The European space agency spent 10 years and 7 billion dollars to develop
the Ariane-5 rocket. On June 4, 1996, the first Ariane-5 was launched.
At 39 seconds after liftoff it exploded, destroying the rocket and cargo
valued at half a billion dollars. So what happened? It turns out that the
explosion was caused by activation of the self-destruct mechanism built
into the rocket. The self-destruct was triggered by unusually large aerodynamic
forces that were ripping off the boosters. These forces were due to an
abrupt course correction made by the on-board steering computer, which
was in compensation for a wrong turn off course that in fact never took
place. The inertial guidance computer had told the steering computer
that the rocket had gone way off course, when in fact it was not off course
at all.
What caused this turn of events? It seems that what happened was that
in the computations done by the inertial guidance computer it was converting
a 64-bit floating point number into a 16-bit signed integer number. At
about 36 seconds into the flight, a number was encountered that was larger
than 32768, which is the largest possible 16-bit signed integer, so the
conversation failed. Thus, erroneous numbers were sent to the steering
computer, causing it to think the missile was off course and leading to
the explosion at 39 seconds into the flight. Again a very costly disaster
due to bad computer arithmetic.
