Why We All Use the Same Units

Here is a map of the nations of the world that use the two systems of measurement.  Metric shown in blue and imperial shown in red.

Metric vs. Imperial measurement systems
Metric vs. Imperial measurement systems

While it’s not always good to go with the crowd, there is a reason why more nations use the metric system. An often-cited passage from the book Wild Thing by Josh Bazell:

“In metric, one milliliter of water occupies one cubic centimeter, weighs one gram, and requires one calorie of energy to heat up by one degree centigrade—which is 1 percent of the difference between its freezing point and its boiling point. An amount of hydrogen weighing the same amount has exactly one mole of atoms in it. Whereas in the American system, the answer to ‘How much energy does it take to boil a room-temperature gallon of water?’ is ‘Go fuck yourself,’ because you can’t directly relate any of those quantities.”

When I was doing my undergraduate degree in Physics, I was glad to have the metric system.

Now it wouldn’t be a big deal that the nation with the most advanced technology on the planet used a system that was completely different than everyone else, as long as all of the scientists and engineers agreed on the system to use and then did their jobs accordingly.  But a problem snuck through in 1999, and it proved to be catastrophic.

NASA’s Mars Climate Orbiter was launched in December of 1998, and was set to study Mars’s climate (duh) and atmospheric patterns.  During orbital insertion, it came into the atmosphere too low, and burned up before taking any scientific data.

The cause of this failure? A computer on the ground at Lockheed Martin was using imperial units when calculating the thrust needed for the orbital insertion maneuver.  It reported the units as pounds-seconds instead of Newton-seconds.  The NASA computer receiving the result thought it was in Newton-seconds, and so the craft was slowed down too much (1 lb is about 4.5 Newtons) causing it to burn up low in the Martian atmosphere.

The cost of the mission? $326.7 Million.


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