Avoid Problems with Units of Measurement


Engineers need to clearly understand units, and be able to convert, express, document, and communicate them in correspondence, operating instructions, and publications.

On Dec. 11, 1998, the National Aeronautics and Space Administration (NASA) launched the first interplanetary weather satellite, the Mars Climate Orbiter (MCO), to study the climate, atmosphere, and surface of Mars and to serve as a communications relay for the Mars Polar Lander (MPL), which was due to arrive a year later. On Sept. 23, 1999, the spacecraft entered the planet’s atmosphere — 49 seconds earlier than expected, on a trajectory approximately 170 km lower than planned — and NASA lost radio contact with it. The $125-million orbiter was lost and presumed to have disintegrated.

The root cause of this incident? Inconsistent units — specifically, failure to use metric units in the coding of a software file used in trajectory models.


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