On January 28, 1986, Space Shuttle Challenger broke apart 73 seconds into its flight, killing all seven crew members. The spacecraft disintegrated about 46,000 feet (14 km) above the Atlantic Ocean, off the coast of Cape Canaveral, Florida, at 16:39:13 UTC (11:39:13 a.m. EST, local time at the launch site). It was the first fatal accident involving an American spacecraft while in flight.
The mission, designated STS-51-L, was the 10th flight for the orbiter and the 25th flight of the NASA's Space Shuttle program. The crew was scheduled to deploy a commercial communications satellite and study Halley's Comet while they were in orbit, in addition to taking schoolteacher Christa McAuliffe into space under the Teacher in Space Project. This latter task resulted in a higher-than-usual media interest in and coverage of the mission, and the launch and subsequent disaster were seen live in many schools across the United States.
The cause of the disaster was the failure of the primary and secondary O-ring seals in a joint in the right Space Shuttle Solid Rocket Booster (SRB). The record-low temperatures on the morning of the launch had stiffened the rubber O-rings, reducing their ability to seal the joints. Shortly after liftoff the seals were breached, and hot pressurized gas from within the SRB leaked through the joint and burned through the aft attachment strut connecting it to the external propellant tank (ET), then into the tank itself. The collapse of the ET's internal structures and the rotation of the SRB that followed propelled the shuttle stack, traveling at a speed of Mach 1.92, into a direction that allowed aerodynamic forces to tear the orbiter apart. Both SRBs detached from the now-destroyed ET and continued to fly uncontrollably until the range safety officer (RSO) sent a signal to destroy them. The orbiter had no launch escape system.
The crew compartment containing human remains, and many other fragments from the shuttle, were recovered from the ocean floor during a three-month search and recovery operation. The exact timing of the deaths of the crew is not known, but several crew members are thought to have survived the initial breakup of the spacecraft. The impact of the crew compartment with the ocean surface at terminal velocity of 207 mph (333 km/h), with estimated deceleration of 200g, was too violent for human beings to survive.
The disaster resulted in a 32-month hiatus in the Space Shuttle program. President Ronald Reagan created the Rogers Commission to investigate the accident. The commission criticized NASA's organizational culture and decision-making processes that had contributed to the accident. Test data since 1977 had demonstrated a potentially catastrophic flaw in the SRBs' O-rings, but neither NASA nor SRB manufacturer Morton Thiokol had addressed this known defect. NASA managers also disregarded engineers' warnings about the dangers of launching in low temperatures, and they did not report these warnings to their superiors.
As a result of this disaster, NASA established the Office of Safety, Reliability, and Quality Assurance, and arranged for deployment of commercial satellites from expendable launch vehicles, rather than from a crewed orbiter. The construction of a new Space Shuttle orbiter, Endeavour, was approved in 1987 to replace Challenger, and the new orbiter first flew in 1992. Subsequent missions were launched with redesigned SRBs and their crews wore pressurized suits during ascent and reentry. In February 2003 the Space Shuttle Columbia disintegrated during re-entry. The Columbia Accident Investigation Board concluded that NASA had failed to learn lessons from the Challenger disaster, which resulted in the second disaster.
On January 28, 1986, Space Shuttle Challenger broke apart 73 seconds into its flight, killing all seven crew members. The spacecraft disintegrated about 46,000 feet (14 km) above the Atlantic Ocean, off the coast of Cape Canaveral, Florida, at 16:39:13 UTC (11:39:13 a.m. EST, local time at the launch site). It was the first fatal accident involving an American spacecraft while in flight.
The mission, designated STS-51-L, was the 10th flight for the orbiter and the 25th flight of the NASA's Space Shuttle program. The crew was scheduled to deploy a commercial communications satellite and study Halley's Comet while they were in orbit, in addition to taking schoolteacher Christa McAuliffe into space under the Teacher in Space Project. This latter task resulted in a higher-than-usual media interest in and coverage of the mission, and the launch and subsequent disaster were seen live in many schools across the United States.
The cause of the disaster was the failure of the primary and secondary O-ring seals in a joint in the right Space Shuttle Solid Rocket Booster (SRB). The record-low temperatures on the morning of the launch had stiffened the rubber O-rings, reducing their ability to seal the joints. Shortly after liftoff the seals were breached, and hot pressurized gas from within the SRB leaked through the joint and burned through the aft attachment strut connecting it to the external propellant tank (ET), then into the tank itself. The collapse of the ET's internal structures and the rotation of the SRB that followed propelled the shuttle stack, traveling at a speed of Mach 1.92, into a direction that allowed aerodynamic forces to tear the orbiter apart. Both SRBs detached from the now-destroyed ET and continued to fly uncontrollably until the range safety officer (RSO) sent a signal to destroy them. The orbiter had no launch escape system.
The crew compartment containing human remains, and many other fragments from the shuttle, were recovered from the ocean floor during a three-month search and recovery operation. The exact timing of the deaths of the crew is not known, but several crew members are thought to have survived the initial breakup of the spacecraft. The impact of the crew compartment with the ocean surface at terminal velocity of 207 mph (333 km/h), with estimated deceleration of 200g, was too violent for human beings to survive.
The disaster resulted in a 32-month hiatus in the Space Shuttle program. President Ronald Reagan created the Rogers Commission to investigate the accident. The commission criticized NASA's organizational culture and decision-making processes that had contributed to the accident. Test data since 1977 had demonstrated a potentially catastrophic flaw in the SRBs' O-rings, but neither NASA nor SRB manufacturer Morton Thiokol had addressed this known defect. NASA managers also disregarded engineers' warnings about the dangers of launching in low temperatures, and they did not report these warnings to their superiors.
As a result of this disaster, NASA established the Office of Safety, Reliability, and Quality Assurance, and arranged for deployment of commercial satellites from expendable launch vehicles, rather than from a crewed orbiter. The construction of a new Space Shuttle orbiter, Endeavour, was approved in 1987 to replace Challenger, and the new orbiter first flew in 1992. Subsequent missions were launched with redesigned SRBs and their crews wore pressurized suits during ascent and reentry. In February 2003 the Space Shuttle Columbia disintegrated during re-entry. The Columbia Accident Investigation Board concluded that NASA had failed to learn lessons from the Challenger disaster, which resulted in the second disaster.
Background
Space Shuttle
The Space Shuttle was a partially reusable spacecraft operated by the US National Aeronautics and Space Administration (NASA). It flew 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 orbiters were built during the Space Shuttle program. Challenger (OV-099) was the second orbiter constructed after its conversion from a structural test article. 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.
When it launched, the orbiter was connected to the ET, which held the fuel for the SSMEs. The ET consisted of a larger tank for liquid hydrogen (LH2) and a smaller tank for liquid oxygen (LOX), both of which were required for the SSMEs to operate. After its fuel had been expended, the ET separated from the orbiter and reentered the atmosphere, where it would break apart during reentry and its pieces would land in the Indian or Pacific Ocean.
Two solid rocket boosters (SRBs), built by Morton Thiokol at the time of the disaster, provided the majority of thrust at liftoff. They were connected to the external tank, and burned for the first two minutes of flight. The SRBs separated from the orbiter 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.
Each SRB was constructed in four main sections at the factory in Utah and transported to KSC, then assembled in the Vehicle Assembly Building at KSC with three tang-and-clevis field joints, each joint consisting of a tang from the upper segment fitting into the clevis of the lower segment. Each field joint was sealed with two 20-foot (6 m) diameter Viton-rubber O-rings around the circumference of the SRB and had a cross-section diameter of 0.280 inches (7.1 mm). The O-rings were required to contain the hot, high-pressure gases produced by the burning solid propellant and allowed for the SRBs to be rated for crewed missions. The two O-rings were configured to create a double bore seal, and the gap between segments was filled with putty. When the motor was running, this configuration was designed to compress air in the gap against the upper O-ring, pressing it against the sealing surfaces of its seat. On the SRB Critical Items List, the O-rings were listed as Criticality 1R, which indicated that an O-ring failure could result in the destruction of the vehicle and loss of life, and that it was considered a redundant system due to the secondary O-ring.
O-ring concerns
Evaluations of the proposed SRB design in the early 1970s and field joint testing showed that the wide tolerances between the mated parts allowed the O-rings to be extruded from their seats rather than compressed. This extrusion was judged to be acceptable by NASA and Morton Thiokol despite concerns of NASA's engineers. A 1977 test showed that up to 0.052 inches (1.3 mm) of joint rotation occurred during the simulated internal pressure of a launch. Joint rotation, which occurred when the tang and clevis bent away from each other, reduced the pressure on the O-rings, which weakened their seals and made it possible for combustion gases to erode the O-rings. NASA engineers suggested that the field joints should be redesigned to include shims around the O-rings, but they received no response. In 1980, the NASA Verification/Certification Committee requested further tests on joint integrity to include testing in the temperature range of 40 to 90 °F (4 to 32 °C) and with only a single O-ring installed. The NASA program managers decided that their current level of testing was sufficient and further testing was not required. In December 1982, the Critical Items List was updated to indicate that the secondary O-ring could not provide a backup to the primary O-ring, as it would not necessarily form a seal in the event of joint rotation. The O-rings were redesignated as Criticality 1, removing the "R" to indicate it was no longer considered a redundant system.
The first occurrence of in-flight O-ring erosion occurred on the right SRB on STS-2 in November 1981. In August 1984, a post-flight inspection of the left SRB on STS-41-D revealed that soot had blown past the primary O-ring and was found in between the O-rings. Although there was no damage to the secondary O-ring, this indicated that the primary O-ring was not creating a reliable seal and was allowing hot gas to pass. The amount of O-ring erosion was insufficient to prevent the O-ring from sealing, and investigators concluded that the soot between the O-rings resulted from non-uniform pressure at the time of ignition. The January 1985 launch of STS-51-C was the coldest Space Shuttle launch to date. The air temperature was 62 °F (17 °C) at the time of launch, and the calculated O-ring temperature was 53 °F (12 °C). Post-flight analysis revealed erosion in primary O-rings in both SRBs. Morton Thiokol engineers determined that the cold temperatures caused a loss of flexibility in the O-rings that decreased their ability to seal the field joints, which allowed hot gas and soot to flow past the primary O-ring. O-ring erosion occurred on all but one (STS-51-J) of the Space Shuttle flights in 1985, and erosion of both the primary and secondary O-rings occurred on STS-51-B.
To correct the issues with O-ring erosion, engineers at Morton Thiokol, led by Allan McDonald and Roger Boisjoly, proposed a redesigned field joint that introduced a metal lip to limit movement in the joint. They also recommended adding a spacer to provide additional thermal protection and using an O-ring with a larger cross section. In July 1985, Morton Thiokol ordered redesigned SRB casings, with the intention of using already-manufactured casings for the upcoming launches until the redesigned cases were available the following year.
The primary mission of the Challenger crew was to use an Inertial Upper Stage (IUS) to deploy TDRS-B, a tracking and data relay satellite. The crew also planned to study Halley's Comet as it passed near the Sun, and deploy and retrieve the Spartan Halley satellite.
The mission was originally scheduled for July 1985, but was delayed to November and then to January 1986. The launch date was initially set for January 22, but this was delayed several times while the shuttle waited on the launch pad.
On December 23, 1985, due to delays in preparations for STS-61-C, the launch was pushed back to January 23. On January 22, the mission was postponed until January 26 due to dust storms at the shuttle's emergency landing site, Dakar-Yoff International Airport in Senegal. NASA chose Mohammed V International Airport in Casablanca, Morocco, as an alternative emergency site, but a lack of runway lighting there meant that the shuttle now could only launch when it was daytime in Morocco and morning in Florida. The launch was postponed again by one day when thunderstorms near Cape Canaveral were forecast for January 26; the weather turned out to be clear at the intended launch time, but storms came in shortly thereafter. The shuttle was prepared to launch on January 27, but the countdown was halted at T−9 minutes when the crew cabin's door handle could not be removed due to a stuck bolt, preventing the needed insulation tile from being installed in its place. By the time the bolt had been removed, high winds at Cape Canaveral prevented the launch, as they would have made a RTLS abort dangerous if it were needed. The winds continued until the morning's launch window closed at 12:37 p.m. EST, causing one final postponement to January 28.
While all of these delays were necessitated by safety concerns, the increased publicity surrounding the mission because of the Teacher in Space Program led major media outlets to ridicule the repeated postponements of the launch. On the evening of January 27, CBS's Dan Rather called the bolt issue a "high-tech low comedy", while other outlets highlighted the frustration of spectators who had been waiting for the launch that day. This negative media coverage was later cited as one of several factors which pressured NASA officials to ignore potential issues with the O-rings so that Challenger could launch as soon as possible.
Decision to launch
The air temperature on January 28 was predicted to be a record low for a Space Shuttle launch. The air temperature was forecast to drop to 18 °F (−8 °C) overnight before rising to 22 °F (−6 °C) at 6:00 a.m. and 26 °F (−3 °C) at the scheduled launch time of 9:38 a.m. Based upon O-ring erosion that had occurred in warmer launches, Morton Thiokol engineers were concerned over the effect the record-cold temperatures would have on the seal provided by the SRB O-rings for the launch. Cecil Houston, the manager of the Kennedy Space Center (KSC) office of the Marshall Space Flight Center in Alabama, set up a three-way conference call with Morton Thiokol in Utah and the KSC in Florida on the evening of January 27 to discuss the safety of the launch. Morton Thiokol engineers expressed their concerns about the effect of low temperatures on the resilience of the rubber O-rings. As the colder temperatures lowered the elasticity of the rubber O-rings, the engineers feared that the O-rings would not be extruded to form a seal at the time of launch. The engineers argued that they did not have enough data to determine whether the O-rings would seal at temperatures colder than 53 °F (12 °C), the coldest launch of the Space Shuttle to date. During this discussion, Lawrence Mulloy, the NASA SRB project manager, said that he did not accept the analysis behind this decision, and demanded to know if Morton Thiokol expected him to wait until April for warmer temperatures. Morton Thiokol employees Robert Lund, the Vice President of Engineering, and Joe Kilminster, the Vice President of the Space Booster Programs, recommended against launching until the temperature was above 53 °F (12 °C).