Information Technology Reference
In-Depth Information
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FIGURE 8.3
A software bug in error recovery code made AT&T's System 7 switches crash in
1990. (1) A single switch in New York City detects an error condition and reboots. When it
comes back up, it sends an “OK” message to other switches. (2) Switches in Detroit, St. Louis,
and Atlanta are so busy that handling the “OK” message causes them to fail. They detect an
error condition and reboot. When they come back up, they send out “OK” messages to other
switches, causing some of them to fail, and so on.
NASA designed the $125-million Mars Climate Orbiter to facilitate communications
between Earth and automated probes on the surface of Mars, including the Mars Polar
Lander. Ironically, the spacecraft was lost because of a miscommunication between two
support teams on Earth.
The Lockheed Martin flight operations team in Colorado designed its software to
use English units. Its program output thrust in terms of foot-pounds. The navigation
team at the Jet Propulsion Laboratory in California designed its software to use metric
units. Its program expected thrust to be input in terms of newtons. One foot-pound
equals 4.45 newtons. On September 23, 1999, the Mars Climate Orbiter neared the
Red Planet. When it was time for the spacecraft to fire its engine to enter orbit, the
Colorado team supplied thrust information to the California team, which relayed it to
the spacecraft. Because of the units mismatch, the navigation team specified 4.45 times
too much thrust. The spacecraft flew too close to the surface of Mars and burned up in
its atmosphere.
A few months later NASA's Martian program suffered a second catastrophe. The
Mars Polar Lander, produced at a cost of $165 million, was supposed to land on the south
pole of Mars and provide data that would help scientists understand how the Martian
climate has changed over time. On December 3, 1999, NASA lost contact with the Mars
Polar Lander. NASA engineers suspect that the system's software got a false signal from
the landing gear and shut down the engines 100 feet above the planet's surface.
