|
"Show your units!" and "check your work!" were the proverbs
of the engineering and science professors that would resound in the ears and on
the papers of their students. But we (the students) knew better, of course…
It was always faster and simpler to do conversions in your
head and have the right units magically appear at the end. Checking over easy
problems would just waste valuable time, right? And yet, we were always amazed
when an error was found in a problem due to a stupid math or unit mistake. "How
could I have missed that?" we'd say.
Well, maybe it isn't a big surprise then that a prominent
space project failure was due to the failure to spot a unit conversion error.
The Incident
 On December 11, 1998, the Mars Climate Orbiter began its ten
month journey to Mars, where it was intended to be used to study the planet and
provide valuable information on many of its properties. But an unexpected
communications failure during its orbit around Mars was the beginning of the
end of its journey. The probe went out of radio contact roughly a minute earlier
than expected, and the connection was never reestablished.
The Cause
"Our inability to recognize and correct
this simple error has had major implications," said Dr. Edward Stone,
director of the Jet Propulsion Laboratory, in response to the incident. "We
have underway a thorough investigation to understand this issue."
The problem, found through the analysis of the Mars Climate
Orbiter Mishap Investigation Board (MCO MIB), was the result of human error in
unit conversion. The crews were inputting units for thruster performance in
Imperial units (pound-force, lbf) into programs designed to use metric units
(Newtons, N). The error was a factor of ~4.4, the conversion factor between the two units.
 The conflict resulted in an incorrect trajectory which
brought the probe too close to the planet's atmosphere, presumably causing it
to burn up and disintegrate.
The Effect
The error, known as the metric
mixup, has been carefully avoided by NASA in all missions since that
incident. In addition, NASA
declared as of 2007 that all of its future missions would use metric units
to avoid confusion and cooperate with other space agencies.
Lessons Learned?
The failure of the Mars Climate Orbiter was a +$300 million
investment in learning communication, unit conversions, and testing
(double-checking). And while you've probably never made such a costly
investment in your training, perhaps you've made or witnessed a similar mistake
at your company due to a failure to remember the old adage of the college
professor.
References
NASA
- Mars Climate Orbiter Fact Sheet
University
of Wellington - Software Engineering: Mars Climate Orbiter
| 
Re: Lost in Space - Cosmic Unit Blunder
