The Cracked O-Ring

The temperature at Kennedy Space Center on the morning of January twenty-eighth, nineteen eighty-six, was twenty-nine degrees Fahrenheit at launch time. Ice had formed overnight on the fixed service structure. Workers had spent the early morning hours walking the pad, photographing icicles. Inside the solid rocket boosters — two white cylinders each a hundred and forty-nine feet tall, strapped to the sides of the shuttle — were a series of field joints. Each joint was sealed with two rubber O-rings, the primary and the secondary, designed to prevent hot combustion gases from escaping through the joint during the seconds of peak pressure at ignition. The rings had a specified operating temperature range. Below that range, the rubber became less resilient. Less resilient meant less responsive to pressure changes. Less responsive meant a gap might open before the ring fully seated. The night before the launch, engineers at Morton Thiokol, the company that built the boosters, had gathered on a telephone conference with NASA managers to make the case for delay. Roger Boisjoly was one of the engineers on that call.