The Space Shuttle Challenger disaster was a fatal accident in the United States space program that occurred on January 28, 1986, when the Space Shuttle Challenger (OV-099) broke apart 73 seconds into its flight, killing all seven crew members aboard; it was the first fatal accident involving an American spacecraft in flight. The mission carried the designation STS-51-L and was the tenth flight for the Challenger orbiter and twenty-fifth flight of the Space Shuttle fleet. The crew was scheduled to deploy a communications satellite and study Halley's Comet while they were in orbit. The spacecraft disintegrated over the Atlantic Ocean, off the coast of Cape Canaveral, Florida, at 11:39 a.m. EST (16:39 UTC).

The disaster was caused by the failure of the two redundant O-ring seals in a joint in the Space Shuttle's right solid rocket booster (SRB). The record-low temperatures of the launch reduced the elasticity of the rubber O-rings, reducing their ability to seal the joints. The broken seals caused a breach into the joint shortly after liftoff, which allowed pressurized gas from within the SRB to leak and burn through the wall to the adjacent external fuel tank. This led to the separation of the right-hand SRB's aft attachment, which caused it to crash into the external tank, which caused a structural failure of the external tank and an explosion. Following the explosion, the orbiter, which included the crew compartment, was broken up by aerodynamic forces.

The crew compartment and many other vehicle fragments were recovered from the ocean floor after a three-month search-and-recovery operation. The exact timing of the deaths of the crew is unknown; several crew members are known to have survived the initial breakup of the spacecraft. By design, the orbiter had no escape system, and the impact of the crew compartment at terminal velocity with the ocean surface was too violent to be survivable.

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 from as early as 1977 had revealed a potentially catastrophic flaw in the SRBs' O-rings. Neither NASA, nor Morton Thiokol (the SRB manufacturer), addressed or corrected the issue. NASA managers also disregarded warnings from engineers about the dangers of launching in cold temperatures and did not report these technical concerns to their superiors. As a result of the disaster, NASA established the Office of Safety, Reliability and Quality Assurance to address safety concerns better, and commercial satellites would be launched on expendable launch vehicles rather than deployed from the crewed orbiter. To replace Challenger, construction of Endeavour was approved in 1987, and the new orbiter first flew in 1992. Later Space Shuttle missions launched with redesigned SRBs, and crews wore pressure suits during ascent and reentry.