NASA lost a Mars orbiter to pound-seconds

transcript

NASA sent a weather satellite to Mars. One file's units doomed it. One program wrote pound-seconds. The next one read newton-seconds.

On the way, small thrusters keep firing. Each one nudges its path. A ground program writes the size of each nudge to a file. Navigation reads that file to work out where the spacecraft is. The contract said newton-seconds. The program wrote pound-seconds. Navigation read them as metric. Every single nudge, underestimated.

One pound of force is 4 point 4 5 newtons. So every nudge was off by that factor. Tiny on its own. But it fired 10 to 14 times more than expected. Its solar array sat to one side. Sunlight kept twisting it. Nine months of cruise. The small errors kept adding up.

But here's the catch. People did notice. Navigation methods disagreed. It was reported informally. Never resolved.

23 September 1999. It fires its engine to enter orbit. It passes behind Mars. 49 seconds earlier than predicted. It was never heard from again.

The plan: pass 226 kilometres above Mars. The lowest it could survive: 80. Investigators estimate it came in at about 57. Below the line.

The spacecraft's own software used metric correctly. The handoff broke. So put units in the contract and the data. Check them automatically. And chase every disagreement. Don't shrug.

Full breakdown in the lecture.