The Bug That Cost $370 Million

On June 4th, 1996, Europe's brand-new Ariane 5 rocket destroyed itself 37 seconds after liftoff, and the cause traced back to a single integer overflow in reused flight software. This file opens the case of Ariane 5 Flight 501: a number too large for the 16-bit box it was converted into, in code borrowed unchanged from Ariane 4, and the honest engineering reasoning that let it through. I'm the RiddleWanderer. Archivist of mysteries. My consultant is a rubber duck named Archimede. He has never once been helpful. Subscribe, and we'll open the next file together. Chapters 0:00 A number too big to launch 0:55 Consulting the expert 1:06 The rocket that had to be new 1:52 How a rocket knows which way is up 2:52 The number that didn't fit 4:37 Code that had already proved itself 5:18 The gap nobody closed 7:08 Two computers, one mistake 7:58 Thirty-seven seconds 9:03 What the radio saved 10:09 What it cost 11:45 Rebuilding 12:32 Why I like this one 13:42 Close Sources Lions, J.-L. et al. (1996), "ARIANE 5 Flight 501 Failure: Report by the Inquiry Board," Paris, 19 July 1996. Dowson, M. (1997), "The Ariane 5 Software Failure," ACM SIGSOFT Software Engineering Notes, 22(2), p. 84. Jézéquel, J.-M. and Meyer, B. (1997), "Design by Contract: The Lessons of Ariane," IEEE Computer, 30(1), pp. 129–130. ESA (1996), "Ariane 501 · Presentation of Inquiry Board report," press release, 23 July 1996. ESA (1998), "Ariane-502 - results of detailed data analysis," press release, 8 April 1998. ESA (1998), Carré et al., "Europe Ready for Ariane-5 Production," ESA Bulletin 93. ESA (1997), "The Resurrection of the Cluster Scientific Mission," ESA Bulletin 91. ESA, "History of Ariane 4," esa.int - flight record and the 74-launch success run. ESA (2024–2026), Cluster reentry record: Salsa 8 September 2024, Rumba 22 October 2025, Samba 31 August 2026, Tango 1 September 2026, esa.int and the ESA Rocket Science blog. NASA / JPL, "Mars Climate Orbiter Mishap Investigation Board, Phase I Report," 1999, and the NASA NSSDC Master Catalog entry 1998-073A for the 57 km entry altitude. Google / YouTube statement on the 32-bit to 64-bit view-counter upgrade, December 2014, as quoted in contemporaneous reporting; the original post is no longer live. Credits Archive imagery: ESA, esa.int, CC BY-SA 3.0 IGO where noted. Diagrams and animations: original. Music: YouTube Audio Library. #riddlewanderer #technology #ariane5