressure drop | SGTC |
| 16:39:13.055 | 73.045 | Vehicle max -Y lateral acceleration (-.254 g). | SGTC |
| Challenger begins to disintegrate. ||||
| 16:39:13.134 | 73.124 | Circumferential white pattern on ET aft dome (LH2 tank failure). The bottom of the ET is open and liquid hydrogen spills. | E204 camera |
| 16:39:13.134 | 73.124 | Right-hand SRB pressure 19 psi lower than left-hand SRB | SGTC |
| 16:39:13.147 | 73.137 | First hint of vapour at intertank | E207 camera |
| 16:39:13.153 | 73.143 | All engine systems start responding to loss of fuel and LOX inlet pressure. | SSME team |
| 16:39:13.172 | 73.162 | Sudden cloud along ET between intertank and aft dome. The right booster slams into the ET just as the LH2 tank is thrust into the oxygen tank. | E207 camera |
| 16:39:13.201 | 73.191 | Flash between orbiter and LH2 tank | E204 camera |
| 16:39:13.221 | 73.211 | SSME telemetry data interference from 73.211 to 73.303 | Data reduction |
| 16:39:13.223 | 73.213 | Flash near SRB forward attachment strut and brightening of flash between orbiter and ET | E204 camera |
| 16:39:13.292 | 73.282 | First indication intense white flash at SRB fwd attach point | E204 camera |
| 16:39:13.337 | 73.327 | Greatly increased intensity of white flash | E204 camera |
| 16:39:13.387 | 73.377 | Start of RCS jet chamber pressure fluctuations | SGTC |
| 16:39:13.393 | 73.383 | All engines approaching HPFT discharge temp redline limits | SGTC |
| 16:39:13.492 | 73.482 | ME-2 HPFT discharge temperature. Channel A votes for shutdown; two strikes on Channel B. | MEC data |
| 16:39:13.492 | 73.482 | SSME-2 controller last time word update | MEC data |
| 16:39:13.513 | 73.503 | SSME-3 in shutdown due to HPFT discharge temperature redline exceedance | MEC data |
| 16:39:13.513 | 73.503 | SSME-3 controller last time word update | MEC data |
| 16:39:13.533 | 73.523 | SSME-1 in shutdown due to HPFT discharge temperature redline exceedance | Calculation |
| 16:39:13.553 | 73.543 | SSME-1 last telemetered data point | Calculation |
| 16:39:13.628 | 73.618 | Last validated orbiter telemetry measurement | SGTC |
| 16:39:13.641 | 73.631 | End of last reconstructured data frame with valid synchronization and frame count | Data reduction |
| 16:39:14.140 | 74.130 | Last radio frequency signal from orbiter | Data reduction |
| Loss of downlink – Challenger is lost. ||||
| 16:39:14.597 | 74.587 | Bright flash in vicinity of orbiter nose | E204 camera |
| 16:39:16.447 | 76.437 | Right-hand SRB nose cap separation and parachute deployment | E207 camera |
| 16:39:17 | 77 | PAO: "One minute fifteen seconds. Velocity: 2900 feet per second (1977 mph). Altitude: 9 nautical miles. Downrange distance: 7 nautical miles." | NASA TV and MCC |
| 16:39:29 | 89 | Flight Director: "FIDO, trajectories..." FIDO: "Go ahead." Flight Director: "Trajectory, FIDO." FIDO: "Flight, FIDO, filters got discreting sources. We're go." GC: "Flight, GC, we've had negative contact, loss of downlink." Flight Director: "OK, all operators, watch your data carefully." FIDO: "Flight, FIDO, till we get stuff back he's on his cue card for abort modes." Flight Director: "Procedures, any help?" Unknown: "Negative, flight, no data." | MCC |
| 16:39:50.260 | 110.250 | Destruction of right-hand SRB via range safety system | E202 camera |
| 16:39:50.262 | 110.252 | Destruction of left-hand SRB via range safety system | E230 camera |