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Timeline of tragedy: The Columbia disaster

Looks like a blast furnace.
-- shuttle commander Rick Husband, midway through re-entry

Plunging back to Earth after a 16-day science mission, the shuttle Columbia streaked through orbital darkness at 5 miles per second, fast enough to fly from Chicago to New York in two and a half minutes and to circle the entire planet in an hour and a half. For Columbia's seven-member crew, the only hint of the shuttle's enormous velocity was the smooth clockwork passage of entire continents far below.

Commander Rick Husband knew the slow-motion view was misleading, a trick of perspective and the lack of anything nearby to measure against the craft's swift passage. He knew the 117-ton shuttle actually was moving through space eight times faster than the bullet from an assault rifle, fast enough to fly the length of 84 football fields in a single heartbeat.

And Husband knew that in the next 15 minutes, the shuttle would shed the bulk of that unimaginable speed over the southwestern United States, enduring 3,000-degree temperatures as atmospheric friction converted forward motion into a hellish blaze of thermal energy. It had taken nearly 4 million pounds of rocket fuel to boost Columbia and its crew into orbital velocity. Now the astronauts were about to slam on the brakes.

For Husband, a devout Christian who put God and family ahead of his work as an astronaut, flying this amazing machine home from space was a near-religious experience in its own right, one he couldn't wait to share with family and friends gathered at the Kennedy Space Center in Florida. He had served as pilot on a previous shuttle flight, but this was his first as commander, and in the world of shuttle operations, it's the commander who actually lands the spacecraft.

He relished the opportunity. But his life as an astronaut took a back seat to his deep faith in God. Before blasting off on his second spaceflight as commander of Columbia, he videotaped 34 Bible lessons for his two kids, one each for the 17 days he would be away from home.

Looking over his cockpit instruments as he prepared Columbia for entry, the 45-year-old Air Force colonel chatted easily with his crewmates, coming across more as an older brother than as the skipper of a $3 billion spacecraft. But underneath the friendly camaraderie was the steady hand of a commander at ease with leadership and life-or-death responsibility.

It was 8:44 a.m. on Feb. 1, 2003, and Columbia was descending through 400,000 feet northwest of Hawaii.

"OK, we're just past EI," Husband told his crewmates, marking when Columbia, flying wings level, its nose tilted up 40 degrees, finally fell into the discernible atmosphere.

He was referring to "entry interface," the moment the shuttle descended through an altitude of 76 miles. At that altitude -- 11 times higher than a typical passenger jet flies -- the atmosphere is still a vacuum in the everyday sense of the word. But enough atoms and molecules are present to begin having a noticeable effect on a vehicle plowing through them at 25 times the speed of sound.

Wearing bulky, bright- orange pressure suits, Husband, rookie pilot William "Willie" McCool, flight engineer Kalpana Chawla (pronounced KULP-nah CHAV-lah), and Navy physician-astronaut Laurel Clark were strapped into their seats on Columbia's cramped flight deck, working through the final entries on a long checklist.

The shuttle's flight computers, each one taking in navigation data and plugging the numbers into long strings of equations, were doing the actual flying. Husband wouldn't take over manual control until the orbiter was on final approach, 50,000 feet above Kennedy.

Husband was in the front left seat, the command position aboard any aircraft, with McCool to his right on the other side of a switch-studded instrument console. Chawla, a native of India, was a veteran of one previous shuttle flight and an accomplished pilot. Something of a legend in her hometown of Karnal in the Indian state of Punjab, Chawla was a role model in a country where less than half the women were literate. She sat directly behind the central console, calling out and double-checking re-entry tasks. Clark was seated to Chawla's right, almost touching shoulders with the diminutive flight engineer.

Strapped into seats on the split-level crew cabin's lower deck were payload commander Michael Anderson, another shuttle veteran and one of only a handful of African-American astronauts at NASA; physician-astronaut David Brown, a former circus acrobat, jet pilot and amateur videographer; and fighter pilot Ilan Ramon, the first Israeli to fly in space.

They were listening in a half-hour earlier as Husband counted down to de-orbit ignition, when Columbia's flight computers fired up the shuttle's twin braking rockets as the spacecraft flew upside down and backwards 170 miles above the central Indian Ocean. The two-minute 38-second rocket firing slowed the shuttle by just 176 mph. But that small decrease was just enough to lower the far side of Columbia's orbit deep into the atmosphere above Florida's east coast.

For the first half hour of re-entry, Columbia and its crew simply fell through the black void of space on a precisely plotted course toward a runway on the other side of the planet. But now, finally back in the discernible atmosphere, things were about to get interesting.

For McCool, an accomplished Navy carrier pilot and father of three who brought a boyish enthusiasm to Columbia's flight deck, entry interface was a long-anticipated milestone. Veteran astronauts had told him to expect a spectacular light show. Right on cue, the inky blackness outside his cockpit windows began giving way to a faint salmon glow.

At first, the effect was so subtle he wasn't sure it was really there.

"That might be some plasma now," he observed, as if seeking confirmation from Husband.

"Think so already?" asked Clark, seated directly behind McCool and aiming a handheld video camera out the cockpit's overhead windows. [The videotape was later found in the shuttle's wreckage.]

"That's some plasma," Husband finally confirmed.

"Copy, and there's some good stuff outside," Clark replied. "I'm filming overhead right now."

"It's kind of dull," McCool said a moment later, as if disappointed.

"Oh, it'll be obvious when the time comes," Husband reassured him.

McCool didn't have to wait long. During the next minute, the initial faint glow steadily brightened until there was no mistaking that the shuttle was embedded in a fireball.

"It's going pretty good now, Ilan," McCool reported to the three fliers on the lower deck. "It's really neat, just a bright orange-yellow out over the nose, all around the nose."

"Wait until you start seeing the swirl patterns out your left and right windows," Husband said.

"Wow."

"Looks like a blast furnace," Husband said dryly.

Even so, he and his crewmates were not worried. Focused, yes. Aware of the danger, yes -- as any well-trained astronauts would be when contemplating the energies involved in a shuttle launch or re-entry. But not worried. After all, there was no reason for concern. In the 111 previous space shuttle re-entries, there never had been a catastrophic "in-flight anomaly," as NASA refers to out-of-the-ordinary events. The only disaster in the history of the program, the Challenger explosion, had occurred during launch when one of its boosters had ruptured.

The Columbia astronauts already had survived the eight-and-a-half-minute climb to orbit that most experts, with some reason, considered a far more dangerous phase of flight. A fully fueled space shuttle weighs 4.5 million pounds at liftoff, yet accelerates to 100 mph -- straight up -- in about 10 seconds. In the first minute, more than two million pounds of fuel are consumed by the ship's twin solid-fuel boosters and three hydrogen-fueled main engines as the spacecraft plows its way through the dense lower atmosphere. Seven and a half minutes later, the astronauts are in orbit, moving through space at more than 17,000 mph.

Compared with launch, getting home was a walk in the park. Or so it seemed to most, including many at NASA. The astronauts knew better, of course. All of the energy it took to boost Columbia to orbital speed was still there in the form of the craft's enormous velocity. To make it back to Earth, the shuttle would have to give up that energy in the form of heat from atmospheric friction.

"This is amazing. It's really getting, uh, fairly bright out there," McCool observed as the glow around the orbiter continued to intensify.

"Yeah, you definitely don't want to be outside now," quipped Husband.

"What, like we did before?" Clark said pointedly as her crewmates laughed.

"Good point," Husband replied.

It was 8:47 a.m. and just three minutes had passed since entry interface. Approaching the coast of northern California, Columbia was dropping like a rock, its nose-up orientation designed to focus re-entry temperatures of up to 3,000 degrees on the heat-resistant reinforced carbon-carbon [RCC] panels making up the wings' leading edges and the orbiter's nose cap. Thousands of black heat-shield tiles making up the belly of the spacecraft would protect the underlying, vulnerable, aluminum skin from slightly lower, but still extreme, temperatures.

Just before 8:50 a.m., still off the coast of California, Columbia's computers ordered small, right-side steering rockets to fire, moving the shuttle's nose to the right. These roll maneuvers were planned to bleed off velocity before reaching the landing site.

McCool was looking forward to getting a little flying time in the next few minutes. After receiving the go-ahead to fire Columbia's braking rockets to begin the trip home, Husband had notified Mission Control that McCool would be taking the stick briefly, just before touchdown, as the shuttle banked to line up on the runway. Not every commander gave his or her co-pilot a chance to actually fly the shuttle, but that was Husband's nature. McCool couldn't wait.

As Columbia streaked toward the California-Nevada border, mechanical systems officer Jeff Kling, monitoring telemetry from the shuttle's myriad systems at Mission Control at the Johnson Space Center, noticed something unusual. Downward-pointing arrows appeared beside readings from two sensors measuring hydraulic fluid temperatures in the shuttle's left wing.

"What in the world?" another mechanical systems engineer, seeing the same data in a different room, said to Kling.

"This is not funny," Kling replied. "On the left side."

"On the left side," the engineer agreed.

They both tried to find some common thread that might explain the readings. Then, a few seconds later, two more down arrows appeared. It was as if the wiring to the four sensors in question, in hydraulic lines leading to the wing's flaps, or elevons, had been cut. Kling notified Flight Director LeRoy Cain.

"FYI, I've just lost four separate temperature transducers on the left side of the vehicle, hydraulic return temperatures," Kling reported, speaking quickly. "Two of them on system 1 and one in each of systems 2 and 3."

"Four hyd return temps?" Cain calmly asked.

"To the left outboard and left inboard elevon."

"OK, is there anything common to them? . . . I mean, you're telling me you lost them all at exactly the same time?"

"No, not exactly," Kling said. "They were within probably four or five seconds of each other."

"OK," Cain said, mulling over possible explanations. "Where are those, where is that instrumentation located?"

"All four of them are located in the aft part of the left wing, right in front of the elevons, elevon actuators," Kling replied. "And there is no commonality."

Cain pondered that for a moment.

"No commonality," he said after a long pause, his tone of voice indicating bafflement. Multiple sensor failures were rare, usually the result of problems with some common electrical system or component shared by the sensors in question. But these data "dropouts" could not immediately be traced to some single failure point.

Cain's thoughts flashed back to Columbia's launch. He recalled a briefcase-sized piece of foam insulation that broke away from Columbia's external fuel tank 81 seconds after liftoff and slammed into the underside of the left wing. A team of experts had studied the impact and dismissed it as not being a safety-of-flight issue. The Mission Management Team had unanimously accepted the results of the analysis.

An unsettling thought crossed Cain's mind. Was the loss of four left-wing temperature sensors related to the foam strike? It couldn't be, he thought. That had to be a coincidence.

"Tell me again which systems they're for?"

"That's all three hydraulic systems," Kling said. "It's . . . two of them are to the left outboard elevon and two of them to the left inboard."

"OK, I got you," Cain said. It was 8:56 a.m. and Columbia was crossing the Utah-Arizona-New Mexico state lines. Aerodynamic pressure was up to 40 pounds per square foot as the shuttle continued its steep descent.

In the management viewing room above Mission Control, Linda Ham was also worried. About the left wing. About the foam strike earlier in the mission. In her pivotal role as chairwoman of the Mission Management Team, Ham had approved the analysis that concluded the foam strike was not a safety of flight issue. She turned to Ralph Roe, the shuttle's engineering director, a tall ex-football player who also had taken part in the deliberations.

"Ralph, it's the left wing."

"It's not that," he said.

Nothing else in the continuous stream of data from Columbia indicated any signs of trouble. In just three minutes, the shuttle would be out of the region of maximum aerodynamic heating. There was reason to hope that all was well.

Suddenly, Husband called down, his first query since Columbia had entered Earth's atmosphere 15 minutes earlier.

"And, uh, Hou[ston] . . ." he began. His transmission was cut off. Such dropouts were not unusual during re-entry as the shuttle banked left and right, its big vertical tail fin occasionally blocking signals from reaching a NASA communications satellite stationed over the western Pacific.

A few seconds later, Kling saw more down arrows appear on his computer screen, this time signaling a loss of data from the shuttle's left main landing gear tires.

"We just lost tire pressure on the left outboard and left inboard, both tires," he told Cain, a half minute after Husband's interrupted call.

Astronaut Charles Hobaugh, the flight controller responsible for talking directly to the shuttle crew, heard Kling's report to Cain and promptly radioed Husband: "And Columbia, Houston, we see your tire pressure messages and we did not copy your last."

Seconds later, Husband made another attempt to contact Mission Control, replying to Hobaugh with "Roger, uh, buh . . ." He might have been saying "before" or "both," but again, the transmission was suddenly cut off and, along with it, the flow of data from the shuttle.

It was 8:59:32 a.m.

As far as the public knew, everything was proceeding smoothly. Communications dropouts typically lasted several seconds, occasionally longer, but sooner or later communications always resumed. Even so, at the midfield viewing site at Kennedy Space Center, Bill Readdy, NASA's associate administrator for spaceflight and the man ultimately responsible for shuttle operations, started paying attention.

Much of the crowd in Florida -- including spouses, children, parents and friends of the Columbia astronauts -- was oblivious to the drama unfolding in Mission Control. But a couple of reporters huddled around a television set inside the runway's public affairs building had noticed something peculiar.

The big map displayed in Mission Control and broadcast on NASA Television showed Columbia's progress as the ship sped across the continent toward Florida, the red triangle marking the shuttle's position. Inexplicably, that triangle had stopped moving over Central Texas.

Hobaugh radioed Husband to check whether the crew could hear communications from Houston.

"Columbia, Houston, comm check," Hobaugh called at 9:03 a.m. "Columbia, Houston, UHF comm check."

There was no reply.

"Columbia, Houston, UHF comm check," Hobaugh tried again. It was 9:04 a.m.

The shuttle now had been out of communication for nearly five minutes. At Kennedy's shuttle landing strip, a small flurry of activity had begun in the VIP area. Clusters of people had grouped together. The laughter and smiles of just 10 minutes earlier had all but disappeared, erased by anxious looks of concern.

Cell phones starting ringing. Soon, it appeared half the people in the crowd had phones pressed against their ears. Sean O'Keefe, NASA's administrator, heard Readdy say, "This is not right, something is not right on this." O'Keefe was stunned. Readdy, the veteran shuttle commander and former fighter pilot, was visibly trembling, his face ashen.

Kennedy shuttle manager Mike Wetmore was overwhelmed. He turned to look at the astronauts' families, the kids still running around in front of the bleachers, unaware of the sense of dread settling over the managers. He was paralyzed by the sight.

"Columbia, Houston, UHF comm check," Hobaugh radioed again.

There was no reply.

RE-ENTRY REVISITED

The most complicated machine ever built got knocked out of the sky by a pound and a half of foam. I don't know how any of us could have seen that coming. The message that sends me is, we are walking the razor's edge. This is a dangerous business and it does not take much to knock you off.

-- flight director Paul Hill

Shuttle wings are made of aluminum, the upper and lower surfaces separated by spars and trusses that form a boxlike internal framework. The main landing gear wheel well boxes are located toward the front of each wing, nestled up against the side of the orbiter's fuselage just behind the leading edge.

Behind its protective insulation, the front of a shuttle wing is flat, made up of a panel of aluminum honeycomb material known as the leading edge spar. To give the wing its aerodynamic shape, and to protect it from the most extreme temperatures of re-entry, 22 reinforced-carbon carbon [RCC] panels are bolted side by side on that flat front surface, creating a smoothly curving leading edge. So-called spanner beams, made out of a heat-resistant alloy called Inconel, provide rigidity. To seal the gaps between RCC panels, thin carbon-composite strips called T-seals are bolted in place to provide a smooth surface along the entire leading edge.

During re-entry, the shuttle's nose is pitched up 40 degrees, which subjects the lower halves of the RCC panels to the most extreme heating. The fittings used to attach the RCC panels to the main spar are protected by heat-resistant insulation that melts at 3,200 degrees.

Whatever happened to Columbia had utterly destroyed this complex system.

[Twenty-seven truckloads of wreckage were hauled to Kennedy Space Center between Feb. 5 and May 6. More than 25,000 searchers, who scoured a debris "footprint" that was 645 miles long, found 84,900 individual pieces, about 38 percent of the space shuttle. Each piece or component was cleaned, decontaminated, bar-coded, photographed and entered into a computer database.]

Wreckage from Columbia's wings, fuselage, and nose section was laid out on a grid in the Reusable Launch Vehicle Hangar near Kennedy's shuttle runway. The most critical RCC panels and attachment fittings -- those numbered 1 through 13 and nearest the fuselage -- were mounted on a full-scale clear plastic mockup of the rounded leading edge that allowed investigators to see each piece in relationship to its neighbors. It also allowed them to map out exactly where the heat went after it entered the leading edge.

[The work at KSC was buttressed by analysis by Johnson Space Center engineers of data from the orbiter's Modular Auxiliary Data System, or MADS, recorder and amateur video images of Columbia's disintegration. The inch-wide MADS tape contained information from 570 sensors; it was found by searchers in Hemphill, Texas, on March 19, six weeks after Columbia disintegrated.]

Ultimately, the Columbia Accident Investigation Board was able to conclude, without qualification, that the foam impact was the root cause of the accident; that the impact had knocked a 6- to 10-inch hole in the lower half of RCC panel 8 on the shuttle's left wing; and that a plume of super-heated plasma entering through that breach had destroyed the wing and triggered the destruction of the orbiter.

The team concluded the foam broke away from the left bipod ramp 81.7 seconds after liftoff and hit the underside of Columbia's left wing two-tenths of a second later. The foam measured 21 to 27 inches long by 12 to 18 inches wide. It was tumbling at 18 revolutions per second. Before the foam separated, the shuttle -- and the foam -- had a velocity of 1,568 mph, about twice the speed of sound. Because of its low density, the foam rapidly decelerated once in the airstream, slowing by 550 mph in that two-tenths of a second. The foam didn't fall on to the leading edge of the left wing as much as the shuttle ran into it from below. The relative speed of the collision was more than 500 mph, delivering more than a ton of force.

[On July 7, investigators using a nitrogen-powered cannon fired a 1,200-cubic-inch block of foam weighing 1.67 pounds at RCC panel 8, taken from the shuttle Atlantis. Traveling at 530 mph, the foam blew a ragged 16-inch hole in the RCC panel, vividly demonstrating how much damage foam could do.]

With the dramatic foam shot at RCC panel 8, all the pieces of the puzzle were finally in place. There was little doubt about what had doomed Columbia and its crew. A second-by-second timeline of the final working scenario provided a gripping account of the shuttle's final minutes.

At 8:44:09 a.m. Eastern time on Feb. 1, 2003, Columbia was a half-hour from home. The shuttle had just dropped below an altitude of 76 miles, slipping into the discernible atmosphere 900 miles northwest of Honolulu.

During re-entry, the shuttle compresses the thin air in front of it, creating two shock waves. Those shock waves intersect around RCC panel 9, subjecting panels in that area to the most extreme heating. But the compression of the air in front of the shuttle forms a so-called boundary layer, a region just a few inches thick that resists further compression and acts as a natural insulator. A few inches away from the leading edge, just beyond the boundary layer, molecules are torn apart and temperatures can exceed 10,000 degrees. But the boundary layer keeps temperatures on the leading edge RCC panels at around 3,000 degrees.

A smooth surface is essential for the boundary layer to form, and is crucial to a shuttle's survival during the plunge to Earth. If the boundary layer is disturbed for any reason, its insulating effect can be compromised by high-temperature turbulence, subjecting the shuttle's tiles and RCC panels to much more heat than they were designed to handle.

But even as the Columbia astronauts chatted about the light show outside, the hole in Columbia's left wing was disrupting that boundary layer. Ever more air molecules were shooting into the inside of the wing at RCC panel 8 and slamming into the insulation protecting the panel attachment fittings, swirling through the cavity and spreading out to either side. At that altitude, the effect was small. But the shuttle was descending, and the air was getting thicker with each passing second.

With Columbia in a 40-degree nose-up orientation, the plume entering the breach in RCC panel 8 was aimed at the upper attachment fittings and insulation. The insulation began melting, and the front face of the left wing's aluminum honeycomb forward spar -- the only barrier between the plume and the interior of the wing -- began heating up.

At 8:48:39 a.m., just four minutes and 30 seconds after Columbia had dipped into the atmosphere, a sensor mounted behind the forward spar, near the point where RCC panel 9 was bolted to the other side, measured an unusual increase in stress. The spar was softening.

About a minute later -- five and a half minutes after entry interface -- the shuttle's flight computers ordered a turn to the right. Up until this point, the shuttle had simply been falling into the atmosphere, wings level, nose up and pointed straight ahead. Now, the ship's flight computers began actively guiding the shuttle toward Kennedy's runway. The shuttle's nose smoothly swung 80 degrees to the right.

Less than 20 seconds after the maneuver, sensors mounted on Columbia's left rear rocket pod measured an unusual change in temperature. Wind tunnel testing would later show some of the hot air blasting into the RCC cavity was exiting through the vents on the upper surface of the wing, carrying thin clouds of metallic vapor from melted insulation.

The firestorm inside the RCC cavity was rapidly increasing in intensity. The boundary layer around the leading edge breach was severely disrupted, and the flow of super-heated air over the lower surface of the wing exposed the protective tiles there to much higher temperatures than they were designed to withstand. Insulation and RCC panel support fittings behind the breach continued to burn away.

Within a few seconds of 8:52:16 a.m. -- the exact time is unknown -- the deadly plume burned its way through the forward wing spar and into the interior of the wing.

The shuttle was still 300 miles from the coast of California. The crew still had no idea anything was wrong.

But with the boundary layer disrupted, the temperature of the atoms and molecules blasting into the wing probably exceeded 8,000 degrees near the leading edge breach itself. Hot gas began flowing into the wheel well through vents around landing gear door hinges. At 8:52:17 a.m., the first unusual sensor reading flashed on a computer screen in Mission Control: a slight increase in temperature in the hydraulic fluid running through a brake line leading to the left main landing gear.

Columbia's left wing was burning up from the inside out. Twelve seconds after the brake line temperature reading showed up in Mission Control, the shuttle's flight computers noticed the effects of the damage for the first time as a force, or drag, began pulling the shuttle's nose to the left. After assessing the data for a few seconds, the computers sent commands to the elevons on both wings to push the shuttle's nose slightly to the right to balance it out.

On the flight deck, Husband and McCool remained oblivious to their ship's ongoing destruction. They might have noticed the elevon movement on their forward computer displays, but the adjustments were small and would not have caused concern.

Columbia finally crossed the coast of California north of San Francisco at 8:53:28 a.m. at an altitude of 45 miles and a velocity of 15,800 mph. By then, the orbiter was in severe distress.

Scores of amateur shuttle watchers in California and Nevada had gotten up before dawn to watch Columbia's fiery descent. Even first-time observers were struck by the appearance of the shuttle's plasma trail. The super-heated air left in the shuttle's wake glowed in the dark sky like a phosphorescent contrail.

The plume shooting into the wing from the front spar breach may have burned a hole through the upper skin of the wing during this period, perhaps at the same time that many observers on the ground saw a bright flash.

By 8:54 a.m., just 32 seconds after Columbia had crossed the coast -- and just a minute and 44 seconds after the forward spar had been breached -- the outboard wall of the left main landing gear wheel well began melting. A scant 11 seconds after that, the shuttle's flight computers detected another change in the way Columbia's flight path was being affected.

It was as if the left wing had suddenly gained additional lift. The flight computers instantly responded, adjusting Columbia's elevons yet again to exactly counteract the two unwanted motions.

The shuttle stayed on course. Husband and McCool may have noticed the elevon movements as the autopilot responded, but again, they made no attempt to contact Mission Control for an explanation. In all likelihood, they still believed the entry was proceeding normally.

The increased lift initially puzzled investigators until they pieced together the plume's path through the wing's interior. The melting of the support spars and trusses just behind the forward spar caused the upper and lower wing surfaces to lose their rigidity. The lower wing, which was directly affected by the increasing pressure of the air, bowed inward, forming a depression. It started out small, but as the seconds ticked by and the wing's interior got even hotter, it grew alarmingly. Over the next five minutes, the depression probably grew to some 20 feet in length and 4 feet in width, a concave area more than 5 inches deep. Wind-tunnel testing and computer simulations later showed such a depression could explain the reaction of Columbia's flight computers.

In Mission Control, the first clear sign of a problem aboard Columbia was the loss of data from sensors in the left wing's hydraulic system. The wires leading to those sensors had been part of a cable bundle attached to the outboard wall of the left landing gear wheel well.

As Columbia was crossing the border between California and Nevada, the shuttle's attitude was down to 43.1 miles. But its velocity was still a blistering 22.5 times the speed of sound. It was 8:54:25 a.m.

Observers on the ground saw or photographed more than 10 debris-shedding events in the next few moments.

At 8:58:03 a.m., Columbia's flight computers detected a sharp change in the aerodynamic forces acting on the shuttle as the depression in the lower surface of the left wing presumably increased in size. At the same time, the drag acting to pull the nose farther to the left continued to increase. Approaching the Texas border, the flight computers again ordered the elevons to counteract the unwanted forces. Several debris-shedding events, indicating the wing was losing additional insulation and structure, were noticed by ground observers.

Months later, Air Force Lt. Col. Pat Goodman, a CAIB investigator, speculated the sudden change in the shuttle's flying characteristics was caused by a major change in the wing's shape. "I believe you can make a case . . . that the wing begins to collapse," Goodman said.

But the crew still would not have noticed any dramatic change.

They did, however, notice the loss of tire pressure data. The computers triggered an alarm in the cockpit and displayed a message to alert Husband to possible problems with the landing gear. This was the crew's first notification of potential trouble. Husband called Mission Control, presumably to report the message -- "And, uh, Hou[ston]" -- but his transmission was cut off.

Astronaut Charles Hobaugh, sitting to Cain's immediate right, radioed Columbia to let Husband know the flight control team was aware of the alarm and the lost tire data. He added, "And we did not copy your last" to let Husband know he needed to repeat whatever he had been trying to say earlier.

By now, the drag and roll forces acting on Columbia were beginning to reach the point where the elevons could no longer keep the shuttle properly oriented. In seconds, they would reach the limit of their motion.

Husband, perhaps beginning to realize major problems were developing, heard Hobaugh's call and tried to respond.

"Roger, uh, buh . . ."

It was 8:59:32 a.m. and Columbia was approaching Dallas. Seconds earlier, data from the shuttle suddenly froze on the computer screens in Mission Control. Down arrows or the letter S, for "static," had appeared on the screens, indicating the numbers were no longer being updated.

As it turned out, data were, in fact, still flowing down from Columbia. The signals were garbled, however, and the computers in Mission Control were programmed not to display potentially corrupted information. Investigators later would be able to extract some of the data. That information, combined with readings stored in the MADS recorder, and analysis of recovered wreckage, eventually allowed investigators to develop a rough timeline of events stretching another one minute and 50 seconds beyond Husband's final transmission.

For the astronauts, the final sequence was mercifully brief, but no doubt terrifying.

The left wing had suffered so much damage by now that nothing could be done to keep the nose pointed in the right direction. First two and then four right-side rocket thrusters were automatically commanded to fire in a futile bid to offset the forces pulling the nose to the left. A master alarm sounded in the cockpit as the elevon control circuitry failed. Columbia's nose yawed farther to the left, toward Earth, as the spacecraft began rolling to its right.

In all likelihood, all or part of the presumably collapsed wing suddenly folded over and broke off. At 8:59:46 a.m., a large piece of debris was seen separating from the shuttle.

Columbia's backup flight system computer began generating a string of fault messages. Two more large pieces of debris fell away from the shuttle within two seconds of each other starting at 9:00:01 a.m. One of these may have been the shuttle's vertical tail fin ripping off in the hypersonic airstream. The other could have been a large piece of the left-side rocket pod. No one knows.

"Everything just wants to fall over at that point," Cain said. "Because again, this is just like a barn door in wind. If that wing came off as we were falling -- pitching down and falling over . . . it is likely that the vehicle then probably broke apart in mid-body area."

But not immediately.

At 9:00:02 a.m., two seconds of relatively clean data reached the ground, providing a snapshot of Columbia's condition at that moment.

Columbia's three hydraulic power units were still running, along with the ship's three electrical generators. The main engine compartment was intact, and the communications and navigation equipment in the crew module were functioning normally. The shuttle's life support systems were operational. Air pressure was stable, and the temperature was a comfortable 71.6 degrees.

But all three hydraulic power units had lost pressure, and the ship's reservoirs of hydraulic fluid were empty. The shuttle's cooling system had shut down. Multiple alarm messages intended to alert the crew to problems were being generated by the computer system. Extreme temperatures were being recorded by sensors on the belly of the orbiter and along the left side of the fuselage. The electrical system was showing signs of multiple shorts.

As of 9:00:04 a.m., when the final two seconds of telemetry ended, the fuselage was still intact, along with the right wing and the right rear rocket pod. All or part of the left wing was gone. The condition of the vertical tail fin was unknown.

Just before telemetry stopped, data from the backup flight system computer indicated one of the two cockpit "joysticks," used to manually fly the spacecraft on final approach to the runway, was moved beyond its normal position. That's one way for a pilot to deactivate the autopilot. But investigators do not believe Husband or McCool was attempting to take over manual control. More likely, one of the pilots inadvertently bumped his hand controller during those horrifying final few seconds. The shuttle's digital autopilot remained engaged through the final loss of signal.

Finally, at 9:00:19 a.m., the fuselage began breaking apart. The shuttle was 37 miles up and still traveling 18 times the speed of sound.

A study done for the CAIB concluded the shuttle's heavily reinforced crew module and nose section broke away from the fuselage relatively intact, separating at the bulkhead that marks the dividing line between the cargo bay and the forward fuselage. Challenger's crew module had also broken away in one piece when the shuttle disintegrated during launch 17 years earlier. As with Challenger, the forces acting on Columbia's crew during this period were not violent enough to cause injury, and investigators believe the astronauts probably survived the initial breakup of the orbiter.

Like Challenger's crew, the Columbia astronauts met their fates alone and the details will never be known. Clark presumably was still videotaping on the flight deck when the alarms began blaring and the shuttle yawed out of control. But the outer portions of the tape -- the portions that might have shown at least the initial moments of the shuttle's destruction -- were burned away.

Investigators concluded the module fell intact for 38 seconds after main vehicle breakup, plunging 60,000 feet to an altitude of 26 miles before it began to disintegrate from the combined effects of aerodynamic stress and extreme temperatures. From the debris analysis, investigators believe the module was probably destroyed over a 24-second period beginning at 9:00:58 a.m. During that period, the module fell another 35,000 feet, to an altitude of 19 miles or so.

Investigators believe the module began breaking up at the beginning of that window. If any of the astronauts were still alive at that point, death would have been instantaneous, the result of blunt force trauma, including hypersonic wind blast, and lack of oxygen.

About 45 percent of the crew module was recovered near Hemphill, Texas, including pieces of the forward and aft main bulkheads, the frames from the forward cockpit windows, the crew airlock, and all of the hatches. About three-quarters of the flight deck instrument panels were found, along with 80 percent of the mid-deck floor panels and numerous parts from the crew's seats and attached safety equipment.

From an analysis of pressure suit components and helmets, investigators concluded three astronauts had not yet donned their gloves when breakup began and one was not wearing his or her helmet. In the end, however, having sealed pressure suits would have made no difference.

But investigators were struck by the way the crew modules of both Challenger and Columbia broke away relatively intact. The survivability study concluded relatively modest design changes might enable future crews to survive long enough to bail out.

But Columbia's crew had no chance. The astronauts fell to Earth amid a cloud of wreckage and debris.

One of the crew members came to rest beside a country road near Hemphill. The remains were found by a 59-year-old chemical engineer and Vietnam veteran named Roger Coday, who called the sheriff and then watched from the porch of his mobile home as a funeral director drove by to collect them.

"The astronauts have always been my heroes," said Coday, who that afternoon fashioned a cross out of two cedar logs he had cut earlier and erected it at the place where the astronaut had fallen to Earth.

"It's there and we still maintain it," he said eight months after the disaster, still wondering who the astronaut was. "I am a very devout Christian, and I prayed for that person's soul."

Copyright © 2014, The Baltimore Sun
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