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Space Shuttle Challenger disaster

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For further information about Challenger's mission and crew, see STS-51-L.
The iconic image of Space Shuttle Challenger's smoke plume after its breakup 73 seconds after launch. The accident caused the death of all seven crew members of the STS-51-L mission.
The iconic image of Space Shuttle Challenger's smoke plume after its breakup 73 seconds after launch. The accident caused the death of all seven crew members of the STS-51-L mission.
The crew of STS-51-L. Front row, from left to right: Michael J. Smith, Dick Scobee, and Ronald McNair. Back row, from left to right: Ellison Onizuka, Christa McAuliffe, Gregory Jarvis, and Judith Resnik.
The crew of STS-51-L. Front row, from left to right: Michael J. Smith, Dick Scobee, and Ronald McNair. Back row, from left to right: Ellison Onizuka, Christa McAuliffe, Gregory Jarvis, and Judith Resnik.

The Space Shuttle Challenger disaster occurred in the United States, above the state of Florida, at 11:39 a.m. EST (16:39 GMT) on January 28, 1986, when the Space Shuttle Challenger disintegrated 73 seconds into its flight after an O-ring seal in its right solid rocket booster (SRB) failed. The seal failure caused a flame leak from the solid rocket booster, which impinged upon the adjacent external fuel tank. Within seconds, the flame caused structural failure of the external tank, and aerodynamic forces promptly broke up the orbiter. The shuttle was destroyed and all seven crew members were killed. The crew compartment and many other vehicle fragments were eventually recovered from the ocean floor after a lengthy search and recovery operation.

The disaster resulted in a 32-month hiatus in the shuttle program and the formation of the Rogers Commission, a special commission appointed by United States President Ronald Reagan to investigate the accident. The Rogers Commission found that NASA's organizational culture and decision-making processes had been a key contributing factor to the accident. NASA managers had known that contractor Morton Thiokol's design of the SRBs contained a potentially catastrophic flaw, but they failed to address it properly. They also ignored warnings from engineers about the dangers of launching on such a cold day and had failed to adequately report these technical concerns to their superiors. The Rogers Commission offered NASA nine recommendations that were to be implemented before shuttle flights resumed.

Many schoolchildren viewed the launch live due to the presence on the crew of Christa McAuliffe, the first member of the Teacher in Space Project. Media coverage of the accident was extensive: one study reported that 85 percent of Americans surveyed in a poll had heard the news within an hour of the accident. The Challenger disaster has been used as a case study in many discussions of engineering safety and workplace ethics and inspired the 1990 television movie, Challenger.

Pre-launch conditions and delays

For more details on this topic, see Space Shuttle Challenger launch decision.

Ice on the launch tower on the morning of the Challenger launch
Ice on the launch tower on the morning of the Challenger launch

Challenger was originally set to launch from Kennedy Space Center in Florida at 2:43 p.m. Eastern Standard Time (EST) on January 22. However, delays suffered by the previous mission, STS-61-C, caused the launch date to be pushed back to the 23rd and then to the 24th. Launch was then rescheduled for the 25th due to bad weather at the Transoceanic Abort Landing (TAL) site in Dakar, Senegal. NASA decided to use Casablanca as the TAL site, but because it was not equipped for night landings, the launch had to be moved to the morning (Florida time). Predictions of unacceptable weather at Kennedy Space Center (KSC) caused the launch to be rescheduled for 9:37 a.m. EST on the 27th.

The launch was delayed the next day by problems with the exterior access hatch. First, one of the microswitch indicators used to verify that the hatch was safely locked malfunctioned.[1] Then, a stripped bolt prevented the closeout crew from removing a closing fixture from the orbiter's hatch.[2] When the fixture was finally sawn off, crosswinds at the Shuttle Landing Facility exceeded the limits for a Return to Launch Site (RTLS) abort.[3] The crew waited for the winds to die down until the launch window finally ran out, forcing yet another scrub.

Forecasts for the 28th predicted an unusually cold morning, with temperatures close to 31 °F (−0.5 °C), the minimum temperature allowable for launch. The low temperature had prompted concern from engineers at Morton Thiokol, the contractor responsible for the construction and maintenance of the shuttle's SRB. At a teleconference which took place on the evening of the 27th, Thiokol engineers and managers discussed the weather conditions with NASA managers from Kennedy Space Center and Marshall Space Flight Center. Several engineers—most notably Roger Boisjoly, who had voiced similar concerns previously—expressed their concern about the effect of the temperature on the resilience of the rubber O-rings that sealed the joints of the SRBs. They argued that if the O-rings were colder than 53 °F (approximately 11.7 °C), there was no guarantee the O-rings would seal properly. They also argued that the cold overnight temperatures would almost certainly result in SRB temperatures below their redline of 40 °F. However, they were overruled by Morton Thiokol management, who recommended that the launch proceed as scheduled.[4]

Due to the low temperature, a significant amount of ice built up on the fixed service structure that stood beside the shuttle. Although the Kennedy ice team had worked through the night removing ice, engineers at Rockwell International, the shuttle's prime contractor, still expressed concern. They warned that during launch ice might be shaken loose and strike the shuttle, possibly due to the aspiration induced by the jet of exhaust from the SRBs. Managers at Rockwell told shuttle program manager Arnold Aldrich that they could not completely assure that the shuttle was safe to launch, but failed to communicate a firm recommendation against launching. As a result of these discussions, Aldrich decided to postpone the shuttle launch by an hour in order to give the ice team the time to perform another inspection. After that last inspection, during which the ice appeared to be melting, Challenger was finally cleared to launch at 11:38 a.m. EST.[4]

[edit] January 28 launch and failure

[edit] Liftoff and initial ascent
Camera captures grey smoke escaping from the right-hand SRB
Camera captures grey smoke escaping from the right-hand SRB

The following account of the accident is derived from real time telemetry data and photographic analysis, as well as from transcripts of air-to-ground and mission control voice communications.[5] All times are given in seconds after launch and correspond to the telemetry time-codes from the closest instrumented event to each described event.[6]

At 6.6 seconds before liftoff, the three space shuttle main engines (SSME) ignited. Until liftoff actually occurs, the SSMEs can be safely shut down and the launch aborted if necessary. At liftoff time (T=0, which was at 11:38:00.010 EST), the three SSMEs were at 100% of their original rated performance, and began throttling up to 104% under computer control. At this moment, the two SRBs were ignited and hold-down bolts were released with explosives, freeing the vehicle from the pad. With the first vertical motion of the vehicle, the gaseous hydrogen vent arm retracted from the External Tank (ET) but failed to latch back. Review of film shot by pad cameras showed that the arm did not re-contact the vehicle, and thus it was ruled out as a contributing factor in the accident.[6] The post-launch inspection of the pad also revealed that kick springs on four of the hold-down bolts were missing, but they were similarly ruled out as a possible cause.[7]

Later review of launch film showed that at T+0.678, strong puffs of dark grey smoke were emitted from the right-hand SRB near the aft strut that attaches the booster to the ET. The last smoke puff occurred at about T+2.733. The last view of smoke around the strut was at T+3.375. It was later determined that these smoke puffs were caused by the opening and closing of the aft field joint of the right-hand SRB. The booster's casing had ballooned under the stress of ignition. As a result of this ballooning, the metal parts of the casing bent away from each other, opening a gap through which hot gases above 5,000 °F (2,760 °C) leaked out. The primary O-ring was designed to close that gap, but at the lower temperature it could not seal fast enough. The secondary O-ring was not in its seated position due to the metal bending. There was now no barrier to the gases, and both O-rings were vaporized across 70 degrees of arc. However, aluminum oxides from the burned solid propellant sealed the damaged joint, temporarily replacing the O-ring seal before actual flame rushed through the joint.
Challenger lifts off.
Challenger lifts off.

As the vehicle cleared the tower, the SSMEs were operating at 104% of their rated maximum thrust, and control switched from the Launch Control Center (LCC) at Kennedy to the Mission Control Center (MCC) in Houston, Texas. To prevent aerodynamic forces from tearing the shuttle apart, at T+28 the SSMEs began throttling down to limit the velocity of the shuttle in the dense lower atmosphere. At T+35.379, the SSMEs throttled back further to the planned 65%. Five seconds later, at about 19,000 feet (5800 m), Challenger passed through Mach 1. At T+51.860, the SSMEs began throttling back up to 104% as the vehicle approached Max Q, the period of maximum aerodynamic pressure on the vehicle.

[edit] Plume
Camera captures plume on right SRB
Camera captures plume on right SRB

Just as the shuttle approached Max Q, it slammed through the most intense wind shear ever experienced to date in the space shuttle program.

At T+58.788, a tracking film camera captured the beginnings of a plume near the aft attach strut on the right SRB. Unknown to those on Challenger or in Houston, ignited gas had begun to leak through a growing hole in one of the right-hand SRB's joints. The force of the wind shear shattered the temporary oxide seal that had taken the place of the damaged O-rings, removing the last barrier to flame rushing through the 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+60.238 there was visual evidence of flame coming through the joint and impinging on the external tank.[5]

At T+64.660, the plume suddenly changed shape, indicating that a leak had begun in the liquid hydrogen tank, located in the aft portion of the external tank. The nozzles of the main engines pivoted under computer control to compensate for the unbalanced thrust produced by the booster burn-through. The pressure in the shuttle's external liquid hydrogen tank began to drop at T+66.764, indicating the effect of the leak.[5]

At this stage the situation still seemed normal both to the astronauts and to flight controllers. At T+68, the CAPCOM informed the crew that they were "go at throttle up", and Commander Dick Scobee confirmed the call. His response, "Roger, go at throttle up," was the last communication from Challenger on the air-to-ground loop.

[edit] Vehicle breakup
View of the Space Shuttle Challenger disaster from Cocoa Beach
View of the Space Shuttle Challenger disaster from Cocoa Beach

At T+72.284, 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, 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, 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 ET. 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 and at an altitude of 48,000 feet (14.6 km).[8] With the external tank disintegrating, Challenger veered from its correct attitude with respect to the local air flow and was immediately torn apart by abnormal aerodynamic forces resulting in a load factor of up to 20g – well over its design limit. The two SRBs, which can withstand greater aerodynamic loads, separated from the ET and continued in uncontrolled powered flight for another 37 seconds. The SRB casings were made of half an inch (12.7 mm) 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.[7]

[edit] Post-breakup flight controller dialog
Jay Greene at his console after the breakup of Challenger
Jay Greene at his console after the breakup of Challenger

In Mission Control, there was silence for a few seconds after the accident. Television screens showed a cloud of smoke and vapor where Challenger had been, with pieces of debris falling toward the ocean. At about T+89, flight director Jay Greene prompted his flight dynamics officer for information. The response was that "fillters [radar] got discreting sources," a further indication that Challenger had broken into multiple pieces. The ground controller reported "negative contact, loss of downlink" of radio and telemetry data from Challenger. Greene ordered his team to "watch your data carefully" and look for any sign that the Orbiter had escaped.

At T+110.250, the Range Safety Officer (RSO) at the Cape Canaveral Air Force Station sent radio signals that activated the range safety system's "destruct" packages on board both solid rocket boosters. This was a normal contingency procedure, undertaken because the RSO judged the free-flying SRBs a possible threat to land or sea. The same destruct signal would have destroyed the External Tank had it not already disintegrated.[9]

"Flight controllers here looking very carefully at the situation," reported public affairs officer Steve Nesbitt. "Obviously a major malfunction. We have no downlink." After a pause, Nesbitt said, "We have a report from the Flight Dynamics Officer that the vehicle has exploded."

Greene ordered that contingency procedures be put into effect at Mission Control; these procedures included locking the doors of the control center, shutting down telephone communications with the outside world, and following checklists that ensured that the relevant data was correctly recorded and preserved.

[edit] No "explosion"
Challenger begins to disintegrate.
Challenger begins to disintegrate.

Contrary to the flight dynamics officer's initial statement, the shuttle and external tank did not actually "explode". Instead they rapidly disintegrated under tremendous aerodynamic forces, since the shuttle was near "Max Q", or maximum aerodynamic pressure. When the external tank disintegrated, the fuel and oxidizer stored within it were released, producing the appearance of a massive fireball. However, according to the NASA team that analyzed imagery after the accident, there was only "localized combustion" of propellant.[7] Instead, the visible cloud was primarily composed of vapor and gases resulting from the release of the shuttle's liquid oxygen and liquid hydrogen propellant. Stored in cryogenic conditions, the liquid hydrogen could not have ignited rapidly enough to trigger an "explosion" in the traditional sense. Had there been a true explosion, the entire shuttle would have been instantly destroyed, killing the crew at that moment. The more robustly constructed crew cabin and SRBs survived the breakup of the launch vehicle; while the SRBs were subsequently detonated remotely, the detached cabin continued along a ballistic trajectory, and was observed exiting the cloud of gases at T+75.237.[7] Twenty-five seconds after the breakup of the vehicle, the trajectory of the crew compartment peaked at a height of 65,000 feet (19.8 km); the breakup had occurred at only 48,000 feet (14.6 km).[8]

[edit] Cause and time of death

During vehicle breakup, the robustly-constructed crew cabin detached in one piece and slowly tumbled. NASA estimated separation forces at about 12 to 20 times the force of gravity g very briefly; however, within two seconds, the forces on the cabin had already dropped to below 4 g, and within ten seconds the cabin was undergoing free fall. These forces were likely insufficient to cause major injury. At least some of the astronauts were likely alive and briefly conscious after the breakup, because three of the four Personal Egress Air Packs (PEAPs) on the flight deck were found to have been activated. Investigators found their remaining unused air supply roughly consistent with the expected consumption during the 2 minute 45 second post-breakup trajectory. Whether the astronauts remained conscious long after the breakup is unknown, and largely depends on whether the detached crew cabin maintained pressure integrity. If it did not, time of useful consciousness at that altitude is just a few seconds; the PEAPs supplied only unpressurized air, and hence would not have helped the crew to retain consciousness. The crew cabin impacted the ocean surface at roughly 207 miles per hour (334 km/h), causing an instantaneous deceleration of over 200 g, far beyond the structural limits of the crew compartment or crew survivability levels.[8]

On July 28, 1986, Rear Admiral Richard H. Truly, NASA's Associate Administrator for Space Flight and a former astronaut, released a report from Joseph P. Kerwin, biomedical specialist from the Johnson Space Center in Houston, relating to the deaths of the astronauts in the accident. Dr. Kerwin, a veteran of the Skylab 2 mission, had been commissioned to undertake the study soon after the accident. The Space Shuttle Columbia disaster occurred on February 1, 2003, when the Space Shuttle Columbia disintegrated over Texas during re-entry into the Earth's atmosphere, shortly before concluding its 28th mission, STS-107. All seven crewmembers were killed.

The loss of the Columbia was caused by damage sustained during launch when a piece of foam insulation the size of a small briefcase broke off the main propellant tank under the aerodynamic forces of launch. The debris struck the leading edge of the left wing on the number 8 reinforced-carbon-carbon (RCC) tile, damaging the Shuttle's thermal protection system (TPS). While Columbia was still in orbit, some engineers suspected damage, but NASA managers limited the investigation on the grounds that little could be done even if problems were found.

NASA's own Shuttle safety regulations stated clearly that external tank foam shedding and subsequent debris strikes upon the Shuttle itself were safety issues that needed to be resolved before a launch was cleared, but launches were repeatedly given the go-ahead as engineers unsuccessfully studied the foam shedding problem. The majority of Shuttle launches recorded such foam strikes and thermal tile scarring, all in violation of safety regulations. During re-entry, the damaged area allowed the hot gases to penetrate and destroy the internal wing structure, eventually causing the in-flight breakup of the vehicle. A massive ground search in parts of Texas, Louisiana and Arkansas recovered crew remains and many vehicle fragments.

The Columbia Accident Investigation Board's recommendations addressed both technical and organizational issues. The Space Shuttle program was set back over two years by the disaster, a delay comparable only to that resulting from the Challenger disaster.
Contents
[hide]

* 1 Crew
* 2 Debris strike during launch
* 3 Flight risk management
* 4 Destruction during re-entry
* 5 Response from the President
* 6 Recovery of debris
* 7 Onboard video
* 8 Initial investigation
* 9 Possible emergency procedures
o 9.1 Rescue
o 9.2 Repair
* 10 Columbia Accident Investigation Board
* 11 Miscellaneous items
o 11.1 Unfounded fears of terrorism
o 11.2 "Purple streak" image
* 12 Memorials
* 13 Impact for space programs
* 14 References
* 15 External links

[edit] Crew
The crew of STS-107.
The crew of STS-107.

* Commander: Rick D. Husband, a US Air Force colonel and mechanical engineer, who piloted a previous shuttle during the first docking with the International Space Station (STS-96).
* Pilot: William C. McCool, a US Navy commander
* Payload Commander: Michael P. Anderson, a US Air Force lieutenant colonel and physicist who was in charge of the science mission.
* Payload Specialist: Ilan Ramon, a colonel in the Israeli Air Force and the first Israeli astronaut.
* Mission Specialist: Kalpana Chawla, an Indian-born aerospace engineer on her second space mission.
* Mission Specialist: David M. Brown, a US Navy captain trained as an aviator and flight surgeon. Brown worked on a number of scientific experiments.
* Mission Specialist: Laurel Clark, a US Navy captain and flight surgeon. Clark worked on a number of biological experiments.

[edit] Debris strike during launch
Columbia lifting off on its final mission. The light-colored triangle visible at the base of the strut is the Left Bipod Foam Ramp. Video
Columbia lifting off on its final mission. The light-colored triangle visible at the base of the strut is the Left Bipod Foam Ramp. Video

Despite its designation as the 107th Space Shuttle mission, STS-107 was actually the 113th to launch. It had been delayed 18 times over the two years from its original launch date of 11 January 2001 to its actual launch date of 16 January 2003. A well-publicized launch delay due to cracks in the shuttle's propellant distribution system occurred one month before a 19 July 2002 launch date, but the Columbia Accident Investigation Board (CAIB) determined that this delay had nothing to do with the catastrophic failure six months later.

The Left Bipod Foam Ramp is an approximately three-foot (one-meter) piece made entirely of foam, as opposed to being a metal ramp that is merely coated with foam. As such, the foam, not normally considered to be a structural material, is required to bear some aerodynamic loads. Because of these special requirements, the casting-in-place and curing of the ramps may be performed only by a senior technician. Apparently, even the best technical servicing available could not compensate for this fatal design flaw.

Bipod Foam Ramps had fallen off on at least three previous flights, with at least one previous strike that caused no serious damage. The euphemism used by NASA management to refer to this phenomenon was "foam shedding." As with the O-ring erosions that ultimately doomed the Challenger, NASA management seemed to grow complacent and accustomed to these phenomena when no serious consequences resulted from these earlier episodes. This phenomenon became known as "normalization of deviance."

Video taken during lift-off was routinely reviewed two hours later and revealed nothing unusual. The following day, higher-resolution film that had been processed overnight revealed that a piece of insulation foam fell from the external fuel tank 81.9 seconds into the launch sequence and appeared to strike the shuttle's left wing, potentially damaging the thermal protection on the Space Shuttle. The exact location where the foam struck the wing could not be determined due to the low resolution of the tracking camera footage.

[edit] Flight risk management
Close-up of the Left Bipod Foam Ramp that broke off and damaged the Shuttle wing.
Close-up of the Left Bipod Foam Ramp that broke off and damaged the Shuttle wing.

In a risk-management scenario similar to the Challenger disaster, NASA management failed to recognize the relevance of engineering concerns for safety. Two examples of this were failure to honor engineer requests for imaging to inspect possible damage, and failure to respond to engineer requests about status of astronaut inspection of the left wing. Engineering made three separate requests for Department of Defense (DOD) imaging of the shuttle in orbit to more precisely determine damage. While the images were not guaranteed to show the damage, the capability existed for imaging of sufficient resolution to provide meaningful examination. In fact, the CAIB recommended subsequent shuttle flights be imaged while in orbit using ground-based or space-based Department of Defense assets.[1] NASA management did not honor the requests and in some cases intervened to stop the DOD from assisting.

NASA's chief thermal protection system (TPS) engineer was concerned about left wing TPS damage and asked NASA management whether an astronaut would visually inspect it. NASA managers never responded.

Throughout the risk assessment process, senior NASA managers were influenced by their belief that nothing could be done even if damage was detected, hence this affected their stance on investigation urgency, thoroughness and possible contingency actions. They decided to conduct a parametric "what-if" scenario study more suited to determine risk probabilities of future events, instead of inspecting and assessing the actual damage. The investigation report in particular singled out NASA manager Linda Ham for exhibiting this attitude.[2]

Much of the risk assessment hinged on damage predictions to the thermal protection system. These fall into two categories: damage to the silica tile on the wing lower surface, and damage to the reinforced carbon-carbon (RCC) leading-edge panels.

Damage-prediction software, known as "Crater", was used to evaluate possible tile and RCC damage. The software predicted severe penetration of multiple tiles by the impact, but engineers downplayed this, believing that results showing that the software overstated damage from small projectiles meant that the same would be true of larger Spray-On Foam Insulation (SOFI) impacts. The program used to predict RCC damage was based on small ice impacts the size of cigarette butts, not larger SOFI impacts. Under 1 of 15 predicted SOFI impact paths, the software predicted an ice impact would completely penetrate the RCC panel. Engineers downplayed this, too, believing that impacts of the less dense SOFI material would result in less damage than ice impacts. In an e-mail exchange, NASA managers questioned whether the density of the SOFI could be used as justification for reducing predicted damage. Despite engineering concerns about the energy imparted by the SOFI material, NASA managers ultimately accepted the rationale to reduce predicted damage of the RCC panels from complete penetration to slight damage to the panel's thin coating.[3]

NASA managers assumed a rescue or repair was impossible, so there was no point in trying to inspect the vehicle for damage while on orbit. However, the CAIB determined either a rescue mission or on-orbit repair, though risky, might have been possible had NASA verified severe damage within five days into the mission.[4][5]

Ultimately the NASA Mission Management Team felt there was insufficient evidence to indicate that the strike was an unsafe situation, so they declared the debris strike a "turnaround" issue (not of highest importance) and denied the requests for the Department of Defense images.

[edit] Destruction during re-entry

The following is a timeline of Columbia's re-entry. The shuttle was scheduled to land at 9:16 a.m. EST.

* 2:30 a.m. EST, Saturday, February 1, 2003 – The Entry Flight Control Team began duty in the Mission Control Center.

The Flight Control Team had not been working on any issues or problems related to the planned de-orbit and re-entry of Columbia. In particular, the team had indicated no concerns about the debris impact to the left wing during ascent, and treated the re-entry like any other. The team worked through the de-orbit preparation checklist and re-entry checklist procedures. Weather forecasters, with the help of pilots in the Shuttle Training Aircraft, evaluated landing-site weather conditions at the Kennedy Space Center.

* 8:00 – Mission Control Center Entry Flight Director Leroy Cain polled the Mission Control room for a GO/NO-GO decision for the de-orbit burn.

All weather observations and forecasts were within guidelines set by the flight rules, and all systems were normal.

* 8:10 – The Capsule Communicator notified the crew that they are GO for de-orbit burn.
* 8:15:30 (EI-1719) – Commander Husband and Pilot McCool executed the de-orbit burn using Columbia’s two Orbital Maneuvering System engines.

The Orbiter was upside down and tail-first over the Indian Ocean at an altitude of 175 statute miles (282 km) when the burn was executed. The de-orbit maneuver was performed on the 255th orbit, and the 2-minute, 38-second burn slowed the Orbiter from 17,500 mph (7.8 km/s) to begin its re-entry into the atmosphere. During the de-orbit burn, the crew felt about 10% of the effects of gravity. There were no problems during the burn, after which Husband maneuvered Columbia into a right-side-up, forward-facing position, with the Orbiter's nose pitched up.

* 8:44:09 (EI+000) – Entry Interface (EI), arbitrarily defined as the point at which the Orbiter enters the discernible atmosphere at 400,000 feet (120 km), occurred over the Pacific Ocean.

As Columbia descended from space into the atmosphere, the heat produced by air molecules colliding with the Orbiter typically caused wing leading-edge temperatures to rise steadily, reaching an estimated 2,500 degrees Fahrenheit (1400 °C) during the next six minutes.

* 8:48:39 (EI+270) – A sensor on the left wing leading edge spar showed strains higher than those seen on previous Columbia re-entries.

This was recorded only on the Modular Auxiliary Data System, and was not telemetered to ground controllers or displayed to the crew.

* 8:49:32 (EI+323) – Columbia executed a pre-planned roll to the right. Speed: Mach 24.5.

Columbia began a banking turn to manage lift and therefore limit the Orbiter's rate of descent and heating.

* 8:50:53 (EI+404) – Columbia entered a 10-minute period of peak heating, during which the thermal stresses were at their maximum. Speed: Mach 24.1; altitude: 243,000 feet (74 km).

Columbia at approximately 0857. Debris is already starting to come off from the left wing.
Columbia at approximately 0857. Debris is already starting to come off from the left wing.

* 8:52:00 (EI+471) – Columbia was approximately 300 miles (500 km) west of the California coastline.

The wing leading-edge temperatures usually reached 2,650 degrees Fahrenheit (1450 °C) at this point.

* 8:53:26 (EI+557) – Columbia crossed the California coast west of Sacramento. Speed: Mach 23; altitude: 231,600 feet (70.6 km).

The Orbiter's wing leading edge typically reached more than 2,800 degrees Fahrenheit (1540 °C) at this point.

* 8:53:46 (EI+597) – Signs of debris being shed were sighted. Speed: Mach 22.8; altitude: 230,200 feet (70.2 km).

The superheated air surrounding the Orbiter suddenly brightened, causing a noticeable streak in the Orbiter's luminescent trail. Observers witnessed another four similar events during the following 23 seconds.

* 8:54:24 (EI+613) – The Maintenance, Mechanical, and Crew Systems (MMACS) officer informed the Flight Director that four hydraulic sensors in the left wing were indicating "off-scale low." In Mission Control, re-entry had been proceeding normally up to this point.

"Off-scale low" is a reading that falls below the minimum capability of the sensor.
The Entry Team continued to discuss the failed indicators.

* 8:54:25 (EI+614) – Columbia crossed from California into Nevada airspace. Speed: Mach 22.5; altitude: 227,400 feet (69.3 km).

Witnesses observed a bright flash at this point and 18 similar events in the next four minutes.

* 8:55:00 (EI+651) – Nearly 11 minutes after Columbia re-entered the atmosphere, wing leading-edge temperatures normally reached nearly 3,000 degrees Fahrenheit (1650 °C).
* 8:55:32 (EI+683) – Columbia crossed from Nevada into Utah. Speed: Mach 21.8; altitude: 223,400 ft (68 km).
* 8:55:52 (EI+703) – Columbia crossed from Utah into Arizona.
* 8:56:30 (EI+741) – Columbia initiated a roll reversal, turning from right to left over Arizona.
* 8:56:45 (EI+756) – Columbia crossed from Arizona to New Mexico. Speed: Mach 20.9; altitude: 219,000 feet.
* 8:57:24 (EI+795) – Columbia crossed just north of Albuquerque.

Columbia debris (in red, orange, and yellow) detected by National Weather Service radar over Texas and Louisiana.
Columbia debris (in red, orange, and yellow) detected by National Weather Service radar over Texas and Louisiana.

* 8:58:00 (EI+831) – At this point, wing leading-edge temperatures typically decreased to 2,880 degrees Fahrenheit (1580 °C).
* 8:58:20 (EI+851) – Columbia crossed from New Mexico into Texas. Speed: Mach 19.5; altitude: 209,800 feet (64 km).

At about this time, the Orbiter shed a Thermal Protection System tile, the most westerly piece of debris that has been recovered. Searchers found the tile in a field in Littlefield, Texas, just northwest of Lubbock.

* 8:59:15 (EI+906) – MMACS informed the Flight Director that pressure readings had been lost on both left main landing-gear tires. The Flight Director then told the Capsule Communicator (CAPCOM) to let the crew know that Mission Control saw the messages and was evaluating the indications, and added that the Flight Control Team did not understand the crew's last transmission.

A makeshift memorial at the main entrance to the Johnson Space Center in Houston, Texas
A makeshift memorial at the main entrance to the Johnson Space Center in Houston, Texas

* 8:59:32 (EI+923) – A broken response from the mission commander was recorded: "Roger, uh, bu - [cut off in mid-word, possibly the word "but"] ..." It was the last communication from the crew and the last telemetry signal received in Mission Control.
* 9:00:18 (EI+969) – Videos made by observers on the ground revealed that the Orbiter was disintegrating. In Mission Control, while the loss of signal was a cause for concern, there was no sign of any serious problem.
* 9:05 – Residents of north central Texas reported a loud boom, a small concussion wave, smoke trails and debris in the clear skies above the counties southeast of Dallas.
* 9:12:39 (EI+1710) – After hearing of reports of the shuttle being seen to break apart, the NASA flight director declared a contingency (events leading to loss of the vehicle) and alerted search and rescue teams in the debris area. He told the Ground Controller to "lock the doors", and two minutes later put Mission Control contingency procedures into effect. Nobody was permitted to enter or leave the room, and flight controllers had to preserve all the mission data for later investigation.

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