Information Technology Reference
In-Depth Information
Mariner 1 Example
The July 22, 1962, launch of the Mariner 1 space probe to Venus gives a particu
larly good illustration of how a seemingly trivial typographical error can have
dramatic consequences in computer programs. The program controlling that
rocket should have contained the line
Do 3 I
1, 3
instructing the computer to repeat a particular sequence of instructions several
times. Instead, the comma was replaced by a period:
Do 3 I
1.3
which assigned the value 1.3 to a variable called Do3I. As a result, the probe veered
off course shortly after launch and had to be destroyed by mission control officers.
Here a simple typographical error caused the loss of an $18.5 million probe and
launch vehicle. Unfortunately, when programs are tens of thousands of lines long,
finding a mistake in a single character is extremely difficult, if not impossible.**
** From page 103 of
The Limits of Computing
. That reference cites
Annals of the
History of Computing
, Volume 6, Number 1, 1984, p. 6, reported in
Software
Engineering Notes
, Volume 8, Number 5, October 1983, p. 4, with further editor
ial comment in Volume 11, Number 5, October 1985, p. 17.
correct, what can go wrong?
Even if code is correct for distinct aspects of a program, dif
ferent parts of a program sometimes interact in unexpected ways.
For example, suppose a program is constructed for a school so
that only people in a Treasurer's Office or Financial Aid Office
are allowed to access a student's account to view financial infor
mation, while only those in the Registrar's Office or Student
Counseling Office can access accounts to view grade information.
In this case, a common processing approach might be to check a
user's privileges at the beginning of a session, and then remember
that information for future use. For the most part, such a system
could work without difficulty. For each user, an initial check is
made, and access is allowed or denied appropriately. However,
