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Extinction by planetoid/comet impacts.

                           THE ELECTRONIC JOURNAL OF
                       Volume 5, Number 8 - March 1994
                              TABLE OF CONTENTS
          * ASA Membership and Article Submission Information
          * A Personal Adventure in Home Computing: The Origin of Comet 
            Shoemaker-Levy 9 - Andrew J. LePage
          * Planetoid Impacts: Devastating Shapers of Earth's History
               - Nicholas M. Burk 
          * Hunting for Comets: One Observer's Success Story - Michael Janes
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                    Copyright (c) 1994 by Andrew J. LePage
        The author gives permission to any group or individual wishing 
        to distribute this article, so long as proper credit is given 
        and the article is reproduced in its entirety. 
        Despite recent trends in our society, I never felt inclined to
    purchase a Personal Computer (PC) for home use.  This is not to say
    that I do not use computers.  Nearly every work day during the past
    decade I have sat in front of a computer terminal or workstation of
    one sort or another.  I program them, transfer and analyze data with
    them, send Electronic Mail (E-Mail), and use them to perform a
    multitude of other useful tasks. 
        Still, with every cryptic error message that appears on the
    screen, I cannot help but wonder what twist of fate sentenced me to a
    career that requires my using these autistic minions of Mephistopheles.  
    I just do not like computers and the last thing I wanted to do was let 
    one of these little beasts into my home. 
        At work there exist those wizard-like individuals known as System
    Managers.  These saints of the late Twentieth Century are the ones who
    have to deal with all the mundane and mystical tasks required to keep
    computers and networks running.  I knew just enough about their
    activities to confidently state that I wanted nothing to do with it. 
    I wanted to be nothing more than a User.  Owning a PC meant that I
    would have to become, in some sense, a System Manager. 
        For half of my professional career I worked for a major computer
    company where PC was a four-letter word.  There were none to be found
    in my lab whether I needed one or not.  As a result, I had absolutely
    no experience with PCs, which just added to their mystery.  If there
    was ever something I really wanted to calculate for an article or just
    out of curiosity, I would use my handy-dandy programmable calculator. 
    If it were a rather large task, I would derive the needed equations,
    write my own programs, and run them on the computers at work on my 
    own time.  All in all, I had little need for a PC. 
        Things have changed over the past several years.  I still tend to
    hate computers to the point where it is almost a phobia.  However, I
    have changed jobs and now have access to PCs, so I have learned to do
    the basics with them.  Many members of my family are very PC-literate
    and are more than willing to help me with my PC problems.  Today's PCs
    are as powerful as the minicomputers I was using in the lab ten years
    ago.  Combined with the array of quality software that is available on
    the market today, one can do more on a PC than I could ever hope to do
    if I were to program a workstation from scratch in my spare time.  It
    was beginning to look as though a PC might be necessary after all. 
        The Plunge
        On July 29, 1993, I took the plunge and bought a PC.  Within a
    couple of days - with the help of my brother - we got the little beast
    up and running.  Around this same time word was spreading about the
    impending impact of comet Shoemaker-Levy 9 (SL-9) on Jupiter in July 
    of 1994.  The amateur orbital dynamicist in me could not resist.  I 
    had to simulate this orbit, but it would take forever to write my own 
    program to do it.  Fortunately, there is PC-compatible software that 
    will do the trick. 
        The first major software purchase I made was one of the new
    commercially available solar system simulators.  After I installed the
    program, I immediately called up the preloaded orbital elements of
    SL-9 and ran things forward in time.  The little comet disappeared in
    a pleasing flash of light as it plunged into the Jovian atmosphere on
    the predicted day.  The next day I loaded the latest orbital elements
    I obtained from the Internet and performed the simulation again with
    similar results. 
        Normally, this would have been the end of it.  After all, from 
    how many angles or levels of magnification can one watch a simulated
    impact, even if the graphics are pretty?  However, the experimentalist
    in me naturally wanted to do a thorough job.  The only other thing 
    to do was to run the simulation backwards in time.  I used two test
    bodies, each using one of the two sets of elements I had in hand.  I
    ran the simulation backwards and encountered the bane of every classical 
    Newtonian determinist:  Chaos.  At first the two test bodies followed 
    similar paths.  Then, as the years passed, they slowly diverged.  By 
    the time both were free of Jupiter's influence, they were in similar 
    orbits but in totally different parts of the sky.  After a few checks 
    to ensure that I was not observing some sort of program or machine 
    induced error, I became convinced that the orbit of comet SL-9 was 
        The Experiment
        Despite this little setback, I was still curious as to where 
    SL-9 came from.  Chaos would make it impossible for myself and even
    professional astronomers to precisely determine the position of this
    strange comet, particularly before its capture by Jupiter.  However, I
    thought perhaps that I could make some sort of statistical statement
    as to the whereabouts of SL-9 at any given point in time.  So I
    designed an experiment. 
        First, I obtained Don Yeoman's and Paul Chodas' orbital element
    solution number 22 (and later number 28) and their uncertainties and 
    produced a swarm of test bodies scattered fairly evenly in parameter 
    space.  Hopefully, this would be a realistic sample of all the possible 
    orbits of SL-9.  Second, I assumed that any computational errors would 
    be random and that over a long enough time span - with a large sample 
    of test objects - these errors would average out to be zero.  Finally, 
    I had to assume that a chaotic system such as this could be meaningfully 
    quantified by what is essentially a Monte Carlo simulation. 
        After 1,500 hours of computer time (that's right, two months at
    twenty-four hours a day) and numerous checks, I had a group of eighteen 
    test bodies which I felt were representative of all possible SL-9 orbits.  
    Being painfully aware of statistics, I realized that such a small test 
    population would be a pollster's nightmare and result in statistical 
    uncertainties of give-or-take about twenty-five percent.  Thankfully, 
    though, the test bodies' orbits did follow a pattern and I felt some 
    meaningful conclusions could be drawn.  At the very least, I was able 
    to glimpse the majesty and grandeur of the forces that shape the orbits 
    of the countless unseen bodies that inhabit the outer parts of our 
    solar system. 
        The Results - Capture
        The first obvious question about comet SL-9 is when was it captured 
    by Jupiter?  Based on the results of my simulation, I would say around 
    the year 1970.  Actually, I should state that there is an eighty-three 
    percent probability that SL-9 was captured by 1970.  Reading between 
    the lines of some of the professional papers and abstracts I have seen 
    on SL-9 orbital simulations, it seems that 1970 was about when the 
    capture likely took place.  So it appeared that my simulation was on 
    the right track. 
        The following table lists the probabilities that SL-9 was in orbit 
    around Jupiter by a certain date: 
                                  Table 1
                      Year                   Probability
                      1980                   100%
                      1975                   94%
                      1970                   83%
                      1965                   39%
                      1960                   39%
                      1940                   28%
                      1920                   17%
                      1900                   6%
                      1880                   0%
        According to my simulation, capture could have occurred sometime
    between the late 1880s and the late 1970s.  This may seem to be a
    large range of dates, but we are dealing with the unpredictability of
    chaos.  Still, given the statistically significant forty-four percent
    transition that took place in the late 1960s (a 1.9 sigma change, to
    use the technical jargon), it is statistically most likely the capture
    took place around 1970. 
        The next question is what sort of orbit did SL-9 have before its
    capture?  There is an eleven percent probability that SL-9 was captured
    from a low inclination orbit with a perihelion near Jupiter and an
    apohelion near Saturn.  With an eight-nine percent probability, it
    seems more likely that SL-9 was captured from a low inclination orbit
    with an apohelion near Jupiter and a perihelion buried in the main
    Planetoid Belt no closer than about 2.5 astronomical units (AU) from
    the Sun.  One AU equals the average distance between the Sun and Earth, 
    or about 150 million kilometers (93 million miles).
        This sort of orbit looks much like that of a short-period comet. 
    I naturally wondered if perhaps SL-9 was observed before its capture?
    Unfortunately, unless the comet is fresh from the outer solar system,
    they will rarely display any significant cometary activity at 2.5 AU. 
    The Sun's feeble heat at such a distance will vaporize only the most
    volatile of ices.  This makes discovery less likely but not impossible. 
    A search of a cometary orbit data base, however, showed no likely can-
    didates.  The average three-degree inclination of the precapture orbits 
    was just too low.  If SL-9 graced our skies before its discovery, it 
    happened centuries or millennia ago, if ever. 
        The Results - The Past
        For my statistical studies of SL-9's possible orbits, I decided to
    make several broad classes of orbits and determine how their populations 
    change with time.  These classes are as follows: 
        Class 1 - Jupiter-Planetoid Belt:  Bodies in this class had a
                  perihelion of between one and about four AU and an 
                  apohelion near Jupiter.  The period of revolution ranged 
                  between five and ten years. 
        Class 2 - Jupiter-Co-Orbital:  Bodies in this class of orbit had
                  periods ranging from ten to fourteen years and orbited 
                  near Jupiter's orbit.  This class was very unstable and 
                  was typically occupied by a test body for only a few 
                  decades as it was making a transition from orbit Class 1 
                  to Class 3, or vice versa. 
        Class 3 - Jupiter-Saturn:  Bodies in this class of orbit had periods
                  ranging from fourteen to twenty-eight years.  The perihelion 
                  was near Jupiter and the apohelion was near the orbit of 
        Class 4 - Jupiter-Uranus:  Bodies in this class had periods ranging
                  from twenty-eight to fifty-six years.  As you have probably 
                  already guessed, the perihelion is near Jupiter and the 
                  apohelion near Uranus. 
        Class 5 - Jupiter-Neptune:  Bodies in this class had periods ranging
                  from fifty-six to ninety-five years.  I know you can figure 
                  out the rest. 
        Class 6 - Saturn-Co-Orbital:  Like Class 2, these bodies circle near 
                  the orbit of Saturn with periods in the twenty-four to 
                  thirty-four-year range.  Also like Class 2, this orbit is 
                  unstable and occupied for only a few centuries at most. 
                  This orbit is typical for bodies making the transition 
                  from Class 3 to Class 7, or vice versa.
        Class 7 - Saturn-Uranus:  Bodies in this class had periods ranging
                  from thirty-four to sixty-eight years.  As expected, the 
                  perihelion is near Saturn and the apohelion near Uranus. 
        Of course there are many other possible classes of orbits.  I just
    did not observe them during my simulation.  With more test bodies and
    simulated run times on the order of one million years, I am sure I
    would have seen a fair number of bodies in Uranus-Kuiper Belt orbits.
    Unfortunately, due to time and program constraints, I could only
    simulate back about seven thousand years. 
        Still, let us see what results I did get.  Below is an abridged 
    list of test body population distributions going back to 5000 B.C.: 
                              Table 2
                            Orbit Class
         Year        1    2     3     4    5     6     7
         1900 AD     72%  0%    22%   0%   0%    0%    0%
         1800 AD     61%  0%    39%   0%   0%    0%    0%
         1700 AD     50%  0%    50%   0%   0%    0%    0%
         1600 AD     50%  0%    50%   0%   0%    0%    0%
         1500 AD     50%  0%    50%   0%   0%    0%    0%
         1000 AD     17%  17%   61%   6%   0%    0%    0%
         500  AD     28%  0%    56%   17%  0%    0%    0%
         1    BC     28%  0%    33%   28%  0%    6%    6%
         500  BC     28%  6%    17%   39%  0%    6%    6%
         1000 BC     22%  0%    33%   33%  6%    0%    6%
         2000 BC     17%  0%    39%   17%  17%   0%    11%
         3000 BC     11%  6%    28%   33%  11%   6%    6%
         4000 BC     11%  11%   11%   39%  6%    6%    17%
         5000 BC     6%   0%    33%   39%  11%   0%    11%
        It should be remembered that the survey represents just a series
    of population snapshots.  Looking at the trends in the data, one might
    think that bodies move orderly from one class of orbit to the next. 
    For example, starting at 1000 B.C., a particular test body is in a
    Jupiter-Neptune orbit, followed by a Jupiter-Saturn, then Jupiter-
    Planetoid Belt, then finally capture.  The process is actually much 
    more complex.  A body could start in a Jupiter-Neptune orbit, changing 
    to a Jupiter-Co-Orbital, then Jupiter-Saturn, Jupiter-Planetoid Belt, 
    then back to Jupiter-Saturn, and so on.  There is a randomness about 
    it all.
        The Results - The Distant Past and the Origin
        Despite the randomness of individual bodies, there appear to 
    be trends in the population data.  The number of bodies in Jupiter-
    Planetoid Belt orbits, for example, seems to decrease logarithmically 
    as we go back in time.  Extrapolating back, one can imagine a time 
    around 8000 B.C. when there would be no bodies left in this class 
    orbit.  As time progresses, the population seems to move from one 
    class of orbit to the next higher one.  We can easily see that as 
    the population in Jupiter-Planetoid Belt orbits decreases, the 
    population in Jupiter-Saturn orbits increases.  As this population
    reaches its apparent peak and starts to decline, we see an increase 
    in populations in Jupiter-Uranus orbits followed by Jupiter-Neptune 
        Unfortunately, one has to be very careful about the conclusions
    drawn.  The uncertainties in these measurements is about plus or 
    minus twenty-five percent.  Statistically speaking, twenty percent is
    indistinguishable from fifty percent at a one sigma confidence level. 
    Scientists typically want something closer to three sigma confidence
    level to be sure of their results.  To that level of confidence, this
    simulation can only tell the difference between zero percent and one
    hundred percent.  The observed population trends could be nothing 
    more than statistical noise. 
        The simulation's population of test bodies is too small to tell
    the difference between pure random scattering, deliberate trends, or
    some combination of the two.  In reality, this is nothing more than a
    detail in the grand scheme of things.  The maximum observed orbital
    inclination was ten degrees.  Most are less than one-third of that. 
    According to the latest literature, this, along with a prograde orbit,
    points to SL-9 originating in the Kuiper Belt, which is now believed
    to be the source of the majority of periodic comets. 
        With this in mind, I now present the following simplified scenario
    of the life of comet Shoemaker-Levy 9: 
        After the formation of our solar system about five billion years
    ago, SL-9 orbited serenely around the Sun near the inner edge of the
    Kuiper Belt beyond Pluto.  As the years passed, the orbit slowly
    changed under the gravitational influence of Neptune.  After billions
    of years of perturbations, SL-9 finally experienced a close encounter
    with Neptune, which wrenched it permanently out of the Kuiper Belt. 
        During the hundreds of thousands to millions of years that
    followed, the Jovian planets played pinball with the comet. 
    Eventually, Jupiter grabbed hold of it and gradually decreased the
    period of the comet's orbit over thousands to tens of thousands of
    years.  A final encounter with Jupiter some hundreds to thousands of
    years ago flung SL-9 into the inner solar system, where it may have
    spent time as an active comet. About twenty-five years ago, SL-9 was
    finally captured by Jupiter. After one dozen or so irregular orbits,
    the twenty-one known parts of SL-9 will dive into the Jovian
    atmosphere to their destruction. 
        So ends not only the life of comet Shoemaker-Levy 9 but my first
    adventure with a PC.  I am a bit more at ease with the little beast
    now, but I am still concerned about what it might try to pull in the
    future.  In the mean time, I will muster up a little courage and try
    something new.  Stay tuned.
        Related EJASA Articles -
        "Cometary Conundrums", by M. Leon Knott - June 1993
        "In Pursuit of Comet Swift-Tuttle", by Harry Taylor - September 1993
        "Hunting for Comets: One Observer's Success Story", by Michael Janes
    March 1994 (this issue)
        "Planetoid Impacts: Devastating Shapers of Earth's History", by 
    Nicholas M. Burk - March 1994 (this issue)
        About the Author -
        Andrew J. LePage is a scientist at a small R&D company in the 
    Boston, Massachusetts area involved in space science image and data 
    analysis.  He has written many articles on the history of spaceflight 
    and astronomy over the past few years that have been published in many 
    magazines throughout North America and Europe.  Andrew has been a 
    serious observer of the Soviet/CIS space program for over one dozen 
        Andrew's Internet address is:  lepage@bur.visidyne.com 
        Andrew is the author of the following EJASA articles:
        "Mars 1994" - March 1990 
        "The Great Moon Race: The Soviet Story, Part One" - December 1990
        "The Great Moon Race: The Soviet Story, Part Two" - January 1991
        "The Mystery of ZOND 2" - April 1991
        "The Great Moon Race: New Findings" - May 1991 
        "The Great Moon Race: In the Beginning..." - May 1992
        "The Great Moon Race: The Commitment" - August 1992
        "The Great Moon Race: The Long Road to Success" - September 1992
        "Recent Soviet Lunar and Planetary Program Revelations" - May 1993
        "The Great Moon Race: The Red Moon" - July 1993
        "The Great Moon Race: The Tide Turns" - August 1993
        "The Great Moon Race: The Final Lap - November 1993
                              By Nicholas M. Burk
        If there has been one catastrophic astronomical event which has
    helped to shape the history of life on the planet Earth, it is the
    impact of planetoids, also known as asteroids.  This phenomenon, 
    which may have given birth to Earth's only natural satellite and 
    destroyed its most colossal life forms, may now be prevented by its 
    most advanced species - humanity. 
        In the embryonic stages of the solar system's formation, billions
    upon billions of planetesimals aggregated into larger bodies.  In time, 
    the largest of these mammoth planetoids became the predecessors of 
    today's planets.  For millions of years, collisions with planetoidal 
    material were almost continuous.  Today we can see the chaos of these 
    early impacts engraved on the faces of Mercury, our Moon, and many 
    other worlds in our solar system where little erosion has taken place 
    in millions and billions of years. 
        Astronomer William K. Hartmann has theorized that in the early
    years of Earth, our planet was hit by a Mars-sized planetoid.  Material 
    spewed out from Earth's mantle into space and formed a ring around the 
    planet.  Just as embryonic material had aggregated before to form new 
    planets, this ring of debris eventually formed our Moon. [1] 
        Later, Earth was still bombarded by planetoids.  Hartmann suggests
    that primordial oceans may have been vaporized and with them early
    forms of life never to be seen again. [2]  As time progressed,
    planetoid impacts occurred with lesser frequency. 
        Life moved from the sea onto land.  Eventually amphibians and
    reptiles surfaced.  Then, in the time span just before the rise of
    the dinosaurs, ninety percent of all species vanished.  A definite
    answer as to why this happened has yet to be uncovered, though a
    devastating planetoid impact with Earth is one theory. 
        For the next two hundred million years, dinosaurs dominated
    Earth's soils, skies, and oceans.  Few species have ever prospered 
    so abundantly and for such a long period of time as these giant
    creatures.  Yet in an abrupt twist of geologic time, all dinosaurs
    were wiped out in a quick ten million years. 
        Science grappled with this fantastic mystery for decades before
    significant evidence emerged that hinted at a catastrophic ending for
    these mighty creatures.  In the early 1980s, a titanic impact crater
    was uncovered near the Yucatan Peninsula. 
        The layer of rock which corresponds to the sixty-five million
    year-old crater paints a grim picture of an apocalyptic catastrophe. 
    Blanketing our planet is a layer of metallic planetoidal residue, the
    element iridium.  Local shock quartz crystals hint at the intense heat
    generated by an impact.  Perhaps most compelling is a layer of soot
    which is apparently the result of global forest fires caused by
    planetoid debris raining down upon the rich Cretaceous forests. 
        Other estimates of the damage caused by the impact hint at massive
    tidal waves and a blackening of the sky caused by airborne dust.
    Although the dinosaurs were not immediately destroyed, the planetoid
    set climatic and biological forces into motion which helped to forever
    change Earth's history. [3] 
        The Cenozoic Era experienced minor extinctions and, perhaps not
    uncoincidentally, layers of iridium at those levels have been found. [4] 
    Even in the age of human civilization - a tiny fraction of geologic 
    time - evidence of powerful planetoid impacts has been brought to light. 
        Perhaps the most famous event is the Tunguska impact which occurred
    in the Siberian forests in 1908.  Had this object struck a thickly
    inhabited area, the results would have been far more disastrous.
    Perhaps the hazards of planetoid impacts would have been permanently
    engraved in the consciousness of humanity. [5]
        As this millennium comes to a close, astronomers have confirmed
    that there are between 750 and 1,000 Earth-crossing planetoids at
    least 0.8 kilometer (0.5 mile) in size or larger roaming through the
    solar system. [6]  Probabilities say that the odds of an impact on
    Earth occurring soon are minimal.  Yet odds cannot predict chaos. 
    For example, in 1991, a ten-meter (thirty-three-foot) wide planetoid
    dubbed 1991 BA quietly zipped halfway the distance between Earth and
    the Moon.  A similar incident occurred on March 15, 1994 with another
    Earth-crossing planetoid.
        For all of the potential catastrophic implications caused by
    Earth-crossing planetoids, deflection has been deemed quite feasible.
    In 1981, the Spacewatch Workshop, a meeting of invited astronomers,
    planetary geologists, space mission specialists, and nuclear weapons
    authorities from the United States Defense Department, addressed the
    issue of deflecting planetoids that could pose a hazard to Earth. [7] 
        The majority of participants at the meeting agreed that not only
    was the possibility of deflecting a planetoid feasible, but work on
    systems to do so could begin right away. [8]  Nuclear devices and even
    TNT could be used to nudge a menacing planetoid off its impending
    course, thereby preventing a tremendous disaster.  In a statement to
    the U.S. Congress, NASA's Dr. Wesley T. Huntress, Jr., stated that the
    civilian space program will continue to monitor Near-Earth Asteroids
    (NEA), but the issue of the actual deflection must be left up to the
    Defense Department. 
        In terms of cost, developing deflection probes and enhancing
    already existing detection mechanisms would be relatively inexpensive
    compared to some other space efforts.  The importance of this safety
    net is priceless if a large planetoid approaches Earth. 
        Planetoid impacts have helped to shape the history of our planet
    and its life forms.  As the second millennium approaches, Earth should
    finally be able to avoid this catastrophic phenomena which it has
    endured for over four billion years. 
        Annotated Bibliography -
    1.  William K. Hartmann and Ron Miller, A HISTORY OF EARTH, Workman 
        Publishing Company, Inc., New York, pages 44-57, 1991.  An 
        excellent resource for understanding the scope of planetoid 
        impacts on Earth history. 
    2.  Ibid., pages 90-93.
    3.  Ibid., pages 158-174.  Hartmann and Miller give a fascinating
        and beautifully illustrated description of the impact.  There is 
        a tremendous amount of information on this subject, including the 
        September 17, 1993 issue of SCIENCE magazine, which states that the 
        size of the planetoid was even larger than previously thought.
    4.  Ibid., pages 175-177.
    5.  Hartmann and Miller give an account of the Tunguska impact on pages
        207-209.  There is also a very informative article in the December,
        1993 issue of ASTRONOMY magazine. 
    6.  Shannon Brownlee, "How to Prevent the Extinctions", page 30,
        DISCOVER, 5:5, May, 1984. 
    7.  Brownlee, op. cit, page 31.
    8.  Brownlee, op. cit, page 31.
    9.  Statement of Dr. Wesley T. Huntress, Jr., before the Subcommittee
        on the Space Committee on Science, Space, and Technology, House of 
        Representatives, March 24, 1993.  Obtained through NASA Spacelink 
        via Internet.  A very substantive description of NASA's role in 
        detecting Near Earth Objects. 
        About the Author -
        Nicholas M. Burk is a Northampton High School student in
    Massachusetts seeking a career in the administration of space public 
    policy.  Nicholas has written a great deal on the economic and
    environmental benefits of an expanded space program and is seeking 
    to have some of this work published.  His related interests include
    cosmology and the effects of space travel on human psychology. 
        Nicholas may be reached on the Internet at: nburk@smith.smith.edu
                              HUNTING FOR COMETS:
                         ONE OBSERVER'S SUCCESS STORY
                               by Michael Janes
          Courtesy of Paul Dickson (p.dickson@asu.edu), Editor of the 
      Saguaro Astronomy Club's newsletter, SACNews, in Phoenix, Arizona.
        The September, 1992 Labor Day weekend reached a high point for
    some valley amateur astronomers on Sunday.  Leon Knott, who recently
    moved to the valley and became a new member with SAC, hosted a small
    get-together in Mesa, Arizona.  Among the people there was a friend of
    Leon's visiting for the weekend from New Mexico, Howard Brewington. 
        Howard and his wife live in Cloudcroft, New Mexico, at an elevation 
    of 2,200 meters (7,400 feet).  Out in front of their home is Howard's 
    observatory, which houses a forty-centimeter (sixteen-inch) f/4.5 
    reflector on an Alt./Az. mounting.  The primary mirror was figured by 
    Howard and the telescope design was done by Leon.  Piggybacked on that 
    telescope is a twenty-centimeter (eight-inch) f/4.3 reflector.  The 
    design of the observatory does not allow for good viewing to the north. 
    However, it does provide good views of both the western and eastern 
        In the winter of 1987, Howard was actively photographing comet
    Bradfield.  By the end of its apparition, he was "bit by the comet
    hunting bug."  The first half of 1988 was spent conducting a
    photographic search with a twenty-centimeter f/1.5 Schmidt camera.
    This type of search was not effective, considering the time involved
    to take the photographs in relation to the amount of sky covered plus
    the expense.  So in the summer of 1988, Howard converted to a visual 
    search method. 
        Don Machholz of California searches for comets by dividing the sky
    into about forty quadrants, examining each for an interloper.  David
    Levy will sweep up and down slowly across the sky.  Howard takes a
    different approach from the methods used by these accomplished
    astronomical observers.
        When asked about his search methods, Howard replied:  "I have only
    four quadrants, two in the evening and two in the morning.  I just
    make sweeps in azimuth sixty degrees long and I just wait for the
    object to come into the eyepiece."  This motion in azimuth is coupled
    with the rotation of our Earth, allowing for a shift of one field in
    altitude after the sixty-degree sweep.  The skies over Cloudcroft, New
    Mexico, seem to be similar to our own here in Arizona during 1992. 
    Says Howard:  "When the skies are good I spend anywhere from twenty to
    twenty-five hours a month at the eyepiece.  But this year I've been
    lucky for ten hours." 
        In November of 1989, after ninety-three sessions, fourteen months,
    and 230 search hours, Comet 1989a1, Aarseth-Brewington, was discovered.  
    Howard's first comet rose to third magnitude by December and is reviewed 
    by David Levy in his Star Trails column in the April, 1990 issue of SKY 
    & TELESCOPE magazine.  Many photographs are also included in that issue. 
        The searches continued for over one year until January 7, 1991. 
    This next object turned out to be comet Metcalf, which had been lost
    after its discovery in the winter of 1906-1907.  Periodic comet
    Metcalf-Brewington, 1991a, has a period of eight years, though its
    last orbit took it towards Jupiter, which increased its distance by 
    one astronomical unit (AU), making any future returns unlikely. 
        Comet 1991a exhibited an outburst of ten magnitudes over a period
    of thirty hours.  Two nights before Christmas of that same year, Comet
    1991g1 was co-discovered by Mauro Zanotta of Italy just twelve hours
    prior to Brewington's observation. 
        The date of August 29, 1992 brought Howard's fourth discovery and
    his first morning comet.  At magnitude 11.5, it was outward bound at a
    distance of two AU.  This comet is now too faint for amateur telescopes. 
        Although there have been a handful of good comets in recent years,
    I feel that we are due for a *great* comet.  According to Howard
    Brewington:  "I plan to find the next great comet.  It'll be the
    biggest disappointment of my life if I don't."  When asked if there
    was one comet observation that stands out from the rest, Howard
    replied:  "Well, the best comet I've seen in my life was mine." 
        Related EJASA Articles -
        "Cometary Conundrums", by M. Leon Knott - June 1993
        "In Pursuit of Comet Swift-Tuttle", by Harry Taylor - September 1993
        "A Personal Adventure in Home Computing: The Origin of Comet 
    Shoemaker-Levy 9", by Andrew J. LePage - March 1994 (this issue)
        About the Author -
        Michael Janes has been a serious amateur astronomer for seven
    years and currently observes with a forty-centimeter reflector. 
    Michael's observing programs include the Herschel 400 list, cata-
    clysmic variables, and the planet Mars.   In addition to being a
    member of the Saguaro Astronomy Club (SAC), Michael is also a member 
    of the American Association of Variable Star Observers (AAVSO). 
                          March 1994 - Vol. 5, No. 8
                           Copyright (c) 1994 - ASA