On February 1, 2003, Space Shuttle Columbia disintegrated as it re-entered the atmosphere over Texas and Louisiana, killing all seven astronauts on board. It was the second of two Space Shuttle missions to end in disaster, after the loss of Challenger and crew in 1986.
The mission, designated STS-107, was the 28th flight for the orbiter and the 113th flight of the Space Shuttle fleet. It was dedicated to research in various fields, mainly on board the SpaceHab module inside the shuttle's payload bay.
During launch, a piece of the insulating foam broke off from the Space Shuttle external tank and struck the thermal protection system tiles on the orbiter's left wing. Similar foam shedding had occurred during previous Space Shuttle launches, causing damage that ranged from minor to near-catastrophic, but some engineers suspected that the damage to Columbia was more serious. When Columbia re-entered the atmosphere of Earth, the damage allowed hot atmospheric gases to penetrate the heat shield and destroy the internal wing structure, which caused the orbiter to become unstable and break apart.
After the disaster, Space Shuttle flight operations were suspended for more than two years, as they had been after the Challenger disaster. Construction of the International Space Station (ISS) was paused until flights resumed in July 2005 with STS-114. NASA made several technical and organizational changes to subsequent missions, including the addition of an on-orbit inspection to determine how well the orbiter's thermal protection system (TPS) had endured the ascent, and continual readiness of designated rescue missions in case irreparable damage was found. Except for one mission to repair the Hubble Space Telescope, subsequent Space Shuttle missions were flown only to the ISS to allow the crew to use it as a haven if damage to the orbiter prevented safe re-entry. The remaining three orbiters were retired after the building of the ISS was completed.
Background
Space Shuttle
The Space Shuttle was a partially reusable spacecraft operated by the US National Aeronautics and Space Administration (NASA). It flew in space for the first time in April 1981, and was used to conduct in-orbit research, and deploy commercial, military, and scientific payloads. At launch, it consisted of the orbiter, which contained the crew and payload, the external tank (ET), and the two solid rocket boosters (SRBs). The orbiter was a reusable, winged vehicle that launched vertically and landed as a glider. Five operational orbiters were built during the Space Shuttle program. Columbia was the first space-rated orbiter constructed, following the atmospheric test vehicle Enterprise. The orbiter contained the crew compartment, where the crew predominantly lived and worked throughout a mission. Three Space Shuttle main engines (SSMEs) were mounted at the aft end of the orbiter and provided thrust during launch. Once in space, the crew maneuvered using the two smaller, aft-mounted Orbital Maneuvering System (OMS) engines.
The orbiter was protected from heat during re-entry by the thermal protection system (TPS), a thermal soaking protective layer around the orbiter. In contrast with previous US spacecraft, which had used ablative heat shields, the reusability of the orbiter required a multi-use heat shield. During re-entry, the TPS experienced temperatures up to 3,000 °F (1,600 °C), but had to keep the orbiter vehicle's aluminum skin temperature below 350 °F (180 °C). The TPS primarily consisted of four sub-systems. The nose cone and leading edges of the wings experienced temperatures above 2,300 °F (1,300 °C), and were protected by the composite material reinforced carbon–carbon (RCC). Thicker RCC was developed and installed in 1998 to prevent damage from micrometeoroid and orbital debris. The entire underside of the orbiter vehicle, as well as the other hottest surfaces, were protected with black high-temperature reusable surface insulation. Areas on the upper parts of the orbiter vehicle were covered with white low-temperature reusable surface insulation, which provided protection at temperatures below 1,200 °F (650 °C). The payload bay doors and parts of the upper wing surfaces were covered with reusable felt surface insulation, as the temperature there remained below 700 °F (370 °C).
Two solid rocket boosters (SRBs) were connected to the ET, and burned for the first two minutes of flight. The SRBs separated from the ET once they had expended their fuel and fell into the Atlantic Ocean under a parachute. NASA retrieval teams recovered the SRBs and returned them to the Kennedy Space Center (KSC), where they were disassembled and their components were reused on future flights.
When the Space Shuttle launched, the orbiter and SRBs were connected to the ET, which held the fuel for the SSMEs. The ET consisted of a tank for liquid hydrogen (LH2), stored at −423 °F (−253 °C) and a smaller tank for liquid oxygen (LOX), stored at −297 °F (−183 °C). It was covered in insulating foam to keep the liquids cold and prevent ice forming on the tank's exterior. The orbiter connected to the ET via two umbilicals near its bottom and a bipod near its top section.
After its fuel had been expended, the ET separated from the orbiter and reentered the atmosphere, where it would break apart during re-entry and its pieces would land in the Indian or Pacific Ocean.
Debris strike concerns
During the design process of the Space Shuttle, a requirement of the ET was that it would not release any debris that could potentially damage the orbiter and its TPS. The integrity of the TPS components was necessary for the survival of the crew during re-entry, and the tiles and panels were only built to withstand relatively minor impacts. On STS-1, the first flight of the Space Shuttle, the orbiter Columbia was damaged during its launch from a foam strike. Foam strikes occurred regularly during Space Shuttle launches; of the 79 missions with available imagery during launch, foam strikes occurred on 65.
The bipod connected the ET near the top to the front underside of the orbiter via two struts with a ramp at the tank end of each strut; the ramps were covered in foam to prevent ice from forming that could damage the orbiter. The foam on each bipod ramp was approximately 30 by 14 by 12 inches (76 by 36 by 30 cm), and was carved by hand from the original foam application. Bipod ramp foam from the left strut had been observed falling off the ET on six flights prior to STS-107, and had created some of the largest foam strikes that the orbiter experienced. The first bipod ramp foam strike occurred during STS-7; the orbiter's TPS was repaired after the mission but no changes were made to address the cause of the bipod foam loss. After bipod foam loss on STS-32, NASA engineers, under the assumption that the foam loss was due to pressure buildup within the insulation, added vent holes to the foam to allow gas to escape. After a bipod foam strike damaged the TPS on STS-50, internal NASA investigations concluded it was an "accepted flight risk" and that it should not be treated as a flight safety issue. Bipod foam loss occurred on STS-52 and STS-62, but neither event was noticed until the investigation following Columbia's destruction.
During STS-112, which flew in October 2002, a 4-by-5-by-12-inch (10 by 13 by 30 cm) chunk of bipod ramp foam broke away from the ET bipod ramp and hit the SRB-ET attachment ring near the bottom of the left SRB, creating a dent 4 inches (10 cm) wide and 3 inches (8 cm) deep. Following the mission, the Program Requirements Control Board declined to categorize the bipod ramp foam loss as an in-flight anomaly. The foam loss was briefed at the STS-113 Flight Readiness Brief, but the Program Requirements Control Board decided that the ET was safe to fly.
A debris strike from the ablative material on the right SRB caused significant damage to Atlantis during the STS-27 launch on December 2, 1988. On the second day of the flight the crew inspected the damage using a camera on the remote manipulator system. The debris strike had removed a tile; the exposed orbiter skin was a reinforced section, and a burn-through might have occurred had the damage been in a different location. After that mission, the NASA Program Requirements Control Board designated the issue as an in-flight anomaly, which was later corrected with the planned improvement for the SRB ablator. The Columbia Investigative Review Board noted the difference in how the STS-27 and STS-107 debris strike events were treated. Upon the debris strike discovery on STS-27, the crew was immediately directed to inspect the vehicle. Fourteen years later, when the debris strike on STS-107 was discovered, Shuttle Program management declined to have the crew inspect the orbiter for damage or to request on-orbit imaging, ultimately discounting the possibility of a burn-through. The Board viewed the failure of Shuttle management to consider the threat to STS-107 posed by a strike similar to STS-27 as a sign that NASA was not functioning as a learning organization.
Flight
Space Shuttle mission
For STS-107, Columbia carried the SpaceHab Research Double Module, the Orbital Acceleration Research Experiment, and an Extended Duration Orbiter pallet. The mission passed its pre-launch certifications and reviews, and began with the launch. The mission was originally scheduled to launch on January 11, 2001, but it was delayed thirteen times, until its launch on January 16, 2003.
The seven-member crew of STS-107 were selected in July 2000. The mission was commanded by Rick Husband, who was a colonel in the US Air Force and a test pilot. He had previously flown on STS-96. The mission's pilot was William McCool, a US Navy commander who was on his first spaceflight. The payload commander was Michael Anderson, a US Air Force lieutenant colonel who had previously flown on STS-89. Kalpana Chawla served as the flight engineer; she had previously flown on STS-87. David Brown and Laurel Clark, both Navy captains, flew as the mission specialists on their first spaceflights. Ilan Ramon, a colonel in the Israeli Air Force and the first Israeli astronaut, flew as a payload specialist on his first spaceflight.
Launch and debris strike
Columbia was launched from the Kennedy Space Center Launch Complex 39A (LC-39A) on January 16, 2003 at 10:39:00 am. At T+81.7 seconds, a piece of foam approximately 21 to 27 inches (53 to 69 cm) long and 12 to 18 inches (30 to 46 cm) wide broke off from the left bipod on the ET. At T+81.9 seconds, the foam struck the reinforced carbon–carbon (RCC) panels on Columbia's left wing at relative velocity of 625 to 840 feet per second (426 to 573 mph; 686 to 922 km/h). The foam's low ballistic coefficient caused it to lose speed immediately after separating from the ET, and the orbiter ran into the slower foam. Neither the mission nor ground crew noticed the debris strike at the time. The SRBs separated from the ET at T+2 minutes and 7 seconds, followed by the ET's separation from the orbiter at T+8 minutes 30 seconds. The ET separation was photographed by Anderson and recorded by Brown, but they did not record the bipod with missing foam. At T+43 minutes, Columbia completed its orbital insertion as planned.
Flight risk management
After Columbia entered orbit, the NASA Intercenter Photo Working Group conducted a routine review of videos of the launch. The group's analysts did not notice the debris strike until the second day of the mission. None of the cameras that recorded the launch had a clear view of the debris striking the wing, leaving the group unable to determine the level of damage sustained by the orbiter. The group's chair contacted Wayne Hale, the Shuttle Program Manager for Launch Integration, to request on-orbit pictures of Columbia's wing to assess its damage. After receiving notification of the debris strike, engineers at NASA, United Space Alliance, and Boeing created the Debris Assessment Team and began working to determine the damage to the orbiter. Intercenter Photo Working Group believed that the orbiter's RCC tiles were possibly damaged; NASA program managers were less concerned over the danger caused by the debris strike.
Boeing analysts attempted to model the damage caused to the orbiter's TPS from the foam strike. The software models predicted damage that was deeper than the thickness of the TPS tiles, indicating that the orbiter's aluminum skin would be unprotected in that area. The Debris Assessment Team dismissed this conclusion as inaccurate, because several previous predictions under similar conditions had forecast greater problems than actually occurred. Further modeling specific to the RCC panels used software calibrated to predict damage caused by falling ice. The software predicted only one of 15 scenarios that ice would cause damage, leading the Debris Assessment Team to conclude there was minimal damage due to the lower density of foam to ice.