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