Welcome to the Lounge

Bjarne Stroustrup wrote:

On 23 September 1999, NASA lost its US$654 million
Mars Climate Orbiter due to a navigation error. “The root
cause for the loss of the MCO spacecraft was the failure
to use metric units in the coding of a ground software
file, ‘Small Forces,’ used in trajectory models. Specifically,
thruster performance data in English units instead
of metric units was used.”5
The amount of work lost was
roughly equivalent to the lifetime’s work of 200 good engineers.
In reality, the cost is even higher because we’re
deprived of the mission’s scientific results until (and if) it
can be repeated. The really galling aspect is that we were
all taught how to avoid such errors in high school: “Always
make sure the units are correct in your computations.”

Why didn’t the NASA engineers do that? They’re indisputably
experts in their field, so there must be good reasons.
No mainstream programming language supports units,
but every general-purpose language allows a programmer
to encode a value as a {quantity,unit} pair. We can
encode enough of the ISO standard SI units (meters, kilograms,
seconds, and so on) in an integer to deal with all
of NASA’s needs, but we don’t because that would almost
double the size of our data. Furthermore, checking the
units in every computation would more than double the
amount of computation needed.
Space probes tend to be both memory and compute
limited, so the engineers—just as essentially everyone else
in their situation has done—decided to keep track of the
units themselves (in their heads, in the comments, and
in the documentation).

In this case, they lost.