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Fig. 4.15. Photo of the destruction of the first launch of the Ariane 5 Flight 501 rocket. Just
thirty-seven seconds into the launch, the trajectory suddenly tilted by almost 90 degrees
and the rocket self-destructed. The software error occurred during data conversion from a
64-bit floating-point number to a 16-bit signed integer. This led to a sequence of events that
resulted in a complete loss of the guidance system.
NASA's Mars Climate Orbiter (1999)
The root cause for the loss of the Mars Climate Orbiter (
Fig. 4.16
) spacecraft was the failure to use met-
ric units in the coding of the software file, “Small Forces,” used in trajectory models. Instead of reporting the
thruster data in metric units of Newtonseconds (N-s), the data was reported in English units of pound-seconds
(lbf-s). Subsequent processing of this thruster data by the navigation software algorithm underestimated the
effect on the spacecraft trajectory by a factor of 4.45, which is the required conversion factor from force in
pounds to Newtons. An erroneous trajectory was then computed using this incorrect data.
Fig. 4.16. Artist's impression of the Mars Climate Orbiter. The space probe was lost at
the first attempt to enter the orbit around Mars on 3 September 1999. Putting a probe
into a final planetary orbit is a long process during which the initial orbit is gradually
reduced until the probe reaches its permanent orbit. Because of a software error the
orbiter entered the Martian atmosphere at too high a velocity and consequently burnt
up. The cost of this failure was $125 million.
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