In September nineteen ninety-nine, the Mars Climate Orbiter completed a nine-month journey across hundreds of millions of kilometres of space and disappeared behind Mars — and never came back out. The spacecraft had been sent to orbit the planet and study its atmosphere. Instead it entered the Martian atmosphere far too low and was destroyed. The cause was a unit mismatch: one engineering team had been providing navigation data in pound-seconds while the software expected newton-seconds. That discrepancy had been quietly accumulating through every small course correction for nine months.

In this episode of 5 Minute Strange, we cover the full story of the Mars Climate Orbiter — the mission that was supposed to open a new chapter in low-cost planetary science, the signs during the cruise phase that the trajectory was slightly off, and the investigation that traced a hundred-and-twenty-million-dollar loss back to a conversion that was never checked. It arrived at Mars. It was lost the moment its work was supposed to begin. The Mars Polar Lander followed it to destruction three months later, making nineteen ninety-nine the worst year in NASA's Mars programme.

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