The Apollo 13 spacecraft carried a computer built to calculate its way through space. Its guidance system used a 2.048 MHz crystal oscillator, digital memory and software designed to help astronauts navigate a journey to the Moon. Yet during one of the most dangerous moments of the mission, that sophisticated system was deliberately switched off, and a much older piece of technology took its place: a mechanical wristwatch.
On April 16, 1970, astronaut Jack Swigert had to help time a critical engine burn as Apollo 13 headed back toward Earth. The lunar module Aquarius was being used as a lifeboat after the spacecraft's oxygen-tank explosion had crippled the service module. With power and cooling resources running dangerously low, the crew could not simply leave every system operating. The burn therefore became a remarkable exercise in human navigation, manual control and precise timekeeping.
