Challenger Space Shuttle – An Incident Investigation Exercise for our Online IOSH Managing Safely Course

The Space Shuttle Challenger disaster was a fatal incident in the United States space program that occurred on January 28th 1986, when the Space Shuttle Challenger broke apart 73 seconds into its flight, killing all seven crew members aboard. The crew consisted of five NASA astronauts including a civilian schoolteacher, who was meant to be the first schoolteacher in space. 

NASA hoped that sending a teacher into space would increase public interest in the Space Shuttle program at a time when the agency was under continuous pressure to find financial support. 

The disintegration of the vehicle was caused by the failure of rubber O-ring seals used in a joint on a solid booster rocket (SRB) that were not designed to handle the unusually cold conditions on the day. The seals’ failure caused pressurised gases to leak through a rocket joint, causing a breach. This caused the external fuel tank to fail structurally and explode. 

Morton-Thiokol was the contractor responsible for the design, construction and maintenance of the shuttle’s SRBs. The contract for building the SRBs was estimated to be worth $800 million. A Morton-Thiokol 1977 test showed that leaks through the joint were possible and this would cause the joint to burst, destroying the shuttle. 

Engineers at the Marshall Space Flight Center wrote to the SRB project manager several times suggesting that Thiokol’s joint design was unacceptable. The manager of the project did not forward these memos to Thiokol, and the joints were accepted for flight in 1980. 

Evidence of serious gas leaks through the O-rings was discovered regular after many shuttle missions, even as early as the second space shuttle mission. The Marshall Center did not report this problem to senior management at NASA, but opted to keep the problem within their reporting channels with Thiokol. Even after the O-Rings were classed as safety critical, no one at Marshall suggested that the shuttles be grounded until the flaw could be fixed. Thiokol engineers found that the amount of gas escaping was relatively small and had not impinged upon the secondary O-ring. They concluded the damage was an acceptable risk. 

By 1985, with seven of nine shuttle launches that year using boosters displaying O-ring erosion or hot gas blow-by, Marshall and Thiokol began the process of redesigning the joint. They did not call for a halt to shuttle flights until the joints could be redesigned, but rather treated the problem as an acceptable flight risk. For example, Lawrence Mulloy, Marshall’s manager for the SRB project since 1982, issued and waived launch constraints for six consecutive flights. Thiokol even went as far as to persuade NASA to declare the O-ring problem “closed”. 

Challenger was originally set to launch from Kennedy Space Center in Florida on January 22nd 1986. However, there were repeated delays to previous missions and the launch was repeatedly put back, along with all future launches. This would have a negative impact on commercial and military missions. 

Forecasts for January 28th predicted an unusually cold morning, with temperatures close to −1°C (30°F), the minimum temperature permitted for launch. The Shuttle was never certified to operate in temperatures that low. There was no test data to prove the rubber O-rings would work at these low temperatures. 

By mid-1985, Thiokol engineers worried that others did not share their concerns about the low temperature effects on the boosters. Engineer Bob Ebeling in October 1985 wrote a memo—titled “Help!” so others would read it—of concerns regarding low temperatures and O-rings. 

After the weather forecast for January 28th, NASA personnel remembered Thiokol’s warnings and contacted the company. When a Thiokol manager asked Ebeling about the possibility of a launch at 18°F (−8°C), he answered “We’re only qualified to 40° [40°F or 4°C] … ‘what business does anyone even have thinking about 18°, we’re in no-man’s land.'” 

Thiokol immediately called NASA recommending a postponement until temperatures rose in the afternoon. NASA demanded that Thiokol state a minimum safe temperature to launch. 

At the teleconference on the evening of January 27th, Thiokol engineers and managers discussed the weather conditions with NASA managers. Several engineers reiterated their concerns about the effect of low temperatures on the rubber O-rings and recommended a launch postponement. They argued that they did not have enough data to determine whether the joints would properly seal if the O-rings were colder than 54°F (12°C). It was clear that, if the O-Rings failed, the shuttle would be destroyed. 

Thiokol management initially supported its engineers’ recommendation to postpone the launch, but NASA staff opposed a delay. During the conference call, a NASA manager told Thiokol, “I am appalled. I am appalled by your recommendation.” Mulloy, Marshall’s manager, said, “My God, Thiokol, when do you want me to launch—next April?” 

A second conference call was scheduled with only NASA and Thiokol management, excluding the engineers. For reasons that are unclear, Thiokol management disregarded its own engineers’ warnings and now recommended that the launch proceed as scheduled. NASA did not ask why.