Lost to inches
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On 23 September 1999 — the same calendar date as Neptune's discovery, 153 years later — NASA lost contact with Mars Climate Orbiter. The spacecraft was supposed to enter orbit. It passed behind the planet and never came back. Later reconstruction said it had come in about 170 kilometers too low and hit the atmosphere.
The cause was not a mystery rock or a failed thruster. It was a unit. Ground software at Lockheed Martin produced thruster impulse in pound-force seconds. The navigation software at JPL expected newton-seconds, as the interface specification required. A pound-force second is not a newton-second. The numbers were off by a factor of about 4.45. The spacecraft was not where the model said it was.
A NASA mishap investigation board wrote this down in public English. The phrase that stuck was a failure to use metric units. Cost estimates sit around $327 million for the spacecraft, launch, and operations. The board's point was not "programmers are sloppy." It was that two teams can both be careful inside their own files and still ship a silent conversion error across the boundary.
That is a software story wearing a planetary costume. Mars did not care. The atmosphere did what atmospheres do to a vehicle that arrives too low. The bug was the number that meant one thing in one office and another thing in the next.
People still make the joke: it is not a bug, it is an undocumented feature. The orbiter is the counterexample. The mismatch was in the documentation. The feature was an unplanned lithobraking. If you want the excuse on a shirt, we print it. If you want the lesson, it is: specify the unit, test the interface, do not assume the other team meant what you meant.
September 23 is crowded. In 1846 the math found a planet. In 1999 the math, with the wrong unit, lost a spacecraft. Both are bookkeeping. One of them got a telescope confirmation the same night. The other got a report.
There is no Mars-probe tee in the shop. The Lab Pick is the documented-bug shirt — the joke inverted. Wear the excuse. Remember the orbiter.