On September 23, 1999, NASA lost contact with the Mars Climate Orbiter as it went behind Mars to begin the first of four aerobraking passes. It never came out. The spacecraft had come in too low, about 57 kilometres above the surface instead of the intended 226, and either burned up or was torn apart by atmospheric forces. It had cost about 125 million dollars to build and launch.
The cause was in the navigation software. It was simple. Lockheed Martin's ground software in Colorado calculated the thruster firings in pound-force and wrote the result in newtons into the file that the navigation team at JPL read. The number was correct as a number and wrong as a unit, and the small errors accumulated over the nine-month cruise without anyone noticing. By the time the spacecraft arrived, its trajectory was about 170 kilometres off.
The mission had been designed to be cheap. It was part of the Mars Surveyor programme that followed the loss of Mars Observer in 1993, and the strategy was faster, cheaper, better, with small spacecraft, short development times and fixed budgets. The Mars Climate Orbiter was to study the atmosphere and act as a relay, and it was launched with the Mars Polar Lander on two separate rockets in January 1999.
What went wrong was not only the units. There had been a warning: the navigation team had noticed the trajectory was drifting, and had raised it in meetings, and had been told the numbers were fine. There was also a review that found no independent check on the software's output, and the project had lost its own propulsion engineer to budget cuts. The accident board's report said the failure was not the result of a complicated error but of an unwarranted lack of discipline.
The Mars Polar Lander was lost three months later, on December 3, 1999, probably because a sensor on the landing legs had generated a false signal that shut the descent engines off at about 40 metres. Two failures in one year ended the faster-better-cheaper approach for Mars and produced a complete reorganisation: NASA cancelled the 2001 lander, and the programme that followed went back to larger and more expensive spacecraft.
Mars exploration recovered. Mars Odyssey arrived in 2001 and is still the longest-serving spacecraft at Mars. The Spirit and Opportunity rovers landed in 2004. The orbiter's replacement, the Mars Reconnaissance Orbiter, has been mapping the planet since 2006, and it relayed for the Curiosity and Perseverance rovers.