The ship was underweight on reentry because of a unit conversion error from pound-forces to newtons in the navigation software, leading it to descend too close to Mars and be destroyed by atmospheric forces.
The ship was underweight on reentry because of a navigation error due to a measurement unit conversion mistake. Specifically, the software that controlled the rotation of the ship's thrusters expected measurements in newtons but instead received data in pound-forces. A pound-force is equal to about 4.45 newtons, and this discrepancy led the Mars Climate Orbiter to approach Mars at too low of an altitude, resulting in the spacecraft being destroyed by atmospheric friction and other forces.
Arthur Stephenson, chairman of the investigation board, identified the root cause as a 'failed translation of English units into metric units' in a segment of ground-based, navigation-related mission software. This unit conversion error had catastrophic consequences, preventing the Mars Climate Orbiter from completing its mission to relay signals from the Mars Polar Lander.