Why the Mars Probe Went Off Course -- James Oberg
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1/10/05 4:04 PM
Why The Mars Probe Went Off Course
By James Oberg
AIRCRAFT ACCIDENT INVESTIGATORS
have a special term for a particularly
insidious type of accident--CFIT, or controlled flight into terrain. It occurs when human
error in the cockpit, in the traffic control tower, or in the flight planning process in effect
flies a perfectly good airplane right into the ground.
In the past 40 years, space flight has encountered all sorts of failure modes. Propulsion
systems have leaked and exploded. Power systems have short-circuited. Observation
instruments have failed to work or have been pointed in wrong directions. But until this
year no CFIT had occurred in outer space.
Then, on 23 September, through a series of still-baffling errors, flight controllers at the Jet
Propulsion Laboratory, a California Institute of Technology facility under contract to
NASA, sent erroneous steering commands to the Mars Climate Orbiter as it neared the
target planet. Obeying blindly like all true robots, the probe, metaphorically speaking,
marched off the cliff and was destroyed.
NASA assigned three separate teams to investigate the embarrassing, US $125 million
debacle and determine its cause. Preliminary public statements faulted a slip-up between
the probe's builders and its operators, a failure to convert the English units of measurement
used in construction into the metric units used for operation.
After six weeks, on 10 November, NASA officials released their preliminary findings.
investigation had been going on separately, using unofficial
sources associated with the program and independent experts.
that far more had gone wrong than just a units conversion error. A critical flaw was a
program management grown too confident and too careless, even to the point of missing
opportunities to avoid the disaster.
As reconstructed by
ground controllers ignored a string of indications that
something was seriously wrong with the craft's trajectory, over a period of weeks if not
months. But managers demanded that worriers and doubters "prove something was wrong,"
even though classic and fundamental principles of mission safety should have demanded
that they themselves, in the presence of significant doubts, properly "prove all is right"
with the flight. As a result, the probe was about 100 kilometers off course at the end of its
500-million-kilometer voyage--more than enough to accidentally hit the planet's
atmosphere and be destroyed.
Edward Stone, director of the Jet Propulsion Laboratory (JPL), in Pasadena, Calif., did not
try to dodge responsibility for events. "Our inability to recognize and correct this simple
error has had major implications," he stated in a 24 September press release from the lab.