Freezing cold made Challenger's rubber seals too stiff to seal

The Space Shuttle Challenger did not explode because a big part broke. It exploded because a small rubber ring got too cold to do its springy job.

Each of Challenger's two solid rocket boosters, the tall white rockets strapped to the shuttle's sides, was built in sections. Where two sections joined, two tiny circular rubber rings called O-rings were supposed to press outward and plug the joint, holding in hot, pressurized gas.

Rubber seals that way because it is elastic. As pressure builds and the metal joint flexes open, the rubber expands with it and keeps the gap closed. Cold stops the rubber from doing this.

When rubber gets cold, it turns hard and sluggish, and it responds too slowly to fill a gap that is opening in fractions of a second.

Morton Thiokol mechanical engineer Roger Boisjoly had flagged the danger the previous summer. After inspecting a booster from a previous flight, he warned in a memo that one ring had stopped working entirely while the backup was harmed. His point was blunt: if that second ring had also failed, the booster would have exploded.

His later analysis tied the damage to cold weather at launch, which made the rubber hard, inflexible, and more likely to fail. On the night before launch, Thiokol vice president Joe Kilminster recommended against launching because of the cold. NASA leaders pushed back, citing repeated delays and a lack of definitive proof of risk, and Thiokol managers then reversed their decision under pressure and approved the launch.

The launch went ahead the next morning at 11:38 a.m. in a record low of 36 degrees Fahrenheit. Seventy-three seconds into flight, Challenger broke apart, killing all seven crew members. The rubber seals were scorched away, so the joint could no longer hold and set off a catastrophic chain reaction.

Cold had stiffened the rings so they could not move with the joint, and hot gas leaked through.