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Lost in Space - Cosmic Unit Blunder

Posted January 03, 2012 10:13 AM by cheme_wordsmithy

"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

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#1

Re: Lost in Space - Cosmic Unit Blunder

01/03/2012 11:11 AM

"In addition, NASA declared as of 2007 that all of its future missions would use metric units"

Yes, moving to the metric system inch by inch.

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#2
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Re: Lost in Space - Cosmic Unit Blunder

01/04/2012 12:30 PM

If you give them an inch---They take a kilometer.

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#3

Re: Lost in Space - Cosmic Unit Blunder

01/04/2012 12:47 PM

I learnt the hard way at school a long time ago.

An early year end science exam posed a question where from the results of a previous school lab experiment the specific gravity of a lump of metal had to be calculated by immersing it in tap water in a vessel graduated in a pints and noting the displacement from which the volume could be calculated.

The conversion of pounds, pints, grammes, cubic centimetres etc, from the given data was relatively easy (if you had been paying attention to the lessons that is!) but from which the my answer came out at 20.

But having paid attention to the practical science during the year, I thought it had to be wrong, because the SG of mercury was about 13; and everything floated on it; except gold. I did not know the SG of gold at the time, but I doubted that the school had a block of gold to play with.

So I spent ages going over the calculations again and again because I did not believe the result - but always to get the same answer. I did not pass the exam because I ran out of to finish all the questions.

In mathematical terms 20 was the correct answer to the question, but the given weight was a misprint, hence my assumed error.

I was told I should have left the question and moved on to the next one. A little knowledge is dangerous it seems.

Generally speaking I prefer to rely on my instincts and double checking figures to ensure I get the answer I expect. Perhaps that is why I have not passed many exams.

Another example, in terms of accuracy, again at school, we had to produce an isometric technical drawing of a simple scaffold having a circular base, a vertical post, a horizontal cross arm with an angled gusset strut to support the cross arm. Constructing the correct angles for the gusset was the tricky bit, but after lots of tries and rubbing out, I eventually got it right. However, on checking with other kids I I found I got less marks than some who got it wrong. I was told it was because my drawing was messy and I lost marks for lack of neatness.

If nothing else, I would not be surprised to learn the Mars Climate Orbiter drawings were neatness themselves - but not necessarily accurate.

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#4

Re: Lost in Space - Cosmic Unit Blunder

01/04/2012 1:49 PM

Falls back to my dad, measure twice, cut once.

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Re: Lost in Space - Cosmic Unit Blunder

01/05/2012 1:03 AM

...and measuring a third or fourth time is good too.

Make sure you cut the longest piece first.

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