Mission-critical Data

October 6, 1999

Dale Dougherty

The loss of the Mars Climate Orbiter gives new meaning to the term "mission-critical" data.  This mission would seem to be a cautionary tale for all of us.  Here's the significant portion of the press release from NASA.

On December 11, 1998, a rocket with NASA's Mars Climate Orbiter lifts off.

Photo: NASA

A failure to recognize and correct an error in a transfer of information between the Mars Climate Orbiter spacecraft team in Colorado and the mission navigation team in California led to the loss of the spacecraft last week, preliminary findings by NASA's Jet Propulsion Laboratory internal peer review indicate.

"People sometimes make errors," said Dr. Edward Weiler, NASA's Associate Administrator for Space Science. "The problem here was not the error, it was the failure of NASA's systems engineering, and the checks and balances in our processes to detect the error. That's why we lost the spacecraft."

The peer review preliminary findings indicate that one team used English units (e.g., inches, feet and pounds) while the other used metric units for a key spacecraft operation. This information was critical to the maneuvers required to place the spacecraft in the proper Mars orbit.

"Our inability to recognize and correct this simple error has had major implications," said Dr. Edward Stone, director of the Jet Propulsion Laboratory.  (JPL, Press Release, 9/30/99)

Here's the relevant aspects of this case:

  • The problem was an interchange of data between two different organizations, one in Colorado and the other in California.
  • Each organization was victimized by its own assumptions, which were implicit in its data and inadequately documented.
  • The units used were not explicitly labelled.
  • The software obviously was not able to validate the data and assumed the data was valid.
  • The problem was not discovered when the data was exchanged.  It did not surface until the spacecraft was lost, some 10 months after launch. 

In the end, while the media found the confusion over metric units amusing, the lesson is that a number is not just a number. For programs and organizations to exchange data successfully, there must be explicit agreements made about such things as what system of measurement is being used. This mission- critical data interchange problem highlights the importance of XML as a standard and why many applications require DTDs or schemas as a means of validating data. It is, of course, a hard lesson that bad data, like bad news, travels fast and far.