Tales from the Bare Metal — Episode 04
23 September 1999. The Mars Climate Orbiter fires its main engine to enter orbit around Mars, passes behind the planet as planned, and is never heard from again. The spacecraft cost 193 million dollars to build, part of a 327.6 million dollar mission. It travelled 670 million kilometres across nine months of deep space, and it was lost to a number with no unit written on it.
This is the cleanest example in the engineering record of why a quantity is not the same thing as a number.
The Incident
The Mars Climate Orbiter launched on 11 December 1998. Its job was to enter a stable orbit around Mars and study the planet's atmosphere, also serving as a communications relay for the Mars Polar Lander that would follow.
