The Mars Climate Orbiter (MCO) was a NASA spacecraft intended to study the Martian climate and serve as a communications relay. On September 23, 1999, as it fired its main engine to enter orbit around Mars, the spacecraft passed behind the planet and was never heard from again. NASA’s Mars Climate Orbiter Mishap Investigation Board released its Phase I Report on November 10, 1999, identifying the cause as a navigation error that brought the orbiter far too close to Mars during orbit insertion.
The root cause was a units mismatch between two pieces of ground software. The Mishap Investigation Board determined that the immediate cause was “the failure to use metric units in the coding of a ground software file.” A ground program that computed the small forces from the spacecraft’s angular-momentum-desaturation thruster firings produced its output in English units of pound-force seconds, while the downstream navigation software that consumed that data expected the value in metric newton-seconds. Because one newton-second is only about 0.225 pound-force seconds, the impulse values fed into the navigation model were off by roughly a factor of 4.45, and the error accumulated over many thruster firings during the long cruise to Mars.
The consequence was a steadily corrupted estimate of the spacecraft’s trajectory. The navigation team’s calculations indicated the orbiter would pass Mars at a safe altitude, but the true closest-approach altitude was far lower than intended, low enough that the spacecraft likely passed through atmospheric regions it could not survive, or skipped off the atmosphere and was lost. The Board’s report describes the proximate failure as the spacecraft entering the Martian atmosphere at an altitude well below the survivable limit.
Crucially, the Board did not blame a single typo. The report frames the units error as the trigger, while the deeper failure was a process that allowed the error to go undetected for nine and a half months of cruise. There were inconsistencies between the navigation team’s solutions and the spacecraft team’s expectations, but the warning signs were not pursued to resolution, communication between the two teams was inadequate, and the navigation software was not adequately verified against the interface specification (see software-testing).
Mars Climate Orbiter is frequently shortened in retellings to “they mixed up metric and imperial,” but the primary report makes clear it was a systems and verification failure: an unverified software interface, missing end-to-end checks, under-staffed navigation, and unresolved anomalies. Together with the loss of the Mars Polar Lander later the same year, it prompted NASA to overhaul its review and verification practices for low-cost planetary missions.