|Ariane 5 G|
|Launch vehicle||Ariane 5 G|
|Launch site||Guiana Space Center, Kourou, French Guiana|
|Description||First stage total failure|
|Cause||Software design flaw|
|Payload||4 x Cluster (Solar research satellites, European Space Agency)|
|Desired orbit||Highly elliptical, polar orbit|
About 40 seconds after initiation of the flight sequence, at an altitude of about 3,700 m, the launcher veered off its flight path, broke up and exploded.
The report of the independent Inquiry Board said that analysis of the flight data indicated:
This failure was caused by what some nowadays regard one of the costliest computer bugs of all times. The root cause was that the Ariane 5 software reused the specifications from the Ariane 4--for a totally different trajectory beyond the range for which the code had originally been designed. Ariane 5 has a high initial acceleration and a trajectory which leads to a build-up of horizontal velocity which is five times more rapid than for Ariane 4.
The greater acceleration caused the back-up and primary inertial guidance computers to crash--the higher-than-expected values caused an overflow at data conversion from 64-bit floating point to 16-bit integer. This exception could have been caught, but the code for that was disabled as it was thought "proven" that such an overflow never could occur. This may have been true for the Ariane 4 but most definetly not for the Ariane 5, as events have shown. After the failure of the inertial guidance system, the launcher's nozzles were directed by meaningless data and began to swivel irregularly, in the end leading to the destruction of the rocket.
Ironically, the part of the software which crashed the inertial guidance system wasn't needed after lift-off at all. On the Ariane 4, it enabled a rapid realignment of the launcher in case of a late hold in the countdown, so inertial guidance was left operating for some time after lift-off. According to the Inquiry Board's final report, "this realignment function, which does not serve any purpose on Ariane 5, was nevertheless retained for commonality reasons and allowed, as in Ariane 4, to operate for approx. 40 seconds after lift-off."
"Somebody should have found the error," acknowledged Jean-Marie Luton, then director general of ESA. But pre-flight tests had never been performed on the re-alignment code under simulated Ariane 5 flight conditions, so the error was not discovered before launch.