The Explosion of the Ariane 5
On June 4, 1996 an unmanned Ariane 5 rocket launched by the European Space Agency
exploded just forty seconds after its lift-off from Kourou, French Guiana.
The rocket was on its
first voyage, after a decade of development costing $7 billion. The
destroyed rocket and its cargo were valued at $500 million. A board of
inquiry investigated the causes of the explosion and in two weeks issued
a report. It turned out that the cause of the failure was a software
error in the inertial reference system. Specifically a 64 bit floating
point number relating to the horizontal velocity of the rocket with
respect to the platform was converted to a 16 bit signed integer. The
number was larger than 32,767, the largest integer storeable in a 16 bit
signed integer, and thus the conversion failed.
The following paragraphs are extracted from
the
report of the Inquiry Board.
An interesting
article on the accident and its implications by James Gleick
appeared in The New York Times Magazine of 1 December 1996.
The CNN
article reporting the explosion, from which the above graphics
were taken, is also available.
On 4 June 1996, the maiden flight of the Ariane 5 launcher ended in a
failure. Only about 40 seconds after initiation of the flight sequence,
at an altitude of about 3700 m, the launcher veered off its flight path,
broke up and exploded.
The failure of the Ariane 501 was caused by the complete loss of
guidance and altitude information 37 seconds after start of the main
engine ignition sequence (30 seconds after lift-off). This loss of
information was due to specification and design errors in the software
of the inertial reference system.
The internal SRI* software exception was caused during execution of a
data conversion from 64-bit floating point to 16-bit signed integer
value. The floating point number which was converted had a value greater
than what could be represented by a 16-bit signed integer.
*SRI stands for Système de
Référence Inertielle or Inertial Reference System.
More disasters attributable to bad numerics
Last modified August 23, 2000 by Douglas N. Arnold