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1999: Mars Climate Orbiter

An illustration of Mars Climate Orbiter. The mission failed when English units weren’t converted to metric.
An illustration of Mars Climate Orbiter. The mission failed when English units weren’t converted to metric. Illustration: NASA/JPL-Caltech

This one might be the most cringe-inducing entry on the list. Launched in 1998, NASA’s Mars Climate Orbiter was meant to study the planet’s atmosphere, collecting data on dust and water vapor and creating a record of Mars’ day-to-day weather in order to get a sense of climatic trends.

The orbiter reached Mars in September 1999 and was supposed to ping the team on Earth when it got around the planet’s far side. The NASA scientists heard nothing… and nothing… and nothing. The eventually determined that the orbiter got some garbled instructions—navigational commands were sent in English units, rather than the internationally standardized metric. This error meant the probe missed the sweet spot for orbiting and instead fell into Mars’ atmosphere, where it disintegrated.

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