Mars Orbiter Lost Over Metric Conversion Error

Buxley writes "NASA is reporting that they've found the likely cause for last week's loss of the Mars Climate Orbiter. It seems one of the engineering teams was using English units of measurement while another team was using Metric units. Getting this straight is rather important when designing navigation sytems for interplanetary spacecraft, one would think." A lot of people sent this one in; thanks to all. And if this event doesn't prove that it's time for the U.S. to go 100% metric, I don't know what will. Oy!

Not just spacecraft

I wonder how much it costs companies who sell precision equipment to the US as well as to the rest of the world to maintain two bookkeeping systems? This particular incident is splashy, but there is no doubt a day-to-day cost of living a double measurement life that we never think of. Definitely, the US should convert; the argument was already there, this is just a nice incident to pin the argument on.

Re:the right tool for the job

It is much easier to imagine and understand what it means to say "I am six feet tall" than it is to say "I am one hundred and eighty three centimeters tall" or "I am one point eight meters tall".

The hell you say. It may be easier for you because you have been raised used feet and inches, but there is definitely nothing intrinsically more intuitive about it.

The Fahrenheit scale uses (aproximately) the normal human body temperature as 100. Since most real world temperatures (like weather) are done around this temperature and above 0 (the temperature of salted ice), the Fahrenheit scale is a more convenient scale for human life.

Again, I take that you mean a more convenient scale for Americans. From my point of view, the Fahrenheit scale is a counterintuitive horror. After ten years of exposure to the Imperial system, I can deal with inches, miles, gallons and pounds, but I still can't get my head around Fahrenheit. Besides, the most significant transition point in "the real world" is that of liquid water to ice. The Celsius scale springs from this. How much more intuitive you want to get?

I guess we all have our cultural biases and try to justify them, but from a standpoint of making sense, the Imperial System doesn't have much of a leg to stand on, definitely not in the 21st Century. Take my word (and that of all the other Europeans in /.) for it, the Metric system does work extraordinarily well in "everyday" situations.

Re:US Metric? Easier said than done.

Hello?? This happened in England a few decades ago. The metric system was not in widespread use until the last few decades. If most of the world can do it, I'm sure Americans can too, unless they have a problem with the "not invented here" syndrome, which I believe they do.

Re:the right tool for the job

I am amazed by the utter stupidity of your post.

Don't you realize that all your pseudo-facts are completely biased by your education?

The English system is focused on the idea that humans only want to / can deal with multiples of two or three things comfortably.
Then metric wins, as there is only 1 "thing" to measure against, and 1 scale. Orthogonal design. Good.

It is much easier to imagine and understand what it means to say "I am six feet tall" than it is to say "I am one hundred and eighty three centimeters tall" or "I am one point eight meters tall"
Guess what I am 1m85cm tall and I "know" what 1m50cm tall means or 1m90cm tall. Why? because I have been raised in a metric country. 6ft. 1in doesn't mean anything to me.

The English units of volumes are done in binary (and a couple, like tablespoons, in ternary). One foot equals twelve inches and one gross equals 144 objects because these are useful numbers: lots of other numbers divide them. Humans don't do floating point arithmatic with great ease. Use a system that bumps everything up into integers.
Are metric-raised people superior then ? Because metric-raised handle non-integeral measures quit well, thank you. Ten has always been a stupid choice for the base of a number system.
I would gladly agree with you but for a small detail: every day numbers are expressed in base 10, which makes handling a base-10 measurement system so much easier. Base 10 is drilled into everyone's mind since their kindergarten. SARCASM and yes base 60 is so much easier, no wonder it's so easy to teach children how to read a clock ! /SARCASM

[Comparisonn of Fahrenheit versus Celsius]
Again, you are biased by your upbringing. Metric-people have none of the problems that you attribute to the Celsius scale. And I boil water multiple times per week, I have a very good understanding of what 100C means. I have ice in my freezer and understand what 0C means. I certainly do not have a good perception of my body temperature, so 100K does not mean anything to me.

t ranks right up there with the prostate as one of the fundamental ways in which humans are "designed". We should've been built with 12 fingers instead of 10. We'd all be much better off.
Yeah right. We'd be better off without morons like you.

Reality check: metric is as easy to use (or easier) as english. You are in denial because of your education, stop deluding yourself with BS explanations of why english is "superior" or "more natural".

Why English is better in Machining

It turns out that a thousandth of an inch is an extraorinarily usefully sized increment when dealing with machined parts. (Not to mention that when you need to check clearances, a one mil shim/feeler gauge is as close as the cellophane cigarrette wrapper from the nearest smoker - try it for yourself. No idea what thickness foreign cigarrette wrappers are, but I'd chortle if they're .001"!)

By comparison, metric units tend to be either too large or too small: A millimeter is huge (39 thousandths), a tenth of a millimeter is still way too big (3.9 thousandths) but a hundredth of a millimeter is overkill at .39 thousandths, and makes accurate dial calipers, etc. more difficult and expensive. This also results in a unit that does not line up well with engineering notation, where exponents are multiples of three to help avoid errors - to fix this you either need to write .001" as .0254 mm or 25.4um (micrometers) - difficult because it's notoriously difficult to type the proper "mu" on non-greek keyboards, and micrometers are both little bitty and the device you use to measure small things with. On top of that, all of these folks "think" clearances and allowances in thousandths, their suppliers provide all their engineering information primarily in English units, and parts, tools, etc. are much more available (and cheaper) in English units than in Metric. Not to mention that everything they need to hook up to is English, so they can't just switch over.

Most machinists and designers are proficient in both, but it's a pain. From what I've seen, the vast majority of US design engineers prefer to design in english and if the design has to go overseas for fabrication, they'll then convert and let the other guys deal with the horribly odd numbers that result.

With computers to help us, there's no real reason to HAVE to change anymore. Seriously, it's still a hassle to convert, but it's much less trouble now than it used to be. Converting would be very expensive, cause many more NASA-type foul-ups, and offer little or nothing in return. It makes about as much sense for the US to convert to Metric units as it does for the rest of the world to adopt the English language. Sure, it makes things easier, but is it worth the trouble? And the cultural issues ARE similar.

Finally, there's still a very real stigma attached to using the effete and wimpy Metric units in many US industries. I'll never forget the withering look I got a few years back when explaining to an oil tool executive that a gap was "about two millimeters" as he rejoindered, "MILLIMETERS? What the HELL?"

FWIW, I think both systems are wrong and we should base length on "Dublins" (where the Dublin would be defined as the distance between a yard and a meter at which the acceleration due to earth's gravity is exactly 10 Dublins/s^2)! THAT would make life easier for a lot of people, and is easier than changing the second, which after all is perfect at 1/86400 of a day! [grin]

Oh, and we'd have to build houses out of 5.08x10.16's (2x4's) that actually measure a nominal 4.445x8.890 (1.75x3.5)??

Re:US Conversion to Metric Overdue

Under Article I, section 8, the Congress has the authority to "fix the Standard of Weights and Measures." In 1866 the U.S. Congress passed a law establishing the legality of the metric system in the United States. (No other system of measurements has been established by the Congress.) We were one of the original 17 signatories to the Treaty of the Meter in 1875. In 1893, the metric measurement standards were adopted as the fundamental standards for length and mass in the United States. Congress passed the Metric Coversion Act in 1975. The Omnibus Trade and Competitiveness Act Of 1988 designated the metric system as the "preferred system of weights and measures for United States trade and commerce," and required that all federal agencies use the metric system by 1992.

And it isn't a problem of corporations. GM and Ford both use metric (theoretically) exclusively internally. When gasoline companies tried to switch to liters, consumers rejected it. Multinational buisnesses have been major backers of metric conversion.

So, given that we are already legally metric, how are we going to do this 2-5 year conversion you speak of? Six months in jail for small buisnessmen using Customary units? $100 fine per incident of civilians calling themselves 5'10"? Life imprisonment without parole for tecahing you child ounce-to-pound conversions?

Science and Metric

There's been a lot of talk here about scientists using the metric system--that's only sort of true. At least in physics, there are those who insist on SI units (Systeme International, e.g. meter,kilogram,Kelvin), some who hang on to CGS (centimeter,gram) units, and a bunch of other unrelated units that are incredibly useful.

What is called the metric system seems to mash together the SI and CGS units, plus the celsius temperature scale and the use of liters for volume. In order to get the easy conversions to work right, you have to stick to either CGS or SI else powers of ten end up missing. The celsius scale (my least favorite of the metric units), by the way, doesn't give any of the easy conversion advantages that the rest of the metric units has; its thermodynamically irrelevant. The celsius scale doesn't work for equations (e.g. PV=nRT) involving temperature. The SI unit of temperature, of course, is the Kelvin.

I'm not aware of any weather forecasts, anywhere, that use Kelvin. I also think kilowatt-hour is in widespread use worldwide for electric power, even though Joule would be the strict SI unit. Likewise kilometers per hour instead of meters per second.

Units really get hairy in electrodynamics. Experimental physicists work with SI units almost exclusively, as that's what lab equipment is made in. Except that magnetic fields are sometimes measured in gauss instead of Tesla. Theorists, on the other hand, often use CGS because the calculations are more elegant. But everyone uses liter, even though its not really SI.

Torr, the unit of pressure, is still in widespread use (1 torr = 1 mm of mercury), although bar and millibar are creeping in. Angstroms are in widespread use and likely always will be.

Physicists would be lost without the electron volt (1eV=1.602e-19 Joule). Atomic mass units (amu), astronomical units (au=Earth-sun distance), light-years, and parsecs are similarly very useful and in widespread use. Furthermore, if you look at graphs in physics articles, you'll often find the raw quantities multiplied or divided by constants such as the Bohr magneton, Planck's constant, or the Boltzmann constant.

By far the most useful way to state the speed of light is one foot per nanosecond. Likewise, the speed of sound in air is roughly one foot per millisecond.

The choice between Kelvin and Celsius is mostly a matter of convenience. Chemists use Celsius almost exclusively, physicists--especially those working at low temperatures or with thermodynamics--use Kelvin.

The point of all this is that practicing scientists use units that are appropriate to the problem. I think we should be comfortable in a variety of units and be able to convert when necessary. I'm perfectly happy with camera lenses in millimeters, monitors in inches, the weather in Farenheit, my experiment in Kelvin, cars in horsepower, light bulbs in watts, and everything else in units that makes the quantity measured easier to think about.

Well...

.... these people aren't exactly Rocket Scientists after all..... oh, wait...nevermind....

US Metric? Easier said than done.

And if this event doesn't prove that it's time for the U.S. to go 100% metric, I don't know what will. Oy!

This statement is all right and good except that it is a bit naive. Although the US is non-standard by following it's own measurement systems for lenght, volume, and weight, it is not necessarily bad. Ask just about any professional machinist what measurement system (s)he uses and you'll find more often than not that they use the US "Standard" system over the Metric. A machinst friend of mine claimed, "it's more accurate." Now, as someone acquainted with the more complicated mathematics, I understand measurement systems as nothing more than numbers. Accuracy in and of itself does not lie in the system used to express the measurement. Speaking practically, however, it seems that the US has found its niches. Its not surprising either, we've had a couple hundred years to get used to it.

As a scientist, I realize that the US mesurement system is not used in all aspects of life in the US. Step into any chemist's lab and you'll find beakers labled liters and milliliters. You'll find scales labeled Kilograms, grams, and micrograms. Factors of 10 are convenient indeed when it comes to science.

Hey, I'm all for such a conversion, but such a conversion would never be 100%; at least not for quite a few years. Just think about how the less honest business "entrepeneaurs" would take advantage of the mass confusion caused by such a conversion. Gas would be sold in liters instead of gallons, a two or ten cent raise in price per liter may go unnoticed (initially). Everything would be thrown out of context for US residents. Every industry would be impacted. For the price of conversion, the rewards would be ill received.

Yes, it's simple to say that the US should convert to metric, just not so simple to implement.

the right tool for the job

The English system is focused on the idea that humans only want to / can deal with multiples of two or three things comfortably. Therefore, each class of jobs gets its own particular unit of measurement. It's one philosophy, and it's the one that ultimately wins out with human nature. It is much easier to imagine and understand what it means to say "I am six feet tall" than it is to say "I am one hundred and eighty three centimeters tall" or "I am one point eight meters tall". Combine that with the fact that most people misuse kilograms as a unit of force, and we're all better off using pounds or stones anyway.

The English units of volumes are done in binary (and a couple, like tablespoons, in ternary). One foot equals twelve inches and one gross equals 144 objects because these are useful numbers: lots of other numbers divide them. Humans don't do floating point arithmatic with great ease. Use a system that bumps everything up into integers.

Ten has always been a stupid choice for the base of a number system. The Babylonians had a much better idea with base 60; look at all the numbers that evenly divide into 60 (and look at our system of time and try to think whether it would be better if we only had 10 "hours" in the day instead of 24 or 100 "minutes" instead of 60.

The convenient thing about the metric system is that it is focused on the idea of orders of magnitude, and since lots of science is also concerned with orders of magnitude, that makes the metric system appropriate for scientific applications. The problem with using the metric system in our normal daily existence is that most humans only have to deal with one or maybe two orders of magnitude as they go about their day. For the few instances where that stops being true, using a different system of units wouldn't kill anyone.

Why does the celsius scale define 100 to be the boiling point of water at one atmosphere? Because it's a convenient constant and lots of laboratory conditions are done near that temperature. But notice how it's done in terms of 1 atm and not 1000 pascals or some such power of 10. There exist these fundamental constants in nature, and lots of them don't have anything to do with each other. Yes we can measure charges in units of coulombs, but since most of nature is constructed in multiples of the electron charge, you still have factors of 1.6x10^-19 in your equations. You're not eliminating complexity; you're just pushing it around. No system of units will eliminate all of this kind of complexity. You might as well use units that are convenient for humans.

The Fahrenheit scale uses (aproximately) the normal human body temperature as 100. Since most real world temperatures (like weather) are done around this temperature and above 0 (the temperature of salted ice), the Fahrenheit scale is a more convenient scale for human life. An analogy: it's like the differene between using an analog car speedometer that goes between 0 and 85 or one that goes between 0 and 140 mph. Since most of your driving is done between 0 and 60, and rarely if ever above 80, all that part of the scale above 80 is just wasted and it squeezes the part we care about into a smaller space that complicates interpolation and other visual interpretation of the value displayed.

It ranks right up there with the prostate as one of the fundamental ways in which humans are "designed". We should've been built with 12 fingers instead of 10. We'd all be much better off.