Mars Climate Orbiter

The units mix-up everyone has heard of, and the part most people miss. Lockheed Martin's ground software reported thruster impulse in pound-force seconds, while NASA's navigation software expected newton-seconds, a factor of 4.45. Small errors added up over the cruise, and on 23 September 1999 the orbiter reached Mars far lower than the planned 226 km and was lost. The arithmetic was not the real failure. Navigators had noticed the trajectory looked wrong, and an extra correction burn was discussed but never carried out. NASA's Ed Weiler put it down to systems engineering and the checks and balances. Interface specifications, end-to-end verification and a culture that acts on doubts are the lessons here.