NASA lost a $327M Mars probe because two teams used different units

The Mars Climate Orbiter spent nine months flying 669 million kilometers, flawlessly, toward the wrong place. Lockheed Martin's ground software reported thruster impulses in pound-force seconds, the imperial unit its engineers had always used. NASA's navigation software read those same numbers as newton-seconds, the metric unit the interface spec actually required. Every course correction was therefore off by a factor of 4.45.

The errors were tiny and they compounded quietly. Navigators noticed the trajectory drifting on the way to Mars and filed concerns, but the discrepancy was never escalated to a formal review. On September 23, 1999, the orbiter began its insertion burn 170 kilometers lower than planned — inside the Martian atmosphere instead of above it — and was destroyed or skipped off into space. Signal was never reacquired.

The mishap board's finding was blunt: the root cause wasn't the unit mix-up itself but the process that let it survive — an unverified interface between two organizations, and warning signs that were noticed, discussed, and not acted on.