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And you thought your computer was flakey. Here is a story about
mans greatest probe ever launched into space and its flakes. This
is an actual account of Voyager 1 and the problems it encountered
December 13'th, 1979. Written by Fredric L. Rice, August 1985.
Original reference material may be found at Griffith Observatory,
located at 2800 East Observatorty Road, Los Angeles, California.
90027. Request back issue of Griffith Observer, May 1980. Page 11
for Stephen S. Fentress, "Lost In Space". Direct requests to Dr.
Edwin C. Krupp and staff. You may aquire subscriptions to the
Griffith Observer through the same address. It provides a great
quantity of understandable information concerning astronomy.
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There is a concept making the rounds describing the attitude of
electronic equipment when it desides to do something out of the
ordinary, by itself, without being asked to. The concept is titled
"Digi-nerds". It may include just about anything you care to name.
It strikes once, leaving much damage.
The cause of digi-nerds is not known. Perhaps cosmic rays,
which bombard us constantly, strike our equipment, mutating a zero
into a one, or a one into a zero. The result may never be noticed,
or it may spell disaster for your bank account.
When Voyager 1 completed its mid-course correction, December
13'th, 1979, it met up with a digi-nerd at 48,000 miles an hour,
and 619 million miles from home.
The course correction required a 37 minute burn to effect a
change in speed of eleven miles an hour. During that time, the
radio dish had to be turned away from its Earth-Line so that the
engine would be aligned according to its flight plan. The Voyager
vehicles were designed to carry out complicated maneuvers like
this without requiring any instructions from Earth.
All went according to plan; Saturn had been treated to a rare
and beautiful sight of a new star tracking through its distant
skys. After the main burn, an inhabitant of Saturn, (if he had had
a good telescope), might have seen some additional flashes as
Voyager attempted to realign itself to its Earth-Line using it
attitude control thrusters.
Voyager 1 regains its Earth orientation by locating the Sun and
the star Canopus. When the Sun tracker is locked onto the Sun, and
the star tracker is locked onto Canopus, the radio disk is aligned
exactly at Earth. When contact was not restored at 3:13 p.m.
P.S.T. on December 13, it was known that something has gone wrong.
To find the Sun, the vehicle rotates itself a few degrees at a
time until the Sun tracker lockes onto the Sun. There is only one
stellar object that can be as bright at the Sun, (even at 711
million miles the Sun is an impressive sight). When the Sun is
positivly identified, the vehicle rotates itself along another
axis until the star tracker locates Canopus.
The Deep Space Network Antenna located in Madrid heard a faint
signal from Voyager. This gave the scientist the idea that the
probe was basicly healthy but somehow simply misaligned. Even if
this be the case, if the device was too badly misaligned, it might
not be able to read a command from Earth telling it how to find
Earth again.
Voyager 1 was on the verge of being lost forever. Adrift in the
heavens with no possibility of being recovered. Unable to report
its posistion and the cause of its ailments.
Dr. Jones and his Spacecraft Team knew that Alpha Centauri and
Rigel could deceive the star tracker. Based on the possibility
that one of these stars was locked onto, the team beamed
instructions through the Deep Space Network at Madrid to the lost
spacecraft in the hopes that a strong enough signal could be read.
Dr. Jones directed the spacecraft to align itself with the
assumption that it was locked onto Alpha Centauri.
Voyager 1 did receive the instructions, and it did attempt to
realign itself according to its new instructions. Alpha Centauri
was the wrong star. Radio contact was not improved after the
spacecraft completed its instructions.
Next, Voyager was instructed to realign itself base on the
assumption it was locked onto Rigel. This did not improve radio
reception, causing much disappointment to the Spacecraft Team.
Though they did not know what star Voyager was locked on, they
did know that from its point of view the Sun and Earth appeared
eight degrees apart. If the spacecraft could be made to wobble out
an eight degree cone, the signal from the spacecraft could be made
to sweep accross the Earth every now and then, and they would be
able to learn more information about where the spacecraft was
pointing.
The maneuver worked. On December 16'th, almost complete contact
was regained through the Canberra, Australia, tracking station.
Total loss of signal time exceeded 71 hours. In order to learn why
the spacecraft has gone astray, Dr. Jones and his team ordered it
to replay all information it had on what had happened for the last
three days. Records showed an error in communications between two
on board computers, and there was nothing showing to restrict
another attempt to regain normal contact. The spacecraft was
instructed to go through its Earth-Find maneuver December 19'th,
and on December 20'th, Voyager was again in full contact with the
Earth.
Reconstruction of the detailed data Voyager offered showed that
the spacecrafts master computer had ordered a secondary computer
to shut down the engines at the end of the course correction.
Commands such as this are requested twice, and it was the second
instruction that got garbled between the two computers. The first
instruction had indeed shut down the engines yet the second
corrupted instruction was not understood by the secondary
computer. This computer reported the strange instruction to the
master computer who declaired an abort.
When a spacecraft abort is executed, all operations are thrown
away and the Earth-Find maneuver is executed. Voyager did this,
and in fact did find the Sun. It was while the spacecraft was on
its search for Canopus that another emergency was detected.
The attitude control system reported a leak in the primary
thrusters. Actually, the master computer had requested from 1026
to 1094 "shots" from the attitude control thrusters, while the
attitude control computer interprets more than 1000 as evidence of
a leak. It reported a problem and the star search was aborted.
So there it stood, with only a minimal contact with Earth; its
star tracker not pointing at any known object. The spacecraft was
compleatly healthy but for no known reason a garbled transmission
from the master computer to the slave had triggered an emergency.
There had been more than five hundred thousand instructions to
cross its data bus, and it had already executed six previous Earth
Find maneuvers.
Sometimes our failures turn out to be our biggest triumphs. To
defeat a problem which might end our achievements is a better
boost to our moral than the defeating of a known hazard, (Remember
Apolo 13 and the problems circumvented by those aboard).
The space shuttle will no doubt encounter digi-nerds on one of
its many scheduled flights. We con only hope it wont be over 600
million miles away when it does.
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Additional information:
1) Voyager 1 was 56 light minutes away when the emergency started.
2) Using the Earth-Find maneuver, the entire sky can be searched
in about four hours, eighteen minutes.
3) Voyager 2 will encounter Uranus in 1986, and Neptune in 1989.
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Additional reading:
1) Edelson, R. E. et al., "Voyager Telecommunications: The Broad-
cast From Jupiter", Science, 204, 913, (June 1979).
--------------------
For information on the Holmann transfer, read:
1) Melbourne, W.G. , "Navigation Between the Planets", Scientific
American, 234, 58, (June 1976). [Authors note: If you want to
read "Navigation", don't forget your calculator and paper. This
article offers simple formula taht is fun to use].