Why getting your units right matters

In December 1998, NASA launched the Mars Climate Orbiter. Its mission was to report data on the Martian atmosphere and climate. The probe was intended to function until 2004 and the construction of the orbiter and its lander were reported to have cost $330 million.

In September 1999, as the spacecraft was about to enter Martian orbit, communication was lost. A subsequent report determined that the Orbiter had most likely been destroyed because it had entered Mars’ atmosphere at the wrong angle. The loss of the Orbiter has been attributed to a mismatch of units. The software that sent commands to the spacecraft from Earth used imperial units whilst the software on the Orbiter worked in metric units.

The Mars Climate Orbiter Mishap Investigation Board reported that the spacecraft was inserted into orbit at the dangerously low altitude of 35 miles above Mars, rather than 140 miles. This was because the imperial unit of force, the pound-force, is equal to 4.45 Newtons so the thrust produced by the orbiter to begin its orbit was incorrect by a factor of 4.45. Ooops!