English/metric measurement error in the Mars Climate Orbiter

[Jon]

The Mars Climate Orbiter was a space probe launched by NASA in 1998. It was loaded with scientific measurement instruments, and it was supposed to enter orbit around Mars, gather data, and then communicate its findings back to Earth. Instead, it disintegrated.

Here's the Mars Climate Orbiter:



The orbiter completed a 286-day journey to Mars, and then fired its engine to push itself into orbit. However, something went wrong - on September 23, 1999, NASA lost contact with the orbiter.

What happened? NASA organized an investigation with a Mishap Investigation Board, which produced a detailed report. The suspected root cause? Failure to use the metric system. Here's a highlighted section from the NASA report:



All systems were supposed to use metric measurement, but a single software file was using English units instead. As a result, thruster measurements were miscalculated, and the orbiter was hundreds of kilometers off course, causing it to (most likely) disintegrate in the Martian atmosphere.

NASA handled the failure pretty well, at least externally. Rather than point the finger at Lockheed, whose engineers had programmed the software, they acknowledged that, while the software measurement error was the root problem, it was their responsibility to validate and verify the measurement systems to identify any such errors.



But when you read between the lines, the vague phrasing about communication structures and auditing processes seems to point at the age-old conflict between engineers and management. Some relevant industry magazines (like Aviation Week and Spectrum) published a good bit of anonymously sourced i-told-you-so's (this Spectrum article is especially notable). It looks like some of the involved engineers knew that the official story was being spun, and they leaked the dirty details to the industry press.

NASA's official claim was that nobody knew that the orbiter was off course until communication was lost. However, Spectrum's position was that some of the engineers voiced strong concerns, but were pushed aside by management.

This is the remaining inconsistency between NASA's official version of what happened and the one reconstructed by Spectrum. Our conclusion is that adequate doubts had been raised to require the TCM-5 burn, even in an emergency mode. Further, according to participants in this tragedy of errors, by the time the probe reached Mars, those most "in the know" were persuaded it was already doomed by its sick trajectory--but by then it was too late.

The incident is a fascinating combination of measurement errors, programming errors, management errors, and public relations errors. These mistakes collectively resulted in the loss of a $328 million spacecraft, and demonstrated that, unlike people, errors are great at working together.

Previously:

International Space Station tools
astronaut loses $100,000 tool bag during spacewalk