005CASE / Spaceflight
The number that crossed without its units
A navigation calculation consumed impulse data under a different unit convention from the system that produced it.
SOURCE STATE · The Last Undo, Chapter 2, “A number with no units,” note 1; NASA, Mars Climate Orbiter Mishap Investigation Board Phase I Report, 10 November 1999, section 4.
This record reconstructs the argument in its cited manuscript draft. It is not a certified procedure or an independent audit.
THE JUDGMENT MACHINE
Trace the chain.
- Object
- Thruster impulse data passed from ground software into the spacecraft navigation process.
- Observation
- Numerical thruster-performance values produced by SM_FORCES.
- Distinction
- A plausible numerical value is not an established physical quantity unless its unit convention is known.
- Representation
- The interface specification required newton-seconds; the producer supplied pound-force-seconds.
- Criterion
- The receiving calculation required impulse expressed in the specified metric units.
- Judgment
- The chapter argues for establishing the unit contract at the interface rather than assuming it from a plausible number.
- Authority
- The engineering interface specification declared the requirement; the investigation board later identified its violation.
- Last undo / seam
- For the interface-design lesson, the intervention point is before incompatible data enters a navigation calculation. This is an editorial boundary, not a reconstruction of the last physically recoverable mission maneuver.
- Consequence
- The navigation error contributed to the loss of Mars Climate Orbiter on arrival in 1999.
EVIDENCE & LIMITS
Keep the denominator attached.
The cited board report identifies failure to use metric units in the ground-software file as the root cause and records a factor-of-4.45 mismatch.
The board also identified contributing organizational and verification failures. A unit check would address one named failure, not certify the mission. The combined orbiter-and-lander program cost is not the loss attributable to this error.
SOURCE TRAIL · The Last Undo, Chapter 2, source note 1; NASA Phase I report, section 4. Site reconstruction of the manuscript draft.